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61
Geological Society of AmericaSpecial Paper 423
2007
The continuum between Cadomian orogenesis and opening of the Rheic Ocean: Constraints from LA-ICP-MS U-Pb zircon dating
and analysis of plate-tectonic setting (Saxo-Thuringian zone, northeastern Bohemian Massif, Germany)
Ulf Linnemann*Staatliche Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Königsbrücker Landstrasse 159,
D-01109 Dresden, Germany
Axel GerdesInstitut für Geowissenschaften, Mineralogie, Johann Wolfgang Goethe-Universität Frankfurt am Main, Senckenberganlage 28,
D-60054 Frankfurt am Main, Germany
Kerstin DrostStaatliche Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Königsbrücker Landstrasse 159,
D-01109 Dresden, Germany
Bernd BuschmannTU Bergakademie Freiberg, Geologisches Institut, Bernhard-von-Cotta-Strasse 2, D-09599 Freiberg, Germany
This paper is dedicated to Jean-Jacques Chauvel (1935–2004)
ABSTRACT
Sediment provenances and magmatic events of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician rock complexes from the Saxo-Thuringian zone are con-strained by new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from fi ve sandstones and magmatic zircons from an ignimbrite and one tuffi te. These geochronological results in combination with the analysis of the plate-tectonic setting constrained from fi eld observations, sedimentological and geochemical data, and trends of the basin development are used to reconstruct Cadomian orogenic processes during the Late Neoproterozoic and the earliest Cambrian. A continuum between Cadomian orogenesis and the opening of the Rheic Ocean in the Cambro-Ordovician is supported by the data set.
Linnemann, U., Gerdes, A., Drost, K., Buschmann, B., 2007, The continuum between Cadomian orogenesis and opening of the Rheic Ocean: Constraints from LA-ICP-MS U-Pb zircon dating and analysis of plate-tectonic setting (Saxo-Thuringian zone, northeastern Bohemian Massif, Germany, in Linnemann, U., Nance, R.D., Kraft, P., and Zulauf, G., eds., The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision: Geological Society of America Special Paper 423, p. 61–96, doi: 10.1130/2007.2423(03). For permission to copy, contact [email protected]. ©2007 Geological Society of America. All rights reserved.
62 Linneman et al.
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INTRODUCTION
The Cadomian orogeny comprises a series of complex sedi-mentary, magmatic, and tectonometamorphic events that spanned the mid-Neoproterozoic (ca. 650 Ma) to the earliest Cambrian (ca. 540 Ma). Rock units formed by the Cadomian orogeny are commonly referred to collectively as Cadomian basement. Owing to similar contemporaneous orogenic processes in the Avalonian microplate, the collective term Avalonian-Cadomian orogeny has also been used in the modern literature. Peri-Gondwanan ter-ranes, microcontinents, and crustal units in central, western and eastern Europe and in north Africa are affected by the Cadomian orogeny. Related orogenic events known as the Avalonian orog-eny, are known from the Appalachians (eastern United States and Atlantic Canada) and from the non-Laurentian part of Ire-land and the British Isles. Baltica escaped Avalonian-Cadomian orogenic activity, although late Precambrian orogenic events of “Cadomian affi nity” have been recognized in the Urals and the Timanides on the periphery of Baltica (Roberts and Siedlecka 2002; Glasmacher et al., 2004).
The Cadomian orogeny was fi rst defi ned in the North Armorican Massif in France on the basis of the unconformity that separates deformed Precambrian rock units from their Early Paleozoic (Cambro-Ordovician) overstep sequence. In central and western Europe this unconformity is commonly referred to as the Cadomian unconformity. The youngest metasedimentary rocks affected by Cadomian deformation may be earliest Cam-brian in age, and many geologists assume that the fi nal stages of Cadomian orogenesis were spatially diachronous, lasting from the latest Neoproterozoic to the earliest Cambrian. From this viewpoint the term Cadomian basement includes Neoproterozoic (Ediacaran) to Early Cambrian sedimentary, igneous, and meta-morphic complexes, although the stratigraphic range of the rocks involved changes from region to region.
The Cadomian unconformity was fi rst described from Rocreux near Caen (Normandy) by Bunel (1835), although it is often attributed to Dufrenoy (1838), who published the fi rst
drawing. The wider geographic extent of the unconformity in central Brittany was recognized by Dufrenoy (1838) and Barrois (1899). The fi rst illustration of the unconformity from Brittany was published by Kerforne (1901).
Cadomus and Cadomum are old Latin terms for the modern city of Caen and are the source of the name of the orogeny. The term discordance cadomienne was fi rst used by Bertrand (1921). The type locality of the Cadomian unconformity is located on the northern edge of the village of Jacob Mesnil (Rocreux), close to Bretteville sur Laize near Caen (Normandy). The best illustration of the unconformity at Rocreux was published by Graindor (1957).
In some publications, the term Pan-African orogeny is used in the same sense as the Cadomian orogeny, because both events were related to the Gondwana supercontinent in the late Precam-brian and occurred at more or less the same time. The main differ-ence between the two orogenic events is their position within the confi guration of the Gondwana supercontinent in Neoproterozoic time. The crustal units affected by the Pan-African orogeny are located between the cratons that assembled Gondwana and, in most cases, refl ect continent-continent collision (see compilation of Windley, 1995). In contrast, the Cadomian orogen, or alterna-tively, the Avalonian-Cadomian orogenic belt, was a peripheral orogen at the edge of the Gondwanan supercontinent and is char-acterized by orogenic processes similar to those of the present-day Andes and Cordilleran chains of the Americas and western Pacifi c (Murphy and Nance, 1991; Nance and Murphy, 1994; Buschmann, 1995; Linnemann et al., 2000, 2004; Nance et al., 2002).
On the basis of provenance studies based on U-Pb ages of detrital zircon grains from sedimentary rocks and inherited zir-cons in igneous rocks, in combination with Nd-Sr-Pb isotope analyses and paleomagnetic and paleobiogeographic data, most geologists accept that the largest part of the Cadomian basement of central and western Europe was formed at the periphery of the West African craton of the Gondwana supercontinent (e.g., Linnemann et al., 2004; Murphy et al., 2004). Remnants of old cratonic basement are represented only by the Icartian basement
In our model, the early stage of the Cadomian evolution is characterized by a Cor-dilleran-type continental magmatic arc, which was established at the periphery of the West African craton between ca. 650 and 600 Ma. Subsequently, at ca. 590–560 Ma, a back-arc basin was formed behind the Cadomian magmatic arc. The back-arc basin was closed between ca. 545 and 540 Ma, leading to the development of a short-lived Cadomian retroarc basin. Subsequently, a mid-oceanic ridge was subducted under-neath the Cadomian orogen. Slab break-off of the subducted oceanic plate resulted in increased heat fl ow, leading to voluminous magmatic and anatectic events that culmi-nated at ca. 540 Ma. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian. This process con-tinued from the Middle to Upper Cambrian, fi nally caused the opening of the Rheic Ocean in the Lower Ordovician.
Keywords: Peri-Gondwana, Cadomian orogeny, Bohemian Massif, Saxo-Thuringian zone, Cadomia, Avalonia, Rheic Ocean, U-Pb zircon dating, provenance.
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 63
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(2.06–2.01 Ga) of the Armorican Massif, and the Svetlik (2.1–2.05 Ga) and Dobra gneisses (1.38 Ga) of the Bohemian Mas-sif. These Meso- to Paleoproterozoic gneiss complexes coupled with abundant Archean to Paleoproterozoic detrital zircon grains in Neoproterozoic sediments indicate that most of the Cadomian “basement” developed on thinned older cratonic crust and that the Neoproterozoic to Cambrian siliciclastic sediments result from eroded older basement slices.
Nance et al. (2002) proposed a Cordilleran model for the evolution of the Neoproterozoic to Cambro-Ordovician rock com-plexes in the Avalonian part of the “Avalonian-Cadomian orogenic belt,” drifted off as a separate microcontinent during the late Cam-brian. They suggested that the formation and separation of Ava-lonia was controlled by a plate-tectonic evolution similar to that presently observed at the western margin of the North American plate in Baja California. Over the past 30 Ma, this area has been affected by terrane accretion, subduction-related processes, ridge-trench collision, and rifting processes. As shown in this article, these processes may also account for the sedimentological and magmatic evolution observed in the Neoproterozoic-Paleozoic basement complexes of the Saxo-Thuringian zone, which lies at the northeastern periphery of the Bohemian Massif. Parts of this zone were less affected by the Variscan orogeny and, thus, forms an ideal area to study sedimentological and magmatic events that occurred during the Neoproterozoic and Cambro-Ordovician.
In this article, we present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb data for detrital zircon grains to constrain the provenance of selected sediments of the Saxo-Thuringian zone. In addition, data from zircons in well-defi ned igneous rocks provide new time markers for the area’s magmatic evolution. These geochronological data, in conjunction with fi eld relations and published results, are used to defi ne the plate-tectonic setting(s) of Neoproterozoic to Cam-bro-Ordovician rock units of the Saxo-Thuringian zone. Finally, the combined data set is used to present a tentative model for the evolution of the Cadomian basement of the Saxo-Thuringian zone, which starts with the formation of a marginal Cadomian orogen between ca. 650 and 540 Ma and ends with the opening of the Rheic Ocean in the Cambro-Ordovician.
GEOLOGIC SETTING
Bohemian Massif
The Bohemian Massif forms the central part of the European Variscides and is subdivided into two principal zones, the Saxo-Thuringian zone to the north and Moldanubian zone to the south (Kossmat, 1927). In addition, the marginal Moravo-Silesian zone rims the Bohemian Massif in its eastern part. To the northwest, the massif is bordered by the Mid-German Crystalline zone, which is assumed to represents an important Variscan suture zone, perhaps the Rheic suture (Kroner et al., 2003, this volume; Zeh and Wunderlich, 2003; Zeh et al., 2003, 2005; Linnemann et al., 2004). The latter was closed by oblique collision between the
Saxo-Thuringian zone (Cadomia) and the Rhenohercynian zone (East Avalonia) during the Late Devonian to Early Carbonifer-ous (Oncken, 1997; Kroner et al., 2003, this volume; Zeh et al., 2003, 2005). To the south and southeast the Bohemian Massif is overthrust by Meso-and Cenozoic rocks of the Alps and the Car-pathians. Additional components of the Bohemian Massif are the Teplá-Barrandian unit and the Moravo-Silesian zone. The oldest units of the Bohemian Massif are remnants of Paleo- to Meso-proterozoic cratonic basement slivers, such as the Dobra gneiss (1.38 Ga) and the Svetlik gneiss (2.1–2.05 Ga).
The Bohemian Massif is the most prominent inlier of basement rocks in central Europe (Fig. 1) and records a com-plex Neoproterozoic to Paleozoic history that includes the Cadomian and Variscan orogenies. Some rock units (e.g., the Erzgebirge Mountains) locally experienced ultra-high pressure metamorphic conditions during the Variscan orogeny with the formation of diamond-bearing rocks (Massonne, 1998). Most marginal rock units and inliers, however, were essentially less affected by the Variscan tectonometamorphic overprint. These rock units comprise Neoproterozoic to Paleozoic successions at the northern margin of the Saxo-Thuringian zone and the Teplá-Barrandian unit (Fig. 1).
The Bohemian Massif has been traditionally interpreted to be part of the Armorica microplate as defi ned by Van der Voo (1979). However, more recent studies (Tait et al., 1997; McKer-row et al., 2000) have assumed that the Armorican microplate was not a coherent block, but comprises several units. Thus, Tait et al. (1997) suggested the term Armorican terrane assemblage, which includes Neoproterozoic and Paleozoic basement units exposed in northern and southern France and in central Europe.
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Figure 1. Location of prominent basement rocks, sutures, continents, and terranes pertinent to this study. AM—Armorican Massif; BM—Bo-hemian Massif; FCM—French Massif Central; M—Moravo-Silesian zone; MZ—Moldanubian zone; S—Sudetes; SXZ—Saxo-Thuringian zone; TBU—Teplá-Barrandian unit.
64 Linneman et al.
spe423-03 page 64
For the late Neoproterozoic to Cambro-Ordovician base-ment rocks relevant to Cadomian and post-Cadomian orogenic processes, we use the term Cadomia (sensu Nance and Murphy, 1996), which comprises Cadomian basement rocks (with interca-lated older cratonic blocks) of the Armorican Massif, the French Massif Central, and the Bohemian Massif with the exception of the Brunovistulian of the Moravo-Silesian zone (Fig. 1).
Based on new geochronological results, the Cadomian base-ment of the Bohemian Massif can be subdivided into Avalonian-type and Cadomian-type units sensu Murphy et al. (2004). Ava-lonian-type units contain detrital zircon grains of Mesoprotero-zoic age, which are assumed to have formed in a juvenile crust between 1.3 and 1.0 Ga. In contrast, Cadomian-type rock units show few or no zircons with ages in the range 1.7–.75 Ga and are dominated by detritus and inherited zircons derived from the West African craton (2.05 Ga and older; Murphy et al. 2004).
To date, all U-Pb zircon provenance studies indicate that the Neoproterozoic to Paleozoic sediments of the Saxo-Thuringian zone, the Teplá-Barrandian unit, and the Moldanubian zone have a west African provenance (Linnemann et al. 2000, 2004; Gehm-lich, 2003; Tichomirowa, 2003; Drost et al., 2004). Thus, they belong to Cadomia sensu Murphy et al. (2004). However, proto-lith ages of ca. 2.1 Ma indicate that the Svetlik granite gneiss of the Bohemian Massif was emplaced during the Paleoproterozoic (Wendt et al., 1993, 1994). From its geological position it seems likely that this gneiss represents part of the Eburnian basement derived from the West African craton.
However, Finger et al. (2000) demonstrated that the Bru-novistulian unit in the Moravo-Silesian zone of the Bohemian Massif (Fig. 1) shows strong affi nities with Avalonia. Rocks from the Brunovistulian unit are assumed to have been derived from the recycled margin of the Amazonian craton, as suggested by U-Pb ages of inherited zircon grains (Friedl et al., 2000). The 1.38-Ma Dobra gneiss is assumed to represent a cratonic inlier that also belongs to the Avalonian part of the Bohemian Massif (Gebauer and Friedl, 1994; Friedl et al., 2004). The available ages suggest that the Bohemian Massif is divided by a suture (likely the Rheic suture) into Avalonian and Cadomian parts (Fig. 1). The Cadomian part comprises the Saxo-Thuringian and Moldanubian zones, the Teplá-Barrandian unit, and the Sudetes, whereas the Avalonian part includes the Brunovis-tulian unit of the Moravo-Silesian zone. The former “Rheic” suture is probably hidden under and/or incorporated into the Variscan fault-and-thrust belt between the Moravo-Silesian and the Moldanubian zones (Fig. 1).
Saxo-Thuringian Zone
The Saxo-Thuringian zone forms the northeastern part of the Bohemian Massif. It consists of Cadomian basement units, which are overlain by a Paleozoic overstep sequence (Fig. 2). The parau-tochthonous part of the Saxo-Thuringian zone forms a northeast–east-trending fold-and-thrust belt, which consists of the Schwarz-burg antiform, the North Saxon antiform, the Berga antiform, and
the Lausitz antiform, and the Torgau-Doberlug and Ziegenrück-Teuschnitz synclines. In addition, the Saxo-Thuringian zone is transected by the northwest–southeast-trending Elbe zone and the Franconian line (Fig. 2). In this study we use the neutral word antiform instead of the traditional term anticline, because none of the tectonostratigraphic units are typical anticlines. For example, the Lausitz antiform is a tilted horst block.
The Saxo-Thuringian zone in this article is subdivided into an internal and external domain, which show signifi cant differences with respect to their Cadomian basement evolu-tion and Paleozoic overstep sequences. The external domain is composed of the Cadomian volcanosedimentary units of the Rothstein Formation in the Torgau-Doberlug syncline and the Altenfeld Formation in the northwestern part of the Schwarz-burg antiform (Figs. 2 and 3). Both are characterized by rock units containing thick layers of massive black chert (Fig. 4A) and are assumed to have originated in a back-arc setting (Bus-chmann, 1995; Linnemann et al., 2000). These sediments are dominated by dark-gray to black distal turbidites composed of an intercalation of graywacke and mudstone bedsets. All known sedimentological and geochemical data point to an origin for the Rothstein Formation in the center of a back-arc basin devel-oped on thinned continental crust (Buschmann, 1995). Owing to its similar spatial position in the Saxo-Thuringian zone and its similarity in lithology and geochemistry, we assign the Altenfeld Formation to the same plate-tectonic setting. Zircon data suggest deposition of the Rothstein and Altenfeld forma-tions at ca. 570–565 Ma (Linnemann et al., 2000; Buschmann et al., 2001).
The Rothstein Formation is overlain by Lower to Middle Cambrian sediments (Fig. 5), whereas the Altenfeld Formation is covered by Lower Ordovician siliciclastics (Fig. 6). In contrast to the internal domain, ca. 540-Ma magmatism in the external domain is very scarce. Only a single small pre-Variscan granitoid body (the Milchberg granite) crops out in the northwestern part of the Schwarzburg antiform, where it intrudes the Altenfeld For-mation. Recent U-Pb zircon datings place that granite to the base of the Ordovician (ca. 490 Ma; U. Linnemann and A. Gerdes, unpublished data). Another important component of the external domain is the Vesser complex of Middle to Upper Cambrian age. This unique complex is characterized by rocks related to the for-mation of the oceanic crust (Bankwitz et al., 1992; Kemnitz et al., 2002). The relationship between the external domain and the Mid-German Crystalline zone to the north is unclear because of coverage by Cenozoic sediments. The bounding element of the internal domain is the Blumenau Shear Zone, which divides the Schwarzburg antiform into a northwestern and southeastern part. In our view, the Blumenau Shear Zone continues to the southern border of the Torgau-Doberlug syncline, which is also covered by Cenozoic deposits. The shear zone is a structural feature that likely originated in the Cadomian orogeny during the tectonic change from a back-arc basin to a retroarc basin setting (see below). During the Variscan orogeny, the Blumenau Shear Zone was reactivated as a sinistral shear zone (Heuse et al., 2001).
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 65
spe423-03 page 65
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its
Sur
face
outc
rops
ofth
eM
id-G
erm
ancr
ysta
lline
zone
Nor
ther
nph
yllit
ezo
ne(C
ambr
o-O
rdov
icia
nro
ckco
mpl
ex)
Ves
ser
com
plex
(Upp
erC
ambr
ian)
Sou
ther
nph
yllit
ezo
ne(C
ambr
o-O
rdov
icia
nro
ckco
mpl
ex)
Sur
face
outc
rops
Low
tohi
ghgr
ade
orth
o-an
dpa
ra-r
ocks
Ord
ovic
ian,
Silu
rian
and
Dev
onia
nvo
lcan
o-se
dim
enta
ryro
ckco
mpl
exes
Cam
bro-
Ord
ovic
ian
toLo
wer
Car
boni
fero
usvo
lcan
o-se
dim
enta
ryro
ckco
mpl
exes
Var
isca
nan
dp
ost
-V
aris
can
ign
eou
sro
cks
(ca.
330
-30
0M
a)
Cad
om
ian
ign
eou
sro
cks
(Lo
wer
Cam
bri
an,
c.54
0-
530
Ma)
Ord
ovi
cian
gra
nit
oid
s(c
a.49
0M
a)
Laus
itzan
atex
ite
Lstgrio auizantidome cplx
Wes
tern
Laus
itzgr
anito
ids
Eas
tern
Laus
itzgr
anito
ids
Gra
nito
ids
ofth
eE
lbe
zone
(Doh
na&
Laas
gran
odio
rites
)
Gra
nito
ids
ofth
eLe
ipzi
gar
ea
Rum
burk
gran
ite(L
ausi
tzan
tifor
m)
Tour
mal
ine
gran
ite(E
lbe
zone
)
Gra
nito
ids
ofth
eS
chw
arzb
urg
antif
orm
shea
rzo
ne-r
elat
edor
thog
neis
ses
(Gro
ssen
hain
gnei
ss)
Per
mo-
Car
boni
fero
usgr
anito
ids
Upp
erC
arbo
nife
rous
rhyo
litoi
ds
Upp
erC
arbo
nife
rous
maj
orgr
anito
iddy
kes
oe
Zn
Zone
Zo
ne
12
BC
7
8
6 A
D
E
GH
4
5
3
Dre
sden
Lausit
z
H
H
(Geo
log
ical
map
wit
ho
ut
any
stra
ta y
ou
ng
er t
han
Lo
wer
Car
bo
nif
ero
us)P
raha
Cad
om
ia
i
(
on
sv
l
t
a
Ea)
aA
ure
utS
eci
Rh
Gerany m
ltB
aic
a
map
Ro
thst
ein
Kam
enz
Lei
pzi
g
oC
dia
an
m ms
iva
gin
pa
ser
F
a
b
c
d
e
Figu
re 2
. Geo
logi
cal
map
of
the
Saxo
-Thu
ring
ian
zone
in
the
nort
heas
tern
par
t of
the
Boh
emia
n M
assi
f, s
how
ing
units
of
Low
er C
arbo
nife
rous
and
old
er a
ges
and
the
dist
ribu
tion
of r
ocks
th
at r
epre
sent
the
diff
eren
t sta
ges
of C
adom
ian
basi
n de
velo
pmen
t (m
odifi
ed f
rom
Lin
nem
ann
and
Scha
uer,
1999
; Lin
nem
ann
and
Rom
er, 2
002)
. Tec
tono
stra
tigra
phic
uni
ts: 1
—Sc
hwar
zbur
g an
tifor
m (
sout
heas
tern
par
t);
2—Sc
hwar
zbur
g an
tifor
m (
nort
hwes
tern
par
t);
3—B
erga
ant
ifor
m;
4—N
orth
Sax
on a
ntif
orm
(L
eipz
ig a
rea)
; 5—
Nor
th S
axon
ant
ifor
m (
Cla
nzsc
hwitz
are
a);
6—To
rgau
-Dob
erlu
g sy
nclin
e; 7
—E
lbe
zone
; 8—
Lau
sitz
ant
ifor
m. N
eopr
oter
ozoi
c vo
lcan
o-se
dim
enta
ry c
ompl
exes
: A—
Rot
hste
in F
orm
atio
n; B
—A
ltenf
eld
Form
atio
n; C
—Fr
ohnb
erg
For-
mat
ion;
D—
Cla
nzsc
hwitz
Gro
up; E
—R
öder
n G
roup
; F—
Wee
sens
tein
Gro
up; G
—L
eipz
ig F
orm
atio
n; H
—L
ausi
tz G
roup
. Sam
ple
loca
tions
(sm
all t
ype
in s
tars
): a
—sa
mpl
e Pu
r-1
(Pur
purb
erg
quar
tzite
of
the
Wee
sens
tein
Gro
up, N
eopr
oter
ozoi
c);
b—W
ett-
1 (m
icro
cong
lom
erat
e of
the
Lau
sitz
Gro
up, N
eopr
oter
ozoi
c);
c—R
oth-
1–16
41H
/18
(gra
ywac
ke o
f th
e R
oths
tein
For
mat
ion,
N
eopr
oter
ozoi
c); d
—K
am-1
–120
9/1
(san
dsto
ne o
f th
e Z
wet
hau
Form
atio
n, L
ower
Cam
bria
n); e
—L
bq-1
(m
icro
cong
lom
erat
e, L
ange
r B
erg
Form
atio
n, T
rem
adoc
, Low
er O
rdov
icia
n).
66 Linneman et al.
spe423-03 page 66
488 Ma
Neo
pro
tero
zoic
(Ed
iaca
ran
)C
amb
rian
Low
erO
rdov
icia
n
datedsubvolcanicintrusion
dated detritalzircon
datedgranitoidpebble
dated tuff
LeipzigFormation
Schildaumassif
NorthSaxon
antiform(Leipzig)
530+/-8
LausitzGroup
Lausitzgranitoidcomplex
Lausitzantiform
Rumburkgranite
noOrdoviciansediments
490+/-3
539+/-6
RothsteinFormation
Torgau-Doberlugsyncline
566+/-10
589+/-9
AltenfeldFormation
Milchberggranite
Vessercomplex
Sc r -hwa zurb g
nti o ma f r(NW t)-Par
Dohnagranodiorite
Elbezone
Tourmalinegranite
FrohnbergFormation
Glasbach& Laubachgranites
w z urSc a b gh ra i ont f rm
Pa )(SE- rt
Blambachrhyolite
479+
/-2
487+/-6
533+/-4541+/-7
605+/-4
483+/-3
569+/-2
570+/-4
629+/-4
485+/-6
486+
/-4
537+/-7
Laasgranodiorite
NorthSaxon
antiform(Clanzschwitz)
531+/-7
Cadomianbackarc basin (N)
Cadomianbackarc basin (S)
External domain
Parautochthonous part of the Saxo-Thuringian zoneInternal domain
Cadomian retroarc basin
Strike-slipand spreadingzones in the
backarc basinon thinned
continental crust
WeesensteinGroup
568+/-4
ClanzschwitzGroup
577+/-3
Passive margin of the backarc basin on
thinned cratoniccrust and remnants
of an older Cadomian magmatic arc
Upper section (~100m):
remnant basinof the retroarc
against thepassive margin
& cratonic crust
1
1
44
22
2 2
2
2
2 2
2
2
2
3
222
2
2
2
L
MU
109521 3 4 6 7 8
502+/-2
Cd
ge
rc
ao
mia
nO
ron
yse
nsu
sti
to(
fe
he
/d
eo
rmat
ion
clo
sto
tP
rec.
Cam
b.-
un
yg
ai
nc
aB
od
ar,m
am
tc
eve
tat
.540
M)
Cad
om
ian
bas
emen
tse
nu
stri
cto
542 Ma
relativepaleo-position:
North
relativepaleo-position:
South
Pur-1
Wett-1
Roth-1577+/-10
555+/-9551+/-8
?
Proximate part ofthe retroarc basin
against the magmatic arc
Lower section:part of the distalretroarc basin
against the magmatic arc
*
*
Figure 3. Generalized lithostratigraphic profi les of parautochthonous units of the Saxo-Thuringian zone, with published geochronological data of the Cadomian basement and its Cambro-Ordovician overstep sequence. Circles designated “Roth-1”, “Wett-1” and “Pur-1” indicate position of samples studied in this article. 1—Cambro-Ordovician rift-related igneous rocks; 2—Cadomian granitoids of the ca. 540-Ma magmatic event; 3—Lower Ordovician siliciclastic sediments; 4—Late Neoproterozoic debris fl ows and glaciomarine tillites; 5—igneous rocks and metasedi-ments of the Upper Cambrian Vesser complex (predominantly mafi c rocks); 6—Neoproterozoic hydrothermal black cherts; 7—Lower to Middle Cambrian sediments; 8—conglomerates, quartzites, and quartzitic shales of the Purpurberg quartzite (Weesenstein Group) and its equivalent in the Clanzschwitz Group; 9—graywackes and mudstones; 10—predominantly mudstones. Sources of geochronological data (numbered circles): 1—SHRIMP U-Pb (Buschmann et al., 2001); 2—thermal ionization mass spectrometer (TIMS) Pb-Pb (Linnemann et al., 2000); 3—TIMS U-Pb (Kemnitz et al., 2002); 4—SHRIMP U-Pb (Linnemann et al., 2004).
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 67
spe423-03 page 67
B
D
E
C
A
Figure 4. Photographs of Cadomian basement rocks of the Saxo-Thuringian zone (Bohemian Massif). (A) Bedded black cherts of the “Rothstein Rock” near Bad Liebenwerda, which are interpreted to have formed close to the spreading center of the Cadomian back-arc basin (Neoproterozoic, ca. 570 Ma, Rothstein Formation, Cadomian basement of the Torgau-Doberlug syncline). (B) Steeply dipping bed of the Purpurberg quartzite with internal cross bedding and impressions of wave ripples (left freestanding bedding plane of block above hammer). The quartzite occurs in the lower part of the Weesenstein Group, which is interpreted to represent the passive margin of the Cadomian backarc basin (Neoproterozoic, ca. 570 Ma, Weesenstein Group, Purpurberg near Niederseidewitz, Cadomian basement of the Elbe zone). (C) Stretched granitoid pebble in a mudstone matrix derived from an eroded magmatic arc during the formation of the Cadomian back-arc basin (upper part of the Weesenstein Group). The pebbly mudstone facies of the Weesenstein Group is interpreted to be in part glaciomarine (Neoproterozoic, ca. 570 Ma, Weesenstein Group, 100 m east of the Weesenstein railway station, Cadomian basement of the Elbe zone). (D) Microconglomerate of the Lausitz Group containing clasts of cherts and felsic rocks. This sediment demonstrates the redeposition of black cherts and arc volcanics during the formation of the Cadomian retroarc basin (Neoproterozoic, ca. 570–545 Ma, Lausitz group, Petershain near Kamenz, Cadomian basement of the Lausitz anticline). (E) Water escape structure at top of the a-interval of a graywacke turbidite of the Lausitz Group, demonstrating rapid sedimentation in the Cadomian retroarc basin (Neopro-terozoic, ca. 570–545 Ma, Lausitz Group, Wetterberg quarry near Ebersbach, Cadomian basement of the Lausitz anticline).
68 Linneman et al.
spe423-03 page 68
The internal domain of the Saxo-Thuringian zone contains Cadomian rock units from two different depositional settings (Fig. 3). Voluminous Cadomian plutons intruded at ca. 540–530 Ma, and a thick and widely distributed Ordovician overstep sequence (Fig. 6) also distinguish the internal domain. Cambrian deposits are restricted to the Heinersdorf 1 and 2 drill holes in the Berga antiform and to large olistolithes in a Lower Carboniferous wild fl ysch matrix in the Görlitz synform.
The fi rst group of Neoproterozoic sedimentary units in the internal domain comprises passive margin sequences character-ized by highly mature quartzites, sandstones, and quartz-rich shales deposited in a shallow marine environment (Linnemann, 1991). The most prominent deposit of this type is represented by the Purpurberg quartzite in the lower part of the Weesenstein Group (Fig. 4B). Facies analysis of the Purpurberg quartzite has shown that its deposition was caused by an extreme drop of sea level that is interpreted to be of glacioeustatic origin (Linnemann, 1991). The stratigraphic equivalent of the Purpurberg quartzite also occurs in the Clanzschwitz Group (North Saxon antiform). Other parts of the Weesenstein and Clanzschwitz groups are like-wise passive margin deposits but comprise quartz-rich mud- and siltstones. In the upper part of the two groups diamictites and lay-ers with isolated pebbles (Fig. 4C) may be glaciomarine in origin (Linnemann and Romer, 2002). These passive margin deposits are situated in the North Saxon antiform and the Elbe zone (Fig. 2). Based on the spatial arrangement of the passive margin deposits in the Saxo-Thuringian zone, we assign these units to the pas-sive margin of the same Cadomian back-arc basin in which the Rothstein and the Altenfeld formations were deposited. Detrital zircons and dated pebbles point to a minimum age of sedimenta-tion of ca. 570 Ma for the passive margin units (Linnemann et al., 2000; Fig. 3).
The second group of Neoproterozoic sedimentary units in the internal domain is represented by the Lausitz Group (Lausitz antiform), the Leipzig Formation (Northsaxon antiform), and the Frohnberg Formation (southeastern part of the Schwarzburg anti-form). All three units are characterized by monotonous, fl ysch-like sections of proximal to distal dark-gray to black colored tur-bidites composed of graywacke and mudstone couplets (Fig. 4E). Seismites indicate an active tectonic setting during basin forma-tion. Intercalations of conglomerates contain fragments of black cherts (Fig. 4D) and other debris from older Neoproterozoic sedi-ments and igneous rocks. The pervasive occurrence of fragments of black chert in both the graywackes and the conglomerates suggests that deposits from the Cadomian back-arc basin of the external zone became eroded, recycled, and redeposited in the Cadomian retroarc basin, remnants of which are represented by the Lausitz Group and the Leipzig and the Frohnberg formations. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dat-ing of detrital zircon grains in the Lausitz Group and the Frohn-berg Formation indicates that they are younger than 555 ± 9 Ma and 551 ± 8 Ma, respectively (Linnemann et al., 2004). Because of the occurrence of debris derived from the back-arc basin, the presence of distinct sedimentary features (see below), and their
?
?
?
?
?
?
?
?
?
?
siliciclasticdebris flows
limestone
no record
dolostone
claystone mafic volcanics
siltstone
sandstone
Zwet
hau
Form
atio
n
Low
erC
ambr
ian
Mid
dle
Cam
bria
nTr
öbitz
Form
atio
nD
elitz
sch
Form
atio
n
Torg
auM
embe
r(>
500
m)
Ros
enfe
ldM
br.
(>30
0m
)D
I(>
126
m)
LS1/
63I(
>57
5m
) DIV
(>41
m)
Kam-1
Figure 5. Generalized lithostratigraphic profi le of Cambrian sediments of the Torgau-Doberlug syncline with stratigraphic position of sample Kam-1–1209/1. The Lower Cambrian profi le is documented in numer-ous drill cores, whereas the Middle Cambrian profi le is derived from reference profi les (boreholes D I, LS 1/63, D IV). The Lower to Middle Cambrian sediments in the column overlie the Rothstein Formation (Neoproterozoic). Mbr—member. From Buschmann et al. (2006).
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 69
spe423-03 page 69
00
10001000
2000
3000
(m)
m
UntererFrauen-bach-quartzite
Frauen-bach-Wechsel-lagerung
ObererFrauen-bach-quartzite
Dach-schieferFm.
Go
ldis
-th
alF
m.
hi
erat
Ph
yco
den
scef
Fo
rmio
nt
.hy
code
nqua
rzi
FmP Qua
rzit-
plat
ten
Mbr
.
Bre
iten-
berg
Mbr
.
Qu
arzi
tban
kM
emb
er
Griffel-schieferFm.
Leder-schieferFm.
gap and/orcondensed
sedimentation
Schmie-defeld Fm.
rau
enG
r.F
bac
hc
eu
ho
sG
pP
yd
rore
ac
Tm
do
ent
alG
.G
räf
hi
shA
gll
Aren
ig
Conglomeratic tuffite(”Konglomeratische Arkose”)and yellow tuffites
topmost Neoproteroz. QuartziteCadomian basement:Cadomian basement:
Frohnberg Group (Neoprot.)Altenfeld Group (Neoproterozoic)black cherts
Glasbach granite (538+/-4 Ma*1)Milchberg granite
Lower Magnetite quartzite
Upper Magnetite quartzite
Lower ore horizon
Phycodes quartzite
Phycodes shale
Lower Frauenbach quartzite
interbedding of shaleand quartzite beds
Upper Frauenbach quartzite
“Phycodes Dachschiefer”
Upper ore horizon“Kalkbank” (limestone layer)Banded Lederschiefer
glaciomarine diamictiteof the Sahara glaciation= Lederschiefer
gapgapgap
no outcrop
gap
Bärentiegel porphyroid(479 +/-5 Ma*1)
Middle ore horizon
Blambach rhyolite (487+/-5 Ma*1)
Dacitic pyroclastite508+/-2 Ma *2
Gabbro502+/-2 Ma *2
Ros
en-
berg
Mbr
.G
öri
tzb
erg
Mem
ber
Lau
sch
enst
ein
Mem
ber
i sG ller dorf Fm.
Neu
wer
kF
m.
Volc
anic
un
itQ
uar
tzit
eu
nit
Ro
llko
pf
Fo
r mat
i on
Hu
nd
srü
ckG
.V
e ss e
rG
rou
p
low
e ru
pp
er
Cadomianbasement
Ord
ovi
cian
Trem
ado
cU
pp
erC
amb
rian
Mid
dle
Cam
br .
dark shales
KArc-1
1
2
4
6
7
10
11
12
9
8
5
3
Lbq-1
Ves-1
?
La er Be .ng rg Fm
Ves
ser
com
ple
x
NW-part of theSchwarzburg
antiformVesser
complex
SE-part of the Schwarzburg antiform
Figure 6. Lithostratigraphic profi les of the Middle and Upper Cambrian and Ordovician rocks of the Vesser complex and the northwestern and southeastern parts of the Schwarzburg antiform. Ellipses Ves-1, Lbq-1, and KArc-1 indicate approximate position of samples studied in this article. 1—Upper Cambrian Rollkopf Formation of the Vesser complex: predominantly mafi c subvolcanic rocks (tholeiitic dolerites and gabbros) and minor subalkaline basalts, dacitic tuffs, rhyolitic ignimbrites, granites, and graphitic metasediments; 2—Upper Cambrian Neu-werk Formation of the Vesser complex: interbedded tholeiitic basaltic, dacitic to trachyandesitic lavas and intermediate to rhyolitic pyroclastics and metasediments; 3—Tremadocian volcanic unit of the Hundsrück Group in the Vesser complex: rhyolitic pyroclastic rocks, and arkoses; 4—rhyolites and porphyroids; 5—Cadomian granites; 6—Neoproterozoic sediments (predominantly graywacke turbidites); 7—conglomerates, microconglomerates, and conglomeratic tuffi tes; 8—sandstones and quartzites; 9—mudstones and silty shales; 10—shales; 11—diamictite (gla-ciomarine tillite?); 12—sedimentary iron ores. Fm—formation; G—group; Mbr—member. Sources of geochronological data: *1—TIMS Pb-Pb (Linnemann et al., 2000); *2—TIMS U-Pb (Kemnitz et al., 2002). Modifi ed after Bankwitz et al. (1992), Linnemann (1996), Linnemann and Heuse (2000), and Kemnitz et al. (2002).
70 Linneman et al.
spe423-03 page 70
maximum ages of sedimentation between ca. 555 and 551 Ma, we classify the Lausitz Group and the Leipzig and Frohnberg formations as relicts of a Cadomian retroarc basin.
In general, all Neoproterozoic sections within the Cado-mian basement of the Saxo-Thuringian zone seem to be rootless because of Variscan stacking of the crust, and an underlying cra-tonic basement is not known. However, Neodymium depleted-mantle (NdT
DM) model ages for the Late Neoproterozoic sedi-
ments range between 1.9 and 1.3 Ga (Linnemann and Romer, 2002), clearly indicating that the source area of the Neoprotero-zoic sediments was dominated by old cratonic crust.
With exception of the Rothstein Formation and maybe the Altenfeld Formation, all Neoproterozoic sedimentary sequences within the Cadomian basement in the Saxo-Thuringian zone were intruded by Early Cambrian post-kinematic granitoid plu-tons in the interval ca. 540–530 Ma (Linnemann et al. 2000; Gehmlich 2003; Tichomirowa, 2003). These plutonic suites are composed of granites, syeno- and monzogranites, granodiorites, and tonalites (Hammer, 1996), whereas granodiorites dominate in most plutons.
The Cadomian basement of the Saxo-Thuringian zone is overlain, usually unconformably, by Lower Paleozoic sedi-ments. Transgression and the development of Lower to Middle Cambrian overstep sequences—including the deposition of con-glomerates, carbonates, siliciclastics, and red beds, with a dep-ositional gap in the lowermost Cambrian (ca. 540–530 Ma)— characterize the fi rst post-Cadomian sedimentary sequence. A second widely distributed gap in sedimentation occurred in the Upper Cambrian (ca. 500–490 Ma), although the Vesser com-plex is composed of mid- to Upper Cambrian magmatic rocks and metasediments related to an oceanic setting (Bankwitz et al., 1992; Kemnitz et al., 2002).
Special features—such as the occurrence of a Cadomian unconformity; peri-Gondwanan Cambro-Ordovician faunas; gla-ciomarine diamictites of the Hirnantian glaciation in the upper-most Ordovician; and the absence of any Salinic, Acadian, and Caledonian orogenic infl uences—paleogeographically link the Saxo-Thuringian zone to Gondwana in the Neoproterozoic and Lower Paleozoic (Linnemann et al., 2000, 2004).
SAMPLES AND METHODS
For provenance studies, detrital zircons were collected from three Neoproterozoic siliciclastic sedimentary rock units, which were deposited in three distinct settings of Cadomian basin devel-opment. Sample Pur-1 was taken from the Purpurberg quartzite of the Weesenstein Group in the Elbe zone. This sediment was deposited in a passive continental margin setting of the Cado-mian back-arc basin distal from the arc. Sample Roth-1 is a gray-wacke of the Rothstein Formation in the Torgau-Doberlug syn-cline taken from the drill hole WisBaW 1641H/80 near the city of Herzberg. Sediments of the Rothstein Formation were deposited in the Cadomian back-arc basin proximal to the arc on the oppo-site side to that of the passive margin.
The third Neoproterozoic sample is a chert-bearing micro-conglomerate (Wett-1) of the Lausitz Group from the Lausitz antiform. This sample was collected from the Wetterberg quarry near the village of Ebersbach. The Lausitz Group is dominated by graywacke turbidites with intercalations of microconglomer-ates deposited in the Cadomian retroarc basin or foreland basin.
In addition, a Lower Cambrian sandstone (Kam-1) and an Ordovician microconglomerate (Lbq-1) were sampled from the Saxo-Thuringian zone. These samples represent Cambro-Ordo-vician shelf sediments, which overlie the Cadomian basement. Kam-1 was taken from a drill core of the Zwethau Formation in the Torgau-Doberlug syncline. The sample was collected from drill hole WisBaW 1209/78 near the village of Falkenberg and is representative of the Lower Cambrian overstep sequence overly-ing the deformed Cadomian sediments of the Rothstein Forma-tion. The Lower to Middle Cambrian sediments of this formation were deposited in an asymmetric rift basin. Lbq-1 is a Lower Ordovician microconglomerate sampled from the Langer Berg Formation close to the village of Willmersdorf in the northwest-ern part of the Schwarzburg antiform. The Langer Berg Formation is a section of highly mature quartzites and conglomerates typi-cal of the widely distributed Lower Ordovician shallow marine sedimentation of the Gondwanan realm. The Lower Ordovician overstep sequence in the Saxo-Thuringian zone was deposited in a rifted shelf basin in a passive margin setting.
To set additional lithostratigraphic time markers for the Cam-brian and Ordovician sedimentation, an Upper Cambrian ignim-brite (Ves-1) and a Lower Ordovician tuffi te (KArc-1) were sam-pled. Sample Ves-1 was taken from a rhyolitic ignimbrite from the Vesser complex (Fig. 6). Sample KArc-1, a pebble-bearing rhyolitic tuffi te, was collected in the valley of the Blambach close to Sitzendorf, from the base of the >3000-m-thick Ordovician sedimentary succession exposed in the southeastern part of the Schwarzburg antiform. This pyroclastic sediment is referred to in traditional German literature as “Konglomeratische Arkose” (=conglomeratic arkose). Additional information concerning the lithostratigraphy and coordinates of the sample locations is given in Tables 1, 2, and 3.
Zircon concentrates were separated at the Museum für Mineralogie und Geologie (Staatliche Naturhistorische Samm-lungen Dresden). Fresh samples were crushed in a jaw crusher and sieved for the fraction 63–400 μm. Density separation of this fraction by a heavy liquid was realized using sodium heteropoly-tungstate in water (“LST fast fl oat”) and followed by magnetic separation of the extracted heavy minerals in a Frantz isodynamic separator. Final selection of the zircon grains for U-Pb dating was achieved by hand-picking under a binocular microscope. Zircon grains of all grain sizes and morphological types were selected, mounted in resin blocks, and polished to half their thickness.
Zircons were analyzed for U, Th, and Pb isotopes by LA-ICP-MS techniques at the Institute of Geosciences, Johann Wolf-gang Goethe-University Frankfurt, using a Thermo-Finnigan Ele-ment II™ sector fi eld ICP-MS coupled to a New Wave™ UP-213 ultraviolet laser system. A teardrop-shaped, low-volume laser cell
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 71
spe423-03 page 71
TAB
LE 1
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
NE
OP
RO
TE
RO
ZO
IC Q
UA
RZ
ITE
S O
F T
HE
ELB
EZ
ON
E,
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ %R
ho**
207 P
b23
5 U±
2σ(M
a)
206 P
b23
8 U±
2σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Pur
-1 (
Loca
tion:
Pur
purb
erg
quar
tzite
, Neo
prot
eroz
oic,
Edi
caria
n, W
eese
ntei
n gr
oup,
Elb
e zo
ne, P
urpu
rber
g ne
ar O
bers
eide
witz
, Eas
ting:
42
3117
, Nor
thin
g: 5
6 40
750)
P-L
10-1
5820
160
640.
310.
3770
0.9
6.96
61.
40.
1340
1.1
0.62
2107
2520
6231
2151
3996
P-L
10-2
2614
868
128
40.
440.
3820
0.8
6.77
51.
00.
1287
0.6
0.81
2083
1720
8528
2080
2010
0P
-L10
-363
2420
699
1.85
0.30
911.
05.
406
1.3
0.12
680.
90.
7118
8623
1736
2920
5434
85P
-L10
-410
2423
125
0.72
0.09
570.
90.
7893
2.2
0.05
982.
00.
4159
120
589
1059
888
99P
-L10
-537
2396
461.
150.
3739
0.9
6.43
31.
60.
1248
1.2
0.61
2037
2720
4733
2026
4410
1P
-L10
-618
901
857
197
0.13
0.22
811.
03.
704
1.1
0.11
780.
60.
8515
7218
1325
2319
2221
69P
-L10
-710
3373
141.
400.
1293
1.3
2.30
53.
20.
1293
2.9
0.42
1214
4678
420
2088
103
38P
-L10
-895
873
101.
070.
1167
1.0
1.04
12.
70.
0647
2.5
0.38
725
2971
214
765
107
93P
-L10
-918
098
145
101
0.26
0.60
111.
022
.33
1.3
0.26
940.
90.
7331
9826
3034
4733
0229
92P
-L10
-10
3941
840
519
40.
470.
3566
1.0
13.3
81.
70.
2721
1.3
0.62
2707
3219
6635
3318
4159
P-L
10-1
111
1424
825
0.58
0.09
251.
00.
7705
2.1
0.06
041.
80.
4958
018
571
1161
778
92P
-L10
-12
9860
182
211.
150.
0925
1.1
0.74
861.
70.
0587
1.4
0.62
567
1557
012
555
6010
3P
-L10
-13
1504
922
214
01.
010.
4907
0.9
11.8
51.
10.
1751
0.7
0.80
2592
2125
7439
2607
2399
P-L
10-1
417
3917
017
0.67
0.09
241.
00.
7566
2.1
0.05
941.
90.
4757
219
570
1158
181
98P
-L10
-15
1010
668
0.93
0.09
811.
20.
8093
2.4
0.05
982.
10.
5160
222
603
1459
791
101
P-L
10-1
627
258
174
141
0.51
0.64
810.
826
.72
1.0
0.29
910.
70.
7833
7320
3221
4134
6520
93P
-L10
-17
4213
6035
0.62
0.49
181.
112
.25
1.6
0.18
071.
20.
6626
2430
2578
4526
5940
97P
-L10
-18
1704
2411
0.92
0.40
041.
77.
900
2.5
0.14
311.
80.
6822
2046
2171
6422
6564
96P
-L10
-19
5850
162
670.
680.
3513
0.9
6.09
81.
40.
1259
1.1
0.61
1990
2519
4129
2041
4095
P-L
10-2
019
3425
928
0.86
0.09
260.
80.
7673
2.1
0.06
011.
90.
3857
819
571
960
884
94P
-L10
-21
5516
9747
0.98
0.34
090.
85.
642
2.7
0.12
002.
60.
2919
2247
1891
2619
5791
97P
-L10
-22
4759
6244
1.38
0.50
831.
113
.00
1.5
0.18
561.
10.
7026
8029
2649
4627
0336
98P
-L10
-23
7390
8253
0.51
0.55
051.
014
.87
1.5
0.19
591.
10.
6528
0728
2827
4427
9237
101
P-L
10-2
417
0224
827
0.75
0.09
701.
00.
7922
2.0
0.05
921.
80.
4959
218
597
1157
678
104
P-L
10-2
557
171
371
322
0.82
0.65
310.
926
.09
1.0
0.28
980.
40.
9033
5020
3240
4634
1613
95P
-L10
-26
2487
216
913
90.
460.
6679
1.1
26.3
61.
40.
2863
0.7
0.84
3360
2732
9859
3398
2397
P-L
10-2
712
4319
721
0.32
0.09
191.
40.
7537
2.5
0.05
952.
00.
5657
022
567
1558
588
97P
-L10
-28
1118
221
270.
560.
1005
1.6
0.82
802.
50.
0597
1.9
0.63
613
2361
719
595
8410
4P
-L10
-29
2173
369
632.
080.
0979
1.4
0.81
561.
70.
0604
1.1
0.79
606
1660
216
618
4697
P-L
10-3
016
7929
648
1.50
0.10
331.
20.
8878
2.0
0.06
231.
50.
6264
519
634
1568
665
92P
-L10
-31
1506
6024
0.68
0.31
341.
74.
730
2.3
0.10
941.
50.
7517
7339
1758
5317
9055
98P
-L10
-32
3245
9641
0.63
0.33
211.
25.
350
1.6
0.11
681.
00.
7518
7727
1849
3819
0837
97P
-L10
-33
988
7513
1.69
0.10
111.
20.
8390
2.6
0.06
022.
30.
4761
924
621
1461
099
102
P-L
10-3
589
817
420
0.43
0.09
611.
70.
8117
3.3
0.06
132.
80.
5360
330
592
2064
812
191
P-L
10-3
632
2744
459
0.86
0.10
361.
20.
8752
1.8
0.06
131.
30.
6663
817
635
1464
958
98P
-L10
-37
3112
415
520.
440.
1064
1.1
0.92
522.
10.
0631
1.8
0.52
665
2165
214
710
7792
P-L
10-3
812
438
267
121
0.43
0.36
371.
26.
371
1.5
0.12
700.
90.
8220
2827
2000
4320
5731
97P
-L10
-39
1015
8113
0.90
0.09
951.
40.
8480
2.6
0.06
182.
10.
5762
424
612
1766
790
92P
-L10
-40
1235
4318
1.05
0.33
011.
55.
319
2.5
0.11
692.
00.
5918
7243
1839
4719
0971
96P
-L10
-41
683
123
150.
470.
0997
1.2
0.83
892.
00.
0610
1.7
0.58
619
1961
214
641
7196
Con
tinue
d
72 Linneman et al.
spe423-03 page 72
TAB
LE 1
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
NE
OP
RO
TE
RO
ZO
IC Q
UA
RZ
ITE
S O
F T
HE
ELB
EZ
ON
E,
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
(co
ntin
ued)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ %R
ho**
207 P
b23
5 U±
2σ(M
a)
206 P
b23
8 U±
2σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Pur
-1 (
cont
inue
d)
P-L
10-4
216
1631
233
0.23
0.09
351.
10.
7746
1.7
0.06
011.
40.
6358
216
576
1260
758
95P
-L10
-43
5059
6540
0.37
0.48
321.
111
.40
1.5
0.17
110.
90.
7725
5728
2541
4825
6932
99P
-L10
-44
1543
8719
3.06
0.10
421.
50.
8843
2.9
0.06
162.
50.
5164
328
639
1865
910
697
P-L
10-4
512
0175
100.
640.
0992
1.5
0.84
003.
00.
0614
2.6
0.49
619
2860
917
655
113
93P
-L10
-46
1277
9712
0.58
0.10
171.
40.
8696
2.4
0.06
202.
00.
5663
523
624
1667
587
93P
-L10
-47
1250
112
131
0.45
0.14
591.
81.
426
2.2
0.07
091.
30.
8190
027
878
3095
453
92P
-L10
-48
1460
133
290.
920.
1626
1.2
1.64
21.
80.
0733
1.4
0.63
987
2397
121
1021
5895
P-L
10-4
930
561
178
168
0.59
0.66
041.
225
.14
1.5
0.27
600.
90.
8133
1429
3269
6133
4127
98P
-L10
-50
1147
150
200.
600.
1049
1.4
0.88
232.
40.
0610
2.0
0.57
642
2364
317
639
8510
1P
-L10
-51
3728
9150
1.22
0.37
441.
16.
417
1.6
0.12
431.
10.
7120
3529
2050
4020
1940
102
P-L
10-5
210
6828
130.
770.
3359
1.3
5.50
41.
80.
1189
1.2
0.73
1901
3118
6743
1939
4496
P-L
10-5
310
912
303
108
0.45
0.32
661.
35.
258
1.6
0.11
680.
90.
8118
6227
1822
4119
0733
96P
-L10
-54
9971
284
980.
450.
3167
1.4
5.07
21.
70.
1161
1.0
0.82
1832
3017
7444
1898
3593
P-L
10-5
521
3662
240.
690.
3370
1.5
5.49
62.
10.
1183
1.4
0.72
1900
3618
7248
1931
5197
P-L
10-5
614
5351
70.
590.
1296
1.4
1.17
92.
80.
0660
2.4
0.51
791
3178
621
805
101
98P
-L10
-57
8669
170
800.
680.
4035
1.4
7.45
91.
60.
1341
0.9
0.85
2168
3021
8552
2152
3010
2P
-L10
-58
5387
110
440.
370.
3636
1.5
6.51
82.
70.
1300
2.2
0.57
2048
4819
9953
2098
7895
P-L
10-5
913
890
286
118
0.46
0.36
851.
46.
955
1.6
0.13
690.
70.
9021
0628
2022
4921
8824
92P
-L10
-60
999
9810
0.38
0.09
521.
40.
7844
2.9
0.05
972.
60.
4858
826
586
1659
411
199
P-L
10-6
121
756
413
157
0.98
0.31
003.
36.
783
3.4
0.15
870.
90.
9620
8462
1741
101
2442
3271
P-L
10-6
211
9224
225
0.56
0.09
571.
30.
7881
2.4
0.05
972.
00.
5359
022
589
1559
488
99P
-L10
-63
1300
8629
0.07
0.35
011.
75.
634
2.6
0.11
672.
00.
6519
2145
1935
5619
0671
102
P-L
10-6
410
484
251
124
1.12
0.36
591.
46.
155
1.5
0.12
200.
70.
8819
9827
2010
4719
8626
101
P-L
10-6
598
921
620
0.36
0.09
041.
50.
7364
2.8
0.05
912.
40.
5256
024
558
1657
010
598
P-L
10-6
679
3372
561.
060.
5863
1.3
17.1
51.
50.
2122
0.8
0.85
2943
3029
7463
2922
2610
2P
-L10
-67
5692
107
470.
390.
3871
1.8
8.61
22.
40.
1614
1.6
0.75
2298
4421
0964
2470
5385
P-L
10-6
813
9623
025
0.79
0.09
811.
50.
8144
2.7
0.06
022.
30.
5460
525
603
1761
198
99P
-L10
-69
3073
7333
0.81
0.37
141.
36.
529
2.0
0.12
751.
50.
6620
5035
2036
4620
6453
99P
-L10
-70
5608
6840
0.54
0.51
891.
413
.46
1.8
0.18
821.
00.
8227
1333
2695
6327
2633
99P
-L10
-71
1230
859
0.04
0.11
012.
00.
9314
8.4
0.06
148.
20.
2366
884
673
2565
235
310
3P
-L10
-74
4643
168
860.
340.
4687
1.3
11.0
11.
60.
1704
0.9
0.81
2524
3024
7854
2561
3297
P-L
10-7
641
824
386
247
0.43
0.55
231.
616
.19
1.6
0.21
270.
40.
9628
8831
2835
7229
2614
97P
-L10
-77
3440
533
610.
550.
1040
1.5
0.89
291.
80.
0623
1.1
0.81
648
1763
818
683
4693
P-L
10-7
846
7510
549
0.71
0.39
111.
47.
344
1.8
0.13
621.
10.
8021
5432
2128
5221
7937
98
Wet
t-1
(Loc
atio
n: m
icro
cong
lom
erat
e, N
eopr
oter
ozoi
c, E
diac
aria
n, L
ausi
tz g
roup
, Lau
sitz
ant
iform
, Wet
terb
erg
near
Ebe
rsba
ch, E
astin
g: 4
0 52
84, N
orth
ing:
56
8022
6)
W-s
1-u1
3134
698
0.41
0.10
510.
70.
8834
1.3
0.06
101.
10.
5564
312
644
963
846
101
W-s
1-u2
6397
159
160.
440.
0956
0.7
0.77
711.
20.
0589
1.0
0.54
584
1158
98
565
4610
4W
-s1-
u362
6415
713
0.18
0.08
780.
60.
7057
1.3
0.05
831.
20.
4654
211
542
654
251
100
W-s
1-u4
5911
100
110.
620.
0959
0.7
0.78
511.
60.
0594
1.5
0.44
588
1459
08
580
6310
2W
-s1-
u511
710
255
280.
420.
1051
0.6
0.87
741.
00.
0605
0.9
0.57
640
1064
47
622
3710
4W
-s1-
u612
003
281
300.
530.
0997
0.6
0.81
621.
00.
0594
0.8
0.58
606
961
37
581
3610
5W
-s1-
u795
089
108
580.
130.
4985
0.6
13.7
80.
90.
2004
0.6
0.71
2734
1626
0726
2830
2092
Con
tinue
d
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 73
spe423-03 page 73
TAB
LE 1
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
NE
OP
RO
TE
RO
ZO
IC Q
UA
RZ
ITE
S O
F T
HE
ELB
EZ
ON
E,
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
(co
ntin
ued)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ %R
ho**
207 P
b23
5 U±
2σ(M
a)
206 P
b23
8 U±
2σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Wet
t-1
(con
tinue
d)
W-s
1-u7
c19
5089
309
214
0.35
0.60
991.
620
.97
1.8
0.24
940.
90.
8831
3736
3070
7831
8127
96W
-s1-
u817
962
376
460.
860.
1013
0.7
0.85
431.
00.
0611
0.6
0.75
627
962
29
644
2797
W-s
1-u9
8833
199
210.
460.
0966
0.6
0.78
511.
00.
0589
0.8
0.57
588
959
56
565
3510
5W
-s1-
u10
1056
522
424
0.38
0.10
120.
70.
8557
1.1
0.06
130.
90.
6262
810
621
865
137
95W
-s1-
u11
2635
294
370.
790.
3309
0.8
5.22
81.
00.
1146
0.6
0.81
1857
1618
4325
1873
2098
W-s
1-u1
234
944
120
440.
350.
3450
0.7
5.51
90.
90.
1160
0.5
0.84
1904
1519
1025
1896
1710
1W
-s1-
u13
7051
425
596
0.65
0.32
750.
65.
089
0.8
0.11
270.
50.
8018
3414
1826
2018
4317
99W
-s1-
u14
1860
541
143
0.56
0.09
600.
80.
7817
1.0
0.05
910.
60.
7858
69
591
957
028
104
W-s
1-u1
519
303
263
450.
430.
1627
0.8
1.60
81.
10.
0717
0.7
0.78
973
1397
215
976
2710
0W
-s1-
u16
2891
6639
822
90.
240.
5290
0.6
13.4
00.
70.
1838
0.3
0.86
2708
1327
3726
2687
1110
2W
-s1-
u17
1954
280
280.
670.
2945
1.0
4.53
01.
20.
1116
0.6
0.84
1736
2016
6429
1825
2391
W-s
1-u1
840
604
6135
0.72
0.47
170.
611
.46
0.8
0.17
620.
50.
7525
6114
2491
2426
1817
95W
-s1-
u19
4567
9212
0.64
0.11
690.
71.
0112
1.4
0.06
271.
30.
4870
915
713
969
954
102
W-s
1-u2
059
2214
315
0.52
0.09
610.
60.
7856
1.2
0.05
931.
00.
5358
911
591
757
944
102
W-s
1-u2
112
664
312
320.
490.
0956
0.6
0.77
831.
10.
0590
0.9
0.54
585
1058
97
568
4110
4W
-s1-
u22
3586
818
253
0.33
0.27
441.
24.
111
1.3
0.10
870.
50.
9116
5721
1563
3317
7720
88W
-s1-
u23
5172
115
766
0.58
0.36
730.
76.
174
0.9
0.12
190.
50.
8420
0116
2017
2619
8418
102
W-s
1-u2
425
772
6926
0.45
0.33
321.
25.
627
1.9
0.12
251.
40.
6619
2032
1854
4019
9250
93W
-s1-
u25
6473
148
211.
600.
0987
0.8
0.81
261.
20.
0597
0.9
0.66
604
1160
79
594
3910
2W
-s1-
u26
4103
610
6198
0.27
0.09
230.
80.
7495
0.9
0.05
890.
40.
9156
88
569
956
217
101
W-s
1-u2
714
934
267
350.
740.
1001
0.7
0.85
091.
00.
0617
0.7
0.74
625
961
59
663
2993
W-s
1-u2
835
2882
111.
580.
0956
0.6
0.79
551.
50.
0603
1.4
0.42
594
1458
97
616
6096
W-s
1-u2
929
962
8134
0.42
0.38
600.
86.
859
1.0
0.12
890.
60.
8020
9318
2104
2820
8321
101
W-s
1-u3
077
679
7458
1.12
0.56
870.
618
.01
0.7
0.22
970.
40.
8529
9014
2903
2930
5012
95W
-s2-
u31
1294
229
035
1.24
0.09
231.
10.
7625
1.3
0.05
990.
80.
7957
512
569
1160
036
95W
-s2-
u32
1482
051
210.
950.
3328
0.9
5.23
71.
30.
1141
0.9
0.72
1859
2218
5230
1866
3299
W-s
2-u3
326
0309
762
307
0.30
0.37
991.
06.
542
1.0
0.12
490.
40.
9320
5218
2076
3420
2713
102
W-s
2-u3
427
117
405
640.
490.
1498
0.8
1.41
41.
20.
0685
0.9
0.63
895
1590
013
883
3910
2W
-s2-
u35
1953
839
747
0.70
0.10
560.
90.
8925
1.2
0.06
130.
80.
7564
812
647
1165
035
100
W-s
2-u3
653
5463
110.
590.
1613
0.8
1.59
051.
20.
0715
0.9
0.68
966
1596
415
972
3699
W-s
2-u3
737
659
105
470.
840.
3629
0.8
6.35
40.
90.
1270
0.4
0.88
2026
1619
9627
2057
1597
W-s
2-u3
840
5674
90.
320.
1151
0.9
0.99
761.
50.
0628
1.2
0.57
703
1570
211
703
5210
0W
-s2-
u39
5770
140
140.
560.
0924
0.8
0.76
301.
30.
0599
1.0
0.63
576
1257
09
600
4595
W-s
2-u4
025
160
8236
1.03
0.35
060.
85.
626
1.0
0.11
640.
60.
8019
2018
1937
2819
0122
102
W-s
2-u4
161
665
8256
0.81
0.53
920.
914
.03
1.1
0.18
860.
60.
8527
5120
2780
4027
3018
102
W-s
2-u4
211
239
138
230.
440.
1537
1.1
1.45
61.
30.
0687
0.8
0.80
912
1692
218
890
3310
4W
-s2-
u43
1579
037
138
0.51
0.09
600.
90.
7963
1.1
0.06
020.
70.
7959
510
591
1060
930
97W
-s2-
u44
6575
148
180.
830.
1038
0.9
0.86
671.
40.
0606
1.1
0.62
634
1363
711
624
4810
2W
-s2-
u45
3595
514
752
0.56
0.31
940.
94.
725
1.1
0.10
730.
70.
7517
7219
1787
2717
5427
102
W-s
2-u4
620
520
5226
0.92
0.39
200.
97.
314
1.2
0.13
530.
70.
7921
5121
2132
3321
6824
98W
-s2-
u47
8530
201
200.
670.
0884
0.9
0.72
061.
40.
0591
1.1
0.61
551
1254
69
571
4896
Con
tinue
d
74 Linneman et al.
spe423-03 page 74
TAB
LE 1
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
NE
OP
RO
TE
RO
ZO
IC Q
UA
RZ
ITE
S O
F T
HE
ELB
EZ
ON
E,
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
(co
ntin
ued)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ %R
ho**
207 P
b23
5 U±
2σ(M
a)
206 P
b23
8 U±
2σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Wet
t-1
(con
tinue
d)
W-s
2-u4
815
685
370
370.
540.
0936
1.0
0.76
741.
20.
0595
0.7
0.83
578
1057
711
585
2999
W-s
2-u4
931
4944
914
323
0.12
0.34
891.
16.
117
1.2
0.12
720.
30.
9619
9320
1929
3720
5912
94W
-s2-
u50
3838
798
0.52
0.08
750.
80.
7031
1.6
0.05
831.
40.
5154
113
541
854
060
100
W-s
2-u5
162
7712
414
0.67
0.10
290.
90.
8740
2.6
0.06
162.
40.
3463
824
631
1166
110
395
W-s
2-u5
268
9884
1239
498
0.16
0.37
071.
49.
602
1.8
0.18
791.
00.
8123
9733
2033
5027
2434
75W
-s2-
u53
6300
152
140.
350.
0878
0.8
0.70
391.
30.
0582
1.1
0.57
541
1154
28
536
4810
1W
-s2-
u54
3523
708
0.51
0.10
481.
00.
8761
1.8
0.06
071.
40.
5963
917
642
1362
762
102
W-s
2-u5
518
814
393
350.
140.
0932
1.0
0.76
011.
30.
0591
0.9
0.76
574
1257
511
572
3710
0W
-s2-
u56
4091
505
0.57
0.08
811.
10.
7016
1.8
0.05
781.
50.
5854
015
544
1152
166
104
W-s
2-u5
734
192
868
810.
350.
0923
1.0
0.74
661.
20.
0587
0.6
0.86
566
1056
911
555
2610
3W
-s2-
u58
1111
828
327
0.31
0.09
370.
80.
7631
1.1
0.05
910.
80.
7457
610
577
957
033
101
W-s
2-u5
954
8813
613
0.28
0.09
530.
90.
7819
1.7
0.05
951.
40.
5658
715
587
1058
559
100
W-s
2-u6
088
4319
924
0.86
0.10
271.
00.
8485
1.3
0.05
990.
90.
7262
417
630
1260
240
105
Rot
h-1
(Loc
atio
n: g
rayw
acke
, Neo
prot
eroz
ic, E
diac
aria
n, R
oths
tein
For
mat
ion,
Tor
gau-
Dob
erlu
g sy
nclin
e, d
rill W
isB
aW 1
641H
/80
near
Her
zber
g, d
epth
of s
ampl
ed c
ore:
507
.0 m
, E
astin
g: 3
8 39
04, N
orth
ing:
57
2741
9)
Rot
-129
1357
60.
550.
0917
1.0
0.74
161.
60.
0587
1.2
0.65
563
1456
511
555
5210
2R
ot-2
1339
133
840
0.93
0.10
111.
00.
8465
1.3
0.06
070.
80.
7962
312
621
1263
035
99R
ot-3
4375
112
120.
660.
0996
1.0
0.84
021.
30.
0612
0.9
0.75
619
1361
212
645
3895
Rot
-456
9015
115
0.43
0.09
671.
10.
8132
1.4
0.06
100.
90.
7860
413
595
1263
937
93R
ot-5
2711
646
0.31
0.09
261.
20.
7511
2.0
0.05
881.
60.
6056
918
571
1356
171
102
Rot
-634
6197
90.
370.
0951
1.2
0.78
702.
10.
0600
1.7
0.59
589
1858
614
603
7297
Rot
-770
8719
522
0.83
0.09
861.
00.
8162
1.4
0.06
011.
00.
7160
613
606
1160
543
100
Rot
-846
2612
115
1.70
0.09
651.
00.
7967
1.5
0.05
991.
10.
6959
513
594
1159
946
99R
ot-9
7383
207
241.
520.
0929
1.0
0.76
431.
40.
0597
1.0
0.72
577
1357
311
592
4397
Rot
-10
2220
497
1.01
0.11
571.
41.
001
1.7
0.06
271.
00.
8270
417
706
1869
941
101
Rot
-11
1576
6759
323
30.
600.
3531
1.1
5.93
61.
20.
1219
0.4
0.93
1966
2019
4936
1985
1598
Rot
-12
5806
162
160.
580.
0912
1.0
0.73
701.
30.
0586
0.7
0.81
561
1156
311
552
3310
2R
ot-1
345
8812
214
0.90
0.09
941.
10.
8227
1.6
0.06
001.
20.
6961
015
611
1360
550
101
Rot
-14
3948
106
121.
030.
0922
1.0
0.75
001.
80.
0590
1.5
0.58
568
1656
811
567
6310
0R
ot-1
581
8439
140.
580.
3073
1.0
4.62
31.
50.
1091
1.0
0.72
1753
2517
2732
1785
3797
Rot
-16
1896
325
0.57
0.13
041.
11.
172
1.6
0.06
521.
20.
6778
718
790
1678
050
101
Rot
-17
4317
127
130.
610.
0942
1.2
0.76
992.
10.
0593
1.7
0.58
580
1858
013
577
7310
1R
ot-1
843
1712
713
0.61
0.09
431.
20.
7735
2.2
0.05
951.
80.
5558
220
581
1358
480
99R
ot-1
946
7713
314
0.46
0.09
911.
00.
8165
1.4
0.05
980.
90.
7460
613
609
1259
541
102
Rot
-20
1760
646
654
1.09
0.09
581.
10.
8061
1.4
0.06
100.
70.
8360
012
590
1363
932
92R
ot-2
154
1088
121.
030.
1045
1.0
0.87
661.
60.
0609
1.2
0.64
639
1564
112
634
5210
1R
ot-2
216
527
437
500.
830.
0992
1.1
0.83
201.
30.
0608
0.7
0.86
615
1261
013
634
2896
Rot
-23
3700
210
5910
70.
550.
0932
1.1
0.77
921.
20.
0606
0.5
0.92
585
1157
513
626
2192
Rot
-24
1812
389
290.
360.
3136
1.0
4.70
91.
20.
1089
0.7
0.83
1769
2117
5832
1781
2599
Rot
-25
6256
922
183
0.19
0.36
741.
06.
546
1.3
0.12
920.
80.
7920
5223
2017
3620
8828
97
Con
tinue
d
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 75
spe423-03 page 75
TAB
LE 1
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
NE
OP
RO
TE
RO
ZO
IC Q
UA
RZ
ITE
S O
F T
HE
ELB
EZ
ON
E,
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
(co
ntin
ued)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ %R
ho**
207 P
b23
5 U±
2σ(M
a)
206 P
b23
8 U±
2σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Rot
h-1
(con
tinue
d)
Rot
-26
3860
8511
1.18
0.11
131.
20.
9605
1.7
0.06
261.
20.
7168
417
680
1669
451
98R
ot-2
719
366
5931
1.34
0.38
471.
17.
280
1.2
0.13
720.
70.
8521
4622
2098
3821
9323
96R
ot-2
825
5959
70.
760.
1048
1.4
0.87
982.
30.
0609
1.8
0.60
641
2264
317
634
7910
1R
ot-2
910
046
219
280.
560.
1161
1.0
1.02
21.
30.
0638
0.9
0.76
715
1470
814
735
3696
Rot
-30
8780
231
250.
600.
0991
1.1
0.82
421.
40.
0603
0.9
0.78
610
1360
913
616
3899
Rot
-31
8396
131
213
60.
730.
3770
0.9
6.57
21.
00.
1264
0.5
0.87
2056
1820
6231
2049
1810
1R
ot-3
239
0248
462
283
0.15
0.55
420.
917
.585
1.0
0.23
020.
30.
9629
6719
2842
4330
539
93R
ot-3
332
8154
70.
410.
1218
1.3
1.08
821.
80.
0648
1.2
0.72
748
1974
118
768
5296
Rot
-34
7044
199
210.
520.
0988
1.0
0.81
551.
40.
0599
1.0
0.70
606
1360
712
600
4410
1R
ot-3
514
7742
40.
470.
0948
0.9
0.77
482.
30.
0593
2.1
0.40
582
2058
410
578
9110
1R
ot-3
626
5668
80.
870.
1051
0.9
0.89
461.
40.
0617
1.1
0.65
649
1464
411
665
4697
Rot
-37
9091
272
260.
460.
0925
1.1
0.75
251.
40.
0590
0.9
0.76
570
1257
012
567
4110
1R
ot-3
848
5614
215
0.63
0.09
341.
00.
7783
1.5
0.06
041.
10.
6658
513
576
1161
947
93R
ot-3
929
6289
111.
140.
0965
0.9
0.79
821.
40.
0600
1.0
0.69
596
1259
411
603
4299
Rot
-40
1139
030
642
1.36
0.10
551.
30.
8858
1.6
0.06
090.
90.
8364
415
646
1663
638
102
Rot
-41
6613
8717
0.99
0.16
051.
01.
595
1.5
0.07
211.
20.
6596
819
959
1798
947
97R
ot-4
257
107
114
600.
390.
4688
1.0
12.1
711.
20.
1883
0.6
0.88
2618
2224
7843
2727
1891
Rot
-43
1189
535
539
0.86
0.09
650.
90.
7985
1.3
0.06
000.
90.
7359
611
594
1060
337
98R
ot-4
483
4335
140.
720.
3517
1.0
5.58
81.
40.
1153
1.0
0.72
1914
2419
4334
1884
3510
3R
ot-4
569
989
155
790.
340.
4540
1.0
10.9
701.
20.
1752
0.6
0.84
2521
2224
1340
2608
2193
Rot
-46
3745
118
130.
800.
0945
1.2
0.77
221.
60.
0592
1.1
0.75
581
1458
213
576
4610
1R
ot-4
737
5610
011
0.58
0.10
611.
10.
9046
1.5
0.06
191.
00.
7665
414
650
1466
941
97R
ot-4
835
8045
50.
840.
0974
0.9
0.80
431.
90.
0599
1.7
0.49
599
1859
911
600
7310
0R
ot-4
955
8117
018
0.69
0.09
461.
10.
7800
1.4
0.05
980.
90.
7858
513
583
1259
738
98R
ot-5
024
7145
50.
970.
0946
1.0
0.77
631.
70.
0595
1.3
0.60
583
1558
311
586
5899
Rot
-51
4638
155
160.
900.
0895
1.0
0.73
081.
30.
0592
0.9
0.74
557
1155
210
576
3896
Rot
-52
2468
404
0.47
0.10
281.
20.
8624
1.5
0.06
080.
90.
7763
114
631
1463
441
100
Rot
-53
8619
528
411
80.
060.
4132
1.0
7.71
71.
10.
1355
0.5
0.90
2199
1922
2936
2170
1610
3R
ot-5
489
8127
131
0.84
0.10
131.
10.
8433
1.4
0.06
040.
80.
8162
113
622
1361
635
101
Rot
-55
1676
960
240.
420.
3725
1.1
6.50
81.
40.
1267
0.9
0.75
2047
2520
4138
2053
3399
Rot
-56
1069
330
236
1.01
0.10
011.
00.
8384
1.4
0.06
070.
90.
7361
813
615
1263
040
98R
ot-5
749
118
159
690.
580.
3834
0.9
7.10
41.
00.
1344
0.5
0.88
2125
1820
9232
2156
1797
Rot
-58
1797
405
0.63
0.12
251.
01.
079
1.5
0.06
391.
00.
7274
315
745
1573
743
101
Rot
-59
1853
643
842
0.30
0.09
511.
20.
7890
1.4
0.06
020.
80.
8359
113
585
1361
134
96R
ot-6
0c21
0159
311
168
0.51
0.46
143.
09.
874
7.4
0.15
526.
70.
4124
2314
024
4612
424
0422
810
2R
ot-6
0r28
3975
488
193
0.26
0.34
491.
85.
897
2.4
0.12
401.
60.
7419
6142
1910
5920
1557
95
Not
e: C
oord
inat
es a
re U
TM
Wor
ld G
eode
tic S
yste
m 8
4. C
onc.
—co
ncor
danc
e.*W
ithin
-run
bac
kgro
und-
corr
ecte
d m
ean
207 P
b si
gnal
in c
ount
s pe
r se
cond
.† U
and
Pb
cont
ent a
nd T
h/U
rat
ios
wer
e ca
lcul
ated
rel
ativ
e to
GJ-
1 an
d ar
e ac
cura
te to
~10
%.
§ Cor
rect
ed fo
r ba
ckgr
ound
, mas
s bi
as, l
aser
indu
ced
U-P
b fr
actio
natio
n an
d co
mm
on P
b (if
det
ecta
ble,
see
text
on
anal
ytic
al m
etho
d) u
sing
the
Sta
cey
and
Kra
mer
s (1
975)
m
odel
Pb
com
posi
tion.
207
Pb/
235 U
cal
cula
ted
usin
g 20
7 Pb/
206 P
b/(23
8 U/20
6 Pb
× 1/
137.
88).
Err
ors
are
prop
agat
ed b
y qu
adra
tic a
dditi
on o
f with
in-r
un e
rror
s (1
sta
ndar
d er
ror)
and
the
repr
oduc
ibili
ty o
f GJ-
1 (1
sta
ndar
d de
viat
ion)
.**
Rho
is th
e er
ror
corr
elat
ion
defi n
ed a
s er
r206 P
b/23
8 U/e
rr20
7 Pb/
235 U
. See
text
for
deta
ils.
76 Linneman et al.
spe423-03 page 76
TAB
LE 2
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b, A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
PA
LAE
OZ
OIC
QU
AR
ZIT
ES
OF
TH
E E
LBE
ZO
NE
, S
AX
O-T
HU
RIN
GIA
N Z
ON
E, B
OH
EM
IAN
MA
SS
IF
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ (%)
Rho
**20
7 Pb
235 U
±2σ
(Ma)
206 P
b23
8 U±2
σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Kam
-1 (
Loca
tion:
san
dsto
ne, L
ower
Cam
bria
n, Z
wet
hau
For
mat
ion,
Tor
gau-
Dob
erlu
ng s
yncl
ine,
dril
l Wis
BaW
120
9/78
nea
r Fa
lken
berg
, dep
th o
f sam
ple
core
: 455
.5 m
,E
astin
g: 3
7 40
90, N
orth
ing:
57
1891
1)
Kam
-127
3340
61.
480.
1046
1.1
0.87
472.
20.
0606
1.9
0.49
638
2164
113
626
8210
2K
am-2
7205
022
286
0.26
0.36
371.
16.
4764
1.1
0.12
910.
40.
9220
4320
2000
3620
8616
96K
am-3
8653
201
301.
710.
1018
1.1
0.85
691.
40.
0610
1.0
0.74
628
1462
513
641
4298
Kam
-462
4911
816
0.78
0.11
311.
00.
9648
1.7
0.06
191.
40.
5968
617
691
1367
059
103
Kam
-576
6418
620
0.49
0.10
001.
20.
8387
1.6
0.06
091.
00.
7861
815
614
1463
442
97K
am-6
3002
080
074
0.22
0.09
431.
10.
7658
1.3
0.05
890.
70.
8557
712
581
1356
430
103
Kam
-754
0813
417
1.27
0.09
661.
10.
8008
1.5
0.06
011.
00.
7459
713
594
1260
843
98K
am-8
2415
455
0.44
0.10
441.
10.
8836
2.0
0.06
141.
70.
5464
319
640
1365
271
98K
am-9
4321
106
120.
510.
1017
1.1
0.84
761.
70.
0604
1.3
0.65
623
1662
513
619
5710
1K
am-1
038
8080
80.
490.
0982
1.1
0.82
721.
70.
0611
1.3
0.63
612
1660
412
641
5794
Kam
-11
2940
495
0.62
0.09
671.
20.
8097
1.8
0.06
071.
40.
6260
217
595
1363
062
94K
am-1
241
770
8749
0.97
0.44
151.
19.
868
1.2
0.16
210.
50.
9224
2322
2357
4424
7816
95K
am-1
376
368
340
116
0.62
0.30
201.
24.
639
1.3
0.11
140.
50.
9317
5622
1701
3618
2317
93K
am-1
480
1318
823
0.96
0.10
081.
10.
8391
1.5
0.06
041.
00.
7261
914
619
1361
845
100
Kam
-15
2506
3166
727
50.
380.
3749
1.2
7.07
41.
30.
1369
0.5
0.93
2121
2320
5243
2188
1794
Kam
-16
1450
040
538
0.41
0.08
851.
00.
7168
1.2
0.05
870.
60.
8554
910
547
1155
728
98K
am-1
728
4354
70.
910.
1095
1.2
0.92
212.
40.
0611
2.0
0.52
663
2367
016
643
8710
4K
am-1
889
961
850
142
0.04
0.17
421.
12.
255
1.5
0.09
391.
00.
7311
9821
1035
2015
0638
69K
am-1
980
1817
922
0.84
0.10
421.
10.
8847
1.6
0.06
161.
10.
7164
415
639
1465
948
97K
am-2
080
1817
922
0.84
0.10
421.
10.
8865
1.5
0.06
171.
00.
7464
414
639
1466
543
96K
am-2
174
0019
320
0.72
0.09
281.
10.
7667
1.4
0.05
991.
00.
7357
813
572
1260
143
95K
am-2
233
4282
90.
750.
0983
1.2
0.80
521.
80.
0594
1.4
0.63
600
1760
513
582
6110
4K
am-2
335
884
109
541.
240.
3742
1.1
6.65
41.
20.
1290
0.6
0.87
2067
2220
4937
2084
2198
Kam
-24
5711
132
160.
680.
1080
1.3
0.91
321.
80.
0613
1.2
0.74
659
1766
116
651
5110
2K
am-2
536
8086
90.
480.
0959
1.1
0.79
571.
50.
0602
1.1
0.72
594
1459
112
609
4697
Kam
-26
7305
174
210.
860.
1011
1.2
0.84
371.
50.
0605
0.9
0.82
621
1462
114
622
3710
0K
am-2
797
5519
926
1.99
0.10
181.
20.
8564
1.7
0.06
101.
20.
7062
816
625
1463
952
98K
am-2
889
9523
725
0.62
0.09
451.
20.
7728
1.5
0.05
930.
90.
7958
113
582
1357
840
101
Kam
-29
6203
814
066
0.60
0.40
521.
07.
639
1.5
0.13
671.
10.
6821
9027
2193
3721
8638
100
Kam
-30
4117
9412
0.98
0.10
181.
20.
8521
1.8
0.06
071.
30.
6962
617
625
1562
955
99K
am-3
179
5417
324
1.38
0.10
410.
80.
8845
1.2
0.06
160.
90.
6564
311
638
966
139
97K
am-3
239
8855
70.
800.
1098
0.7
0.94
651.
40.
0625
1.2
0.52
676
1467
110
692
5397
Kam
-33
5413
132
171.
100.
1030
0.8
0.85
471.
80.
0602
1.6
0.44
627
1763
29
610
6910
4K
am-3
442
0211
211
0.31
0.09
560.
90.
7930
1.8
0.06
021.
60.
4959
316
589
1060
968
97K
am-3
544
1212
312
0.58
0.09
180.
80.
7536
1.4
0.05
951.
10.
6057
012
566
958
647
97K
am-3
610
866
311
290.
280.
0928
0.8
0.75
121.
10.
0587
0.8
0.67
569
1057
28
556
3610
3K
am-3
753
8611
814
0.56
0.10
910.
80.
9149
1.5
0.06
081.
20.
5766
014
667
1163
451
105
Kam
-38
3379
8513
1.71
0.09
990.
80.
8255
1.7
0.05
991.
50.
4961
116
614
1060
064
102
Kam
-39
3660
104
100.
460.
0866
0.9
0.70
601.
50.
0591
1.2
0.59
542
1353
59
572
5294
Kam
-40
3327
445
0.88
0.09
241.
20.
7713
3.0
0.06
062.
80.
3958
127
570
1362
312
091
Kam
-41
4189
100
120.
630.
1070
0.8
0.90
181.
30.
0611
1.1
0.60
653
1365
510
644
4610
2K
am-4
239
1460
70.
680.
0971
0.8
0.80
661.
80.
0602
1.6
0.46
601
1659
79
612
6898
Con
tinue
d
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 77
spe423-03 page 77
TAB
LE 2
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b, A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
PA
LAE
OZ
OIC
QU
AR
ZIT
ES
OF
TH
E E
LBE
ZO
NE
, S
AX
O-T
HU
RIN
GIA
N Z
ON
E, B
OH
EM
IAN
MA
SS
IF (
cont
inue
d)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ (%)
Rho
**20
7 Pb
235 U
±2σ
(Ma)
206 P
b23
8 U±2
σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Kam
-1 (
cont
inue
d)
Kam
-43
4659
129
140.
770.
0966
0.8
0.79
791.
40.
0599
1.1
0.60
596
1359
410
601
4999
Kam
-44
9519
3215
1.04
0.36
750.
86.
424
1.3
0.12
681.
00.
6320
3624
2018
2920
5437
98K
am-4
520
7350
60.
980.
0951
0.9
0.78
661.
60.
0600
1.3
0.55
589
1458
610
603
5897
Kam
-46
7097
141
140.
630.
0889
1.0
0.71
581.
60.
0584
1.3
0.59
548
1454
910
545
5710
1K
am-4
736
3016
20.
860.
1051
1.0
0.87
132.
80.
0601
2.6
0.34
636
2764
412
608
114
106
Kam
-48
3850
9112
1.38
0.09
920.
80.
8177
1.8
0.05
981.
70.
4460
717
610
959
772
102
Kam
-50
3271
374
0.39
0.09
890.
90.
8306
1.9
0.06
091.
70.
4761
418
608
1063
672
96K
am-5
126
7366
91.
590.
1080
0.8
0.91
961.
60.
0618
1.4
0.46
662
1666
19
666
6299
Kam
-52
1515
317
230
0.35
0.16
940.
91.
727
1.1
0.07
390.
70.
7910
1915
1009
1710
4028
97K
am-5
383
6119
624
0.99
0.10
360.
90.
8820
1.2
0.06
170.
80.
7364
211
636
1066
435
96K
am-5
418
610
352
441.
180.
0978
0.8
0.81
091.
00.
0602
0.7
0.75
603
1060
19
609
3099
Kam
-55
3730
013
352
0.38
0.36
151.
26.
208
1.3
0.12
450.
60.
8820
0623
1989
4020
2223
98K
am-5
671
5417
219
0.59
0.09
970.
90.
8244
1.6
0.06
001.
40.
5461
015
613
1060
259
102
Kam
-57
7995
207
230.
520.
1015
0.9
0.85
251.
30.
0609
1.0
0.68
626
1262
311
636
4298
Kam
-58
3800
818
0.56
0.09
060.
80.
7467
2.5
0.05
982.
40.
3156
622
559
859
510
594
Kam
-59
6430
3915
0.30
0.36
270.
86.
457
1.3
0.12
911.
00.
6320
4023
1995
2820
8635
96K
am-6
027
906
756
770.
580.
0950
0.9
0.78
571.
00.
0600
0.6
0.82
589
958
510
602
2697
Lang
erbe
rg Q
uart
zite
(Lo
catio
n: m
icro
cong
lom
erat
e, L
ower
Ord
ovic
ian,
Tre
mad
oc, L
ange
r B
erg
For
mat
ion,
Sch
war
zbur
g an
tifor
m [n
orth
wes
tern
par
t], L
ange
r B
erg
near
W
illm
ersd
orf,
Eas
ting:
64
3489
, Nor
thin
g: 5
6 09
502)
Lbq_
110
3842
247
950.
030.
3873
1.3
7.46
71.
80.
1398
1.2
0.74
2169
3221
1048
2225
4195
Lbq_
211
930
148
150.
510.
1058
1.0
0.88
334.
10.
0605
4.0
0.25
643
4064
812
623
172
104
Lbq_
310
574
239
220.
620.
0958
1.0
0.77
451.
30.
0587
0.8
0.78
582
1159
011
554
3510
6Lb
q_4
2675
810
734
0.57
0.29
791.
24.
528
1.4
0.11
020.
70.
8517
3623
1681
3418
0326
93Lb
q_5
5224
316
863
0.42
0.34
761.
05.
628
1.1
0.11
740.
50.
9119
2020
1923
3519
1717
100
Lbq_
615
342
360
320.
490.
0910
1.0
0.73
691.
10.
0587
0.6
0.85
561
1056
211
557
2610
1Lb
q_7
2171
1215
714
91.
080.
7058
1.2
28.3
71.
30.
2915
0.6
0.89
3432
2634
4362
3426
1910
1Lb
q_8
5306
110
110.
450.
1011
1.0
0.84
191.
90.
0604
1.6
0.53
620
1862
112
617
6910
1Lb
q_9
2472
970
280.
800.
3720
1.1
6.27
51.
20.
1223
0.6
0.87
2015
2220
3937
1991
2210
2Lb
q_10
1401
433
128
0.14
0.08
611.
30.
6943
1.5
0.05
850.
90.
8353
513
532
1354
838
97Lb
q_11
1786
240
440
0.42
0.09
271.
20.
7409
1.4
0.05
800.
70.
8856
312
571
1352
929
108
Lbq_
1226
4462
60.
420.
0930
1.1
0.75
421.
80.
0588
1.5
0.58
571
1657
312
561
6510
2Lb
q_13
3910
778
0.45
0.09
221.
00.
7727
3.5
0.06
083.
40.
2958
131
568
1163
214
590
Lbq_
1434
784
864
700.
310.
0795
1.2
0.63
871.
30.
0583
0.7
0.87
502
1149
311
539
2991
Lbq_
1510
562
239
240.
250.
1003
1.1
0.82
281.
50.
0595
1.0
0.74
610
1461
613
585
4310
5Lb
q_16
1115
324
725
0.43
0.09
581.
00.
7707
1.3
0.05
840.
80.
7858
012
590
1254
436
108
Lbq_
1758
3011
111
0.32
0.10
451.
10.
9071
1.9
0.06
301.
60.
5665
518
640
1370
766
91Lb
q_18
1375
130
030
0.66
0.10
301.
00.
8443
1.3
0.05
940.
80.
8062
212
632
1358
434
108
Lbq_
1911
9431
353
128
0.15
0.35
881.
25.
811
1.3
0.11
750.
70.
8719
4823
1977
3919
1824
103
Lbq_
2030
878
7635
0.74
0.37
951.
06.
896
1.1
0.13
180.
50.
9020
9820
2074
3521
2217
98Lb
q_21
5795
375
430.
390.
4966
1.0
13.6
51.
10.
1994
0.5
0.87
2726
2125
9942
2821
1892
Lbq_
2255
794
145
620.
550.
3761
1.0
6.53
61.
10.
1260
0.5
0.90
2051
2020
5836
2043
1810
1
Con
tinue
d
78 Linneman et al.
spe423-03 page 78
TAB
LE 2
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
b, A
ND
Th
DAT
A O
F D
ET
RIT
AL
ZIR
CO
N G
RA
INS
FR
OM
PA
LAE
OZ
OIC
QU
AR
ZIT
ES
OF
TH
E E
LBE
ZO
NE
, S
AX
O-T
HU
RIN
GIA
N Z
ON
E, B
OH
EM
IAN
MA
SS
IF (
cont
inue
d)
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ (%)
Rho
**20
7 Pb
235 U
±2σ
(Ma)
206 P
b23
8 U±2
σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Lang
erbe
rg Q
uart
zite
(co
ntin
ued)
Lbq_
2314
6269
329
148
0.44
0.40
811.
07.
611
1.1
0.13
520.
40.
9221
8619
2206
3721
6715
102
Lbq_
2412
6831
30.
590.
0915
1.0
0.73
882.
40.
0585
2.2
0.40
562
2156
510
550
9710
3Lb
q_25
6566
118
120.
660.
1024
1.0
0.86
391.
40.
0612
1.0
0.72
632
1462
812
647
4397
Lbq_
2614
8581
182
109
0.28
0.53
861.
014
.06
1.1
0.18
940.
40.
9427
5420
2777
4627
3712
101
Lbq_
2712
729
265
290.
420.
1021
1.0
0.83
191.
50.
0591
1.1
0.68
615
1462
712
571
4911
0Lb
q_28
1295
7325
911
70.
340.
4159
1.0
7.94
21.
20.
1385
0.6
0.87
2224
2222
4239
2208
2010
2Lb
q_29
6445
891
470.
080.
4964
1.1
12.9
21.
30.
1888
0.7
0.86
2674
2525
9848
2732
2295
Lbq_
3040
378
964
960.
610.
0882
1.1
0.72
441.
30.
0595
0.7
0.86
553
1154
512
587
2893
Lbq_
3155
1562
90.
320.
1461
1.5
1.40
92.
00.
0700
1.4
0.73
893
2487
924
927
5795
Lbq_
3293
4018
422
0.89
0.09
890.
80.
8140
1.1
0.05
970.
80.
7560
510
608
1059
433
102
Lbq_
3315
446
339
330.
460.
0916
0.9
0.75
231.
20.
0596
0.9
0.70
570
1156
59
588
3796
Lbq_
3414
116
262
270.
520.
0946
0.8
0.77
611.
20.
0595
0.8
0.70
583
1158
39
585
3710
0Lb
q_35
6388
861
765
0.32
0.08
531.
00.
6956
2.1
0.05
911.
80.
4853
617
528
1057
279
92Lb
q_36
3083
297
390.
640.
3407
0.9
5.74
91.
10.
1224
0.6
0.83
1939
1918
9030
1991
2295
Lbq_
3722
6574
167
127
0.40
0.63
660.
822
.40
0.9
0.25
520.
50.
8732
0118
3175
4032
1714
99Lb
q_38
2440
857
058
0.72
0.08
860.
90.
7189
1.1
0.05
880.
70.
7955
010
547
956
131
98Lb
q_39
1078
012
014
0.28
0.09
980.
80.
8294
1.0
0.06
020.
70.
7661
39
614
961
229
100
Lbq_
4027
007
529
580.
540.
1012
0.8
0.85
780.
90.
0615
0.5
0.83
629
962
19
657
2395
Lbq_
4180
5019
419
0.61
0.08
840.
70.
7173
1.1
0.05
890.
80.
6354
99
546
756
237
97Lb
q_42
2250
127
228
0.47
0.09
550.
70.
7759
2.1
0.05
892.
00.
3358
319
588
856
486
104
Lbq_
4384
932
236
920.
500.
3473
0.6
7.18
70.
70.
1225
0.4
0.87
1956
1319
2121
1993
1396
Lbq_
4482
4526
110.
330.
3971
0.9
0.65
61.
40.
1313
1.1
0.65
2135
2521
5634
2115
3810
2Lb
q_45
2060
713
612
0.43
0.08
120.
90.
6557
2.0
0.05
861.
80.
4551
216
503
955
077
91Lb
q_46
3704
8948
822
70.
020.
4461
1.0
12.0
11.
10.
1952
0.4
0.95
2605
2023
7841
2787
1285
Lbq_
4713
1963
173
118
1.14
0.51
170.
813
.09
0.8
0.18
550.
30.
9226
8616
2664
3327
0311
99Lb
q_48
2950
272
300.
460.
3665
0.7
6.93
00.
90.
1371
0.6
0.79
2103
1720
1326
2191
2092
Lbq_
4931
150
701
640.
480.
0880
1.2
0.71
371.
70.
0588
1.2
0.73
547
1454
413
561
5197
Lbq_
5069
1114
215
0.59
0.09
640.
70.
8044
1.2
0.06
051.
00.
5859
911
593
862
343
95Lb
q_51
1637
835
937
0.62
0.09
300.
90.
7566
1.2
0.05
900.
90.
7157
211
574
1056
638
101
Lbq_
5339
7784
100.
790.
0965
0.7
0.80
321.
30.
0604
1.1
0.55
599
1259
48
616
4896
Lbq_
5410
025
181
180.
660.
0880
1.2
0.70
371.
80.
0580
1.3
0.68
541
1554
413
529
5810
3Lb
q_55
1569
633
032
0.40
0.09
170.
80.
7641
1.2
0.06
050.
80.
7157
611
565
962
037
91Lb
q_56
2526
757
752
0.28
0.08
870.
80.
7167
1.0
0.05
860.
60.
7954
98
548
855
327
99Lb
q_57
2590
6241
921
50.
510.
4447
1.0
10.1
41.
10.
1654
0.6
0.85
2448
2123
7138
2511
2094
Lbq_
5813
896
152
290.
820.
1607
0.9
1.57
71.
20.
0712
0.9
0.68
961
1696
115
963
3710
0Lb
q_59
7482
156
201.
180.
1001
0.7
0.83
451.
30.
0605
1.1
0.56
616
1261
59
620
4799
Lbq_
6030
8864
70.
830.
0873
0.8
0.70
411.
80.
0585
1.6
0.42
541
1554
08
547
7199
Not
e: C
oord
inat
es a
re U
TM
Wor
ld G
eode
tic S
yste
m 8
4. C
onc.
—co
ncor
danc
e.*W
ithin
-run
bac
kgro
und-
corr
ecte
d m
ean
207 P
b si
gnal
in c
ount
s pe
r se
cond
.† U
and
Pb
cont
ent a
nd T
h/U
rat
io w
ere
calc
ulat
ed r
elat
ive
to G
J-1
and
are
accu
rate
to ~
10%
.§ C
orre
cted
for
back
grou
nd, m
ass
bias
, las
er in
duce
d U
-Pb
frac
tiona
tion
and
com
mon
Pb
(if d
etec
tabl
e; s
ee te
xt o
n an
alyt
ical
met
hod)
usi
ng th
e S
tace
y an
d K
ram
ers
(197
5)
mod
el P
b co
mpo
sitio
n. 2
07P
b/23
5 U c
alcu
late
d us
ing
207 P
b/20
6 Pb/
(238 U
/206 P
b ×
1/13
7.88
). E
rror
s ar
e pr
opag
ated
by
quad
ratic
add
ition
of w
ithin
-run
err
ors
(1 s
tand
ard
erro
r) a
nd
the
repr
oduc
ibili
ty o
f GJ-
1 (1
sta
ndar
d de
viat
ion)
.**
Rho
is th
e er
ror
corr
elat
ion
defi n
ed a
s er
r206 P
b/23
8 U/e
rr20
7 Pb/
235 U
. See
text
for
deta
ils.
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 79
spe423-03 page 79
TAB
LE 3
. LA
SE
R A
BLA
TIO
N-I
CP
-MS
U, P
B, A
ND
TH
DAT
A O
F Z
IRC
ON
FR
OM
PY
RO
CLA
ST
IC S
ED
IME
NT
S O
F T
HE
SA
XO
-TH
UR
ING
IAN
ZO
NE
, BO
HE
MIA
N M
AS
SIF
Isot
opic
rat
ios§
Age
s
Sam
ple
207 P
b*(c
ps)
U†
(ppm
)P
b†
(ppm
)T
h†
U
206 P
b23
8 U1σ (%
)
207 P
b23
5 U1σ (%
)
207 P
b20
6 Pb
1σ (%)
Rho
**20
7 Pb
235 U
±2σ
(Ma)
206 P
b23
8 U±2
σ(M
a)
207 P
b20
6 Pb
±2σ
(Ma)
Con
c(%
)
Ves
1 (L
ocat
ion:
rhy
oliti
c ig
nim
brite
, Upp
er C
ambr
ian,
Rol
lkop
f For
mat
ion,
Ves
ser
com
plex
, old
qua
rry
clos
e to
the
ski-j
ump
at S
chm
iede
feld
, Eas
ting:
62
8631
, Nor
thin
g: 5
6 06
911)
Ves
1_1
1117
429
124
0.41
0.07
980.
60.
6268
1.2
0.05
701.
10.
5149
410
495
649
147
101
Ves
1_2
1502
340
736
0.71
0.07
980.
50.
6335
1.0
0.05
760.
90.
5449
88
495
551
537
96V
es1_
318
762
505
450.
710.
0800
0.7
0.63
461.
10.
0576
0.8
0.63
499
849
67
513
3797
Ves
1_4
1511
031
730
0.76
0.08
160.
80.
6496
1.2
0.05
770.
90.
6850
89
506
852
037
97V
es1_
566
1217
815
0.55
0.07
980.
80.
6255
1.2
0.05
691.
00.
6349
310
495
848
742
102
Ves
1_6
1132
127
025
0.79
0.08
020.
60.
6254
1.1
0.05
660.
90.
5949
38
497
647
539
105
Ves
1_7
3239
087
084
0.53
0.08
130.
70.
6436
1.1
0.05
740.
80.
6650
59
504
750
636
100
Ves
1_8
2147
560
257
0.96
0.08
020.
50.
6340
1.2
0.05
741.
10.
4549
99
497
550
547
98V
es1_
910
974
262
230.
510.
0804
0.8
0.63
401.
40.
0572
1.2
0.58
499
1149
98
499
5110
0V
es1_
1065
1218
115
0.53
0.08
070.
80.
6312
1.2
0.05
670.
90.
6549
710
500
848
141
104
Ves
1_11
1751
752
345
0.57
0.08
020.
80.
6293
1.2
0.05
690.
90.
6349
69
497
748
841
102
Ves
1_12
1731
245
940
0.59
0.08
070.
80.
6404
1.2
0.05
760.
80.
7350
39
500
851
435
97V
es1_
1344
335
1259
121
0.27
0.07
880.
80.
6221
1.2
0.05
721.
00.
6349
110
489
850
043
98V
es1_
1464
1017
415
0.48
0.08
060.
90.
6385
1.3
0.05
750.
90.
7150
110
500
951
040
98V
es1_
1524
741
665
620.
820.
0800
0.9
0.63
591.
30.
0577
0.9
0.73
500
1049
69
518
3996
kArc
1 (L
ocat
ion:
peb
ble-
bear
ing
rhyo
lithi
c tu
ffi te
(tr
aditi
onal
term
in th
e ol
der
Ger
man
ref
eren
ces:
“K
ongl
omer
atis
che
Ark
ose”
), L
ower
Ord
ovic
ian,
Tre
mad
ocia
n, G
oldi
stha
l For
mat
ion,
S
chw
arzb
urg
antif
orm
[sou
thea
ster
n pa
rt],
valle
y of
the
Bla
mba
ch n
ear
Sitz
endo
rf, E
astin
g: 6
5 24
47, N
orth
ing:
56
1173
2)
kArc
-16
5093
127
110.
640.
0773
1.8
0.60
292.
40.
0566
1.6
0.74
479
1848
016
475
7010
1kA
rc-1
753
0312
511
0.65
0.07
901.
40.
6245
2.3
0.05
731.
90.
6149
318
490
1450
382
98kA
rc-1
860
4212
612
0.68
0.07
951.
50.
6197
1.8
0.05
661.
00.
8349
014
493
1447
444
104
kArc
-19
5438
152
130.
760.
0746
1.7
0.58
452.
10.
0569
1.3
0.80
467
1646
415
486
5695
kArc
-20
6145
265
210.
590.
0784
1.6
0.61
801.
90.
0572
1.1
0.81
489
1548
615
499
5098
kArc
-21
5877
879
460.
680.
4927
1.4
11.9
81.
60.
1764
0.8
0.85
2603
3025
8358
2619
2899
kArc
-22
1023
829
022
0.14
0.08
001.
70.
6294
2.1
0.05
701.
30.
7949
617
496
1649
358
101
kArc
-23
8023
333
170.
770.
0412
3.2
0.31
643.
30.
0558
1.0
0.96
279
1626
016
443
4359
kArc
-24
7313
241
201.
240.
0772
2.1
0.60
862.
50.
0572
1.3
0.85
483
1947
920
498
5996
kArc
-25
6387
160
150.
280.
0934
1.6
0.75
932.
20.
0589
1.5
0.73
574
1957
618
565
6510
2kA
rc-2
640
5013
49
0.64
0.06
231.
60.
4888
2.0
0.05
691.
30.
7640
414
389
1248
958
80kA
rc-2
780
8129
715
0.42
0.04
912.
50.
3764
3.0
0.05
571.
60.
8432
417
309
1543
973
70kA
rc-2
858
9416
715
0.73
0.07
901.
40.
6230
1.8
0.05
721.
10.
7849
214
490
1350
049
98kA
rc-2
914
059
342
300.
640.
0791
2.0
0.62
493.
10.
0573
2.4
0.63
493
2449
119
503
106
98kA
rc-3
026
8371
60.
700.
0791
1.9
0.62
642.
30.
0574
1.3
0.82
494
1849
118
507
5997
kArc
-31
7614
210
170.
430.
0772
1.3
0.59
721.
90.
0561
1.4
0.67
475
1547
912
458
6410
5kA
rc-3
244
2711
910
0.66
0.07
921.
50.
6242
1.7
0.05
720.
90.
8449
214
491
1449
841
99kA
rc-3
311
356
318
230.
410.
0707
1.5
0.54
012.
30.
0554
1.8
0.64
438
1644
012
429
7910
3
Not
e: C
oord
inat
es a
re U
TM
Wor
ld G
eode
tic S
yste
m 8
4. C
onc.
—co
ncor
danc
e.*W
ithin
-run
bac
kgro
und-
corr
ecte
d m
ean
207 P
b si
gnal
in c
ount
s pe
r se
cond
.† U
and
Pb
cont
ent a
nd T
h/U
rat
io w
ere
calc
ulat
ed r
elat
ive
to G
J-1
and
are
accu
rate
to ~
10%
.§ C
orre
cted
for
back
grou
nd, m
ass
bias
, las
er in
duce
d U
-Pb
frac
tiona
tion
and
com
mon
Pb
(if d
etec
tabl
e; s
ee te
xt o
n an
alyt
ical
met
hod)
usi
ng th
e S
tace
y an
d K
ram
ers
(197
5) m
odel
Pb
com
posi
tion.
207
Pb/
235 U
cal
cula
ted
usin
g 20
7 Pb/
206 P
b/(23
8 U/20
6 Pb
× 1/
137.
88).
Err
ors
are
prop
agat
ed b
y qu
adra
tic a
dditi
on o
f with
in-r
un e
rror
s (1
sta
ndar
d er
ror)
and
the
repr
oduc
ibili
ty o
f G
J-1
(1 s
tand
ard
devi
atio
n).
**R
ho is
the
erro
r co
rrel
atio
n de
fi ned
as
err20
6 Pb/
238 U
/err
207 P
b/23
5 U. S
ee te
xt fo
r de
tails
.
80 Linneman et al.
spe423-03 page 80
was used to enable sequential sampling of heterogeneous grains (e.g., growth zones) during time-resolved data acquisition (see also Janoušek et al., 2006). Each analysis consisted of ~20 s back-ground acquisition followed by 35 s data acquisition, using a laser spot size of 30 and 40 μm, respectively. A common-Pb correction based on the interference- and background-corrected 204Pb signal and a model Pb composition (Stacey and Kramers, 1975) was car-ried out if necessary. The necessity of the correction is based on whether the corrected 207Pb/206Pb lies outside of the internal errors of the measured ratios. Discordant analyses were generally inter-preted with care. Raw data were corrected for background signal, common Pb, laser-induced elemental fractionation, instrumental mass discrimination, and time-dependant elemental fraction-ation of Pb/Th and Pb/U using an Excel® spreadsheet program. Reported uncertainties were propagated by quadratic addition of the external reproducibility (standard deviation) obtained from the standard zircon GJ-1 (n = 18; ~0.6% and 0.5–1.0% for the 207Pb/206Pb and 206Pb/238U, respectively) during individual analyti-cal sessions and the within-run precision of each analyses (stan-dard error). Concordia diagrams (2σ error ellipses) and concordia ages (95% confi dence level) were produced using Isoplot/Ex 2.49 (Ludwig, 2001) and frequency and relative probability plots using AgeDisplay (Sircombe 2004). The 207Pb/206Pb age was taken for interpretation for all zircons >1.0 Ga, and the 206Pb/238U ages was used for younger grains. For further details on analytical protocol and data processing see Gerdes and Zeh (2006).
RESULTS
The results of LA-ICP-MS U-Pb zircon dating are listed in Tables 1–3 and shown on the concordia diagrams in Figures 7, 9, and 11 (see below). Binned frequency and probability density distribution plots are shown in Figures 8 and 10. For the latter two plots only those analyses less than 10% discordant were used. In this study, all systems, erathems, and eonothems are used in accordance with the stratigraphic table of Gradstein et al., (2004). Percentages of zircon ages for each sample are shown in Table 4. For zircons older than 1.0 Ga, the 207Pb/206Pb age is mentioned in the text. Younger zircon ages refer to the 206Pb/238U age.
From sample Pur-1, seventy-four detrital zircon grains were analyzed, sixty-seven of which yielded concordant ages (Figs. 7 and 8). The age of the youngest concordant grain is 558 ± 16 Ma, and that of the oldest is 3465 ± 20 Ma (Table 1). Archean ages make up ~21% of the population. These ages fall into two groups at 3.5–3.3 Ga (six grains) and at 2.9–2.6 Ga (nine grains). About 33% of the grains record Paleoproterozoic ages between 1.8 and 2.3 Ga and nearly 45% record Neoproterozoic ages. The later group show pronounced peaks at ca. 640, ca. 620–590, and ca. 570 Ma and less pronounced peaks at ca. 710, ca. 790, and ca. 1000–950 Ma (Fig. 8, Table 4).
All sixty-one analyses from sample Roth-1 are 90–110% concordant (Figs. 7 and 8). The ages range from 552 ± 11 Ma to 3053 ± 11 Ma. Only 5% of the analyzed grains are Archean in age, ~20% Paleoproterozoic, and ~75% Neoproterozoic.
3200
2800
2400
2000
1600
0.2
0.4
0.6
0 4 8 12 16 20 24 28
1200
206Pb238U
Wett-1 n = 61
900
800
700
600
0.12
0.14
0.16
1.0
1.2
1.4
1.6
207Pb/235U
3000
2600
2200
1800
1400
10000.2
0.4
0.6
0 4 8 12 16 20
Roth-1 n = 61
900
800
700
600
0.12
0.14
0.16
1.0
1.2
1.4
1.6
206Pb238U
0.2
0.4
0.6
3200
2800
2400
2000
1600
0 4 8 12 16 20 24 28
206Pb238U
Pur-1 n = 74
1200
900
800
700
600
0.12
0.14
0.16
1.0
1.2
1.4
1.6
data-point error ellipses are 2σ
Figure 7. Concordia plots of LA-ICP-MS U-Pb analyses of detrital zir-con grains from Late Neoproterozoic (Ediacaran) sedimentary rocks of the Saxo-Thuringian zone: Purpurberg quartzite from the Weesenstein Group (Pur-1), microconglomerate from the Lausitz Group (Wett-1), and graywacke from the Rothstein Formation (Roth-1). Error ellipses are 2σ. Insets show enlargement of the younger ages. n—number of analyses. For sample details see Table 1.
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 81
spe423-03 page 81
0
2
4
6
8
10
12
14
500
600
700
800
900
100 0
110 0
1600
18 00
200 0
220 0
2400
26 00
280 0
3000
320 0
34 00F
req
uen
cy
Roth-1 (Neoproterozoic)
n=61/61, 90-110% conc.
n=46/4690–110% conc.
Rla
ie
Pr
abli
ye
tv
ob
it n=15/15
90–110% conc.
Pur-1 (Neoproterozoic)
n=67/74, 90-110% conc.
0
1
2
3
4
5
6
7
8
9
105006007008009 00100 01100120 01 3001 400150 01 600170 018001 9002000210 0220023002 400250 02 6002700280 02 900300 03 100320 03 300340 0
Age (Ma)
Fre
qu
ency
n=31/3290–110% conc.
n=36/4290–110% conc.
500
600
700
80 0
900
100 0
11 00
1600
180 0
200 0
220 0
2400
2600
28 00
300 0
320 0
3400
Ra
ie
ra
iy
elt
vP
ob
bil
tt
vb
it
Rel
ai
eP
roab
liy
Wett-1 (Neoproterozoic)
n=59/61, 90-110% conc.
0
2
4
6
8
10
12
14
16
Fre
qu
ency
n=39/3990–110% conc.
500
600
700
800
90 0
10 00
1 100
160 0
18 00
20 00
22 00
2 400
2 600
2 800
30 00
32 00
3 400
N=20/2290–110% conc.
Figure 8. Binned frequency and probability density distribution plots of detrital zircon grains from Late Neoproterozoic (Ediacaran) sandstones of the Saxo-Thuringian zone. conc.—concordance; n—number of analyses with <10% discordance/total number of analyzed grains.
82 Linneman et al.
spe423-03 page 82
Archean ages (three grains) range from 3.05 to 2.6 Ga, Paleopro-terozoic ages from 2.17 to 1.78 Ga, and the Neoproterozoic ages fall mainly in the interval 650–550 Ma. There are clear clusters at ca. 612 (nine grains) and ca. 642 Ma (six grains), whereas the age range 600–550 Ma is defi ned by a broad peak (Fig. 8). In addi-tion, minor peaks occur at 710, ca. 740, ca. 790, and ca. 960 Ma, similar to those of Pur-1.
From sample Wett-1, sixty-one zircon grains were ana-lyzed, of which fi fty-nine are 90–110% concordant (Figs. 7 and 8). The ages vary from 3181 to 542 ± 27 Ma, of which ~10% are Archean, 24% Paleoproterozoic, and ~60% Neoproterozoic. Archean ages range from 3.2 to 2.6 Ga with a cluster at ca. 2.8–2.6 Ga (fi ve grains). Paleoproterozoic ages (fi fteen grains) defi ne several peaks in the range 2.17–1.76 Ga, whereas Neoprotero-zoic ages fall mostly in the interval 650 –540 Ma, with fi ve rela-tively well-defi ned clusters at ca. 642 (six grains), 625–610 (fi ve grains), ca. 590 (ten grains), ca. 572 (fi ve grains), and ca. 543 Ma (fi ve grains). The latter subpopulation yields a concordia age of 543 ± 4 Ma (see Fig. 11A), which straddles the Precambrian- Cambrian boundary (542 ± 1 Ma; Bowring et al., 2003).
From the Cambrian sandstone Kam-1 we analyzed fi fty-nine zircons, only one of which is more than 10% discordant (Figs. 9 and 10). No Archean and only nine Paleoproterozoic grains (15%) were identifi ed, although the age of the oldest grain (2478 ± 16 Ma), which is close to the Archean boundary, must be con-sidered a minimum age. Paleoproterozoic ages fall mostly in the interval 2.1–2.0 Ga, with one grain yielding a 206Pb/238U age of 1000 ± 18 Ma. Neoproterozoic ages, which make up more than 80% of the population, fall predominantly in the interval 690–550 Ma. The latter population shows clear clusters at ca. 570, ca. 590, ca. 610, ca. 623, ca. 643, and ca. 665 Ma, each of which is represented by six to nine grains. Two grains yielded ages of ca. 547 Ma, and one grain yielded a 206Pb/238U age of ca. 535 ± 9 Ma. The latter analysis is ~6% discordant but is considered to provide a maximum depositional age constraint.
From the Lower Ordovician microconglomerate Lbq-1 we analyzed fi fty-nine zircons; fi fty-seven analyses yielded 90–110% concordant ages, of which 12% are Archean (seven grains), 23% Paleoproterozoic (thirteen grains) and 60% Neo-proterozoic to Cambrian. The Archean grains yielded ages of ca. 2511, ca. 2820–2700 (four grains), ca. 3217, and ca. 3246 Ma. The Paleoproterozoic ages defi ne six peaks in the interval 2200–1800 Ma (Fig. 10) and Neoproterozoic ages (thirty-three grains) fall predominantly in the interval 650–540 Ma, with a broader peak at ca. 636 Ma and clear clusters at ca. 615, ca. 590, ca. 570, and ca. 546 Ma, each defi ned by fi ve to seven analyzed grains. The latter subpopulation of seven grains yielded a concordia age of 546 ± 4 Ma, which straddles the Precambrian-Cambrian boundary like that of Wett-2. In addition, two grains yielded ages close to the Neo-Mesoproterozoic boundary (927 ± 57 and 961 ± 16 Ma), and two others gave a Cambrian concordia age of 530 ± 8 Ma. The two youngest grains (503 ± 8 and 493 ± 12 Ma) straddle the Cambrian-Ordovician boundary close to the deposi-tion age of the sample. The Th/U ratio of most of the measured
TAB
LE 4
. PO
PU
LAT
ION
S A
ND
PE
RC
EN
TAG
ES
OF
ALL
ZIR
CO
NS
CO
NC
OR
DA
NT
BE
TW
EE
N 9
0% A
ND
110
%
Sam
ple
Sed
imen
t. ag
eTo
tal
num
ber
of
grai
ns
Sam
ple
Neo
prot
eroz
oic-
Cam
bria
n tr
ansi
tion
inte
rval
Late
N
eopr
oter
ozoi
cM
id-
Neo
prot
eroz
oic
Neo
-M
esop
rote
rozo
ic
tran
sitio
n in
terv
al
Pal
eopr
oter
ozoi
cN
eo-
to
Mes
oarc
hean
Pal
eoar
chea
n
Rift
ing
Cad
omia
n re
troa
rcC
adom
ian
back
-arc
Rod
inia
br
eak-
upG
renv
illia
nE
burn
ean
Libe
rian
Leon
ian
(ca.
490
–53
5 M
a)(c
a. 5
40–
545
Ma)
(ca.
550
–69
0 M
a)(c
a. 7
00–
790
Ma)
(ca.
900
–10
50 M
a)(c
a. 1
7,05
0–24
80 M
a)(c
a. 2
510–
3180
Ma)
(ca.
322
0–34
65 M
a)
Pur
-1N
eopr
oter
ozoi
c67
00
40 %
3 %
3 %
33 %
13 %
8 %
Rot
h-1
Neo
prot
erzo
ic61
00
66 %
8 %
1 %
20 %
5 %
0W
ett-
1N
eopr
oter
ozoi
c.59
08
%48
%3
%7
%24
%10
%0
Kam
-1Lo
wer
Cam
bria
n58
00
83 %
02
%15
%0
0Lb
q-1
Low
er O
rddo
vici
an57
9 %
5 %
47 %
04
%23
%9
%3
%
Not
e: T
he to
tal r
ange
of m
easu
red
U/P
b ag
es o
f eac
h po
pula
tion
are
show
n in
par
enth
eses
.
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 83
spe423-03 page 83
207Pb/235U
3200
2800
2400
2000
1600
0 4 8 12 16 20 24 28
206Pb238U
Lbq-1 n = 59
900
800
700
600
0.12
0.14
0.16
1.0
1.2
1.4
1.6
0.2
0.4
0.6
data-point error ellipses are 2σ
0.2
0.4
0.63200
2800
2400
2000
1600
0 4 8 12 16 20 24 28
Kam-1 n = 59206Pb238U
1000
800
700
600
0.12
0.14
0.16
1.0
1.2
1.4
1.6
900
207Pb/235UFigure 9. Concordia plots of LA-ICP-MS U-Pb analyses of detrital zircon grains from Lower Cambrian sandstone (Kam-1) and Early Ordovi-cian microconglomerate (Lbq-1) of the Saxo-Thuringian zone. Error ellipses are 2σ. Insets show enlargement of the younger ages. n—number of analyses. For sample details see Table 2.
n=58/59, 90-110% conc.
0
2
4
6
8
10
12
14
4005006007 008009 001000110012001 30 01 4001 5001 6001 70 01 80019002 0002 10 022002 30 02 40 02 500260 02 70 02 8002 9003 000310032003 30034 00
Age (Ma)
Fre
qu
ency
Kam-1 (Lower Cambrian)
n=49/4990–110% conc.
ate
oR
eliv
Pr
bab
ility
500
4 00
60 0
700
80 0
9 00
10 0 0
1 10 0
1600
1 800
2 000
2 20 0
2400
26 00
2800
3 0 00
3200
3 400
n=9/990–110% conc.
0
1
2
3
4
5
6
7
8
9
Fre
qu
ency
Lbq-1 (Lower Ordovician)n=57/59, 90-110% conc.
n=20/2190–110% conc.
n=37/3890–110% conc.
Rel
ativ
eP
rob
abili
ty
500
4 00
6 00
700
800
900
100 0
110 0
16 00
180 0
2000
2200
2400
2600
28 00
3000
3200
3400
Figure 10. Binned frequency and prob-ability density distribution plots of de-trital zircon grains from sample Kam-1 (Lower Cambrian) and Lbq-1 (Early Ordovician). conc—concordance; n—number of analyses with <10% discor-dance/total number of analyzed grains.
84 Linneman et al.
spe423-03 page 84
zircons vary between 0.1 and 1.0 (Tables 1–3), a range that is typical for zircon crystallized from magmas of intermediate to felsic composition (e.g., Hoskin and Schaltegger, 2003). From the Vesser complex ignimbrite Ves-1 we analyzed fi fteen zircon grains, which yielded a concordia age of 497 ± 2 Ma (Fig. 11B). Thus, the upper part of the Rollkopf Formation was most likely deposited during the uppermost Cambrian.
From the pebble-bearing rhyolitic tuffi te KArc-1, eighteen needle-shaped zircon grains were analyzed (Table 3). Nine of these yielded a concordia age of 486 ± 4 Ma, which is identical to the upper-intercept age defi ned by the four discordant analyses (Fig. 11C). The concordia age is interpreted to provide maximum age constraints for tuffi te deposition. In addition, one zircon yielded a concordant Archean age of 2.62 Ga (Table 3).
AGE OF SEDIMENTARY DEPOSTION
In each sample, the youngest concordant zircon age provides a maximum constraint for the deposition of the sampled unit. In the case of the Purpurberg quartzite and the Rothstein Formation,
the youngest ages are 558 ± 16 Ma and 552 ± 11 Ma, respec-tively. However, the uncertainty in the degree of concordance of Neoproterozoic-Paleozoic grains dated by the LA-ICP-MS method is relatively large, and results obtained from just a single analysis have to be interpreted with care. For example, a typical uncertainty of 2–3% (2σ) in 207Pb/206Pb for a Late Neoproterozoic grain (e.g., 560 Ma) relates to an absolute error on the 207Pb/206Pb age of 44–65 Ma. Thus, the youngest grains in both samples can be grouped in the ca. 570-Ma age population. Seven grains that defi ne this population in Pur-1 and Roth-1 defi ne concordia ages of 570 ± 4 Ma (mean squared weighted deviated [MSWD
C+E] =
0.59) and 566 ± 4 Ma (MSWDC+E
= 0.95), respectively. These ages are in agreement with a SHRIMP age of 566 ± 10 Ma for a tuff from the Rothstein Formation (Buschmann et al., 2001). It is therefore likely that the ca. 570-Ma grains in Pur-1 and Roth-1 originated from tuff horizons and that their crystallization ages closely date the depositional ages of the Purpurberg quartzite, Weesenstein Group and Rothstein Formation, respectively. In other words, the deposition of Weesenstein Group and the Roth-stein Formation took place shortly after ca. 570 and ca. 566 Ma,
520
440
400
360
320
280
0.04
0.05
0.06
0.07
0.08
0.35 0.45 0.55 0.65
206Pb238U
481 ±15 Ma(upper intercept age)
MSWD = 0.71
486 ±4 MaMSWDC+E = 0.81
to zero
KArc-1
data-point error ellipses are 2 σ
C
data-point error ellipses are 2 σ
520
510
4800.077
0.079
0.081
0.083
0.085
0.60 0.62 0.64 0.66 0.68
Ves-1206Pb238U
497 ±2 MaMSWDC+E = 0.81
C+E = 0.77Probability
500
490
B
data-point error ellipses are 2 σ
560
206Pb238U
Wett-1
207Pb/235U
543 ±4 MaMSWDC+E = 0.30
C+E = 0.97Probability
0.086
0.088
0.090
0.092
0.68 0.70 0.72 0.74 0.76
550
540
530
youngestzircongrains
only
A
Figure 11. Concordia plots showing LA-ICP-MS U-Pb ages impor-tant for the modeling of the Cadomian orogeny and the opening of the Rheic Ocean in the Saxo-Thuringian zone of the Bohemian Massif. Error ellipses are 2σ. For sample details see Tables 1 and 3. (A) Con-cordia age of 543 ± 4 Ma calculated from the ages of the fi ve youngest detrital zircon grains of microconglomerate sample Wett-1, placing the maximum age of deposition of the Lausitz Group close to the Precam-brian-Cambrian boundary (Lausitz antiform, Saxo-Thuringian zone). (B) Concordia age of 497 ± 2 Ma calculated from the ages of fi fteen magmatic zircon grains from rhyolithic ignimbrite sample Ves-1 from the Vesser complex. (C) Concordia age of 486 ± 4 Ma obtained from the ages of nine magmatic zircon grains from pebble-bearing felsic tuffi te sample Karc-1 from the base of the Ordovician in the southeast-ern part of the Schwarzburg antiform. The upper intercept age of 481 ± 15 Ma, defi ned by four discordant grains, overlaps with the concor-dia age. MSWD
C+E—mean squared weighted deviates of concordance
and equivalence.
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 85
spe423-03 page 85
respectively. A Neoproterozoic age for both units is also indicated by the intrusion of the Dohna granodiorite into the Weesenstein Group at 537 ± 7 Ma (Linnemann et al., 2000; Fig. 3) and the overlaying of the Rothstein Formation by the Zwethau Formation during the Atdabanian (Lower Cambrian) at ca. 534 Ma (Bus-chmann et al., 2006).
The youngest zircon subpopulation in sample Wett-1 yielded an age (543 ± 4 Ma) close to the Neoproterozoic-Cam-brian boundary. This age (Fig. 11A) is interpreted to provide a maximum depositional age for the Lausitz Group. However, shortly after its deposition, the Lausitz Group was deformed and subsequently intruded by voluminous granitoids at 539 ± 6 Ma (Linnemann et al., 2000). Given the uncertainties in the age deter-minations, deposition of the Lausitz Group can be constrained to a narrow time interval of ~10 m.y. (between 547 and 533 Ma) around the Precambrian-Cambrian boundary.
The control for the age of deposition of the Zwethau Forma-tion (Kam-1) comes from paleontological data. Based on trilo-bites and archaeocyatha, the formation is assigned to the Atda-banian, that is, the Lower Cambrian stage starting at ca. 534 Ma (Elicki, 1997, and references therein). The depositional age of the Langer Berg Formation (Lower Ordovician, Lbq-1) is controlled by trace fossils and regional lithostratigraphic correlation, with well-dated sections in the southeastern part of the Schwarzburg antiform (Linnemann, 1996). In both samples the youngest zir-con ages (Kam-1, ca. 535 Ma; Lbq-1, two grains, 498 ± 7 Ma) are very close to the biostratigraphic-controlled deposition age.
The age of 497 ± 2 Ma (Fig. 11B) for Ves-1 zircons is interpreted to date ignimbrite deposition within the upper part of the Rollkopf Formation in the Vesser complex. This part of the Rollkopf Formation is therefore Upper Cambrian, according to Gradstein et al. (2004), who place the upper boundary of the Middle Cambrian at 501 ± 2 Ma. In contrast, the lower part of the Rollkopf Formation is assigned to the Middle Cambrian, based on conventional U-Pb dating of zircon from a dacitic pyroclas-tite (508 ± 2 Ma) and from a gabbro (502 ± 2 Ma) that intrudes the units (Kemnitz et al., 2002; see also Fig. 6). At present no biostratigraphic or geochronological data are available for the Neuwerk Formation in the upper part of the Vesser complex. The overlying Hundsrück Group contains felsic volcanic rocks and highly mature quartzites that are interpreted to be Lower Ordo-vician, based on lithostratigraphic correlation (Bankwitz et al., 1992; Linnemann, 2003a; Fig. 6).
The age of 486 ± 4 Ma for seventeen zircon grains from sample KArc-1 is interpreted to closely date the deposition of the lithostratigraphic unit enclosing the tuffi te. The sample was taken from the level at which the boundary between the Cadomian basement and Lower Paleozoic overstep sequence is suspected. This transition interval is free of key fossils. In this section of the southeastern part of the Schwarzburg antiform no angular unconformity between Cadomian basement and the Lower Paleozoic sediments is observed. The age of the KArc-1 zircons suggest that the Cadomian basement was directly over-lain by highly mature sediments of lowermost Tremadocian age
(lowermost Ordovician; Fig. 6) and, thus, the entire Cambrian is missing.
PROVENANCE OF SEDIMENTS
The analyzed samples were selected to be representative of different successions of distinct ages. However, some care must be taken in the interpretation of the age spectra because a certain degree of sample bias cannot be excluded. Nevertheless, the U-Pb age spectra of the detrital zircons from samples Pur-1, Wett-1, Roth-1, Kam-1, and Lbq-1 show striking similarities (Figs. 8 and 10, Table 4), indicating that they display a common characteristic of the source area. Hence, variations among the samples are inter-preted as indicating variations in the source area. All fi ve samples predominantly contain Late Neoproterozoic (690–550 Ma; 40–83%) and Paleoproterozoic (2.2–1.8 Ga; 15–33%) grains with a smaller fraction of Neo- to Mesoarchean constituents (5–13%; Table 4). In addition, all samples contain a small fraction of ca. 1000- to 900-Ma (1–7%) grains. Only the three Neoproterozoic sediments contain Mid-Neoproterozoic (790–700 Ma; 3–8%) zircons, and Paleoarchean (3–8%) components are present only in Pur-1 and Lbq-1. A common feature of all samples is an “age gap” between 1.75 and 1.0 Ga, which is typical of a Cadomian and/or west African provenance and is diagnostic in distinguish-ing it from East Avalonia and Baltica (e.g., Nance and Murphy, 1994; Friedl et al., 2000). This age gap is in agreement with the characteristic clusters of Paleoproterozoic ages in the interval 2.17–1.78 Ga. Such ages are typical of the western part of the Gondwana supercontinent, which was affected by abundant mag-matic intrusions (ca. 2.2–1.8 Ga) during the Eburnean orogeny (West African craton). Furthermore, Neo- to Mesoarchean zircon ages and, in the case of Pur-1 and Lbq-1, additional Paleoarchean grains, point to recycling of magmatic rocks formed during the Liberian and the Leonian orgenies, respectively. Both orogenies affected the West African craton during the Archean.
The overall dominance of Late Neoproterozoic zircon ages with apparent clusters at ca. 570, ca. 590, ca. 620–610, ca. 640, and ca. 660 Ma point to the fact that the most important source of sedimentary detritus (>55%) was the active magmatic arc of the Cadomian belt (ca. 700–550 Ma; e.g., Nance et al., 2002; Mur-phy et al. 2004; Linnemann et al., 2000, 2004). Taking all 165 Neoproterozoic zircon ages (700–550 Ma) into consideration, the main magmatic activity of this Cadomian arc took place at ca. 615, ca. 590, and ca. 570 Ma (28, 27, and 21%, respectively).
Orthogneisses of Mid-Neoproterozoic protolith age (ca. 755–700 Ma) have been described from the Armorican Massif (Samson et al., 2003) and the Anti-Atlas orogen in Morrocco (D’Lemos et al., 2006). Based on Hf and Nd isotopes these have been interpreted as remnants of an earlier, independent arc sys-tem. Such ages are relative rare in the Avalonian-Cadomian belt. Our results suggest that such sources were present only during the Neoproterozoic and were absent during younger sedimentation.
All samples contain a small fraction of grains with ages close to the Neo- to Mesoproterozoic boundary. Until recently,
86 Linneman et al.
spe423-03 page 86
such “Grenville” ages were interpreted as being derived either from the Amazonian craton (e.g., Friedl et al. 2000; Hegner and Kröner 2000; Fernández-Suárez et al., 2002) or from the Grenville orogenic belt (e.g., Nance and Murphy, 1994). Gren-ville ages typically fall in the range 1.25–1.0 Ga (e.g., Keppie et al., 1998), and an Amazonian provenance is characterized by various Mesoproterozoic age peaks in the 1.8- to 1.0-Ga inter-val (e.g., Nance and Murphy, 1994; Friedl et al., 2000). These ca. 1.05- to 0.9-Ga zircon ages are relatively rare but have been described from various localities within Cadomia (Gebauer et al., 1989; Fernández-Suárez et al., 2002; Gehmlich, 2003; Friedl et al., 2004). They therefore represent a typical Cadomian detrital zircon subpopulation and most likely also have a west African origin. The presented zircon ages strongly suggest a provenance
of the investigated Neoproterozoic rocks from the margin of the West African craton (Fig. 12).
PLATE-TECTONIC MODEL
The data set presented above provides several cornerstones that must be taken into account in the reconstruction of the Neoproterozoic to Cambro-Ordovician evolution of the Saxo-Thuringian zone and adjoining crustal units of the Bohemian Massif. First, all investigated units contain considerable quanti-ties of Archean, Paleoproterozoic, and Neoproterozoic zircons, which provide evidence of intense crustal recycling (Linnemann et al., 2000, 2004; Drost et al., 2004; this study). The detrital age spectra point to a west African provenance and exclude an Ama-
South
pole
Aval
onia
n-
Cad m ano
iA ict ve M r na gi
ca. 570 Ma
Peri Gondwana
-
Per
i-G
on
dw
ana
Ura
ls
E-Ava
lonia
-Aal
ona
Wv
i
Co
lar
ina
Florida
Iber
ia
AM
FMC
Turkish plateAegean
Dobrogea
SXZ T UB
r-A
s
"Poto
lp"
0.55-0.650.9-1.1
1.65-1.852.45-2.7 Ga
*2
0.54-0.71.8-2.22.7-2.9
3.1-3.4 Ga *1
0.54-0.71.0-1.35
1.45-1.752.5-3.1 Ga
*3
0.9-1.21.3-2.2
2.5-2.853.0-3.2 Ga
*4
a
nw
n
Est Go
da
a
eo
dana
Wst G
nw
Yu
cata
n
Oax
aia
qur
isC
ot
Cratons (Archean-Paleoproterozoic)
1.1 - 1.3 Ga Megashear event in Amazonia
Mesoproterozoic mobile belts (Grenville and related events)
Neoproterozoic mobile belts of Gondwana(Pan-African and related events)
Neoproterozoic mobile belts of peri-Gondwana(Cadomian and related events)
CADOMIA
Figure 12. Paleogeography of the Cadomian-Avalonian active margin and related major peri-Gondwanan terranes at ca. 570 Ma. AM—Armori-can Massif; FMC—French Massif Central; SXZ—Saxo-Thuringian zone (part of the Bohemian Massif); TBU—Teplá-Barrandian unit (part of the Bohemian Massif). Numbers in circles: zircon ages from the cratons in Ga. *1—from the compilation of Nance and Murphy (1994 and refer-ences therein); *2—from Avigad et al. (2003); *3—from Schneider Santos et al. (2000); *4—from the compilation of Zeh et al. (2001). Modifi ed after Nance and Murphy (1994, 1996), Linnemann et al. (2000, 2004), Murphy et al. (2000), Linnemann and Romer (2002), Nance et al. (2002); paleogeography of the Gondwanan continental plates after Unrug (1996).
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 87
spe423-03 page 87
zonian and/or Baltic source (Drost et al., 2004; Linnemann et al. 2004; this study). Second, the zircon age spectra indicate that some Late Neoproterozoic sediments were deposited at about the same time (ca. 570 Ma), during, and after intense magmatic activity in the adjacent arcs. Third, most sedimentary basins were deformed between ca. 570 and 540 Ma and intruded by volu-minous granitoid plutons at ca. 540 Ma. The absence of Cado-mian high-grade or high-pressure regional metamorphic rocks like granulite and eclogite suggests that this structural-magmatic event took place without major thickening of the crust and with-out subduction of continental crust (Linnemann et al., 2000). Electron microprobe dating of metamorphic monazite grains from the Teplá-Barrandian unit yielded Th-U-Pb model ages of 540 ± 16, 542 ± 13, and 551 ± 19 Ma, which Zulauf et al. (1999) related to low-pressure/high-temperature metamorphism. Fourth,
the generation of Lower to Middle Cambrian rift-related sedi-ments, Upper Cambrian (ca. 500 Ma) MOR-related mafi c rocks (Bankwitz et al., 1992; Kemnitz et al., 2002; this study) and thick Lower Ordovician successions with high subsidence rates indi-cate the formation of a rift-drift succession in Cambro-Ordovi-cian time (Linnemann and Romer, 2002; Kroner et al., 2003; Linnemann et al., 2004).
Cadomian Back-Arc Basin Evolution
The oldest rocks of the Saxo-Thuringian zone are sediments deposited at ca. 570–565 Ma. Rock units belonging to this age interval make up the Weesenstein and Clanzschwitz groups as well as the Altenfeld and Rothstein Formations (Figs. 3 and 13). The Altenfeld Formation of the Schwarzburg antiform comprises
debris
debris
Strike-slip basinand spreadingzones in the
backarc basin
RothsteinFormation(Roth-1:
566+/-4 Ma)
ClanzschwitzGroup
WeesensteinGroup
Purpurbergquartzite
(Pur-1: 570+/-4 Ma)
AltenfeldFormation
Continent(crust of the
West African craton)
Transitional crust:early Cadomian arc
(c. 600-650 Ma)
Oblique vectorof subduction
MOR-basalts,andesites andhydrothermalblack cherts
closed strike-slipbasin: redepositionof sediments and
arc-related igneousrocks
Cadomian arc(late stage,
c. 550-590 Ma)
Intra-arc basin
stretched and thinnedcontinental and arc crust
byU
lLi
nem
ann
206
fn
0
Cadomian backarc basinc. 590-560 MaEarly Miocene
Euian
rasate
Pl
pJa
aArc
n
f cPaci i Plate
stretchedcontinental
crustoceanic crust
apanJ
Sea
N
debris
Figure 13. Model for the plate-tectonic development of the Cadomian back-arc basin at ca. 590 and 560 Ma, based on data derived from the Saxo-Thuringian zone (Bohemian Massif). Back-arc basin consists of a continentward passive margin, represented by the Weesenstein and Clanzschwitz groups, and an arcward margin, characterized by more strongly stretched continental crust and the accumulation of predominantly arc-derived debris. The back-arc is documented by MOR-related rocks and hydrothermal black cherts recorded in the Altenfeld and the Rothstein formations. Inset: Sketch of analogous plate-tectonic confi guration represented by the opening of the Japan Sea in the western Pacifi c region during the Early Miocene (after Jolivet et al., 1992). The back-arc basin of the Japan Sea is largely fl oored by stretched continental crust.
88 Linneman et al.
spe423-03 page 88
similar facies to those of the Rothstein Formation (black cherts, mafi c igneous rocks), suggesting a similar age and plate-tectonic setting of deposition (Fig. 13). In the same way the Clanzschwitz Group can be correlated with the Weesenstein Group (Pur-1).
We assume that all of these units were deposited in a back-arc basin, which predominantly consisted of thinned continental crust and was fl anked by a magmatic arc to the “north” and by a cratonic source to the “south” (Linnemann et al., 2000; Bus-chmann et al., 2001; Figs. 3 and 13). Back-arc spreading at ca. 570 Ma is best documented by the Rothstein Formation (Bus-chmann, 1995). Field data and geochemical information sug-gest that the 566 ± 10-Ma-old Rothstein Formation comprises a low-grade metamorphic suite of intrusive and effusive enriched mid-ocean ridge basalts (E-MORB), andesites, calc-alkaline metabasalts, and subordinate alkaline metabasalts (Buschmann, 1995; Buschmann et al., 2001). The submarine effusive character of these rocks is indicated by pillow structures that may have formed during seafl oor spreading. The submarine character is additionally supported by black cherts, which are assumed to be the product of hydrothermal activity at a spreading center that caused alteration of the submarine volcanic and sedimentary rocks (Fig. 4A). According to Buschmann (1995), deposition of the Rothstein Formation was accompanied by strike-slip faulting that produced submarine pull-apart basins and led to the re-sedi-mentation of older unconsolidated sediments. A similar age and tectonic regime is assumed for the Altenfeld Formation.
In contrast to the Rothstein and Altenfeld formations, the Weesenstein and the Clanzschwitz groups were most likely deposited at the passive margin of the back-arc basin, the exis-tence of which is indicated by (1) Nd-model ages for the sedi-ments in the range ca. 2.1–1.5 Ga (Linnemann and Romer, 2002), (2) abundant Paleoproterozoic detrital zircon ages (Linnemann et al., 2004; this study), and (3) the presence of highly mature sedi-ments like the Purpurberg quartzite (Linnemann, 1991; Fig. 13).
The existence of a Cadomian magmatic arc can deduced from the geochemical signatures of the Late Neoproterozoic sediments (Buschmann, 1995; Linnemann and Romer, 2002; Drost et al., 2004). These sediments have a felsic provenance pointing to a relatively mature continental arc with a relatively thick root zone.The main phase of arc magmatism occurred between ca. 560 and 600 Ma (Linnemann et al., 2004; this study). Comparable ages and arc-related igneous rocks are also described from the Avalon-ian part of the Bohemian Massif (Finger et al., 2000; Friedl et al. 2004; Fig. 2). Relatively small remnants of the Cadomian arc are known from the Armorican Massif and Iberia. Also in the Bohe-mian Massif arc remnants sensu stricto are scarce (e.g., Kríbek et al., 2000). It therefore appears that the main part of the Cadomian arc and its Avalonian counterpart are preserved in the Avalonia microcontinent (Murphy et al., 2006), whereas the main part of the back-arc basins remained in Cadomia. This arrangement is impor-tant to the subsequent opening of the Rheic Ocean (see below).
Figure 13 shows a possible reconstruction of the Cadomian back-arc basin in the Saxo-Thuringian zone, with deposition of the passive margin sequences of the Weesenstein and Clanzschwitz
groups on the southern fl ank. The Rothstein and the Altenfeld formations are located in the interior portion of the back-arc basin within the external domain of the Saxo-Thuringian zone to the north (Fig. 2). We interpret the present geographical arrangement as refl ecting the original paleoposition on the west African mar-gin. Sample Pur-1 (Weesenstein Group) from the passive margin sequence contains ~40% Late Neoproterozoic detrital zircons derived from the Cadomian arc, which is considerably less than their abundance in Roth-1 from the Rothstein Formation (66%). In addition, the two samples differ in their Neoproterozoic age spectra, ~60% of Late Neoproterozoic grains in Pur-1 falling in the age range 670–600 Ma, whereas ~60% in Roth-1 fall in the range ca. 590–560 Ma. This difference suggests that in the source area of the passive margin, sequence remnants of an earlier stage of the Cadomian magmatic arc were exposed. This arc may rep-resent the transitional zone between the craton and the stretched crust underlying the basin fl oor of the back-arc basin (Fig. 13). Consequently, we assume this early arc stage (ca. 670–600 Ma) was characterized by a Cordilleran-type active margin with sub-duction of the oceanic plate directly under the craton.
The Cadomian back-arc basin probably opened into an expanded marginal basin to allow for the differentiation of its deposits into various facies patterns. The age of granitoid peb-bles (577 ± 3 and 568 ± 4 Ma; Linnemann et al., 2000) from the Weesenstein and Clanzschwitz groups suggests rapid exhuma-tion of granitoids intruded in the source area during or shortly before opening of the back-arc basin. The latter likely took place within a strike-slip regime, with the development of small sub-basins acting as local suppliers of sediments, including material derived from the underlying stretched crust (Fig. 13). On the basis of similar relationships in Avalonia, Nance and Murphy (1994) proposed a model in which the oblique vector of subduc-tion beneath the arc led to strike-slip motions in the back-arc basin. The concept of oblique subduction and its effects in the hinterland, combined with our fi eld observations, is incorporated into the model in Figure 13.
Cadomian Retroarc Basin
The largest part of the weakly metamorphosed and well-pre-served Late Neoproterozoic sediments of the Saxo-Thuringian zone is represented by the Lausitz Group (Wett-1; Lausitz anti-form), the Leipzig Formation (North Saxon antiform), and the Frohnberg Formation (southeastern part of the Schwarzburg antiform) (Figs. 2 and 3). These units are positioned between the deposits of the inner back-arc basin and the passive margin (Figs. 2 and 3) and clearly differ in their fl ysch-like character from the sedimentary units discussed above. They are characterized by monotonous series of dark-gray graywacke turbidite inter-calations of conglomerates and microconglomerates that often contain fragments of granitoids, metasediments, and black cherts (Fig. 4D). Frequent black chert fragments indicate their deriva-tion, in large part, from eroded material derived from the inner back-arc basin. Given their similarity in lithology, sedimentation
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 89
spe423-03 page 89
regime, spatial distribution, and detrital modes, a comparable depositional age is assumed for the Leipzig and Frohnberg for-mations and the Lausitz Group. Depostion most likely took place at the Precambrian-Cambrian boundary, ~20–30 m.y. after the opening of the back-arc basin, as inferred from Wett-1 zircon ages and the intrusion of younger granites (see above).
Sedimentary structures and paleoseismic features, such as water-escape structures (Fig. 4E), soft pebbles, and seismites, suggest rapid sedimentation (Linnemann, 2007). As shown in Figure 14, we interpret the Lausitz Group and the Leipzig/Frohn-berg formations to be parts of a Cadomian retroarc basin or fore-land basin. In our model this basin was formed during closure of the back-arc basin in response to the collision of the Cadomian
arc with the West African craton. Only the inner part of the retro-arc basin was folded and thrusted, which explains why the more northerly portions of the Lausitz Group and the Leipzig Forma-tion were deformed before intrusion of the ca. 540-Ma granitoids (Linnemann et al., 2000). An angular Cadomian unconformity between Late Neoproterozoic rocks and Cambro-Ordovician deposits is documented from different sections of these units (Linnemann and Buschmann, 1995a,b). However, the zircon age of 486 ± 4 Ma from pyroclastic sediments (KArc-1) of the Frohn-berg Formation point to an Early Ordovician onset of Paleozoic sedimentation in the sections from the southeastern part of the Schwarzburg antiform. Cambrian strata are missing here, and the Cadomian unconformity is not an angular one. Instead, it is a
Cadomianretroarc basinc. 545-540 Ma
Retroarc basin
Remnantbasin
inner
outer
Magmatic andanatectic event
at c. 540 Ma
Fold-and thrust belt
FrohnbergFormation
(upper section)
Continent(cratonic crust)with chemical
weatheringsurface
Slab break-off
ol ed a hr tF d nd t us edr ks o heoc f t
a omi nC d aba a s nck rc ba i
Ul
iem
nn00
byf L
nna
26
FrohnbergFormation
(lower section)
Change to atransform margin
Erosion of the black chert-bearingRothstein and Altenfeld formations
in the fold- and thrust belt
LausitzGroup
(Wett-1:543+/-2 Ma)+Leipzig fm.
Figure 14. Model for the plate-tectonic evolution of the Cadomian retroarc basin between ca. 545 and 540 Ma, based on data from the Saxo-Thuringian zone. There is no Cadomian angular unconformity on the continentward outer margin of the retroarc basin because Late Neoprotero-zoic sediments were unaffected by deformation (e.g., upper section of Frohnberg Formation in southeastern part of Schwarzburg antiform). In contrast, closer to the fold-and-thrust belt in the inner part of the retroarc basin, the sediments are deformed and consequently an angular uncon-formity is developed between Neoproterozoic retroarc sediments and overstepping Cambro-Ordovician strata (e.g., Cadomian unconformities at top of the Lausitz Group and Leipzig Formation). fm—formation.
90 Linneman et al.
spe423-03 page 90
disconformity or a simple sedimentation gap (paraconformity), without the occurrence of deformation between the under- and overlying strata. Thus, the undeformed Frohnberg Formation is interpreted to have been deposited in a more distal position rela-tive to the fold-and-thrust belt and more proximal to the cratonic hinterland, whereas the deformed Lausitz Group and Leipzig Formation were situated closer to the colliding arc (Fig. 14).
In contrast to the lower part of the Frohnberg Formation, which is composed mainly of thick-bedded graywackes, the ~100-m-thick upper part was likely deposited in a shallower marine environment (Linnemann, 2003b). This observation sug-gests termination of the retroarc basin regime and the onset of deposition in a remnant basin located in front of the outer retro-arc deposits (Fig. 14). The very topmost quartzite bed of the sec-tion, known as “Basisquarzit” (=basal quartzite), is tradionally interpreted as the base of the Paleozoic overstep sequence (von Gaertner, 1944). It is, however, more likely that this quartzite rep-resents the fi nal bed of a continuous upward-thickening section of the Neoproterozoic remnant basin.
The Cadomian retroarc basin and the related remnant basin were short-lived depositional systems. Based on the youngest detrital zircon ages of Wett-1 (543 ± 4 Ma; Fig. 11A) and the age of the Lausitz granitoid complex (539 ± 6 Ma; Fig. 3), which intrudes these sedimentary units, the ages and time spans of these systems can be confi ned to a time interval of <12 m.y. at the end of the Neoproterozoic and in the earliest Cambrian.
Most Late Neoproterozoic sedimentary were intruded by voluminous granitoids at ca. 540 Ma (Linnemann et al., 2000; Gehmlich, 2003; Tichomirowa, 2003; Fig. 3). The largest exposed body is the Lausitz granitoid complex, which covers an area of ~100 × 50 km2. For this complex, a minimum granitoid volume of ~5000 km3 can be calculated, assuming a thickness of only 1 km. Most granitoids were likely derived from melting of the Late Neo-proterozoic graywackes or similar units, because they contain large numbers of inherited zircons with age spectra comparable to those in the sediments (Linnemann et al., 2000, 2004; Gehmlich, 2003; Tichomirowa, 2003). This interpretation is supported by geo-chemical data and graywacke xenoliths in the granitoids (Hammer, 1996). These ca. 540-Ma granitoids record a relatively short-lived regime of high levels of heat fl ow, which we attribute in our model to slab break-off of the subducted oceanic plate (Fig. 14).
All the processes summarized in Figures 13 and 14, from the early stages of a Cadomian magmatic arc (ca. 650–600 Ma), through opening of the Cadomian back-arc basin and its closure during arc-continent collision with subsequent formation of a ret-roarc basin, to the magmatic-anatectic event at ca. 540 Ma, corre-spond to our present understanding to the Cadomian orogen that formed at the margin of the West African craton.
Opening of the Rheic Ocean
There is no sharp break between the geological history linked to the Cadomian orogen and that of the Cambro-Ordovi-cian, which fi nally led to the opening of the Rheic Ocean. Instead,
the latter is viewed as a logical continuation of the geological his-tory of the dying marginal orogen. Nance and Murphy (1996) and Nance et al. (2002) proposed a Cordilleran model for the fi nal stages of the Avalonian-Cadomian orogen analogous to the Cenozoic history of ridge-continent collision in the area of Baja California in the eastern Pacifi c. Such a model would explain both the geodynamic change from subduction-related processes to the opening of a new ocean and the excision of a long slice of continental crust, like that which formed the microcontinent of Avalonia. We have adapted these ideas to explain the plate-tectonic setting of the Saxo-Thuringian zone during the Cambro-Ordovician. After ridge-continent collision, slab break-off was triggered at ca. 540 Ma by the switch from an active margin to a transform margin setting (Fig. 15, inset).
Cambrian sediments in the Saxo-Thuringian zone are restricted to the Lower and Middle Cambrian, with the onset of sedimentation occurring in the Atdabanian at ca. 530 Ma (Elicki, 1997). These units are characterized by carbonates with archaeocyatha, siliciclastic sediments, and red beds. The last were likely derived from erosion of laterite horizons generated on the denuded Cadomian orogen and the cratonic hinterland at ca. 540–530 Ma (Linnemann and Romer, 2002). These occurrences suggest a general uplift of the Cadomian orogen that was prob-ably due to the rapid changes in plate-tectonic setting. In addi-tion, the laterites and the occurrence of archaeocyatha point to deposition at low paleolatitudes.
The overall change of the plate-tectonic regime is refl ected by the onset of Cambrian sedimentation. Detritus of the Cam-brian deposits was predominantly (~80%) derived from the Cadomian orogen, as inferred from the age spectrum of Kam-1. The plate-tectonic setting may therefore have been similar to that of the present-day Basin and Range Province lying close to Baja California and the San Andreas fault (Fig. 15). In this way, stretching and thinning of the Cadomian crust and transcurrent faulting induced by the activity of the transform margin may have led to the opening of a rift basin fi lled with Lower and Middle Cambrian sediments. As a result, the Cadomian orogen became largely denuded. The rift basin likely developed on the side of the faulted and thrusted orogen, because this location would have been more sensitive to tectonic reactivation than the cratonic hin-terland. The interplay between the more stable cratonic hinter-land and the weaker Cadomian crust is thought to have led to asymmetric rifting (Fig. 15).
Upper Cambrian sediments are relatively scarce in the Saxo-Thuringian zone, and fossiliferous deposits of this age are unknown. However, the lower and upper part of the Rollkopf Formation from the Vesser complex were deposited during the Middle and Upper Cambrian, respectively (see above). This unit belongs to the external domain of the Saxo-Thuringian zone, refl ecting a paleoposition on the outer margin of the eroded and recycled Cadomian orogen. The Vesser complex is dominated by MOR-related igneous rocks associated with metasediments (Bankwitz et al., 1992; Figs. 2 and 6). In our view, this complex records the incision of an oceanic ridge that collided with the
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 91
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periphery of the Cadomian orogen in a situation similar to pres-ent-day Baja California (inset of Fig. 16). Thinning of the litho-sphere and upwelling asthenosphere led to enhanced heat fl ow in the upper lithosphere and the generation of Vesser magmatism. In our model the Vesser complex formed between the outer and inner zone of the asymmetric rift basin, as this location represents its weakest part (Fig. 16). The outer part, the former continental arc, was characterized by relatively thick crust, ongoing subsidence, and Upper Cambrian sedimentation, whereas the inner part, the former Cadomian back-arc basin, was more strongly affected by lithospheric thinning caused by uplift and upwelling of the asthe-nosphere (Fig. 16). Asymmetric rifting typically shows uplift of the remaining, thinned, lower plate and subsidence of the depart-ing heavier upper plate (Wernicke, 1985; Coward, 1986). This asymmetry would explain the ongoing Cambrian sedimentation
on the upper plate, which would later become a part of Avalonia or a related terrane, and the absence of Upper Cambrian deposits on the lower plate, which represents the Cadomian realm at the periphery of the West African craton. This scenario is in agree-ment with the lack of Upper Cambrian sediments in the Saxo-Thuringian zone and the high maturity of Lower Ordovician deposits resulting from intense chemical weathering processes during the Upper Cambrian.
Lower Ordovician deposits in the Cadomian part of cen-tral and western Europe are characterized by thick and wide-spread sandstone deposits, frequently metamorphosed to quartzites. The most prominent example is the ≤700-m-thick Armorican quartzite of the Armorican and Iberian massifs. Its equivalents in the Saxo-Thuringian zone are the quartzites of the 3000-m-thick Frauenbach and Phycodes groups from the
Asymmetric rift basinduring the Lower and
Middle Cambrianc. 530-500 Ma
Rift basin(asymmetric
rifting)
RiftedCadomianbasement
Continent(cratonic crust)
FormerCadomian
continental arc
Lower to MiddleCambriansediments
byU
lL
nem
nn2
0f
in
a0
6
Zwethau,Tröbitz,
& DelitzschFormations
Meandetachment
level
Transform margin
San Andreas Fault
Basin and Range
10 Ma
N
Figure 15. Model for the formation of the asymmetric rift basin during the Lower to Middle Cambrian between ca. 530 and 500 Ma in the Saxo-Thuringian zone. The geological setting (see inset) is assumed to be similar to that of the Basin and Range province of North America 10 m.y. ago. Modifi ed from Atwater (1970), Christiansen and Lipman (1972), Dickinson (1981), Condie (1989), and Nance et al. (2002).
92 Linneman et al.
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southeastern part of the Schwarzburg antiform (Fig. 6). Quartz-ites of the Langer Berg Formation (Lbq-1) and those from the Hundsrück Group (Vesser complex) are their stratigraphic equivalents (Fig. 6). These deposits overstep in places Lower to Middle Cambrian strata (e.g., drill holes Heinersdorf 1 and 2; Wucher, 1967; Linnemann et al., 2000) and in other cases over-lie directly the Cadomian basement, as in the Hohe Dubrau area in the Lausitz antiform (Linnemann and Buschmann, 1995b). Both forms of the Cadomian unconformity are also reported from the Armorican Massif and different parts of Iberia. It is therefore likely that rifting culminated in the Upper Cambrian, with the formation of rift shoulders, tilted blocks, and/or horsts and grabens, such that, in some places the Lower to Middle Cambrian is preserved, whereas in others the underlying Cado-mian basement is exposed.
Extension over the entire paleolandscape and the enormous thickness of the Lower Ordovician overstep sequences classify these siliciclastics as post-rift sediments or deposits of a rift-drift transition. These sedimentary rocks must have been linked to considerable tectonic activity and thermal subsidence, resulting in large systems of detachment faults and escarpments on the sur-face. Lbq-1 (Langer Berg Formation) shows a very similar pre-Ordovician zircon age spectrum to those of the Neoproterozoic deposits (Table 4). The signifi cant number of Paleoproterozoic and Archean grains suggests that rift-related breakaway faults extended into the cratonic hinterland (Fig. 17). In addition, Cado-mian basement and Cambrian igneous rocks were either avail-able to erosion or their zircons were distributed in the weather-ing crust formed during the Upper Cambrian and recycled by the Lower Ordovician transgression.
Upper Cambrian:Incision of theoceanic ridge
c. 500-490 Ma
Uplift
Vessercomplex:bimodal
magmatism& MOR-relatedrocks
(Ves-1:497+/-2 Ma)
Continent(cratonic crust)
Inner Zoneof the
rift basinSubsidenceand ongoing
Upper Cambriansedimentation
Uplift, chemical weatheringand gap during
the Upper Cambrian
Uf
byl
Linn
eman
n20
06
Transform margin
Dehydration& new melts
Incision of theoceanic ridge
High heat flow and magmaticevent caused by thinning
of lower crust and upwellingof the asteosphere
Baja CaliforniaJuan de Fuca Rise
3 Ma
OuterZoneof the
rift basin
N
Figure 16. Plate-tectonic model for the opening of the Rheic Ocean during the Upper Cambrian between ca. 500 and 490 Ma in the Saxo-Thuringian zone. Ocean opening is assumed to have been caused by the oblique subduction of an oceanic ridge similar to the present plate-tectonic situation (see inset) on the west coast of North America. MOR—mid-oceanic ridge. Modifi ed from Atwater (1970), Christiansen and Lipman (1972), Dickinson (1981), Condie (1989), and Nance et al. (2002).
The continuum between Cadomian orogenesis and opening of the Rheic Ocean 93
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Magmatic rocks with an age of ca. 490–480 Ma are widely distributed in the Saxo-Thuringian zone. They range from large plutons (Rumburk granite, Lausitz antiform; Figs. 2 and 3) to subvolcanic porphyroids (Bärentiegel, Schwarz-burg antiform; Fig. 3) and various pyroclastics (e.g., Wurzel-berg tuffi te; Linnemann et al., 2000). An example of the last is sample KArc-1 (“Konglomeratische Arkose”), dated at 486 ± 4 Ma (Figs. 3, 11A). This magmatic event represents the fi nal rift-related magmatism (Fig. 17). After ca. 480 Ma, the Saxo-Thuringian zone is characterized by tectonic and magmatic qui-escence and monotonous shelf sedimentation. We interpret this zone to be the passive margin of the Rheic Ocean, which had
opened as a result of the separation of Avalonia or a related ter-rane (Fig. 18).
The geodynamic evolution outlined above has been deduced as far as possible from fi eld geology, zircon dating, and petrographic and geochemical data. We have attempted to illus-trate the way Cadomian orogenic processes may have operated and have compiled evidence supporting the idea that large parts of the Cadomian magmatic arc are now present in Avalonia or a related terrane, while the back-arc and the retroarc basins remained in Cadomia. We believe the long-lasting subduction processes that produced the arc and related basins were termi-nated by ocean closure in combination with ridge-continent
Rift and drift transition: Openingof Rheic ocean and formation
of passive marginc. 490-480 Ma
Tuffite(KArc-1:
486+/-4 Ma)
Oceanic crust
Oceanic ridge
BuriedVesser
complex
Lower to MiddleOrdovician
overstep sequence
Final rift-relatedmagmatismat c. 490 Ma
Southern passive margin sequenceoverlying Cadomia
Opening of theRheic ocean
Remnants of Lowerto Middle Cambrian
byU
lfLi
nnem
an2
06n
0
Baja CaliforniaJuan de Fuca Rise
3 Ma
Northern passive margin sequenceoverlying driftedarc (Avalonia orrelated terrane)
N
Unconformities onNeoproterozoic and
L.-M. Cambriansediments
Figure 17. Plate-tectonic model for the fi nal opening of the Rheic Ocean and the formation of passive margins at ca. 490–480 Ma. The northern terrane that separated from the Gondwanan margin (Cadomia) could be part of Avalonia or a correlative terrane. Note the different unconformi-ties between Neoproterozoic/Cambrian, Neoproterozoic/Ordovician and Cambrian/Ordovician strata, the general overstep of Lower Ordovician shallow marine sediments, the burial of the Vesser complex, and the renewed exhumation of cratonic crust in the hinterland. Inset shows analo-gous plate tectonic situation in part of the Basin and Range province of North America at ca. 3 Ma. Modifi ed from Atwater (1970), Christiansen and Lipman (1972), Dickinson (1981), Condie (1989), and Nance et al. (2002).
94 Linneman et al.
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collision. We have also tried to show that Cadomian oroenic processes and the subsequent opening of the Rheic Ocean were closely related to each other.
CONCLUSIONS
Sediment provenances and magmatic events of Late Neopro-terozoic (Ediacaran) and Cambro-Ordovician rock assemblages of the Saxo-Thuringian zone have been constrained by new LA-ICP-MS U-Pb ages from detrital zircons of fi ve sandstones and magmatic zircons in an ignimbrite and one tuffi te. These geo-chronological results, in combination with the analysis of plate-tectonic setting constrained from fi eld observations, sedimento-logical and geochemical data, and trends in basin development, have been used to reconstruct Cadomian orogenic processes dur-ing the Late Neoproterozoic and the earliest Cambrian. A contin-uum between Cadomian orogenesis and the opening of the Rheic Ocean in the Cambro-Ordovician is supported by this data set.
The early stage of Cadomian evolution is characterized by a Cordilleran-type continental magmatic arc, which was estab-lished at the periphery of the West African craton between 650 and 600 Ma. Subsequently, at ca. 590–560 Ma, a back-arc basin was formed behind the Cadomian magmatic arc. The formation of this basin was caused by crustal stretching in a strike-slip regime, which is similar to that presently observed in the Japan Sea of the western Pacifi c. Following collision of the Cadomian magmatic arc with the cratonic hinterland, the back-arc basin was closed between ca. 545 and 540 Ma. At this time a short-lived Cadomian retroarc basin was formed. Subsequently, a mid-oceanic ridge was subducted underneath the Cadomian orogen. This process may have been accompanied by slab break-off of the subducted
oceanic plate, which resulted in increased heat fl ow, refl ected in voluminous magmatic and anatectic events culminating at ca. 540 Ma. The subsequent oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian (530–500 Ma). This plate-tectonic scenario is assumed to have developed in a setting similar to that of the Baja California area ca. 3–10 Ma ago. This process contin-ued from the Middle to the Upper Cambrian (ca. 500–490 Ma) and fi nally caused the opening of the Rheic Ocean, an event doc-umented by thick Lower Ordovician siliciclastic sediments and a fi nal rift-related, bimodal magmatic event at ca. 490–480 Ma.
ACKNOWLEDGMENTS
<FLUSH>Prof. Dr. Gerhard Brey, Dr. Heidi Höfer, Jan Heli-osch, Kai Klama, and Dr. Yann Lahaye (Institut für Geowis-senschaften, Facheinheit Mineralogie, Universität Frankfurt am Main) are thanked for their help and fruitful discussions. We thank Damian Nance (Athens, Ohio, United States) for editorial handling and improving the language, and Dr. Jana M. Horák (Cardiff, United Kingdom), Dr. Stanislaw Mazur (Wroclaw, Poland) and Dr. Armin Zeh (Würzburg, Germany) for their crit-ical reviews and inspiring discussions. Funding was provided to AG by the German Science Foundation (DFG; GE 1152/2-2). This paper is a contribution to the International Geological Cor-relation Program Project 497—“The Rheic Ocean: Its origin, evolution and correlatives” (http://www.snsd.de/igcp497/).
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