iNTERNATIONAL JOURNAL OF MARINE GEOLOG~ GEOCHEMISTRY ANO GEOPHYSICS
Calc-alkaline magmatism and rifting of the deep-water volcano of Marsili (Aeolian back-arc, Tyrrhenian Sea)
C. Savelli a, G. Gasparotto b
"CNR, Istituto per la Geologia Marina, Via Gobetti 101, 40129 Bologna, Italy bDipartimento di Scienze Mineralogiche, Universitgz Piazza di Porta S. Donato 1, 40126 Bologna, Italy
(Received June 15, 1993; revision accepted March 29, 1994)
Abstract
Geochemistry and mineral chemistry data indicate that new lava samples recovered from a large depth interval of Marsili volcano (southern Tyrrhenian Sea) are calc-alkaline. The deep to intermediate portions of the volcano are made up by calc-alkaline basalts, whereas the summit consists of high-potassium calc-alkaline andesites. The contents of MgO, CaO, total iron, Cr, Ni and V decrease with increasing silica, while K20, Rb, Ba, Th, Nb and LREE exhibit the opposite behavior. The andesite lavas of the top, at waterdepth shallower than about 800 m, are younger than 0.2 Ma.
Composition of the basalts varies with the waterdepth; overall, the deepest products of Marsili are geochemically more mafic than those erupted in the higher physiographic positions. Geochemical features suggest the existence of a mantle source that was modified by metasomatic acquirement of subduction-derived incompatible elements.
The seamount lies on oceanic crust which formed above subducting lithosphere not earlier than about 2 Ma ago. It is currently undergoing extension along linear faults. The major distensional fractures trend N S to N15°E paralleling the physiographic elongation of the volcano. At the summit, later faults have an oblique NE-SW orientation which is associated with the eruption of high-potassium andesites. The transition from basalts to high- potassium andesites is discussed as a possible response to the extension and rapid subsidence that dominate the volcano and surrounding region.
1. Introduction
Owing to the setting on basaltic crust which precludes significant modification of mantle melts by assimilation of sialic crust during the storage and ascent of magmas in the upper lithosphere, as recognized in the products of the Aeolian arc volcanoes (e.g., Bargossi et al., 1990; Peccerillo and Wu, 1992), information about the geochemical and mineralogical compositions of the Marsili volcanics may contribute to a better understanding of the complex nature and evolution of calc- alkaline (CA) basic parental magmas emplaced at the edge of active continental margins (Fig. 1 ).
The seamount in question is located above a deep seismogenic zone whose length (Fig. 2) is short, about 120 km, with respect to its maximum focal depth, about 500km (Ritsema, 1979; Gasparini et al., 1982; Anderson and Jackson, 1987; Giardini and Velon/t, 1991).
The rocks analyzed were obtained in the course of two expeditions aboard USSR research vessels (R/V Vityaz, 1986, Leg 12 and R/V A.M. Keldysh, 1988, Leg 16) using the Mir 1 and 2 submersibles (Keldysh expedition), as well as dredging and coring. The investigation carried out by the Russian Institute of Oceanology on the Marsili slopes sampled lava from 2590 to 500 m depth.
138 C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157
4 0 °
VAVILOV Smt.
7
0 ~5
ODP 650 •
GLAUCO Smt.
o o c9
\
<
m
ne
Ill
, ,A
s , c , L v
1 3 ° 1 4 ° 1 5 ° 1 6 °
Fig. 1. Bathymetric (contours in meters) and tectonic sketch map of the Southeastern Tyrrhenian volcanic region (Aeolian arc and Marsili Basin), modified after Boccaletti et al. (1984, 1990) and Doglioni (1991). 1 =Major normal faults; 2=ma j o r strike-slip faults (small arrows indicate the sense of motion); 3 = submerged and 4=subaerial volcanic centers; 5=present-day slip vector of the Calabria Peloritani orogenic arc; 6=drill-site 650 of the Ocean Drilling Project; 7(circled numbers): main volcanic centers: (1) = Isl. of Ustica; (2 )= Sisifo smt.; ( 3 )= Enarete smt.; (4 )= Eolo smt.; (5 )= Isl. of Alicudi; (6 )= Isl. of Filicudi; (7 )= lsl. of Salina; (8) = Isl. of Lipari; (9) = Isl. of Vulcano; (10) = Isl. of Panarea; (11 ) = Isl. of Stromboli; (12) = Lametini smt.; (13) = Alcione smt.; (14) =Palinuro smt. Marsili Basin is bounded to the north by the important E W trending shear zones of Palinuro and to the south by the continental slope of Sicily.
These rocks and the samples recovered previously (Maccarrone, 1970; Keller and Leiber, 1974; Selli et al., 1977) allow for an investigation covering practically the entire elevation of the volcano, with the exception of the lowermost flanks which are overlain by sediments.
The scope of this paper is to provide petrochemi- cal data of new lava samples in order to evaluate the evolution of magmatism and the dynamics of the mantle and lithosphere in the study area. Previous investigations of Marsili's magmatic pro- ducts demonstrated the presence of CA lavas. Besides these, basalts geochemically similar to ocean island basalts (OIB) have been recovered in
one dredge station located in the central crest zone (Fig. 3) (Selli et al., 1977). Owing to the same NNE-SSW trending physiographic alignment of all three great central Tyrrhenian volcanoes (Magnaghi, Vavilov and Marsili; Fig. 4), the lower to intermediate parts of Marsili were considered to consist of products similar to OIB, thus sharing petrochemical affinities with the two other volca- noes (e.g., Morrison, 1980; Carta Tettonica d'Italia, 1981; Serri, 1991 ). However, geochemistry and distribution of the new lava samples demon- strate that manifestations of calc-alkaline nature are by far the prevailing ones, in both the deep and shallow sectors of the volcano in question.
C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157 139
v \
,o°½ t ~MAVILOV Smt. o._K
39" ~ o o 30o0
®® ®
;% :o0o- 7
® I 6s0 ® ~ [® "
I (?tAt/CO Sml. MARSILI Smt.
e ® •
TICA I. C3
"'l
~ D AEOLIAN ARC o C~7 O (~)
. J
¢3
[ I S I C Ij L Y J / / ~ _ _ _ j / 13 ° 14 ° 15 ° 16 °
Fig. 2. Distribution of the deep foci in the south Tyrrhenian seismogenic area simplified after Giardini and Velonfi (1991). Depth of foci: 1 = 100 250 km; 2 = 250-350 km; 3 = > 350 km; 4 = main deep-seated structural discontinuity, roughly corresponding to the Pliocene relict volcanic arc between the Marsili and Vavilov basins; black triangle = Site 650 of ODP.
2. Geological setting
Marsili Volcano is a prominent almost N-S trending physiographic feature emplaced in the deep basin of Marsili, the youngest (latest Pliocene-Quaternary) area of the Tyrrhenian Sea (Fig. 3 and inset). The main part of Marsili Seamount is associated with a positive magnetic anomaly, likely belonging to the the Brunhes chron (Savelli and Schreider, 1991). The volcano thus appears to be not older than about 0.7 Ma.
The deep plain around the seamount is bounded to the south and east by the Aeolian arc that includes Palinuro, Alcione and Lametini Seamounts, the Aeolian Islands, and Eolo, Enarete and Sisifo Seamounts (Fig. 1). The vol- canites of this tightly concave arc of Quaternary age (1-0 Ma; Beccaluva et al., 1985) exhibit a
notably wide spectrum of chemical composition varying from the arc-tholeiites of North Lametini Seamount to the leucite tephrites of the Islands of Stromboli and Vulcano (Barberi et al., 1974; Villari, 1980a, Beccaluva et al., 1985). In the Island of Lipari true basalts (SIO2<52%) are absent. The basalts of Stromboli belong to the high-K, shoshonitic and high potassium calc- alkaline (HKCA) series, while basalts have CA affinity in the Islands of Filicudi, Alicudi, and Salina. The Marsili Basin and the Aeolian island arc, are floored with different types of crust. Marsili Seamount rests on thin, oceanic crust, whereas the Aeolian volcanoes are emplaced on continental crust 15-20km thick (Morelli et al., 1975; Nicolich, 1981; Boccaletti et al., 1990). This transitional crust was thinned during the post-collisional evolution of the lithosphere
140 C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157
)
13& C T 6 9 - 3 0
10'
/
j . 140zo ,
1 F--6~
13 ct = F-6-q ~ F--i-q
= F--n-1 87w-q . F - ~ 7 F-z-]
30'
Fig. 3. Bathymetry of Marsili Seamount according to map N. 1602 of the Istituto Idrografico della Marina; location of sampling sites: 1=dredge site o f OIB-like basalt (Station CT69/27 of Selli et al., 1977). Lavas of CA affinity: [3=basalts, ¢¢=andesites; 2 = coring sites of basalt and basaltic andesites; 3 = dredge sites of basalts and basaltic andesites; 4 = sampling sites of basalts recovered during dive no. 21 of the Mir 1 submersible; 5=cor ing sites of andesites; 6=dredge sites of andesites; 7=submersible Mir 2 dive no. 16, sampling sites of H K C A andesites (near the top). Dashed lines=fractures in the northern area of seamount; VB ( inset)= Vavilov Basin.
C Savelli, G Gasparotto/Marine Geology 119 (1994) 137-157 141
NA , 7 ~ 7~7N
3 r " ~ ~ I-~T]
5r'~ 11~ 61 ] 12[ -~
Fig. 4. Sketch map-of volcanic arcs and thrust fronts in the Tyrrhenian Sea and adjoining orogens (modified from Scandone, 1980; Carta Tettonica d'Italia, 1981; Francalanci and Manetti, 1994). Legend: 1 =west-dipping subduction of the Apennines; 2=east-dipping subduction of the western Alps; 3 =inactive, relict front of east-dipping subduction of northern Corsica and western Tyrrhenian Sea; 4=Oligo- Miocene arc volcanism of Sardinia Island (SA); 5= Calabria Peloritani orogen (CPO); 6=faults (presumed faults are indicated by dashed lines); 7=Vavilov Basin; 8=Marsili Basin; 9=Pliocene and Quaternary volcanisms of the central Tyrrhenian Arc and Aeolian Arc (PA=Pliocene arc, A E A = Aeolian Arc, respectively); 10= 3000 m depth contour of the central Tyrrhenian deep basins; 11 = volcanic seamounts of the deep basins (MS=Magnaghi smt., VS=Vavilov smt.); 12= ODP Sites sampling basaltic crust in the deep basins. NA = northern Alps; IL=Insubric lineament; A F S = A t l a s Fold System; IP=Is land of Ponza; AS=Anchise Seamount.
internal to the Calabria-Peloritani orogen, a segment of the Apennines.
To the west, the plain around the seamount is bordered by a gentle convex structure which may be a relict arc (Figs. 2 and 4) of Pliocene age (5 to 2 Ma; Kastens et al., 1988; Savelli and Schreider, 1991; Serri, 1991; Francalanci et al., 1994) extend- ing from the Anchise Seamount, to the south, to the Island of Ponza to the north. The Vavilov Basin, located to the west of the intermediate
volcanic arc, is chronologically distinct from the Marsili Basin. Spreading in the Vavilov Basin began 7-5 Ma, whereas inception of spreading in the Marsili Basin occurred 1.9-1.7 Ma (Hsti et al., 1978; Kastens et al., 1988; Savelli, 1988). Both basins are subcircular in shape, and floored with basaltic basement, as documented by the deep drill holes 373 of Leg 42 of the Deep Sea Drilling Project, and 650, 651, and 655 of Leg 107 of the Ocean Drilling Program. Large volcanic sea- mounts elongated almost N-S occupy the centres of both basins. Magnaghi Seamount, in the Vavilov Basin, is located to the west of the basin centre (Fig. 4).
Fig. 2 shows that the Benioff zone beneath Marsili is deeper than beneath the Aeolian arc. The configuration of the Tyrrhenian deep seismo- genic zone (Ritsema, 1979; Gasparini et al., 1982; Anderson and Jackson, 1987; Giardini and Velon/t, 1991) is, on the whole, anomalous if compared with usual, Pacific-type geometries of slab subduc- tion where a parallelism between volcanic arc and deep loci exists. From a geodynamic point of view, the Tyrrhenian Benioff zone is in an advanced senile stage of evolution characterized by slab detachment and its passive sinking in the litho- sphere (Scandone, 1980; Malinverno and Ryan, 1986; Amato and Alessandrini, 1991; Giardini and Velon/t, 1991; Mantovani et al., 1992).
Overall, after the middle Miocene (15-13 Ma), in the Tyrrhenian Sea and surroundings there has been eastward (sensu lato) displacement of the magmatism (Fig. 4) and its associated rifting as well as migration of the back-arc basin-volcanic arc-accretionary wedge system (Fabbri and Curzi, 1979; Boccaletti et al., 1984; Beccaluva et al., 1985; Kastens et al., 1988; Savelli, 1988; Doglioni, 1991, 1992). The latest Pliocene to Recent volcanic activ- ity of the southeastern Tyrrhenian Sea developed subsequent to the compressional tectonism, and concurrent with rifting and subsidence of notable intensity. The stratigraphic record of the ODP Site 650 in the Marsili Basin suggests a post-collisional collapse rate not lower than 0.7 km/Ma (Wang et al., 1991). The centrally located seamount was affected by vigorous extensional tectonism indi- cated by the occurrence of open fissures and large linear fractures trending N-S to NE--SW (Finetti
142 (2 Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157
and De1 Ben, 1986; Sborshchikov et al., 1988, 1990; Savelli, 1993).
From a structural point of view, the Marsili back-arc volcano and volcanoes of the adjacent Aeolian arc occupy internal positions relative to the Calabria-Peloritani orogen (Fig. 4). The orogen consists of overthrusts of Palaeozoic base- ment rocks, Jurassic ophiolites and Mesozoic to Recent sediments, and was affected by drift motion and compression which have been more intensive in comparison to the other border lands of the Tyrrhenian Basin.
3. Methods of analysis
Mineral analyses were performed with a Philips 515 SEM equipped with an EDS spectrometer at Dipartirnento di Scienze Mineralogiche, Bologna University. Operating conditions were 15 kV accel- erating voltage, 2 nA sample current and 100 live sec counting time at 3000 cps. Natural mineral standards (kindly supplied by the National Museum of Natural History, Smithsonian Institution, Washington D.C.; Jarosewich et al., 1986) were used to minimize matrix effects.
Major, trace and rare earth elements were analyzed at the Service d'Analyses des Roches et Mineraux du CNRS, CRPG, Vandoeuvre- les-Nancy, France with ICP spectrometry (Govindaraju and Mevelle, 1987; Potts, 1987).
4. Petrography and mineral chemistry
The new samples from the Marsili Seamount are 6 basalts, 4 andesites and 1 basaltic andesite. Sites of sampling are listed in Table 1 and shown in Fig. 3. These lavas were recovered during sub- mersible dives from the talus present at depths ranging from 2590 to 500 m depth and by dredge and coring stations. The studied rocks are charac- terized by notable freshness, but one sample has minor amounts of calcite and zeolites (M1-3; Table 1 ).
Basalts have porphyritic, vesicular textures; some samples have a low porphyritic index (P.I.<5%) with phenocrysts of olivine, clino-
pyroxene and plagioclase. Other samples are char- acterized by a higher P.I. (about 20%) with plagioclase as the most abundant phenocryst and low (< 3%) contents of olivine and clinopyroxene. Groundmass textures are intergranular to intersertal.
Andesites are texturally homogeneous; they have a porphyritic texture (mean P.I. around 14%) with plagioclase phenocrysts (10%), and sparse augite (about 1%), orthopyroxene (0.5%) and magnetite (0.6%). The groundmass texture is hyalopilitic with plagioclase, pyroxene and magnetite microcrystals set in brown fresh glass of trachytic composition (Table3A). Very rarely have been observed partially resorbed green amphibole crystals. Glomeroporphyritic clots made up of plagioclase, pyroxene, magnetite and entrapped glass are common. Selected major element analyses of min- erals are reported in Table 2.
Olivine occurs in basalts and basaltic andesites as small (rarely exceeding 2 ram) euhedral phe- nocrysts and groundmass microphenocrysts; less commonly as large (5-6mm), rounded and embayed phenocrysts. In one andesite only has been observed a strongly resorbed olivine crystal. Modal abundance is less than 5%. Olivines from basalts have normal zoning (core-rim range Fo89~2; Table 2) but reverse zoning has also been observed. Olivine compositions become pro- gressively more iron-rich from basalt to basaltic andesite (sample 1691), and disappear with increasing silica contents being replaced by opx in the more Si-rich andesites. The calculations of olivine/liquid equilibrium applying the Fe/Mg exchange coefficient (KD=0.33) determined by Roeder and Emslie (1970) suggest that olivine is in equilibrium with bulk rock both in basalts and basaltic andesite.
Orthopyroxene (opx) appears in basaltic andes- ites as rims on augite phenocrysts and in andesites as tiny (< 1 mm) euhedral phenocrysts and ground- mass microcrystals. Modal abundances are always low (< 1%). In basaltic andesites the scarce opx has an Env9 composition; in andesites opx phe- nocrysts are unzoned and have an En74 content; groundmass microcrystals are En69_64. The appear- ance of opx is related to the silica content of the magma; opx is present in basaltic andesites in very
Tab
le 1
L
oca
tio
n o
f sa
mp
lin
g s
tati
on
s an
d s
chem
e of
mai
n p
etro
gra
ph
ic f
eatu
res
of s
amp
les
fro
m M
arsi
li s
eam
ou
nt
Sta
tion
L
at.
(N)
Lon
g. C
E)
Dep
th (
m)
Ro
ckty
pe
Ph
eno
cry
sts
Gro
un
dm
ass
No
tes
MIR
1-
4 39
°19.
6 '
14°2
4.9
' 15
00
CA
-Bas
alt
O1-
P1
P1-
OI-
G1-
Cpx
v.
p.
MIR
1-
3 39
°20.
0 '
14°2
6.4
' 20
10
CA
-Bas
alt
Pl-
Cpx
-O1
P1-
Cpx
-O1
vv.
V1-
1663
cor
e 39
°24.
2 '
1402
0.5
' 30
80
CA
-Bas
alt
P1-O
I P
1-C
px-O
I-G
1 v.
f. (
155
16
4 cm
b.s
fl )
M22
-103
cor
e 39
°04.
5 '
1402
3.2
' 32
00
CA
-Bas
alt
O1-
Cpx
G
I-P
1-S
p v.
f. c
orer
bo
tto
m
MIR
1-
1 39
°20.
3 '
14°2
7.2
' 25
90
CA
-Bas
alt
O1-
P1
P1-
Cpx
-O1-
G1
v.p.
M
IR
1-2
39°2
0.2
' 14
026.
9 '
2450
C
A-B
asal
t O
I-P
I P
I-O
1-C
px-G
I vv
. V
1-16
61 c
ore
39"1
7.95
' 14
o24.
0 '
920
CA
-Bas
alt
Pl-
Cp
x-O
l P
1-C
px-G
I v.
f. (
30
-60
cm
b.
sit.
)
MA
C-4
D
from
39
°14.
09 '
14°2
2.05
' 67
7 C
A-B
asal
t m
p. P
1-C
px-O
1 P
1-C
px-O
p-G
I-Z
e-C
c a.
v.
to
39°1
4.09
' 14
°22.
05 '
677
T72
-14
from
39
°08.
1 '
1402
1.5
' 13
50
CA
-Bas
-An
d.
P1-
Cpx
-Ol
PI-
Px-
GI
to
39°0
8.4'
14
° 19
.7'
1000
V
1-16
91 f
rom
39
°12.
1 '
14°2
1.1
' 11
00
HK
-Bas
-An
d.
P1-
Cpx
-O1
(Opx
) P
1-C
pxG
I-S
p (O
px)
v.
to
39°1
2.3
' 14
022.
3 '
760
MA
C-4
B f
rom
39
°14.
09 '
14°2
2.05
' 67
7 H
K-B
as-A
nd
. m
p. P
1-C
px-O
l G
1-O
p-Z
e-C
c
to
39°1
4.09
' 14
022.
05 '
677
T72
-17
from
39
°06.
3 '
14°1
7.3
' 26
06
CA
-An
des
ite
Pl-
Cp
x-O
px
-(A
mp
)-O
1
PI-
Px
-GI-
Am
p
to
39°0
7.7'
14
° 17.
3'
2300
M
t V
1-16
57 f
rom
39
°17.
0 '
14°2
2.8
' 11
70
HK
-An
des
ite
P1
-Cp
x-O
px
-Mt
P1
-CP
x-M
t-G
to
39°1
6.9
' 14
o23.
6 '
1270
M
22-1
04 c
ore
39°0
4.8
' 14
°23.
2 '
3300
H
K-A
nd
esit
e C
px-P
l-O
l-(S
p)
GI-
P1
v.f.
cor
er b
ott
om
M
IR 2
-1
39°1
7.2
' 14
o23.
5 '
505
HK
-An
des
ite
PI-
Cp
x-O
px
-Mt
P1-
Cpx
-Mt-
G1
v.pt
.
V1-
1656
fro
m
39-'
17.0
' 14
°23.
1 '
1000
H
K-A
nd
esit
e P
1-C
px
-Op
x-M
t P
I-C
Px
-Mt-
G
to
39 °
17.0
' 14
°24.
1'
700
MA
C-3
fro
m
39°1
6.09
' 14
o24 '
75
0 H
K-A
nd
esit
e P
I-C
px
-Op
x-A
mp
G
I-P
l-C
px
-Op
-Ap
p.
v.
to
39°1
7.06
' 14
°24 '
77
2 M
IR
2-3
39~'
17.2
' 14
°23.
5 '
503
HK
-An
des
ite
P1
-Cp
x-O
px
-Mt
P1-
Cpx
-Mt-
G1
p.v.
pt.
CS
T-6
8-13
co
re
39°1
7.2
' 14
°24.
3 '
651
HK
-An
des
ite
P1
-Cp
x-O
px
-(A
mp
) G
1-P
I-P
x-O
p C
T-6
9-30
fro
m
39°1
6.9
' 14
023.
5 '
628
HK
-An
des
ite
P1-
Cpx
-Opx
-(A
mp)
-(O
1 )
Gl-
P1-
Px-
Op
v.
to
39°1
7.0
' 14
°23.
9 '
618
CT
69/2
7/2A
fro
m
3901
4.2
' 14
~'22
.6 '
771
WP
-bas
alt
PI-
Ol-
(Cpx
)-G
1 G
I-O
p-P
1-P
x
to
39 °
14.1
' 14
°22.
0 '
760
Mt-
Ap
-&
& V
-.q
Sam
ples
lab
elle
d M
IR
wer
e re
cove
red
by
the
su
bm
ersi
ble
s M
ir
1 an
d
2; a
ster
isk
indi
cate
s sa
mp
les
from
li
tera
ture
. D
red
ged
sa
mpl
es a
re i
nd
icat
ed "
fro
m
to".
C
A =
ca
lcal
kali
ne;
HK
=h
igh
p
ota
ssiu
m
calc
alka
line
; O
I=o
cean
is
land
. O
l-o
liv
ine;
P
l=p
lag
iocl
asc;
C
px
=cl
ino
py
rox
ene;
O
px
=o
rth
op
yro
xen
e;
Gl=
gla
ss;
Sp
=sp
inel
; Z
e =
zeol
ites
; C
c =
calc
ite;
Am
p =
amp
hib
ole
; M
t =
mag
net
ite;
A
p =
apa
tite
; O
p-o
paq
ues
; (
)=m
iner
al
in
very
sm
all
amo
un
t;
mp.
=m
icro
ph
eno
cry
sts;
v.
=v
esic
ula
r;
vv. =
ver
y v
esic
ular
; p
.-
porp
hyri
tic;
f. =
fra
gmen
ts;
a. =
aph
yric
; pt
. =
pil
ota
xit
ic t
extu
re;
b.sf
l = b
elow
sea
floo
r.
z
144 C Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157
Table 2 Representative analyses of minerals from Marsili seamount
B=basalt ; BA=basalt ic andesite; A=andesite; gms=groundmass; Fo%=percent forsterite component in olivine; Mg# Mg/ (Mg+Fe) . Fe203*, FeO* recalculated from stoichiometry. (* )=mean of 12 analyses; B=basalt ; BA=basaltic andesite; A andesite; gins = groundmass.
c. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157 145
small quantity and it coexists with olivine; its abundance increases in andesites where olivine disappears.
Clinopyroxene phenocrysts of diopsidic to augitic composition are present in both basalts and andesites always with low modal abundances (< 3%). Augites in basalts are quite homogeneous; the Fe-enrichment is moderate. A1203 and TiO 2 contents increase from core to rim and in ground- mass crystals. Some zoning patterns are reversed. Augites from andesite lavas have homogeneous composition and differ from augites in basalts by being slightly more Fe-rich according to whole- rock composition. Mg/(Mg + Fe) of clinopyroxene cores are similar to those of coexisting olivines indicating equilibrium between these two phases (Sakuyama, 1981).
Plagioclase phenocrysts in basalt are generally clear and relatively unzoned; less abundant are plagioclase crystals with dusty or sieve textures and glass inclusions. Zoning patterns are generally normal but reverse zoning may be present. Core compositions are bimodal with one group clustered around An9o and another around Anvo; rim and groundmass compositions are scattered with a limited sodic enrichment with respect to the phenocrysts.
Plagioclases from andesites are texturally more varied. Crystals with dusty or sieve textures and/or rich in glass inclusions are common. Zoning pat- terns are generally normal; compositional ranges from core to rim in phenocrysts and from phe- nocrysts to groundmass are wide. Subordinate phenocrysts with unzoned high-anorthite cores (An90) overgrown by a rim An41 are present in some rocks. These cores are identical to those of some basalt plagioclases. It is unlikely that these cores crystallized from the andesitic magma because this would require a very high, possibly unrealistic water pressure (PH20 not less than about 5 kbar; Gill, 1981). A possible explanation is that anorthite-rich cores may be xenocrysts inherited from basaltic magma.
Oxides are almost completely absent in basalts being represented only by very small groundmass crystals and tiny Cr-A1 spinels enclosed in olivine. Titanomagnetite appears as scarce micropheno-
crysts in basaltic andesite and as well-developed microphenocrysts (0.3-0.5 mm) in andesites (modal abundance about 0.5-1%). The appearance of magnetite may be related to an increasing water content of magma because water disassociation will increase oxygen fugacity favoring magnetite crystallization (Halsor and Rose, 1991). A single partially resorbed green amphibole crystal was observed in one andesite sample, but was not analysed. The presence of amphibole suggests more water-rich conditions for the magma.
A comprehensive account of the mineral chemis- try of Marsili Seamount lavas is given elsewhere (Gasparotto and Savelli, 1994).
5. Geochemistry
In Table 3A are reported the major, trace and rare earth (REE) element analyses of the new lava samples from the seamount in question. Literature data (Maccarrone, 1970; Keller and Leiber, 1974; Selli et al., 1977) are listed in Table 3B. The recovered rocks have characteristics of orogenic associations, except those from dredge station CT69/27 which are similar to OIB lavas (Selli et al., 1977; Serri, 1991) and are not treated in this work.
On the K20 vs SiO 2 classification diagram (Le Maitre, 1989) (Fig. 5) the samples plot in the fields of the CA and the HKCA series. The plot illustrates that the CA and the HKCA suites consist prevailingly of lavas which have basalt and andesite composition, respectively. The basaltic andesites T72/14/2, ME-4D and VI 1691 plot near the field boundary of the CA basalts. The T72/17/4B lava is the only andesite that falls in the field of the CA series. Both the K20 content and the KzO/Na20 ratio (0.26-0.70) of Marsili lavas increase with increasing SiO 2.
In the major element variation diagrams (Fig. 6) can be observed negative trends for CaO, MgO and total Fe203, and positive trends for the alkalis, whereas A1203, TiO 2 and P205 do not show sig- nificant variations with silica. The analyzed lava samples reveal the existence of a compositional gap between 54% and 58% SiO 2.
In the basalts, TiO/ contents increase with
146 C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157
Table 3A New analyses of major, trace, and rare earth elements of Marsili seamount
B=basa l t ; BA=basa l t i c andesite; A =andesite. Mg# calculated with M g / ( M g + F e ) = 0 . 2 0 (basalts and basaltic andesite) and 0.30 for andesites.
respect to the abundance of SiO2, while they show no correlation in the HKCA andesites. The decrease of MgO values is more pronounced in the mafic volcanites than in the andesites.
Fig. 7 shows the trends of selected trace elements versus Zr. The large ion lithophile elements (LILE)
Rb and Ba, and the high field strength elements (HFSE) Nb and Th are positively correlated with the Zr contents. The correlation is negative for the concentrations of the compatible ferromagnesian elements Ni, Cr and V.
The Sr values show a large dispersion. The
C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157 147
c o n t e n t s o f G a a n d Z n ( n o t s h o w n in Fig . 7) a re
s c a t t e r e d . T h e c h o n d r i t e - n o r m a l i s e d r a r e e a r t h ele-
m e n t ( R E E ) d i s t r i b u t i o n ( F i g . 8) i n d i c a t e s t h e
o r o g e n i c n a t u r e o f t h e C A b a s a l t s a n d H K C A
a n d e s i t e s ; t he l i gh t r a r e e a r t h e l e m e n t s L R E E s h o w
w e l l - d e v e l o p e d f r a c t i o n a t i o n s , w h i l e t he h e a v y o n e s
( H R E E ) a r e n e a r l y f la t ( W i l s o n , 1989) . A n d e s i t e s
h a v e f l a t t e r H R E E p a t t e r n s t h a n b a s a l t s ( t h e
D y n / Y b n r a t i o v a r i e s f r o m 1.1 to 1.3). T h e r e a re
s m a l l n e g a t i v e E u a n o m a l i e s w h i c h a re less p r o -
n o u n c e d in t he m a f i c rocks .
T h e d i a g r a m o f t h e h y g r o m a g m a t o p h i l e ele-
m e n t s ( F i g . 9) n o r m a l i z e d to t he p r i m o r d i a l m a n t l e
c o m p o s i t i o n o f W o o d ( 1 9 7 9 ) d i s p l a y s a f r a c t i o n -
a t e d t r e n d w i t h d i f f e r en t a b u n d a n c e s o f i n c o m p a t i -
b le e l e m e n t s f o r b a s a l t s a n d a n d e s i t e s . T h e ove ra l l
t r e n d o f t he p lo t , e n r i c h m e n t s in L I L E , T h a n d
La , a c c o m p a n i e d b y n e g a t i v e H F S E ( N b , Z r , T i )
a n o m a l i e s a re c h a r a c t e r i s t i c o f n o n - a l k a l i n e vo l -
c a n i t e s e r u p t e d in s u b d u c t i o n - r e l a t e d se t t ings .
H o w e v e r , t h e H K C A a n d e s i t e s l ack t h e Z r a n o m -
a ly e x i b i t e d b y t he b a s a l t s a n d s h o w a n e g a t i v e Sr
p e a k . T h e n e g a t i v e S r a n o m a l y , a n a t y p i c a l f e a t u r e
o f i s l a n d a rc v o l c a n i s m a n d p r o b a b l y a c o n s e -
q u e n c e o f t he l o w - p r e s s u r e f r a c t i o n a l c ry s t a l l i z a -
t i o n o f p l ag ioc l a se , is r e p o r t e d a l so in s o m e
p r o d u c t s o f S t r o m b o l i ' s I s l a n d ( F r a n c a l a n c i
e t al., 1989) .
148 C Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157
~ 3
v
0 ~d
2 {1 tn ~g
m
I f I I I I
0o
J (1) ~
0 I I I I I
4 5 55 65 Si 02 (Wt °1o)
Fig. 5. KzO vs SiO 2 relationship of the Marsili lavas (plotted data are volatile free). Boundary lines of the main volcanic associations according to Le Maitre (1989): ( / ) = b o u n d a r y between low-K (tholeiitic) to medium-K (calcalkaline) series, (2) =boundary berween medium-K to high-K and shoshonitic series. Legend: • = new samples of this study; [] = Selli et al. ( 1977); A = Keller and Leiber ( 1974); x = Maccarrone (1970).
can be derived from the basalt parent by 32% fractionation of an assemblage of plagioclase, oliv- ine and clinopyroxene; the homogeneous andesites of the seamount's top can be produced from the basaltic andesite parent by 42% crystallization of an assemblage of plagioclase, olivine, clinopyrox- ene, titanomagnetite and apatite. The fit of the models, expressed by the sum of squared residuals ( ~ r 2 ) , is good for both.
These results for the major elements have been tested using Rayleigh surface equilibrium models. The trace element crystal/liquid partition coeffi- cients (kD'S)used were those proposed for orogenic rocks by Gill (1981). A model was considered acceptable if the observed value for a trace element fell between the values calculated using high and low kD'S. The results are reported in Table 4. The model 1 (from M1-2 to 1691) fails for the highly incompatible elements Ba and Th, predicting higher values with respect to the observed ones; in the model 2 (from 1691 to mean andesite) the fit is satisfactory with the exception of Ba for which the observed value is higher than the calculated ones.
6. Fractional crystallization 7. Discussion
A model of crystal fractionation was employed in order to test it as a viable mechanism responsible for the evolution of the volcanites. The approach followed by us was first to use a major element fractionation model (Stormer and Nicholls, 1978). The input data were the whole-rock compositions of the selected parent and daughter lavas and compositions of analysed phenocrysts.
Because of the bimodality of the seamount vol- canites with scarce products intermediate between basalts and andesites, the calculations were exe- cuted in two steps; in the first one was tested the derivation of the basaltic andesite 1691 from the basalt parent M 1-2 and in the second the deriva- tion of the andesites (taking the mean values) from the 1691 lava. The M1-2 basalt was chosen due to low P.I. (< 5%), high Mg# value and high Cr and Ni contents.
The results of the models are reported in Table 4. Calculations show that the basaltic andesite 1691
7.1. Geochemistry
Notwithstanding Marsili's structural setting with respect to the neighbouring Aeolian arc volca- noes, the seamount consists mainly of CA rocks. The prevalence of the subduction-related CA vol- canics over OIB-like basalts is documented by the fact that only one dredge station recovered rocks of within-plate nature (Selli et al., 1977).
Previously Marsili Seamount was thought to be mainly the product of OIB-like magmatism, due to its structural setting similar to Vavilov and Magnaghi Seamounts (Carta Tettonica d'Italia, 1981; Sartori, 1988; Serri, 1991). The current sample suite demonstrates that, unlike Vavilov and Magnaghi, Marsili Seamount is made up almost entirely by CA lavas. This orogenic magmatism is characterized by a transition of lava composition from CA basalts to HKCA andesites which consti- tute the top of the seamount.
C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157 149
22
20
18
16
14
12
13
11
9
7
5
3
1
10
8
6
4
2
0 46
I I I I I I
x
[ ]
x o ~ o o
[ ]
i I
A I 2 0 3
[ ]
A~IpO
x
'6
[ ]
CaO
A
O ~ o
0
g
x
I I I I I
48 50 52 54 56
si 02
Mg 0
[]
o~]~ [ ]
I I 1
58 60 62 64
Fig. 6. Harker variation diagrams
I U ! I I I I I
F"e 2 O 3 To t .
x X •
A o o e D x
,d ~ ' []
N a 2 0
• A
• x e
0
A [] []
A
• x
A 0 0 0
x Ti 0 2
[] A
~ o • [ ]
P2 0 5
[] []
I I I I I
48 50 52 54 56
S i 0 2
( w t % ) . S y m b o l s a s i n F i g . 5.
I I
58 60
15
13
11
9
7
5
3
5
4
3
2
1
i 11.3
1
.7
.4
.5
.4
.3
.2
~- .1
62 64
The high Cr and Ni concentrations of some basalts, suggest a generation from melts which were not significantly affected by crystal-liquid fractionation processes. High Cr contents (Table 3A, e.g., MI-1,328 ppm of Cr) are reflected in the presence of tiny Cr-A1 spinels enclosed in olivine phenocrysts. The Cr concentrations higher (average 250 ppm) than in the comparable Aeolian arc basic lavas (average 100 ppm; Villari, 1980a) with similar Ti contents suggest a relatively primi- tive character for the Marsili rocks (Pearce, 1975). This character is indicated also by the Ti-V ratios (22-45). These are higher than those in Aeolian basalts with equivalent K20 contents (i.e., the CA
basic lavas from the Islands of Filicudi, Alicudi, and Salina; Villari, 1980a), which fall in the sug- gested range for arc lavas (10-20; Shervais, 1982).
The geochemistry of the investigated basalts suggests the existence of a mantle heterogeneously enriched in K and the related incompatible ele- ments Rb, Sr, Ba, Th and LREE. The enrichment of the mantle sources can be attributed to the introduction of metasomatic melts and/or fluids derived from subducted lithosphere, similar to enrichments recognized elsewhere in the south Tyrrhenian volcanic region (Beccaluva et al., 1985, 1990; Bertrand et al., 1990; Serri, 1991; Francalanci et al., 1993).
150 C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157
350
300
200
100
0
200
100
0
200
100
0
650
550
4 5 0
350
250 0
, % A ,
O O •
[]
I I I
Cr
[] %o'o
[ ]
.Soo
0
e O : 0 o
Ni
0
• D
I I I 50 100 150
Z r ( p p m )
S r
0
I I 200 250
I I I I
O O D o o r ' l
• /2 [ ]
o e
0 1 3
13 $ • 0
o e •
0
I
Rb
Ba
t l []
• 0
0 Nb
":o • 0
Th
I I I I i
0 50 100 150 200 250 Z r ( p p m )
Fig. 7. Trace element variation diagrams (ppm) versus Zr. Symbols as in Fig. 5.
100
80
60
40
20
0
1000
600
200
0
30
20
10
0
15
10
The studied lavas consist of basalts and andesites while rocks of intermediate composition (basaltic andesites) are relatively few. This bimodal distribu- tion of the eruptive products seems to be a distin- guishing feature of Marsili, compared to the continuum basalt-basaltic andesite-andesite of the Aeolian arc volcanoes as a whole (Gasparotto and Savelli, 1994). In the arc, basaltic andesite lavas prevail, even though with different serial character- istics (K enrichments) in the different islands. None of the islands shows a lower abundance of basaltic andesites relative to basalts and andesites.
In the chondrite-normalised rare earth element (REE) plot (Fig. 10) are shown the distributions
of lavas from different tectonic settings of the southeast Tyrrhenian Sea. These lavas, all of calc- alkaline, medium-K serial affinity, are represented by the basaltic andesites from the mafic basement of Marsili Basin (ODP Site 650; Beccaluva et al., 1990) and by basalts from Marsili Seamount (MI-1, M1-3 and M1-4) and the Aeolian Arc (selected, least evolved basalts from the Islands of Filicudi and Alicudi; Villari, 1980b,c). The dia- gram shows that, albeit some overlap occurs among rocks from different areas, the contents of the LREE (La to Eu) in the seamount products are somewhat higher than in the igneous basement and lower than in arc basalts with comparable
C. Savelli, G. Gasparotto//Marine Geology 119 (1994) 137-157 151
100
"C im
u
io
, , , , , , , , , , , , , ,
- 3
- 4
La Ce Nd Sm Eu Tb Dy Er Yb Lu
Fig. 8. Chondrite-normalized rare earth element patterns in selected Marsili rocks. • =basalts: -3=sample MIR 1/3; -4= sample MIR 1/4; - /=sample MIR 1/1; and basaltic andesite (1691); x=andesi tes (samples MIR 2/1 and VI/1656). Normalization constants from Sun (1982).
, , , , , , , , , , , 1 , , , , , , 1 , , ,
100 1 1 ~ m
~ 1 1 ~ -
L i L i I i i i L i i L i i L h i i i i i
Rb 5a Th K Nb La Ce Sr Nd Zr SmEu Ti Dv Y Er Yb Lu
Fig. 9. Minor and trace element abundances in selected Marsili rocks normalized to primordial mantle values (Wood, 1979). Legend as in Fig. 8.
K 2 0 abundances. The lavas of Marsili volcano and ODP Site 650 exhibit a relatively wide range and some fractionation of the HREE patterns, from Dy to Yb and Tb to Yb, respectively. The HREE distribution of these rocks is somewhat lower than that of the least evolved Aeolian rocks which have no fractionation trend from Tb to Yb.
An examination of the variation of the chemical data vs depth of sample location suggests the occurrence of an overall relationship. For example, the plots of MgO and K20 values against depth (Figs. 11 and 12) indicate that the MgO abun-
dances decrease while those of K20 increase from the base to the top of Marsili. The two diagrams show that the basaltic andesites are located at shallower depth. They also illustrate that the bulk of the volcano, from low to intermediate elevations is made up by basalts with subordinate amounts of CA andesite lava, and the summit area by HKCA andesites. The more primitive basalts gen- erally occur in the lower flanks.
Nature and location of the magmatic rocks of Marsili and Aeolian arc indicate the non-existence of a K-h relationship, a feature which has also been noted elsewhere (Morrison, 1980). If there were a regional relationship between KzO content of the products and depth of the Benioff zone (Keller, 1974; Villari, 1980a; Beccaluva et al., 1985; Ellam et al., 1988), the volcanism in question, being located above an underthrusted lithosphere which is deeper than that beneath the Aeolian Arc, should be richer in potassium than the arc lavas at equivalent SiO2 contents. On the contrary, all the analyzed basalts belong to the CA association, while shoshonitic and HKCA basic lavas occur in the arc, where depths to the deep foci are shallower.
7.2. Fractional crystallization
The observed overall variation of chemical com- position conforms to crystal fractionation model- ling from a common parent magma based on actual mineral compositions and major analyses of low porphyritic lava (MI-2). However, the significant difference of calculated FeO and Na20 values of the fractionation model from the actual values (Table 4), the scattering of the incompatible elements Rb, Ba, Th and Sr (Fig. 7), the increase of K20/Na20 ratios with SiO 2 and the gap of composition existing between basalts and andesites suggest that closed system crystal fractionation is unable to account entirely for the magmatic evolu- tion of Marsili Seamount.
7.3. Tectonics
Linear normal faults, fractures and open fissures indicate that the volcano was affected by intensive, tensional tectonism (Sborshchikov et al., 1988, 1990; Savelli, 1993). The drilling record of ODP
152
Table 4 Fractional crystallization models
C SavellL G. Gasparotto/Marine Geology 119 (1994) 137-157
*Unsatisfactory fit. Numbers in parentheses are 1 standard deviation in ppm.
Site 650 shows a v e r y h igh r a t e o f subs idence o f
the Mars i l i bas in since 1 .9 -1 .7 M a ( K a s t e n s et al.,
1986, 1988). T h i s was the in i t ia l t i m i n g o f basa l t i c
c rus t e m p l a c e m e n t in the basin . T h e ea r ly e rup-
t ions o f ocean ic , d e e p - s e a t e d c rus t t o o k p lace
d u r i n g the absence o f v o l c a n i s m in t he t w o ad jo in -
ing arcs (F ig . 4) . T h e v o l c a n i c ac t iv i ty h a d ceased
in the a rc o f P l i ocene age to the west , w h e r e a s it
h a d n o t b e g u n in the Q u a t e r n a r y A e o l i a n vo lca -
noes to the eas t ( B e c c a l u v a et al., 1985; Savel l i ,
1988; Savel l i a n d Schre ider , 1991).
T h e faul ts o f the i n t e r m e d i a t e to deep p o r t i o n s
C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137 157 153
102 ....
8 OE
101
102
10'
MARSILI Smt.
~ EOLIAN ARC
I I t I I I I I I I I I l [
La Ce Pr Nd Sm Eu Gd To Dy Ho Er Tm Yb Lu
a
b l
EOLIAN ARC
I ~ ODP 650
S i 1 [ 1 1 1 ~ 1 1 1 1 [ 1 1 Ce Pr Nd Sm Eu Gd TO Ov Ho Er Tm ~ Lu
600
1200
1800
~ 2400
3000
360C
- - 0 0
(,) (°) I I I I I I t I [ I I 1 2 3 4 5 6 7 8 9 10 11
Mg O
Fig. l l . Plot of variation of MgO values (% =basalts; • = basaltic andesites; ~ = andesites) versus depth of rock recovery. The brackets indicate two lava specimen which consist of lapilli erupted at a phreato-magmatic depth likely shallower than sampling depth (Keller and Leiber, 1974).
600 o o o •
o o • • o E 1 8 0 0 7-
E W a~ 0 e~
3000 I01
I I I 0 110 210 310
K 2 0
Fig. 12. Plot of variation of K20 values versus depth of rock recovery. Symbols as in Fig. 11.
Fig. 10. Rare earth element (REE) concentrations normalized to chondrite values of Sun (1982). Data plotted are of least evolved rocks of calealkaline serial affinity from the following areas: basalts of Marsili Seamount and the Islands of Alicudi and Filicudi (Aeolian arc; Villari, 1980b,c), and basaltic andesites from the ODP site 650 (Beccaluva et al., 1990).
of the volcano are oriented N-S to NNE-SSW (Selli et al., 1977; Sborshchikov et al., 1988, 1990). The geochemical transition from the basalts of the volcano's base to the andesites of the summit is reflected in a distinct change of the tectonic trend. The late, NE-SW oriented linear faults and open fractures of the summit were produced in response to a clockwise rotation of the seamount's system of spreading (Fig. 3). This re-orientation of the young extension tectonics is widespread in the Tyrrhenian basin (Gennesseaux et al., 1986;
Sartori, 1988; Schreider et al., 1988; Gabbianelli et al., 1990).
In situ observations permitted to recognize (Sborshchikov et al., 1988; Savelli, 1993) that the late fractures affected flows characterized by different morphology. Indeed, while the eruptive manifestations of the basal to intermediate levels of Marsili, at depth greater than about 1100 m, consist of pillow lavas, those of the summit appear to be devoid of these forms, scoriaceous and, on the whole, of rubble-like, irregular appearance.
7.4. Geochemical and struetural evolution
Unlike Aeolian arc magmas which were contam- inated by continental crust (Bargossi et al., 1990; Crisci et al., 1991; Peccerillo and Wu, 1992),
154 C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 137-157
similar processes of contamination are precluded for Marsili magmas, because they were erupted through basaltic crust.
The geochemical characteristics of basalts and HKCA andesites appear to be related to slab- derived metasomatic mantle modifications con- trolled by sinking of subducted, detached litho- sphere in the underlying mantle. Such a geodynamic magmatic evolution may result from an atypical, passive subduction regime (Ritsema, 1979; Giardini and Velona, 1991) attending the tensional tectonics and intense subsidence of the seamount in question (Finetti and Del Ben, 1986; Kastens et al., 1986, 1988; Sborshchikov et al., 1990; Savelli, 1993).
Similarly to Marsili's lava sequence, the eruptive sequences of the different Aeolian arc volcanoes exibit transition from calc-alkaline to more K-rich magmatism with time. The overall evolution of the Aeolian arc magmatism was considered to be a response to sinking of the subducted slab and/or deepening of the magma sources controlling an enhancement of the metasomatizing processes (Barberi et al., 1974; Keller, 1974; Villari, 1980a; Beccaluva et al., 1985).
The unique, tightly concave shape of the Aeolian arc (Fig. 4) and the subcircular shape of the Marsili basin may be a consequence of the east- ward displacement of the recent Tyrrhenian slab whose length has decreased with respect to the Pliocene slab. The eastward shift of the destructive tectonic environment has probably caused signifi- cant deformations, as segmentation and torsion of the subducting lithosphere. From such a mature, segmented and detached slab may have had origin the variably "metasomatized", heterogeneous mantle sources of the Aeolian back-arc and arc magmatisms.
The tectonic evolution of the volcano may have controlled, in a simple way, the fractionation pro- cess if the melt supply from the source to a magma chamber occurred at roughly constant rates. Given the active, rapid subsidence and extension, enlarge- ment and deepening of the volcano's reservoir and feeding system have probably taken place. As a consequence of the change of size and location of the magma chamber, the time necessary for a new eruption to occur after its refilling may become
longer. In this tectonic regime, a greater amount of magma may crystallize due to greater duration of its storage in the chamber and decrease of the overall velocity of ascent to the surface.
As far as it concerns the distinct, tectonic-related magma differentiation processes taking place at shallow and great depths, respectively, it must be observed that they are not mutually exclusive; on the contrary, both mechanisms may have contrib- uted to the evolution of Marsili's rocks.
8. Conclusions
Lavas recovered from Marsili Seamount are calc-alkaline. The high, variable concentrations of K and incompatible trace elements and LREE of the basic lavas suggest heterogeous modifications of mantle composition by subduction-related metasomatic processes. Metasomatic enrichments were recognized elsewhere in the mantle of the south Tyrrhenian volcanic region (Beccaluva et al., 1985, 1990; Serri, 1991). Based on compatible element concentrations, basalts from Marsili are interpreted to be more primitive than basalts from the Aeolian arc volcanoes for rocks having compa- rable K20 contents.
Like all the Aeolian volcanoes, the more evolved lavas of Marsili Seamount occur in higher strati- graphic levels. The seamount magmatism shows a significant change from CA basalts to the HKCA andesites which constitute the summit area.
The andesitic volcanism correlates with a clock- wise rotation of the seamount spreading system. The extensional faults and fractures, initially about N-S oriented, parallel to the axial elongation of the volcano, rotated to NE-SW during the erup- tive construction of the volcanic pile of the top, in the upper Pleistocene-Holocene time (< 0.2 Ma).
The petrochemistry of the Marsili's and Aeolian arc lavas does not correlate with depth to the underlying Benioff zone. Conversely, the magmatic products of the arc and back-arc system of the south Tyrrhenian, manifest a temporal relation- ship. Overall, K-enriched lavas occupy higher posi- tions in the stratigraphical sequences of the subaerial arc volcanoes (Villari, 1980a) and Marsili Seamount.
C. Savelli, G. Gasparotto/Marine Geology 119 (1994) 13~157 155
Linear normal faults, fractures and open fissures indicate that the volcano underwent strong exten- sion and subsidence which may reflect a senile stage of subduction (passive sinking and fragmen- tation of the slab).
A deepening of the subducted lithosphere and displacement of the region of melt generation to greater mantle depths is thought to be an impor- tant mechanism controlling the transition from the "low to high" K20 orogenic rocks of Marsili. Moreover, model calculations suggest that frac- tional crystallization had a role in magma differen- tiation, too. Given the tectonic regime of the south Tyrrhenian region, this process may be favored by change of volume and depth of the volcano's magma reservoir and attendant augment of the ratio between the fractionating and the input melts at intermediate-low pressures.
Acknowledgements
Thanks are due to E.M. Emelyanov, I.M. Sborshchikov, A.A. Schreider and V.S. Yastrebov for the given shares of the studied volcanite samples and for stimulating discussions with one of us (CS) during the Tyrrhenian Sea expeditions of the R/Vs Vityaz (1986) and Keldysh (1988). We are grateful for the critical review and constructive comments of Patchin C. Curtis that helped to improve the paper; Paul M. Holm and two anonymous reviewers also contributed to its improvement.
This is contribution no. 942 of the Istituto per la Geologia Marina--CNR.
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