Geol Rundsch (1997) 86, Suppl.:S286—S295 ( Springer-Verlag 1997

ORIGINAL PAPER

I. Wendt

Inferences from SR-isotope systematics for magma evolutionof NW Bavarian granitic intrusion sequences

Received: 14 June 1996 /Accepted: 11 November 1996

Abstract Many granites occur as sequences of intru-sions which show certain relations between its mem-bers. In this work the systematics of isochron slopes (i.e.isochron ages) and initial 87Sr/86Sr ratios are derivedmathematically for two different types of granite intru-sion sequences: fractional crystallization and magmageneration by partial melting. The equations derived inthis paper are applicable to any radioactive mot-her—radiogenic daughter system as Rb/Sr, Sm/Nd,U/Pb, etc. If granites are produced by magma frac-tionation, it can be shown that in an Rb—Sr isochrondiagram the average isotope ratios y

*"(87Sr/86Sr) and

x*"(87Rb/86Sr) of a member G

*of a granitic intrusion

sequence plot at the intersection of the individualisochrons of the two subsequent granites G

*and

G*`1

which have intrusion ages of t*

and t*`1

. Ina Compston-Jeffery diagram, where the initial ratiosb*

are plotted vs the ages t*

(or isochron slopes a*),

b*increases monotonically with decreasing age t

*. Two

subsequent granites G*and G

*`1with the coordinates

x*, y

*and x

*`1, y

*`1are connected by a line with a

minimum slope of x*and a maximum slope of x

*`1. If

granites G*are produced by progressive partial melting

of a source rock, the present-day average values (x*, y

*)

of all granites G*

plot on an isochron which yieldsthe age and the initial ratio of the source rock. TheCompston-Jeffery diagram, contrary to the fractionalcrystallization model, does not necessarily yielda monotonic curve. The isotopic criteria derived in thispaper, together with geochemical criteria, are appliedto a granitic intrusion sequence in the Fichtelgebirgewhere the different granites are probably producedby fractional crystallization. A granitic intrusion

I. WendtGeoForschungsZentrum, Telegrafenberg A51,D-14473 Potsdam, Germany; Bundesanstalt furGeowissenschaften und Rohstoffe, Hannover, GermanyFax: #05130 36502E-mail: immo.wendt@t-online.de

sequence which covers a large area in the Oberpfalzonly a few kilometres east of the KTB and approxim-ately 30 km south of the Fichtelgebirge, however, prob-ably is derived from magma generated by partial melting.

Key words Isotope systematics · Initial ratio—agerelation · Rb—Sr system · Raleigh fractionation ·Granite · Fichtelgebirge · Oberpfalz

Introduction

Fractionation processes controlling the chemical,especially the trace element composition, have beendiscussed in the literature during the past decades.Emmermann (1968, 1977) has shown that Zr and Ticoncentrations decreased, Sr decreased while Rb in-creased with increasing differentiation of graniticmagma in the Schwarzwald, SW Germany. White andChappel (1977) showed the relationship between sev-eral elements and SiO

2(Harker diagrams) for granitoid

rocks. A review of mathematically derived systematicson trace element concentrations in partial melting andfractional crystallization processes is given by Allegreand Minster (1978). A combined partial melting-frac-tional crystallization model has been developed byWetzel et al. (1989) who derived mathematically theevolution of the chemical composition of a magma.Whereas a pure differentiation model shows the se-quence mafic to sialic, and a pure partial melting modelshows the sequence sialic to mafic, the combined modelof Wetzel et al. (1989) yields a sequence mafic—sia-lic—mafic rocks.

The aim of this paper is to demonstrate the relation-ship between chemical partition effects, especially thesegregation of the element with the radioactive parentisotope from the element with the radiogenic daughterisotope caused by fractional crystallization due to cool-ing of the magma and consequences with respect toinitial ratios and isochron slopes.

Theory of Rb—Sr isotope systematics during fractionalcrystallization and partial melting

Fractional crystallization of a liquid magma, andchemical fractionation and its influence on theRb/Sr ratios

Chemical fractionation is described by the well-knownRayleigh distillation formula:

CM"CO

Mf (D~1) with D"

CS

CM

(1)

where CM

and CSare the concentrations of any element

of the melt (M) and the solid phase (S); D is the bulkpartition coefficient between melt and solid phaseassumed to be independent of f and f"m/m

0the ratio

of the remaining and initial mass of the liquid phase,and CO

Mthe initial concentration of the element in

question. One can obtain an expression relating theconcentrations of two elements, A and B, in the residualmagmatic liquid by solving Eq. (1) for each element,equating the two expressions, and solving for the con-centration of one element to obtain:

In (CB)"In (CO

B)#

DB!1

DA!1

[(In (CA)!In (CO

A))] (2)

i.e. the concentrations of any two elements should ploton a straight line in a log—log plot.

For example, in the case of Rb and Sr with DR"(1

and DS3'1 the ratio of the Rb and Sr concentrations

in the remaining melt Rb/Sr increases with decreasingremaining liquid magma such that according to Eq. (1)

ARbSrB"A

RbSrBof D (3)

with D"DR"!D

S3(Gast 1968; McCarthy and Caw-

thorn 1980). Assuming reasonable values of DR""0.5

and DS3"2.0 (i.e. D"!1.5) the Rb/Sr ratio increases

very rapidly as soon as f(0.3 (McCarthy and Caw-thorn 1980). Two models of magma evolution are disc-ussed in the following sections: fractional crystalliza-tion and partial melting.

Isotope ratios in a fractional crystallization of a magma

If a magma decreases its fraction (f ) vs time (t) and,accordingly, its 87Rb/86Sr (x) increases vs time (Fig. 1),the transition from a magma M

*fraction f

*with an

87Rb/86Sr ratio x*

to a magma M*`1

with f*`1

andx*`1

follows a very irregular and in most cases un-known path over the time interval t

*to t

*`1. But there

are two extreme cases:1. Magma M

*decreases from fraction f

*to f

*`1and

accordingly its Rb/Sr ratio x changes from x*

tox*`1

at t*immediately after separation of a fraction

Fig. 1a–c Evolution paths of a magma M*to M

*`1where the solid

phase reduces from fraction f*

to f*`1

, the parent—daughter ratioincreases from x

*to x

*`1and the radiogenic isotope ratio increases

from y0*

to y0*`1

during time interval t*to t

*`1. Path 1: segregation of

magma M*

into solid phase S*`1

and liquid magma M*`1

afterseparation of granite G

*; path 2: separation of granite G

*immedia-

tely after segregation into S*`1

und M*`1

; path 3: exponential seg-regation f"e!a (5*~5); path 4: linear segregation: f"(1!(t

*!t)/T)

of magma M*which intrudes as a granite G

*(path

1 in Fig. 1) and magma M*`1

remains unchanged tillafter the separation of a fraction which intrudes asa granite G

*`1.

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Fig. 2 Scheme of the differential model: segregation of a magmaM

*into a solid phase S

*`1and a remaining liquid magma

M*`1

immediately after a granite G*has been intruded at time t

*

2. The magma remains unchanged from the time t*of

separation of a fraction of magma and intrusion ofgranite G

*just untill before t

*`1and changes its

87Rb/86Sr ratio to x*`1

immediately before G*`1

isseparated and intrudes (path 2 in Fig. 1). In bothcases the average chemical composition of G

*is

equal to the one of M*. However, in the first case the

initial ratio of G*`1

is equal to the 87Sr/86Sr ratio ofM

*`1at t

*`1, whereas in the second case G

*`1would

have the 87Sr/86Sr ratio of M*at t

*`1. Most pro-

bably, however, this process is a more or less con-tinuous one between t

*and t

*`1, and the results

expected must be somewhere between the resultsderived for these two extreme cases.As examples for evolutions between these two cases

an exponential (Siebel 1993)

f"e~a(51~5) with a"!1

D(t*!t

*`1)In

x*`1x*

and a linear magma reduction

f"A1!t*!tT B with T"

t*!t

*`11!(x

*`1/x

*)1@D

are shown in Fig. 1 as path 3 and path 4 which lead toan 87Sr/86Sr ratio of the magma M

*`1which is between

the ratios of the above-mentioned extreme cases.The following calculation is based on a model

(Fig. 2) which represents the first extreme case of in-stantaneous partial crystallization. A liquid magmaM

1which is cooling segregates at the time t

1into

a solid phase S2

and a remaining liquid phaseM

2immediately after a granite G

1has been withdrawn

from the magma M1. At a later time t

2another granite

G2

is withdrawn from the magma M2

and a newsegregation into a solid phase S

3and a remaining

liquid phase M3

takes place and so on.

First case

If a magma has an 87Sr/86Sr ratio of b1

and a87Rb/86Sr ratio x@

1at time t

1before present (the latter

Fig. 3 Evolution of isochrons b*corresponding to ages t

*and initial

ratios b*of granites G

*with 87Rb/86Sr ratios x

*and 87Sr/86Sr ratios

y*caused by instantaneous differentiation of magmas M

*immediate-

ly after intrusion of G*. For further explanation see text

would be x1

at present and x@1"x

1ek51, where k is the

decay constant of the radioactive isotope, in this case of87Rb), and a granite G

1intruded at an age t

1from this

magma would yield an isochron (Fig. 3)

y"a1x#b

1(4)

with a1"ek51!1,

where y"87Sr/86Sr are present-day 87Sr/86Sr ratios ofsamples of G

1, and x their 87Rb/86Sr ratios.

If we assume partial crystallization right after theintrusion of G

1at t

1the remaining magma with an

87Rb/86Sr ratio x2

would have built up an 87Sr/86Srratio b

2at the time t

2of the intrusion of granite G

2of

b2"b

1#(a

1!a

2)x

2(5)

and samples of granite G2

today would define an iso-chron

y"a2x#b

2. (6)

However the x- and y-value of granite body G2

(x2, y

2)

as a whole would be located at the intersection of theisochrons of G

1and G

2because, according to Eq. (6),

y2"a

2x2#b

2and to Eq. (5),

y2"a

1x2#b

1,

i.e. any two subsequent granites G*and G

*`1are lo-

cated on the G*isochron. In a diagram initial ratio vs

age (or more precisely vs slope a) the bulk granite datapoints G

*and G

*`1are connected by a line with a ne-

gative slope of !x*`1

according to the generalizedEq. (5), as shown by Brewer and Lippolt (1974).

The process described by Eqs. (4)—(6) is showngraphically in Fig. 3: at time t

1from a magma M

1(t1) with

an x-value of x1

and an 87Sr/86Sr ratio of b1, a granite

G1

intrudes into a depth of some kilometres and

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samples from this granite define an isochron corre-sponding to an age t

1and an initial ratio b

1. Immedi-

ately after the intrusion of G1

the remaining magmadifferentiates into a solid phase S

2and a remaining

magma M2

with a mass fraction f2

compared with theprimary mass changes its x-value to x

2. The average

87Sr/86Sr ratio of the magma increases during the timeinterval (t

1—t

2) from b

1to b

2. From M

2at t

2a granite

G2

intrudes and defines a present-day isochron corres-ponding to an age t

2and an initial value b

2.

This process can be repeated several times.

Second case

If an instantaneous crystallization is assumed to havetaken place just before the intrusion of G

*`1, then x

2in Eq. (5) has to be replaced by x

1, and one obtains

y1"a

2x1#b

2, i.e. granites G

*and G

*`1are both

located on isochron G*`1

. In an initial ratio vs age(slope a)—plot the two subsequent granites are connec-ted by a line with negative slope x

*.

General case

In the b vs a (or t) diagram (Fig. 4), a magma of thecomposition G

1evolves linearly towards G

2with the

Fig. 4 Systematics in an initial ratio (b) vs age (t) plot in a fractiona-tion model (upper polygon instant segregation of magma M

*into

solid phase S*`1

and remaining magma M*`1

immediately afterintrusion of granite G

*with an average 87Rb/86Sr ratio x

*; lines

connecting G*and G

*`1have a slope x

*`1; lower polygon segregation

immediately before intrusion of G*: slope"!x

*; dashed line con-

tinuous segregation). After a granite G*has been withdrawn from

a magma M*

which now segregates into a solid phase S*`1

anda remaining magma M

*`1which increases its average 87Sr/86Sr

ratio b to b*`1

at time t*`1

according to the 87Rb/86Sr ratio x*`1

.For further explanation see text

slope of its 87Rb/86Sr ratio which must be at least x1,

but cannot be higher than x2, in which case it would

move towards G*2. For any continuous fractionation

during the time interval t1—t

2the evolution of the

magma must follow a curve between these two straightlines. For example the curved line in Fig. 4 shows theevolution for a continuous exponential fractionationwith time f"e!a (51~5)where t

1't't

2and a is the

relative crystallization rate df/dt/f.

Summary

The following criteria for a differentiation sequencethrough fractional crystallization are summarized asfollows:1. The logarithms of any two average element concen-

trations are linearly related.2. The average 87Rb/86Sr ratios of the granites increase

monotonically with decreasing age.3. In an isochron (x, y) diagram (e.g. y"87Sr/86Sr vs

x"87Rb/86Sr), the G*`1

point representing theaverage value of granite G

*`1as a whole must be

located at the intersection of the two isochronst*and t

*`1originating from G

*, — in an initial ratio vs

age plot (more precisely vs a"(e~k5!1) plot), theG

*`1point with the coordinates (a

*`1, b

*`1) must

be located between lines with slopes x*

and x*`1

originating from G*.

It should be noted that due to analytical uncertain-ties the average x- and y-values (which are highlycorrelated) of one granitic body G

*(1)D)n D) are

ellipses which are almost degenerated to error bars inthe direction of the corresponding isochron. In the b vsa (or vs t) diagram (Fig. 4), the G

*points are rotated

ellipses inside an error box 2pb 2pa. The angle ofrotation and the length of the main axis A and B ofthe ellipses depend on pa and pb and on the correlationcoefficient r

!"between these two quantities

(r!""—x6 /Jx6 2 ). Additionally, the standard deviations

of the maximum and minimum slopes x*and x

*`1must

be considered

Magma production by partial melting

At time t1, a certain fraction F

1of a solid rock with an

age t0

and an initial ratio b0

has been converted intoa liquid phase M

1(Fig. 5) from which a granite G

1is

separated. At a later time t2, when the temperature of

the magma M1

has increased because of deeper burial,another fraction F

2of the rock has been converted into

the liquid phase and added to magma M1

which re-mained after the separation of granite G

1forming

a new magma M2from which a granite G

2is separated.

This process may be repeated several times.

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Fig. 5 The partial melting model: From a solid M0of age t

0a frac-

tion F1

is separated by partial melting and forms a magma M1

fromwhich a granite G

1intrudes at t

1. At time t

2a second fraction F

2is

molten and creates a magma M2

(possibly combined with theremaining magma M

1) and a granite G

2is formed

Extraction of granite G1

The first magma M1

consists of a fraction of massm

1with 87Rb and 86Sr concentrations C

R1and C

S1: i.e.

x1"C

R1/C

S1. The 87Sr/86Sr ratio b

1at the time t

1of

the intrusion of granite G1

is given by

b1"(a

0!a

1)x

1#b

0(9)

Fraction 1, which formed the magma M1, has evolved

from F1(t0) at t

0to F

1(t1)"M

1(t1) at t

1(Fig. 6) and

a granite originating from this magma at t1

defines anisochron

y"a1x#b

1(10)

and the average x-value of the granite G1, which is x

1,

represents a present-day data point

y1"a

0x1#b

0, (11)

i.e. it is located on the source rock isochron (Fig. 6)

y"a0x#b

0.

Extraction of granite G2

At an age t2fraction 2 with an x-value x@

2is molten, has

evolved from F2(t0) to F

2(t2) and the 87Sr/86Sr ratio of

this fraction is given by

y@2(t2)"(a

0!a

2)x@

2#b

0, (12)

whereas the first magma M1has evolved from M

1(t1) to

M1(t2) and its y-value is

y@1(t2)"(a

1!a

2)x

1#b

1"(a

0!a

2)x

1#b

0. (13)

A mixture M2

of these two liquid fractions F1

andF2

has an x-value x2

and at t2

a y-value

b2"(a

0!a

2)x

2#b

0. (14)

Fig. 6 Evolution of isochrons and initial ratios caused by magmaformation due to partial melting. New liquid fraction F

*of the

magma generating rock which has an 87Rb/86Sr ratio x*mixed with

remaining previous magma M*`1

produces a new magma M*from

which at t*a granite G

*is with an initial ratio b

*withdrawn

The granite originating from this magma defines anisochron

y"a2x#b

2(15)

and substituting b2

(Eq. (14)), the data point (x2, y

2) of

the bulk granite is located on the source rock isochron

y2"a

0x2#b

0. (16)

In general, the difference

b*`1

!b*"a

0(x

*`1!x

*)#a

*x*!a

*`1x*`1

may be either positive or negative since mostly, but notnecessarily, x

*`1(x

*, but always a

*'a

*`1, i.e. there is

no monotonic trend of the initial ratios as in the case ofa differentiation model (Fig. 7).

Contrary to the differentiation model described pre-viously, where an areal isochron as defined byKohler and Muller-Sohnius (1980) has no geologicalmeaning, in the case of a partial melting model, a plotof the average 87Rb/86Sr ratios of the individual gran-itic bodies yields an isochron corresponding to the ageof the source rock which has produced the granitemagmas by progressive partial melting. In other words,since all the different fractions of a source rock, if theywould have remained as individual closed systems,would have defined an isochron corresponding to theage of the whole rock, also any mixture of two or morefractions would have remained on this isochron.

Because in most cases age differences are small com-pared with the ages themselves (i.e. t

*!t

*`1;t

*), the

slopes of the isochrons are too similar to be distinguish-able in a conventional isochron plot. Therefore, it ismore informative to plot the differences of the indi-vidual isochron slopes and a: the slope of an average

S290

Fig. 7 Initial ratio vs age plot for a partial melting model as ex-plained in Fig. 6. The granites G

*with ages t

*and initial ratios b

*and

present-day bulk granite 87Sr/86Sr ratios are composed of partialmelt fractions of a source rock of age t

0and initial ratio b

0

isochron (i.e. a best-fit line y"ax#b through the bulkgranite data points G

*)

*y*"(a

*!a)x#(b

*!b) (17)

and the data points of the individual granites G*in such

a diagram are given by

*:(G

*)"y

*!ax

*!b (18)

as shown in Figs. 11 and 13.If, however, two or more different source rocks with

different initial ratios and ages are subject to partialmelting and contribute to various degrees to the seq-uence of magmas, no areal isochron can be expected.

Applications to granite intrusion sequences in NE Bavaria

The granites of the Fichtelgebirge

In NE Bavaria two granitic sequences have been in-vestigated very intensively geochemically as well asgeochronologically. A large number of samples havebeen analysed with respect to major and trace elements.Whole-rock Rb/Sr isochrons with age errors mostlyless than 1%, Rb/Sr as well as K/Ar dates have beenobtained for each of these granites.

In the northern part of the area shown in Fig. 8 thegranites G1, G2, G3 and G4 of the Fichtelgebirge wereinvestigated geochemically and petrographically byRichter and Stettner (1979) and dated by Besang et al.(1976). The samples of G1 were remeasured with higherprecision by Lenz (1986). Some additional samples ofG1, G1R, G1S, G2, G3 and G4 have been measured byCarl and Wendt (1993) and the new results are present-ed in Table 1.

Fig. 8 Sketch of the location of NE Bavarian granites. 1 Fine-grai-ned granite; 2 medium-grained granite; 3 coarse-grained porphyrygranite

Porphyritic granite G1 which covers the largest areain the Fichtelgebirge is the oldest member of the above-mentioned suite G1—G4 (Stettner 1964) and was em-placed into a deep level. G1 is further subdivided in G1sensu strictu a medium-grained biotite granite withsome muscovite and K-feldspars up to 8 cm in size, theweakly porphyritic more fine-grained peripheral faciesG1R (Reut granite) with K-feldspars up to 2 cm sizeand the fine-grained G1S (Selb granite) in the easternpart with dominating muscovite content and only smallbiotite contents. After a long period of time, the ‘‘Rand’’granite G2 was emplaced into a low-temperatureenvironment. G2 shows coarse phenocrysts in a fine-grained matrix. Later on, the ‘‘Kern’’ granite G3 wasemplaced. G3 has a coarse-grained texture and con-tains fragments of G2, indicating that G3 is youngerthan G2. The latest intrusion, G4, is a ‘‘tin’’ granitewith high amounts of fluid phases which resulted inpneumatolytic mineralizations.

The granites of the Oberpfalz

Approximately 30 km south of the Fichtelgebirge, agranitic massif is comprised of the Falkenberg granite

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Table 1 NE Bavarian Granites: Average Rb, Sr, Ba concentrations, Rb-Sr ages and initial ratios. Fichtelgebirge granites: G-1 Weissenstadt-Markleuthener porphyry; G-2 Rand; G-3 Kern; G-4 Tin; Zh Zainhammer; GSF Friedenfelser; Fa Falkenberger; GS Steinwald; Oberpfalzgranites: Ap1. granitic veins in Fa; F1 Flossenburger; Be Barnauer; Le Leuchtenberger

Type N! Rb" Sr Ba" X ($1 rX) a$ T (Ma)% b& r!"

' Reference(ppm)" 10~3 #

G-1 14 220 248 900 2.35 4.645 326.4 0.70818 !0.838 2, 3, 10, 1137 34 237 0.14 30 2.1 13

G-2 20 420 39 266 27.3 4.339 305.0 0.71056 !0.943 2, 3, 1162 8 97 2.1 54 4.0 135

G-3 32 431 29 134 43.7 4.360 306.4 0.71056 !0.931 2, 3, 1158 9 72 2.6 39 2.7 137

G-4 11 720 5.3 11 580 4.107 288.7 0.7202 !0.829 2, 3, 11140 3.3 10 131 23 1.6 61

Zh 4 233 133 708 5.12 — — — 5, 618 8 60 0.25

GSF 21 355 41 135 25.5 4.480 314.8 0.7076 !0.950 4—651 25 34 3.5 32 2.3 11

Fa 26 364 65 289 16.5 4.431 311.3 0.70967 !0.979 325 18 11 0.9 51 4.0 81

GS 20 611 12.8 24 148 4.414 310.1 0.7188 !0.791 5, 5171 9.4 5 26 18 1.3 15

Ap1. 14 354 22 66 48 4.405 309.5 0.71200 !0.810 6, 786 11 10 7 17 2.2 47

F1 119 437 34 113 38.2 4.438 311.9 0.7144 !0.91 8, 1233 8 35 0.9 39 2.7 17

Be 25 431 18 57 72 4.457 313.2 0.7151 !0.90 8, 1252 8 36 7 40 2.8 33

Le 39 219 196 675 3.23 4.565 326.1 0.70776 !0.999 3, 9, 1345 78 248 0.23 0.115 2.2 13

!No. of samples for calculaltion of average chemical concentrations"Average concentration for Rb, Sr and Ba in ppm with standard deviation#Average 87Rb/86Sr ratio X derived from Rb and Sr columns, with $1!r error of the mean$Slop a of Isochron%Calculated age with $1!r error& Initial 87Sr/86 Sr ratio b with $1!r error'Correlation coefficient between a and bReferences: 1 Besang et al. 1976; 2 Richter and Stettner 1979; 3 Wendt et al 1986; 4 Richter and Stettner 1987; 5 Wendt et al. 1988; 6 Wendt etal. 1992a; 7 Wendt et al. 1989; 8 Tavakkoli 1985; 9 Madel 1988; 10 Lenz 1986; 11 Cari and Wendt 1993; 12 Wendt et al. 1995; 13 Siebel 1993

(Fa) which covers the largest area in the central partand two appendices in the south and the southeast: theLeuchtenberger granite (Le) and the Flossenburg (Fl)granite together with the Barnau granite (Be) approx-imately southeast of the Fl. Fa is porphyritic granitelike G1 (Richter and Stettner 1987), with K-feldsparsup to 80 mm, and biotite mostly dark red/brown andonly occasionally weakly chloritized (Wendt et al.1986). Granites Le (dated by Kohler et al. 1976 andSiebel 1993) and Fa were considered to belong to the‘‘older granites’’ together with granite G1 in the Fich-telgebirge, whereas Fl was considered to belong to the‘‘younger granites’’ such as granite G2 to G4 in theFichtelgebirge.

The Friedenfels granite (GSF), located at the NWcorner of the Fa granite, is in direct contact with theSteinwald granite (GS). Based on geochemical results,Richter and Stettner (1987) derived a sequenceFa—GSF—GS with decreasing age. Geochemical invest-igations (Wendt et al. 1988, 1994) have shown thata southern appendix of the GSF near Zainhammer (Zh)must be treated as a separate unit which, because of its

low and almost constant Rb/Sr ratio, could not bedated. These granites were dated by Kohler et al.(1974), Kohler and Muller-Sohnius (1976), Wendtet al. (1986, 1988, 1989, 1990, 1992, 1994). They havebeen investigated geochemically and petrographicallyby Richter and Stettner (1987; Fa, GSF, GS) andTavakkoli (1985; Fl and Be).

The relevant geochemical and isotopic results arecompiled in Table 1. The Ba—Sr log—log plot (Fig. 9)shows a fairly positive correlation. The Rb-Sr log—logplot (Fig. 10) shows good negative correlation for theFichtelgebirge granites G1—G4 (r"!0.997) favouringa fractional crystallization model. However, the gran-ites from the Falkenberg area are less well correlated(r"!0.918), as several granites have almost the sameRb concentration (Fa, GSF, ca. &350 ppm, and Fl, Be&430 ppm), but very different Sr concentrations (65,41, 22 ppm, and 34, 18 ppm). This means that theRb—Sr log—log plot contradicts a simple fractionalcrystallization model. The average x

*"87Rb/86Sr,

y*"87Sr/86Sr ratios of the granites G

*(i"1!4) are

plotted as deviations from an average age-reference

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Fig. 11 Deviations of individualisochrons of the four granites ofthe Fichtelgebirge from best-fitisochron for all granites (arealisochron) including their errorhyperbolas. The average granitedata points are located at theintersections of the twosucceeding granite isochrons

Fig. 9 Ba vs Sr plot (logarithmic scale) of four granites of theFichtelgebirge and eight granites of the Oberpfalz (NE Bavaria)

Fig. 10 Rb vs Sr plot of the granites of Fig. 9

Fig. 12 Initial ratio vs age plot of the granites of the Fichtelgebirge

isochron (slope a and initial ratio b; Eq. (18)) and thecorresponding difference isochrons (Eq. (17)) of theFichtelgebirge granites (Fig. 11) as well as the b vst plot (Fig. 12) agree with the fractional crystallizationsequence model close to path 2 in Figs. 1, 3 and 4.Also, the trend of increasing Rb- and decreasing Srconcentrations with decreasing age supports thismodel.

In contrary to the Fichtelgebirge, the data points ofthe granites of the Falkenberg area (Fig. 13) scatterirregularly about a reference isochron corresponding to315.6 Ma and an initial ratio of 0.70908 as obtained byregressing the unweighted isochron data of the granitesof Table 1. The bulk granite isochron data plot close toan isochron corresponding to an age greater than mostindividual ages, which supports the partial meltingmodel. If a best-fit line (weighting points for x, y errors

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Fig. 13 Deviations of individualisochrons of the eight granites ofthe Oberpfalz from the best-fitisochron

Fig. 14 Initial ratio vs age plot(upper part above the time axis)and average K/Ar muscoviteand biotite dates of the granitesof Fig. 13. (lower part below thetime axis)

and the correlation of r9:&#1.000 is used) an areal

isochron (Kohler et al. 1980) corresponding to an age of325 Ma is obtained. Deviations from this best-fit line,however, exceed the analytical errors (MSWD&16),demonstrating that such a common isochron does notexist. The evolution diagram of the initial ratios alsocontradicts the assumption of a unique fractional crys-tallization sequence, since the Flossenburg and Bernaugranites located east and southeast of the Falkenbergcomplex cannot be derived from a magma of Falk-enberg composition. On the other hand, a fractionalcrystallization sequence Le, GSF, Fa, GS cannot beexcluded. However, two different common areal isoch-rons for Le, GSF, Apl, GS west and south of theFalkenberg granite, and Fa, Fl, Be, i.e. the Falkenberggranite itself and southeastern granite bodies (Fig. 14),

i.e. partial melting of two different source rocks, arepossible. Also, the chemical data (equal Rb- but dif-ferent Sr concentrations) favour a partial meltingmodel.

Conclusion

It can be concluded that the Fichtelgebirge sequenceG1!G4 may represent an example of a fractionalcrystallization sequence; the granites of the Oberpfalzcan be derived neither by fractional crystallization norby partial melting of only one source rock. The Ober-pfalz granites were probably derived by partial meltingof two different source rocks. Different cooling histories

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are also indicated by K/Ar muscovite and biotite ages.The granites in NW direction of the Falkenberg granite(Fa) show systematically decreasing muscovite agesfrom 310 Ma (Fa) through 308 Ma (GSF) to 307 Ma(GS) with considerably lower biotite ages of 300 Ma.The granites SE of the Falkenberg complex show wide-ly scattering muscovite ages from 310 to 300 Ma anda similar wide scatter of the biotite ages from 300 to290 Ma, indicating local reheating and cooling eventswhich may have slightly disturbed the isotopic evolu-tion as assumed in the idealized models.

Acknowledgements The valuable advice and comments of G. Stet-tner (Munich) and the critical and constructive review by H. J.Lippolt (Heidelberg) improved the paper considerably and aregratefully acknowledged.

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