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Early Palaeozoic Acid Magmatism in the Saxothuringian Belt: New Insights from a Geochemical and Isotopic Study of Orthogneisses and Metavolcanic Rocks from the Fichtelgebirge, SE Germany W. SIEBEL1∗, H. RASCHKA2, W. IRBER1, H. KREUZER3, K.-L. LENZ4, ¨ HNDORF2 AND I. WENDT1 A. HO 1
GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, 14 473 POTSDAM, GERMANY ¨ R GEOWISSENSCHAFTEN UND ROHSTOFFE, STILLEWEG 2, 30 655 HANNOVER, GERMANY BUNDESANSTALT FU
2 3
HAMSUNSTRASSE 19, 30 655 HANNOVER, GERMANY
4
GLOCKENHOLZWEG, 29 331 LACHENDORF, GERMANY
RECEIVED DECEMBER 12, 1995 REVISED TYPESCRIPT ACCEPTED AUGUST 21, 1996
rocks range from 316 to 298 Ma for muscovites and from 306 to 280 Ma for biotites, demonstrating thermal influences from Late Visean to Late Stephanian (325–290 Ma) granite intrusions. The involvement of dominantly crustal-derived melts is considered to account for the peraluminous character (A/CNK > 1·08), high initial 87Sr/86Sr (> 0·709) and negative eNd(500 Ma) (–2·9 to –6·4) of the intrusive and volcanic rocks. The generation of the melts is assumed to have taken place within an overall extensional geodynamic setting. The wider range in eNd(500 Ma) for the metavolcanic rocks (–3·8 to –6·4; orthogneisses: –2·9 to –4·0) suggests that differential admixture of a pelagic sedimentary component during emplacement may have occurred. Nd model ages range from 1·5 to 1·7 Ga and are consistent with those of granitoids from the Erzgebirge and the Lausitz but completely different from those of metavolcanic rocks from the Thu¨ringer Wald. In addition, Nd model ages of the Early Palaeozoic granitoids in the Fichtelgebirge are consistent with those of the Late Carboniferous granitoids from the same area. This suggests similar source material for the pre-Variscan and the late Variscan Fichtelgebirge granitoids.
Orthogneisses and acid metavolcanic rocks from the Fichtelgebirge, NE Bavaria, Germany, are predominantly chemically evolved (monzo)granites and rhyodacites–dacites, respectively. The metavolcanic rocks are variably tectonized and include samples with anomalously low CaO, Na2O and MnO (80 wt %) and K2O (>6 wt %) concentrations, implying selective element mobility during post-eruptive events. Sm–Nd isotope data for samples from three main orthogneiss units (Wunsiedel, Selb, Waldershof) yield a composite Sm–Nd whole-rock isochron corresponding to an age of 560 ± 45 Ma. This age estimate is constrained, however, to be less than ~560 Ma by the presence of detrital Cadomian zircons in the country rocks. Fifteen specimens from the Wunsiedel orthogneiss give an Rb–Sr whole-rock isochron of 480 ± 4 Ma with an initial 87Sr/86Sr ratio of 0·7095 ± 0·0007 (MSWD = 2·7). Rb–Sr isotope data from the Waldershof orthogneiss and the metavolcanic rocks suggest, however, that, in general, Sr isotopic equilibrium, if ever reached, was significantly modified during later events. Taking recent geochronological literature data into account, it now appears that Early Palaeozoic acid magmatism in the Fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the Early and Mid Ordovician and ended with the eruption of the volcanic successions during the Late Ordovician. K–Ar ages of the investigated
KEY WORDS: Fichtelgebirge; geochemistry; orthogneisses; metavolcanic rocks; radiogenic isotopes; Saxothuringian belt
∗Corresponding author. e-mail:
[email protected]
Oxford University Press 1997
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between 3·5 and 5·5 kbar (Mielke et al., 1979). Magmatic activity is marked by polyphase plutonic and volcanic cycles. The predominant magmatic sequences, both preVariscan and late Variscan, consist of large volumes of felsic rocks. Metabasites, characterized by amphibolitic and doleritic rocks of alkaline basaltic character, are subordinate (Okrusch et al., 1989). The age of the pre-Ordovician strata, a variegated metasedimentary sequence (metapelites, metabasites, quartzites, marbles, graphite schists, calc-silicate rocks, meta-arkoses, metagreywackes) in the core of the Fichtelgebirge anticline is controversial. In the first half of this century it was correlated with the Precambrian of the Moldanubian terrane (Wurm, 1928). Von Gaertner (1944), however, invoked a mainly Cambrian age and introduced the term ‘Arzberg Series’ for these strata. In agreement with von Gaertner (1944), and Raschka (1967), Stein (1988) interpreted the Arzberg Series as a postCadomian, Early Palaeozoic unit which had been deformed only during the Variscan orogeny. Stettner (1972, 1988, 1993), however, divided the Arzberg Series into an underlying Precambrian Variegated Group and overlying Cambrian Warmensteinach Formation (Fig. 2). He considered the notional Variegated Group to have been deformed during the Cadomian event. Grauert et al. (1973) obtained U–Pb isotope data on detrital zircons from the ‘Plattenquartzite’ of the Upper Arzberg Series (Warmensteinach Fm.) which suggest that the sedimentation age of the quartzites was 1·08, which is a typical S-type signature (Chappell & White, 1974). Chondrite-normalized REE patterns (Fig. 7) for the orthogneiss and metavolcanic samples are typical for those of peraluminous granites (Miller & Mittlefehldt, 1982). These include considerable enrichment in light rare earth elements (LREE) over heavy rare earth elements (HREE) and significant negative Eu anomalies, increasing with decreasing La/Yb ratio. A decrease in RREE with increasing Rb/Sr can best be observed in the orthogneisses of Wunsiedel and Selb, indicating that the REE concentration is controlled by the degree of differentiation. The restricted range of REE variation within the Waldershof gneiss reflects the limited degree of differentiation of this orthogneiss unit. Emphasis should be placed on the REE pattern of the most fractionated Wunsiedel sample because the curvature is typical for more highly evolved S-type granites. A similar pattern can be found in the highly fractionated Variscan granites of the Fichtelgebirge (Siebel et al., 1995), but also in other granite suites (Bau, 1996).
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Fig. 4. Zr/TiO2 vs Nb/Y discrimination diagram for orthogneisses and metavolcanic rocks from the Fichtelgebirge.
In general, depletion of both LREE and HREE is attributed to the removal of accessory phases, whereas Eu depletion is ascribed to removal of plagioclase and alkali feldspar during differentiation. An alternative interpretation of REE patterns in altered rocks is that the REE distribution is partly controlled by fluids. This is suggested from the weak correlation of the Eu anomaly with the La/Yb ratio. Owing to often marked loss of Ca and Sr in the metavolcanic rocks, some mobility of Eu has also to be assumed. Slight negative Ce anomalies in some of the metavolcanic rocks in addition hint at an alteration by oxidizing fluids.
Sm–Nd isotopic data Fourteen samples, including seven orthogneisses and seven metavolcanic rocks, have been analysed for Sm and Nd concentrations and for their Nd isotopic composition (Table 3, Fig. 8). Within the orthogneisses, 147Sm/144Nd reaches exceptionally high ratios (0·13–0·19) suggesting the segregation of an LREE-enriched phase such as allanite or monazite during crystallization. Fractionation
of the Sm/Nd ratio during crystallization can be inferred from positive correlation between 147Sm/144Nd and 87Rb/ 86 Sr (Table 3). The metavolcanic rocks have low and more restricted 147Sm/144Nd ratios ranging from 0·13– 0·14. The present-day values of epsilon Nd, eNd(0), range from −3·8 to –7·7 (orthogneisses) and from −8·1 to –10·7 (metavolcanic rocks). On the Sm–Nd isochron diagram, the orthogneisses of Wunsiedel, Selb and Waldershof define a linear array (Fig. 8). The composite regression line which corresponds to an age of 560 ± 45 Ma with eNd(T) = −3·1 ± 0·3, and an MSWD value of 1·0 is defined by two Waldershof and two Selb samples with lower 147Sm/144Nd ratios and three Wunsiedel samples with higher ratios. A line fit through the three Wunsiedel samples would yield an age estimate of 470±136 Ma. The orthogneisses are characterized by a limited range of eNd(T) (T = 500 Ma) values (−2·9 to −4·0; Table 3). The metavolcanic rocks, in contrast, show a larger range of eNd(T) values which decrease from the western complex (−3·8 to −4·2) through the eastern complex (−4·8 and −4·9) to the southern complex (−6·2 and −6·4). The
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Fig. 5. Zr vs TiO2 for orthogneisses and metavolcanic rocks from the Fichtelgebirge.
eNd(T) values reported here were calculated at 500 Ma, the approximate age as constrained by stratigraphic and geochronological information. Uncertainties in the crystallization ages of about ±50 Ma have little influence on the calculated eNd(T). Depleted mantle Nd model ages (DePaolo et al., 1991) are ~1·5 Ga for the orthogneisses and in the range 1·5–1·7 Ga for the metavolcanic rocks.
Rb–Sr isotopic data
samples, the data scatter around an errorchron corresponding to an age of 377 ± 31 Ma with initial 87Sr/ 86 Sr of 0·7142 ± 0·0015 and an MSWD of 9·5 (Fig. 10). For Waldershof, omission of certain samples would yield better results but we do not feel justified in discounting a larger number of samples. For the metavolcanic rocks, 12 analyses on specimens from the western complex were obtained. However, owing to the strong alteration and possible admixture of clastic material, age calculation based on Rb–Sr whole-rock data is not justifiable.
Rb–Sr isotope compositions were obtained for representative samples from the Wunsiedel and Waldershof gneisses and from the western metavolcanic complex (Table 3). The 87Rb/86Sr-isotopic ratios are in the range 3–6 (Waldershof ), 5–140 (Wunsiedel) and 20–100 (metavolcanic rocks). Excluding sample 551, regression of the remaining 15 whole-rock samples from the Wunsiedel gneiss gives an isochron fit corresponding to an 87Rb–87Sr age of 480 ± 4 Ma, initial 87Sr/86Sr of 0·7095 ± 0·0007, and MSWD = 2·7 (Fig. 9). Taking all 10 Waldershof
Conventional K–Ar dates on mica concentrates from samples collected from the three orthogneiss massifs range from 316 to 298 Ma for muscovites, and from 306 to 280 Ma for biotites (Fig. 11, Kreuzer et al., 1989), depending on the cooling history of the respective neighbouring granites of the Namurian to Early Permian intrusive cycles (Carl & Wendt, 1993; Siebel, 1995a).
K–Ar isotopic data
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Fig. 6. Plot of A/CNK [mol % Al2O3/(CaO+Na2O+K2O)] vs SiO2 for orthogneisses and metavolcanic rocks from the Fichtelgebirge.
However, the model of Cadomian intrusion of the Fichtelgebirge granitoids is incompatible with the younger Rb–Sr isochron age for the Wunsiedel gneiss presented here. It is important to state that our Rb–Sr dating has recently been proven by Rb–Sr whole-rock, U–Pb and Pb–Pb zircon studies of Wiegand (1996) and Mielke et al. (1996): these give ages in the range 455–472 Ma for the gneisses from Wunsiedel, Waldershof and Selb. Intrusive rocks of similar age (i.e. 500–450 Ma) are widely distributed within the Saxothuringian belt and adjacent areas. Zircon ages between 500 and 460 Ma for granitoid rocks have been reported for the western Sudetes (Borkowska et al., 1980; Oliver et al., 1993; Kro¨ner et al., 1994b). Igneous activity between 500 and 480 Ma has been recorded within the geotectonic units situated to the south and to the south-east of the Fichtelgebirge, i.e. Erbendorf–Vohenstrauss Zone, Maria´nske´ La´zneˇ complex and Tepla´–Barrandium (U–Pb zircon and Rb–Sr muscovite, von Quadt, 1990; Bowes & Aftalion, 1991) and for granitoids in western and eastern Bohemia (U–Pb
TIMING AND TECTONIC SETTING OF THE MAGMATISM Age of the orthogneisses For the orthogneisses, the Sm–Nd and Rb–Sr wholerock isotope data provide age estimates of 560 ± 45 Ma and 480 Ma ± 4 Ma, respectively. An upper age limit of 560 Ma for the intrusions is provided from U–Pb analyses of detrital country rock zircons (Grauert et al., 1973). The Sm–Nd age appears to be compatible with a Cadomian event. Within the Saxothuringian belt intrusive rocks of Cadomian age (i.e 600–540 Ma) have recently been reported from the Lausitz and the Erzgebirge (U–Pb and 207Pb–206Pb zircon ages on granitic orthogneisses; Korytowski et al., 1993; Kro¨ner et al., 1994a, 1995). Among the manifestations of Cadomian magmatism within other parts of the Bohemian Massif, U–Pb zircon ages of 585 and 550 Ma (van Breemen et al., 1982) and Rb–Sr ages of 550 Ma (Scharbert & Batik, 1980) have been reported.
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Fig. 7. Rare earth distribution patterns for representative orthogneisses and metavolcanic rocks from the Fichtelgebirge. Arrows indicate the evolutionary trend caused by fractional crystallization. Chondrite concentrations for normalization from Evensen et al. (1978).
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Table 3: Rb–Sr and Sm–Nd whole-rock isotopic data Sample no.
87
Rb
Sr
(p.p.m.)
(p.p.m.)
Rb/86Sr
87
Sr/86Sr
Sm
147
Sm/144Nd
Nd
143
Nd/144Nd
eNd(0)
eNd( T )
TDM(Ga)
(p.p.m.) (p.p.m.)
Orthogneisses
Wunsiedel 683
179·
86·
6·052
739
202·
76·
7·747
0·75072
738
197·
53·8
10·67
0·78265
547
252·
35·6
20·76
0·85333
741
236·
27·7
25·01
0·87969
548
299·
34·8
25·23
0·87710
740
214·
21·7
29·05
0·90875
745
266·
26·1
33·36
0·94315
744
267·
22·6
35·04
0·94810
742
231·
12·2
40·36
0·97740
556
330·
17·5
56·65
1·09670
551
284·
14·3
59·93
1·15964
707
334·
12·4
82·42
1·27478
510
382·
9·2
130·9
1·59084
706
376·
8·6
137·8
1·66011
555
354·
8·1
138·0
1·67644
736
147·
154·
2·779
0·72827
731
146·
140·
3·048
0·73083
726
148·
139·
3·107
0·73012
732
154·
143·
3·124
0·73248
737
178·
155·
3·332
0·73128
734
150·
125·
3·475
0·73381
733
157·
112·
4·054
0·73576
728
167·
114·
4·246
0·73749
730
164·
101·
4·735
0·73979
729
171·
101·
4·889
0·73965
0·76300
2·7
10·1
0·1641
0·512365
–5·3
–3·3
1·5
2·3
8·1
0·1735
0·512413
–4·4
–2·9
1·5
1·7
5·6
0·1885
0·512443
–3·8
–3·3
1·5
515
7·1
32·5
0·1315
0·512246
–7·7
–3·5
1·5
560
5·2
23·8
0·1323
0·512245
–7·7
–3·6
1·5
586
4·4
18·9
0·1410
0·512285
–6·9
–3·3
1·5
603
3·3
13·2
0·1501
0·512280
–7·0
–4·0
1·5
4·4
19·8
0·1336
0·512217
–8·2
–4·2
1·6
Waldershof
Selb
Metavolcanic rocks
Western complex 758
274·
41·2
19·40
0·81650
759
244·
33·6
21·21
0·83011
761
177·
22·6
22·95
0·87294
646
252·
31·8
23·19
0·83446
760
184·
18·8
28·80
0·88268
640
358·
29·7
35·79
0·97608
644
262·
16·1
48·31
0·97089
4·7
22·4
0·1278
0·512216
–8·2
–3·8
1·5
637
329·
12·3
80·64
1·16005
4·0
18·4
0·1322
0·512222
–8·1
–4·0
1·5
638
307·
11·2
83·39
1·19352
643
223·
8·1
83·39
1·22028
642
139·
4·6
92·09
639
309·
9·3
101·7
1·21115 1·25913
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Table 3: Rb–Sr and Sm–Nd whole-rock isotopic data—continued Sample no.
Rb
Sr
(p.p.m.)
(p.p.m.)
87
Rb/86Sr
87
Sr/86Sr
Sm
Nd
147
143
eNd(0)
eNd( T )
TDM(Ga)
Sm/144Nd
Nd/144Nd
(p.p.m.) (p.p.m.)
Eastern complex 568
3·8
16·4
0·1391
0·512198
–8·6
–4·9
1·6
572
8·3
35·4
0·1410
0·512209
–8·4
–4·8
1·6
624
7·6
36·0
0·1270
0·512092
–10·7
–6·2
1·7
655
8·6
37·8
0·1373
0·512114
–10·2
–6·4
1·7
Southern complex
Isotope normalization factors: 86Sr/88Sr = 0·1194, 146Nd/144Nd = 0·7219. Error estimates: 87Rb/86Sr = 1%, 87Sr/86Sr = 0·3‰, 147Sm/144Nd = 0·3%, 143Nd/144Nd = 0·03‰ (1r confidence level). Decay constants: kRb = 1·42 × 10–11 yr −1; kSm = 6·54 × 10–12 yr −1. eNd values were calculated relative to a chondrite present-day 143Nd/144Nd value of 0·512638 and 147Sm/144Nd of 0·1967; eNd( T ) calculated for T = 500 Ma. TDM model ages calculated according to the method proposed by DePaolo et al. (1991, p. 2074).
Fig. 8.
143
Nd/144Nd vs
147
Sm/144Nd diagram for orthogneisses and metavolcanic rocks from the Fichtelgebirge. The regression line is defined by all orthogneiss samples (seven whole-rock analyses). Error bars represent 1r errors.
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SIEBEL et al.
Fig. 9.
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ACID MAGMATISM IN SAXOTHURINGIAN BELT
Sr/86Sr vs 87Rb/86Sr isochron diagram for the Wunsiedel orthogneiss. The regression line is defined by analyses of 15 whole-rock samples. Errors are 1r.
and Pb–Pb zircon, Kro¨ner et al., 1994c; Do¨rr et al., 1995). Granitic augengneisses, metagabbros and eclogites within the Mu¨nchberg Massif, an allochthonous crystalline unit north-west of the Fichtelgebirge, yield concordant U–Pb, Rb–Sr and Sm–Nd ages between 500 and 480 Ma (Gebauer & Gru¨nenfelder, 1979; So¨llner et al., 1981; Stosch & Lugmair, 1990). The magmatic age of granulites from the Saxonian granulite massif has also been determined at ~470 Ma (Sm–Nd whole-rock; von Quadt, 1993). Despite several examples of metamorphic activity around 480 Ma reported for the Saxothuringian and Moldanubian zones (Grauert et al., 1973, 1974; Krentz, 1985; Teufel, 1988; Gebauer et al., 1989) a reset of the Rb–Sr and even the U–Pb isotopic systems in the Fichtelgebirge orthogneisses by an Ordovician event is rather unlikely. The country rocks of the orthogneisses lack evidence of a strong Ordovician metamorphic
221
imprint, which impedes the interpretation of the Early Ordovician ages of the orthogneiss bodies as a metamorphic age. Hence, the ages are taken to reflect the time of intrusion of the orthogneisses. The 560 ± 45 Ma Sm–Nd age does not contradict an Ordovician intrusion age. It is methodically questionable, anyway, because it is defined essentially by combining data from three different orthogneiss bodies. Disregarding this doubt, it is nevertheless consistent with the Rb–Sr age at the 95% analytical confidence level. Reheating of the orthogneisses by Variscan magmatism was sufficient to reset the K–Ar mica system (Kreuzer et al., 1989). The K–Ar ages of the Selb gneiss (muscovite ages of 316 Ma) may be related to reheating by the G1 granite, whereas in the Wunsiedel and Waldershof gneisses reheating occurred in response to the intrusions of both older and younger granites (Fig. 11).
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Sr/86Sr vs 87Rb/86Sr diagrams for the Waldershof gneiss and the metavolcanic rocks with best fit lines through all data points. Errors are 1r.
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and the Northern Phyllite Zone, respectively. In geotectonic terms, these segments are interpreted as part of a Silurian active continental margin with the development of arc magmatism (Meisl, 1990; Sommermann et al., 1992; Dombrowski et al., 1995).
Tectono-magmatic setting
Fig. 11. Summary of previously published K–Ar ages for the orthogneisses [data from Kreuzer et al. (1989)]. (For sample location, see Fig. 1.) Stippled columns: ages of late-Variscan granites [data from Carl & Wendt (1993) and Siebel (1995a)].
Age of the metavolcanic rocks On the basis of regional stratigraphic data the metavolcanic rocks have been considered to be intercalated with Late Cambrian and Early Ordovician strata (Fig. 2). Near Waldsassen (Fig. 1) the upper Phycoden Beds (Fig. 2), which are elsewhere considered as the youngest strata interstratified with the metavolcanic rocks (Emmert & Stettner, 1995), contain acritarchs of Tremadocian– Arenigian age (M. Kling, personal communication, 1996). From these indications, minimum ages of 480 Ma would be expected for the metavolcanic rocks [time scale based on Odin (1994)]. Conventional U–Pb analyses of zircons from a metavolcanic rock specimen from the western complex, however, yielded a lower intercept age of 449 ± 2 Ma, pointing to a Late Ordovician intrusion age (Teufel, 1988). This age was recently confirmed by additional U–Pb zircon studies of metavolcanic rocks from the southern complex (Wiegand, 1996: 443+11 –13 Ma). To account for the Late Ordovician U–Pb ages of the metavolcanic rocks and the Early Ordovician stratigraphic age of the country rocks (see Fig. 13 below), a tectonic rather than a stratigraphic relationship between the two rock types has to be invoked at least for the dated occurrences (see Stettner, 1971). In a regional context, the U–Pb ages of the metavolcanic rocks tend to be slightly older than the emplacement age of the Red gneiss and Haibach orthogneiss units in the Spessart (440–410 Ma, Dombrowski et al., 1995) and of metavolcanic rocks in the southern Taunus (442–426 Ma, Sommermann et al., 1992). Both areas are part of distinct tectonic units of the northern margin of the Saxothuringian Zone and the southern margin of the Rhenohercynian Zone, the Mid-German Crystalline Rise
The tectonic setting of the magmatic activity in the Fichtelgebirge is still open to dispute. The spread of data points across several fields in the tectonic discriminant diagrams of Pearce et al. (1984) does not permit the definition of a distinct tectonic setting. In the Na2O × Zr × Y vs (K2O × Rb)/MgO diagram (Tischendorf & Fo¨rster, 1992) the orthogneiss samples plot in the COLG (collision granitoids) field and in the COLG & VAG (volcanic arc granitoids) field (Fig. 12). Similar characteristics are indicated by the fresh metavolcanic rock samples. However, as mentioned above, a large number of metavolcanic rocks are depleted in Na and enriched in K, thus shifting the data points from the COLG and COLG & VAG field into the VAG field. Besides, the metavolcanic rocks include volcaniclastics, thus any discrimination should be used with caution. Early Palaeozoic basic and acid volcanogenic rocks have been documented in other parts of the Saxothuringian Zone. Cambro-Ordovician volcanic rocks of the Vesser area, northern margin of the Saxothuringian Zone, are thought to have been generated within a backarc setting involving ensialic crust (Bankwitz et al., 1992). The occurrence of Early Palaeozoic alkali basalts and tholeiites within the western Sudetes indicates emplacement in an extensional geodynamic setting (Furnes et al., 1989). Within the Fichtelgebirge, metabasic rocks of Cambro-Ordovician age are rare and their geochemical characteristics show affinities to modern within-plate basalts (Okrusch et al., 1989). Extension of the Saxothuringian belt during the Early Palaeozoic is documented in the sedimentary record. The Cambro-Ordovician sediments of the Fichtelgebirge are part of a neritic to hemipelagic succession which might have been deposited in an intracratonic basin or a ‘failed rift’ (Franke & Oncken, 1995). It is likely that the genesis of the orthogneisses and metavolcanic rocks is related to extensional forces. Lithospheric extension, thinning and heating owing to extra mantle input may have caused the heating of fusible crustal rocks followed by partial melting of these rocks.
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PETROGENESIS OF THE ORTHOGNEISSES AND THE METAVOLCANIC ROCKS On the basis of the available geochemical data and the geological context in which the Fichtelgebirge granitoids
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Fig. 12. Discrimination diagram of Na2O × Zr × Y vs (K2O × Rb)/MgO (Tischendorf & Fo¨rster, 1992). COLG, collision granitoids; VAG, volcanic-arc granitoids; WPG, within-plate granitoids.
and metavolcanic rocks occur, it seems likely that their petrogenesis is dominated by partial melting of crustal source rocks. Initial 87Sr/86Sr isotopic values and A/CNK ratios are in the range of S-type granitoids. The midProterozoic Nd model ages confirm the involvement of old (>1·5 Ga) continental crust. As a paradox, there was no major crust-forming event during the mid-Proterozoic within the Bohemian Massif. Currently, there is no better explanation than that the rocks may have had as their source a mixture of components of different age involving older (Archean to Early Proterozoic) and younger (Cadomian) crustal components. The fact that the Nd model ages are close together should imply a very good mixing of these components. The three orthogneiss bodies analysed in this work have very similar eNd(T) values, which is consistent with their derivation from similar crustal source. The metavolcanic rocks exhibit geographic variations in their eNd(T) values, possibly reflecting more diverse source compositions. Differential admixture of a sedimentary component derived from weathering of older crustal rocks could account for the Nd isotope heterogeneity of the metavolcanic rocks rather than melting of spatially distinct sources. Based on partial melting reactions (e.g. Patin˜o Douce & Johnston, 1991) and the behaviour of Rb and Sr during anatectic melting and fractional crystallization, the orthogneisses and metavolcanic rocks can be derived by variable degrees of partial melting of a predominantly metasedimentary protolith followed by variable degrees of crystal fractionation. The enhanced Rb/Sr ratios of
the orthogneisses of Wunsiedel and Selb are compatible with a low fraction of incongruent melting (~20%) involving a muscovite and/or biotite dehydration reaction in a pelitic or greywacke source with an Rb/Sr ratio of about unity, followed by a strong degree of fractional crystallization. The presence of magmatic tourmaline in the Wunsiedel orthogneisses may be used to argue that the rocks crystallized from a melt having a low water activity, XH2O < 0·3 (Scaillet et al., 1991). To generate crustal granites with low Rb/Sr ratios (Waldershof ) would require a larger degree of non-modal batch melting of a mica-rich sedimentary protolith or a lower degree of fractional crystallization. The southern metavolcanic complex has a somewhat unusual composition: enriched in Ti and Zr (Fig. 5) and low in eNd(T). This could be ascribed to the breakdown of greater quantities of titaniferous minerals, such as biotite, during partial melting, and to the greater involvement of non-volcanogenic sedimentary components. Major element changes in some of the metavolcanic rocks involve preferential depletion of Ca, Na and Mn. This depletion probably resulted in a volume change and allowed the introduction of silica. Enhanced element mobility has given rise to Rb–Sr errorchron systematics within the metavolcanic rocks of the western complex. It is assumed that the erratic Rb–Sr isotopic systematics of these rocks was mainly an effect of depletion of Sr resulting in variable fractionation of the primary Rb/ Sr ratios. Elemental changes corresponding to those observed in the metavolcanic rocks are also encountered in non-metamorphic Permian rhyolites of the Saar–Nahe
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Fig. 13. Time chart showing pre-Variscan magmatic activity within the Saxothuringian belt and adjacent areas. References: Southern Taunus, Sommermann et al. (1992); Spessart, Lippolt (1986), Dombrowski et al. (1995); Fichtelgebirge, Teufel (1988), Wiegand (1996), Mielke et al. (1996), this study; Erzgebirge, Kro¨ner et al. (1992); Lausitz, Korytowski et al. (1993), Kro¨ner et al. (1994a); Western Sudetes, Borkowska et al. (1980), Korytowski et al. (1993) Oliver et al. (1993). Time scale based on Odin (1994).
basin (Wimmenauer, 1985, p. 182) and are common in acid volcanics. Taking into account these similarities, we suggest that the geochemical signatures of the metavolcanic rocks may be attributed not only to metamorphic processes but also to late alteration involving hydrous fluids or deep surface weathering on a palaeo erosion surface.
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IMPLICATION FOR THE MAGMATIC EVOLUTION OF THE SAXOTHURINGIAN BELT Within the Saxothuringian belt and adjacent areas, the petrogenesis of pre-Variscan granitic orthogneisses and acid metavolcanic rocks has been recently summarized
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by Kro¨ner et al. (1992, 1994a), Sommermann et al. (1992), Korytowski et al. (1993), Oliver et al. (1993) and Dombrowski et al. (1995). Figure 13 provides a comparative summary of the available ages. This figure includes data from the northernmost element of the Saxothuringian belt, the Mid-German Crystalline Rise (Dombrowski et al., 1995) and the Northern Phyllite Zone (Sommermann et al., 1992); the last is considered part of the southern margin of the Rhenohercynian belt to the north. The allochthonous Mu¨nchberg Massif, which is characterized by deeper-water environments (Bavarian facies realm), has not been included in this comparison. The geochronological data set is largely based on U–Pb and Pb–Pb analyses of zircons. The intrusive ages of the igneous rocks span ~200 Ma. However, in most subunits the age data fall into restricted ranges. It is important to note that there was a conspicuous lack of magmatic activity during the Cambrian. Considering the whole area, the following pre-metamorphic igneous events can be identified: (1) evidence for a Cadomian magmatic event (600–540 Ma) is provided by plutonic rocks of the Erzgebirge and the Lausitz (Kro¨ner et al., 1992, 1994a; Korytowski et al., 1993), (2) magmatic ages intimately associated with the Caledonian phase (520–480 Ma) are dominant in granitoids of the western Sudetes (Borkowska et al., 1980; Korytowski et al., 1993; Oliver et al., 1993), (3) ages in the 450–410 Ma range are found in granitic gneisses exposed in the Mid-German Crystalline Rise and in metavolcanic rocks from the Northern Phyllite Zone (Sommermann et al., 1992; Dombrowski et al., 1995). Thus, in the Erzgebirge and the Lausitz there is convincing evidence for a Cadomian orogenic event. Obviously, the western Sudetes escaped the Cadomian plutonic activity. Granitoids of the western Sudetes are related to the closure of the Tornquist ocean (Oliver et al., 1993; Kro¨ner et al., 1994c). The granitoids of the Mid-German Crystalline Rise (i.e. Spessart in Fig. 13) have been related to a collision zone tectonic setting by Okrusch & Richter (1986) and were redefined by Dombrowski et al. (1995) as members of a volcanic arc assemblage reflecting Silurian magmatism around 440–410 Ma at an ancient active margin between the Saxothuringian and Rhenohercynian blocks. An active margin setting was also deduced for Silurian metavolcanic rocks (~440–420 Ma) of the Northern Phyllite Zone (i.e. Taunus in Fig. 13) by Meisl (1990). Both areas are distinct from the Saxothuringian sensu stricto, where no Silurian igneous activity has been recorded. Nd model ages for a range of Saxothuringian granitoids are plotted in Fig. 14. Excluding extreme values, rocks from the Erzgebirge and the Lausitz show a variation in T DM, ranging from 1·3 to 1·9 Ga. All Nd model ages determined here fall within this age range. This agreement suggests that the Saxothuringian granitoids from the Lausitz, Erzgebirge and the Fichtelgebirge could
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share a common Proterozoic crustal source. Different sources were involved in the production of the Early Palaeozoic metabasic and meta-acid volcanic rocks of the Thu¨ringer Wald (Bankwitz et al., 1992). The low Nd model ages indicate that these rocks may contain a larger amount of mantle-derived material. For comparison, the Variscan granites of the Fichtelgebirge are also included in Fig. 14. Generated during different magmatic cycles but of close geographic relationship, the pre-Variscan and Variscan granitoids of the Fichtelgebirge show a comparable Nd model age distribution.
CONCLUSION Our work on the pre-Variscan orthogneisses in the Fichtelgebirge leads to the following conclusions: (1) Major and trace element data suggest that the granitic orthogneisses and acid metavolcanic rocks from the Fichtelgebirge are not simply related to each other by variable degrees of partial melting of a common source or fractional crystallization in a crustal magma chamber. This conclusion is reinforced by the Nd isotope data, which attest not only to differences between the orthogneisses and the metavolcanic rocks but also to differences between each of the three metavolcanic complexes. (2) The granitic protoliths of the orthogneisses were formed through variable degree of partial melting and fractional crystallization of a relatively homogeneous crustal source. Many of the metavolcanic rocks are likely to be volcaniclastic in origin, containing an additional crustal component mixed in during sedimentation giving rise to more heterogeneous geochemical and isotopic features. (3) Mid-Proterozoic Nd model ages of 1·5–1·7 Ga may reflect some averaging effect of combining crustal source material of Archaean and Cadomian age. Similar protolith assemblages may also have been the source of the later Variscan granitoid magmatism. (4) The variable distribution of LILE within certain metavolcanic rocks is unlikely to be related to primary igneous processes. The data suggest that these rocks underwent at least one major open system event in which they became progressively depleted in Na, Ca and Mn, and enriched in Si and K. These disturbances, which also affected the Rb–Sr isotopic system of the rocks, might be related to late magmatic alteration (e.g. sericitization), tectonothermal overprint and partly to the volcaniclastic origin of the rocks. (5) The Rb–Sr whole-rock isochron age of the Wunsiedel gneiss of 480 ± 4 Ma is regarded as an estimate for its presumably Early Ordovician age of intrusion. Ordovician ages have been confirmed by recent U–Pb and Rb–Sr analyses for all of the investigated orthogneiss bodies (Wiegand, 1996; Mielke et al., 1996). The Sm–Nd
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Fig. 14. Compilation of Nd model ages for pre-Variscan and Variscan granitoids and metavolcanic rocks from the Saxothuringian belt. References: Thu¨ringer Wald, Bankwitz et al. (1992); Fichtelgebirge G1–G4 granites, Siebel et al. (1995); Erzgebirge, Kro¨ner et al. (1995); Lausitz, Kro¨ner et al. (1994a). (For signatures of metavolcanic rocks and orthogneisses, see Fig. 2.)
age of 560 ± 45 Ma derived from a composite regression line on samples from all three Fichtelgebirge orthogneiss units is consistent with an Early Ordovician intrusion age within twice the standard errors. On the other hand, composite regression of the Sm–Nd data may represent a mixing line without any meaning for a geological age. According to the U–Pb data of Teufel (1988) and Wiegand (1996), extrusion of the metavolcanic rocks occurred at ~450 Ma. Thus, the metavolcanic rocks seem to be younger than the orthogneisses as well as their postulated stratigraphic position. (6) The observation that the orthogneisses yield approximately the same K–Ar ages as adjacent Variscan granites strongly suggests that reheating by these granites was a major cause for the age resetting.
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(7) Different subunits of the Saxothuringian belt are characterized by granitoids with distinct emplacement ages, i.e. Late Precambrian, Cambro-Ordovician, Ordovician and Silurian. This fact alone excludes a common pre-Devonian magmatic evolution of the subunits.
ACKNOWLEDGEMENTS This study originated as part of the KTB pre-site investigations carried out by the Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, BGR, Hannover. Field support by Diplom-Geologe U. Vetter is gratefully acknowleged. Conversion of geochemical data by Ju¨rgen Heil was extremely helpful. Drs H. Fesser and J. Erzinger
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provided analytical facilities. We thank the late DiplomPhysiker Heinz Lenz, Monika Bockrath, Ilse Deneke, Beate Eichmann, Erika Kramer, Julian Lodziak, Margot Metz, Horst Klappert, Detlef Requard, Henryk Schyroky, Lutz Thießwald and Detlef Uebersohn for laboratory work. This research programme was supported by the Deutsche Forschungsgemeinschaft, Bonn (research grant II C 6—Ra 390/1–1). Comments and suggestions by Marjorie Wilson have greatly improved this work. Thanks are due to Paddy O’Brien (Bayreuth) and two anonymous reviewers who made constructive criticisms.
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