72-nasirspesertchronologyr.pdf

Mineralogy and Petrology (1991) 44:39-55 Mineralogy an(1 Petrology © by Springer-Verlag 1991 Printed in Austria Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise* S. Nasir 1'2, M. Okrusch 1, H. Kreuzer 3, H. Lenz~ 3, and A. H 6 h n d o r f 3 1Mineralogisches Institut der Universit~it W/irzburg, Wiirzburg, Federal Republic of Germany 2 Institute of Earth and Environmental Sciences, University of Yarmuk, Irbid, Jordan 3 Bundesanstalt f/ir Geowissenschaften und Rohstoffe, Hannover, Federal Republic of Germany With 6 Figures Received June 28, 1990; accepted February 12, 1991 Summary New Rb-Sr and K-Ar datings help to clarify the geologic history of the Spessart Crystalline Complex, Mid-German Crystalline Rise. The oldest dates, refined by new measurements, are recorded by whole-rock Rb-Sr analyses of the orthogneisses of the Rotgneiss Complex. These confirm a late Ordovician to Silurian age which is interpreted as the time of intrusion of the granitic precursors. Hornblendes, muscovites, and biotites from different lithostratigraphic units and rock types of the Spessart Crystalline Complex yielded K-Ar dates mainly in the range 324 to 318 Ma, an interval which conforms to the analytical precision. Two hornblendes and one muscovite show slightly older dates up to 328 Ma. On the other hand, there is a tail of younger hornblende dates towards 311 Ma, and two hornblendes gave dates as low as 293 and 274 Ma for no immediately obvious reason. The concordant dates around 324 Ma on the three different minerals may be interpreted as marking the time of a rapid uplift and cooling at about the boundary between Early and Late Carboniferous, presumably soon after culmination of the Variscan deformation and amphibolite facies metamorphism. Zusammenfassung Geochronologie des Spessart-Kristallins, mitteldeutsche Kristallinschwelle Neue Rb-Sr- und K-Ar-Datierungen liefern einen Beitrag zum Verst/indnis der geologischen Geschichte des Spessart-Kristallins. Die iilteste radiometrische Datierung, die * Dedicated to Borwin Grauert on the occasion of his 60th birthday. 40 S. Nasir et al.: Geochronology of the Spessart Crystalline Complex bislang im Spessart-Kristallin zur Verffigung steht, leitet sich aus Rb-Sr-Gesamtgesteinsanalysen yon Orthogneisen des Rotgneis-Komplexes ab. Die bereits von frtiheren Bearbeitern gefundenen sp/it-ordovizischen bis silurischen Daten wurden durch neuere Messungen best/itigt. Sie werden als Intrusionsalter des granitischen Ausgangsmaterials der Rotgneise interpretiert. Hornblenden, Muscovite und Biotite aus unterschiedlichen lithostratigraphischen Einheiten und Gesteinstypen des Spessartkristallins erbrachten K-Ar-Daten vorwiegend zwischen 324 und 318 Ma, d. h. einen Streubereich, der etwa der analytischen Genauigkeit entspricht. Zwei Hornblenden und ein Muscovit ergaben etwas/iltere Daten bis 328 Ma. Auf der anderen Seite beobachtet man eine Reihe von jiingereren Hornblendedaten bis 311 Ma, und zwei Hornblenden von nur 293 und 274 Ma, die sich nicht ohne weiteres erkl~ren lassen. Die konkordanten Alterswerte um 320 Ma, die fiir die drei Mineralarten gewonnen wurden, k6nnen als die Zeit einer raschen Hebung und Abkiihlung etwa an der Grenze Unter-/Oberkarbon interpretiert werden, die vermutlich bald nach dem H6hepunkt der variscischen Deformation und amphibolit-faziellen Metamorphose erfolgte. Introduction With the change from a fixistic to a mobilistic picture of the Variscan Orogen, the Mid-German Crystalline Rise--initially recognized by Brandes (1919) and later on defined by Scholtz (1930) and Brinkmann (1948)--has aroused renewed interest. This important structural element is now regarded as a suture zone where the Saxothuringian crystalline basement is thrusted upon the imbricated "Northern Phyllite Zone" and the anchimetamorphic Rhenohercynian (Weber and Behr, 1983; Giese et al., 1983; Behr et al., 1984; Weber, 1984, Franke, 1989), see Fig. 1. The new geological model received strong support by the fascinating results of deep-reflexion seismic recording along the profile D E K O R P 2 south which exhibits a highly laminated upper crust with faulted and southeast dipping reflectors just below the Spessart Crystalline Complex ( D E K O R P Research Group, 1985; Heinrichs, 1986; Behr and Heinrichs, 1987). One of the questions crucial to the geodynamic interpretation of the MidGerman Crystalline Rise is its metamorphic history. In this paper we present results of K-Ar dating on hornblendes, muscovites and biotites from various lithostratigraphic units of the Spessart Crystalline Complex which will give a minimum age of metamorphism. Another important event in the geological history of the Spessart is the intrusion of the granitic protolith of the Rotgneiss complex. The Rb-Sr whole-rock dates published by Kreuzer et al. (1973) and Lippolt (1986) are discussed in the light of new data. Geological Setting The Mid-German Crystalline Rise which extends in SW-NE direction from the Saar area to the Lusatia area is documented in several uplifted blocks, i.e. the crystalline complexes of Bergstrfisser Odenwald, B611stein Odenwald, Spessart, Ruhla and Kyffh~iuser (Fig. 1). Major parts are hidden by a cover of Permian, Mesozoic and Cenozoic sediments. Comprehensive reviews of the Spessart Crystalline Complex are given by Matthes and Okrusch (1977), Okrusch (1983) and Hirschmann and Okrusch (1988). The follow- North _ Sea v ~>'~-~ q ~ ~.~ oMagdeburg HARZf~ ,, 3 £~to.~.z." BRABANT .<,e.% MASSIF ~.~.e <"'~~,' (q.~ ~ Bruxelles 3 ~- 6 ---"~ / Strasbourg 8 ~ ~ / i ~ ÷ BLACK FOREST 9 voso - - 0 __~ KYFFH~USER o ~ 4 BerllnO s Fig. 1. Geological sketch map of the main tectonic units in the Central European Variscides (modified after Franke, 1989). Blank--Sedimentary cover of Permian, Mesozoic and Cenozoic age. 1 Variscan granites (largely post-tectonic). Rhenohercynian Realm 2 Devonian and Carboniferous sedimentary and volcanic rocks with Variscan metamorphism of very low grade. 3 Pre-Devonian sedimentary, volcanic and plutonic rocks with Caledonian metamorphism of low grade. Northern Phyllite Zone 4 Pre-Devonian sedimentary and volcanic rocks with Variscan metamorphism of low grade. Mid-German Crystalline Rise 5 Precambrian to Silurian sedimentary, volcanic and plutonic rocks, with Variscan metamorphism of medium grade: Kyffhiuser, Ruhla, Rh6n (metamorphic xenoliths in Tertiary volcanites), Spessart, Odenwald (Brllstein: NE part, Bergstrisser: main part). Saxothuringian Realm 6 Devonian and Carboniferous sedimentary and volcanic rocks with Variscan metamorphism of very low grade. 7 Precambrian to Silurian sedimentary, volcanic and plutonic rocks with Variscan metamorphism of low, medium and high grade. Moldanubian Realm 8 Paleozoic rocks, undifferentiated, in part slightly metamorphosed. 9 Moldanubian s. str., mostly polymetamorphic gneisses and migmatites, with rare eclogitic relics, last Variscan overprint under low-P/high-T conditions, about 325 Ma ago. 10 1V[iinchbergcrystalline nappes, Erbendorf-Vohenstrauss Zone, Tepl~-Barrandian Zone; mostly polymetamorphic micaschists, gneisses and metabasites, in part with eclogitic relics, last metamorphic overprint under medium P/medium T conditions, about 380 Ma ago. 42 S. Nasir et al. 3;o5 I ~ ~ Permian, Buntsandstein, Quaternary Basalt, phonolite F ~ - ] Rhyolite (quartz porphyry) [~ Leucogranodiorite, aplitic granite Quartz diorite - granodiorite complex Rotgneiss complex Geiselbach formation ~ Micaschists, quartz - micaschists Quartzites I '~bber 3515 a~za] Mombris formation Staurolite - garnet- plagioclase gneiss Garnet-plagioclase gneiss Elterhof formation, Alzenau formation Striated paragneJss with intercalations of amphibohte and marble Schweinheim-Haibach formation Schweinheim micaschist [[~[[~ Haibacher biotite gneisses Metabasite Fig. 2. Geological sketch-map of the Spessart crystalline complex. After Biickin9 (1891), Gabert (1957), Weinelt (1962), Okrusch and Weinelt (1965), Okruseh et al. (1967) and Streit and Weinelt (1971). Except for the metabasites, individual geological units are listed according to their respective, inferred or constrained ages. Sample localities and the new K-Ar dates are indicated; B biotite, H hornblende, M muscovite ing, S W - N E striking units can be distinguished from N W to SE (Fig. 2): 1. The Alzenau formation: paragneisses, aplitic gneisses, amphibolites and subordinate marble. 2. The Geiselbach formation: micaschists and quartzites, with subordinate intercalations of paragneiss, epidote gneiss and amphibolite. Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 43 Fig. 3. Geological sketch map of the central part of the Spessart crystalline complex showing the sample localities for Rb-Sr whole-rock dating (circles with numbers) and the K-Ar dates of Lippolt (1986) 3. The M6mbris formation: predominantly staurolite-bearing paragneisses with minor intercalations of staurolite-free paragneiss, amphibolite, hornblende gneiss and chlorite-amphibole fels. 4. The Rotgneiss complex: orthogneisses with intercalations of meta-aplite, amphibolite, hornblende gneiss and chlorite-amphibole fels. 5. The Schweinheim-Haibach formation: intercalations of micaschists (Schweinheim member) and biotite gneisses (Haibach member). 6. The Elterhof formation: paragneisses with intercalations of marble, calcsilicate gneiss, graphite-bearing quartzite and amphibolite. 7. The quartz diorite-granodiorite complex with minor aplitic granites. According to their respective tectonic positions, their lithologies and their inferred or constrained ages of deposition, the different units of the Spessart Crystalline Complex can be characterized as follows: The Geiselbach and the M6mbris formations are assumed to represent a normal stratigraphic succession of Early Paleozoic age (Matthes, 1954; Bederke, 1957; Weinelt in Okrusch et al., 1967). Recently Reitz (1987) detected the first fossils in a garnet-bearing micaschist in the central part of the Geiselbach formation. The well preserved spores, derived from primitive land plants (Psilophytinae), point to a Silurian age (probably Ludlow) of the sedimentary protolith. This exciting paleontological evidence means that the Geiselbach formation may reach up to the Devonian/Silurian boundary while, judging from lithostratigraphic considerations, part of the Geiselbach and the whole M6mbris formation are assumed to be of Cambro-Ordovician age (Hirschmann and Okrusch, 1988). Both formations are 44 S. Nasir et al. of relatively monotonous lithological character. According to geochemical investigations of Smoler (1987) the staurolite-bearing metapelites and staurolite-free metagraywackes of the M6mbris formation were deposited in a continental-margin environment. Part of the intercalated amphibolites, especially in the northwestern belt between H6rstein and Huckelheirn, at the transition from the M6mbris to the Geiselbach formation, may represent a phase of volcanic activity during the Early Paleozoic sedimentation. The metabasites of the southeastern belt Aschaffenburg-FeldkahlRottenberg are intercalated within the metapelites of the M6mbris formation, but also within the orthogneisses of the Rotgneiss complex. Although the contact relationships are poorly exposed, we assume that at least part of these metabasites are derived from basaltic dikes which intruded the Rotgneiss granite (see below). The Alzenau formation in the northwestern, and the Elterhof formation in the southeastern part of the Spessart crystalline complex are very similar in their variegated lithology, a fact already recognized by Th~rach (1893). Both formations are now interpreted as remnants of one nappe (Heinrichs, 1986; Behr and Heinrichs, 1987). Based on lithostratigraphic correlations within the Variscan Orogen, this variegated group may be of Late Proterozoic (Braitsch, 1957b) or Early Cambrian age (Bederke, 1957; Braitsch, 1957b, Hirschmann and Okrusch, 1988). Braitsch (1957b) showed that the metapelitic micaschists of the Schweinheim member underly the variegated Elterhof formation in a normal stratigraphie succesion and, consequently, might have a Late Proterozoic age (see also Hirschmann and Okrusch, 1988). The protolith of the intercalated biotite gneisses of the Haibach member--igneous vs. sedimentary--is still open to discussion. Our unpublished Rb-Sr whole-rock analyses of the Haibach gneiss did not yield an isochron. The muscovite-biotite flaser-gneisses of the Rotgneiss complex are derived from a relatively shallow intrusion of granitic to granodioritic composition and S-type character (Matthes and Okrusch, 1965; Okrusch and Richter, 1986). Rb-Sr wholerock analyses were interpreted towards a Silurian intrusion age of 414 _ 18 Ma (2s) (Lippolt, 1986, compatible with older, less precise data of Kreuzer et al., 1973) which would conform to the initial age estimate of Bederke (1957). The quartz diorite-granodiorite complex in the southern part of the Spessart crystalline area does not exhibit a typical igneous texture, but reveals a crystalloblastic one as well as an indistinct foliation. Another crucial feature is the extremely inhomogeneous appearance on the scale of an outcrop: lensoid rafts of amphibolite and hornblende gneiss as well as acid veins and schlieren rich in K-feldspar are numerous. Therefore, Okrusch (1963) discarded an igneous origin of the complex (Braitsch, 1957a) and advocated a transformistic model. However, a recent petrofabric investigation speaks in favour of a magmatJc origin (O. Oncken, pers. comm.). Granite-pegmatites are widespread in the central and southeastern part of the Spessart Crystalline Complex, especially in the Haibach and in the Goldbach gneisses. They are syn- to post-tectonic. Metamorphism in the Spessart Crystalline Complex took place under P-T conditions of the lower amphibolite facies. Mineral assemblages in the staurolitebearing paragneisses of the M6mbris formation testify to temperatures of about 600 °C at pressures of about 6 kb (Matthes and Okrusch, 1977; Okrusch, 1983, 1990). The application of various mineral thermometers to the amphibolite intercalations point to somewhat lower temperatures in the northwestern part of the M6mbris formation and in the Geiselbach formation (Nasir, 1986). Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 45 Analytical Results A short description of the investigated samples can be obtained from M. Okrusch on request. Conventional chemical analyses and optical properties of dated hornblendes are listed in Table 1; their designation according to the nomenclature of Leake (1978) is shown in Fig. 4. Except for Table 3, Fig. 3, and the bars in Fig. 6, all error estimates given refer to an interlaboratory analytical precision at a 95% confidence level. The IUGS-recommended constants (Steiger and Jiiger, 1977) were used. K-Ar Dating (Table 2) In Fig. 5 the K-Ar dates of the analyzed hornblendes, muscovites, and biotites are depicted synoptically. Most of them group within the narrow range of 324 to 318 Ma which conforms to the analytical uncertainty. Two hornblendes and one muscovite yielded slightly older dates of 326 to 328 Ma. On the other hand, there is a tail of younger hornblende dates towards 311 Ma. Two hornblendes from Mfinchhof (Sp83-W2) and nearby Sternberg (Sp83-W4) gave distinctly younger dates of 293 and 274 Ma, respectively. These two amphiboles as well as hornblende Sp83-W3 from Miichhof (which, too, gave a relatively young date of 311 Ma) are distinguished by cloudy cores of opaque dust. Otherwise, these three amphiboles have no special chemical composition (Table 1, Fig. 4), and do not show any sign of secondary alteration. In comparison, five hornblendes of variable composition from different rock types of the Rauenthal area revealed concordant dates of 322 __+4 to 328 ___4 Ma (weighted mean 324 _ 2 Ma). A biotite concentrate from one of these samples yielded the same date of 322 _ 4 Ma. The K-Ar dates of Lippolt (1986, Table 6) fit well within the narrow range of the majority of our dates (Fig. 5). This holds true not only for biotites from an orthogneiss of the Rotgneiss complex (sample S 419-B, Steinbach quarry: 322 _ 16 Ma) and a quartz diorite (sample Sp76-B, Stengerts quarry: 321 __+20 Ma), but also for a muscovite from the Grauenstein pegmatite (sample Peg2-76-M: 318 ___20 Ma) as well as a muscovite and a biotite from the Wendelberg pegmatite (sample Pegl-76M: 317 + 20 Ma, sample Pegl-76-B: 322 _ 20 Ma). Rb-Sr Whole-rock Dating Kreuzer et al. (1973) analyzed six muscovite-biotite gneisses and one metaaplite from the Rotgneiss complex. The data points define a regression line in the Nicolaysen (1961) diagram which was interpreted as an isochron yielding a date of 397 _ 22 Ma and an initial 87Sr/a6sr ratio (IR) of 0.712 _ 0.004. However, the authors were aware of the fact that five of the data points clustered within a narrow interval of 87Rb/86Sr around 10. Slope and precision of the regression line depend critically on the data points of the two remaining samples, one highly recrystallized orthogneiss (Sp63-1237) and the meta-aplite (Sp72-105) sampled from a dike 0.5 m in width. Lippolt (1986) published high-precision Rb-Sr analyses on five whole-rock samples of muscovite-biotite gneiss (open circles in Fig. 6), three of which were taken from the same locality (S 423, S 424, S 425). The five data points, with a spread in the 87Rb/86Sr ratios of 3 to 55, defined a regression line corresponding to a date of 414 _ 18 Ma and an IR of 0.7068 + 0.0022. However, the mean square of weighted deviates MSWD = 62 demonstrated that the basic requirements for an interpretation of the regression line as an isochron were violated. 46 S. Nasir et al.: Geochronology of the Spessart Crystalline Complex Table 1. Chemical analyses of dated hornblendes Sample Wt.-~ SiO2 TiO2 AI20~ Fez0~ FeO MnO HgO CaO MazO K20 F H,O Sum Sp83M 13 Hc 4 A 3 Rh 3 Rh 1 Rh 4 Rh 5 44.70 0.65 12.20 4.43 9.46 0.22 12.68 10.96 1.58 0.31 0.02 1.93 99.14 44.50 0.60 13.25 3.91 11.59 0.29 10.62 10.90 1.56 0.42 0.09 1.87 99.60 46.20 0.55 11.75 3.65 9.80 0.24 13.43 10.15 1.40 0.62 0.06 1.76 99.61 49.40 0.46 8.40 1.52 9.98 0.21 15.32 ii.i0 0.95 0.19 0.02 2.15 99.70 44.60 0.60 11.65 3.62 12.88 0.20 11.01 10.90 1.25 0.96 0003 2.08 99.78 42.10 0.89 13.10 5.81 14.31 0.39 7.98 11.25 1.49 0.65 0.06 1.88 99.91 Ions per 24 (O + OH + F) Si 6.561 6.548 6.527 AI(4) 1.439 1.452 1.473 6.709 1.291 7.070 0.930 6.578 1.422 Z 8.000 8.000 8.000 8.000 8.000 AI(6) Ti Fe'+ Hg Fe z+ Hn 0.668 0.055 O.431 2.632 1.214 0.000 0.653 O.071 0.488 2.765 1.023 0.000 0.826 0.066 0.432 2.325 1.351 0.000 0.725 0.060 0.399 2.907 0.909 0.000 Y 5.000 5.000 5.000 Fe 2~ Kn Ca Na 0.049 0.032 1.756 0.163 0.136 0.026 1.718 0.120 0,072 0.036 1.715 0.177 X 45.10 0.50 12.20 3.93 10.37 0.26 12.12 11.25 1.42 0.38 0.03 2.12 99.68 2.000 2.000 2.000 Sp58ADX W 2 W 3 W 4 F 6 44.80 1.22 13.25 4.O1 12.10 0.24 10.04 11.08 1.25 0.68 0.03 1.82 100.52 44.20 1.16 13.80 3.50 i0.00 0.20 11.54 10.85 1.63 0.34 0.03 2.O1 99.26 45.00 0.80 13.35 3.66 9.74 0.22 11.63 11.15 1.25 0~37 O.O2 1.98 99.17 44.50 1.15 13.80 2.66 11.34 0.20 11.49 10.45 1.38 0.45 0.O2 2.04 99.48 47.00 0.63 13.10 4.40 12.16 0.24 9.96 10.84 1.65 0.66 0.06 1.84 99.54 42.45 1.45 14.29 0.50 12.53 0.26 12.46 12.21 1.53 1.42 n.d. 2.08 100.23 42.55 1.81 10.75 3.79 12.O2 0.28 12.06 11.27 1.68 1.25 n~d. 1.98 99.38 6.316 1.684 6.539 1.461 6.444 1.556 6.556 1.444 6.482 1.518 6.506 1.494 6.252 1.748 6.363 1.637 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 0.492 0.050 0.164 3.272 1.022 0.000 0.613 0.067 0.402 2.437 1.481 0.000 0.645 0.i01 0.657 1.787 1.798 0.012 0.824 0.134 0.440 2.185 1.417 0.000 0.826 0.127 0.384 2.511 1.152 0.000 0.854 0.088 O.401 2.526 1.131 0.O00 0.864 0.126 0.292 2.497 1.221 0.O00 0.795 0.070 0.489 2.195 1.451 0.000 0.733 0.161 0.055 2.516 1.535 0.000 0.258 0.204 0.426 2.679 1.433 0.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 0.281 0.030 1.579 0.ii0 0.174 0.025 1.704 0.097 0.074 0.025 1.726 O.175 0.000 0.038 1.810 0.152 0.061 O.030 1.733 O.176 0.068 0.025 1.697 0.210 0.056 0.027 1.740 O.177 0.161 0.025 1.632 0.182 0.052 0.030 1.718 0.200 0.008 0.032 1.927 0.033 0.070 0.035 1.806 0.089 2.000 2.000 2.000 2.000 W 1 2.000 2.000 2.000 2.000 2.000 2.000 CAH 2.000 Na K 0.238 0.071 0.327 0.058 0.267 0.079 0.284 0.115 0.167 0.035 0.183 0.181 0.270 0.125 O.178 O.127 0.251 0.063 0.176 0.069 0.208 0.084 0.272 0.125 0.404 0.267 0.398 0.238 A 0.309 0.383 0.346 0.399 0.202 0.364 0.395 0.305 0.314 0.245 0.292 0.397 0.671 0.636 F OH 0.014 2.060 O.010 1.883 0.042 1.832 0;028 1.705 0.009 2.055 O.014 2.051 0.029 1.884 0.014 1.772 0.014 1.957 O.010 1.924 0.010 1.982 0.028 1.812 2.040 2.000 18 ° 76 ° 16 ° 81 ° 17 ° 19 ° 16° 74° 14 ° 78 ° 17 ° 76° 180 78 ° 16 ° 0.024 0.022 0.021 0.021 0.023 0.024 0.021 590 18 ° 1.659 1.680 0.021 c 680 15 ° 1.660 1.682 0.022 c Optical data 2V= n~'/c n, n( nv-n. 0.021 6 0.023 ccalculated Analysts: Samples Sp 83-, Nasir (1986), samples Sp 58-, Okrusch (1961). For a perfect regression the deviations of the determining analytical points should be in the range of the analytical uncertainties, i.e. the mean square of weighted deviates (MSWD) should be close to unity. For a number of samples n > 6, Wendt (1989) and Wendt and Carl (in press) estimate the standard deviation of the M S W D as s(MSWD) ~ __+ whereby f is the number of freedoms (f = n - 2 in the case of a linear regression of n points). Hence, the M S W D of a five points' regression line should be in the range of i ___2 x / ~ ~ 1 _ 2 for a statistically safe isochron. (Na÷K)A <0.50~ Ti <0.50 i trem.hb/ magnesiotsch. hornblende hbl. tschermakite act. o O~o actinolite hbL 1.o tremolite 0 0.5 ferro- ferroact, actinolite hbl. 0'O.0 ferro- ferrotsch. hbl. tschermakite hornblende I 7.5 6.5 7.0 6.0 .....Si (Na+K) A >0.50; Ti <0.50 1.o silicic LL ferro- % parg. eden. hbl. edenite edenite pargasite hbl. ferroan ferroanparg. pargasite ferro hbl. *0.5 silicic ferro-edenite ferroedenite 0.0 8.0 7.5 270 290 I 7.0 eden. ferro- ferrohbl. parg. pargasite hbl. I 6.5 6.0 Si 510 Fig. 4. Nomenclature (Leake, 1978) of analyzed hornblendes from the metabasite belts Hrrstein--Huckelheim (open triangles) and AschaffenburgFeldkahl-Rottenberg (open circles) and from the quartz diorite-granodiorite complex (closed circles) 550(Mo) METABASITE BELT H¢4 HORSTEIN - HUCKELHEIM F6 ~ w~ ~7-:-/7-/] METABASITE BELT r//l] Rh3 Rhl Rh4 Rh5 I///////I ASCHAFFENBURO - [ ~ FELDKAHL ROTTENBERG w2 w3 ~ ~ A.3 f////I 1112 Peg2* I-I 1166 "--'1111 1167 1330 ~}1 MOMBRIS FORMATION PEGMATITE I ROTGNEISS COMPLEX I s41g* Pegl* EAA Sp76, ADX CAH PEGMATITES, HAIBACH MEMBER r///l AMPHIBOLITE, ELTERHOF FORMATION i OUARTZDIORITE - GRANODIORITE COMPLEX r ~ 270 K-Ar II dates 29O ± 2s: 510 ~ 550(M0) hornblende I i muscovite ---- biotite Fig. 5. K-Ar dates from the Spessart crystalline complex. The dates are arranged according to the lithostratigraphic succession as in Table 2, i.e. from NW (top) to SE (base). The dates for the mica samples of Lippolt (1986) are inserted and marked with an asterix. However, the original dates are increased by 1.8% which is the interlaboratory bias from Heidelberg to Hannover according to our analyses on the interlaboratory standard biotite HD-B1 (Lippolt and Hess, 1989, written commun.). The errors of these dates are given as twice the standard deviation of these 5 dates. All error bars are given at a level 95% intralaboratory confidence. The ruled background represents the interval 4- twice of the standard error of the mean age of the 5 hornblendes from Rauenthal (samples Rh) 48 S. Nasir et al. Table 2. K-Ar mineral data of crystalline rocks from the Spessart, Mid-German Crystalline Rise Sample No. Rock t y p e Locality Metabasite Sp83-Hc4 hornblende Sp83-MI3 hornblende gneiss gneiss Metabasite Sp83-F6 quartz-bearing belt Sp83-W4 quartz-bearing amphibolite quartz-epidote amphibolite Sp83-Rhl hornblende Sp83-Rh4 epidote-hornblende Sp67-Rh amphibolite Sp83-WI quartz-bearing amphibolite amphibolite Sp83-W2 quartz-bearing Sp83-W3 amphibolite Sp83-A3 epidote-bearing hbl 316 ± 6 north hb] 320 ± 5 o f BrUcken amphibolite quartz-plagioclase fels hbl Sp64-1330 biotite gneiss, muscovite-bearing biotite gneiss, muscovite-bearing muscovite-biotite Sp58-EAA chlorite-bearing Sp58-ADX quartz diorite with feldspar blasts amphibolite raft in q u a r t z d i o r i t e 5p62-1167 Sp58-CAH 323 ± 8 hbl 324 ± 4 bi 322 ± 4 322 ~ 4 328 ± 4 MUnchbof hbl 316 ± 4 MUnchhof hbl 295 ± 5 MUnchhof hbl 311 ± 5 Strietwald, near A s c h a f f e n b u r g hbl 311 * 4 Elterhof Quartz 322 ± 4 hbl - 0.12 2 0.24 2 0.326 5 0.293 ± 6 0.457 ± 5 0.114 ± 3 0.725 ± 7 6.44 ± 8 0.660 ± 7 0.711 ± 7 0.443 ± 5 0.253 ± 4 0.243 ± 4 0.283 ± 4 4.36 4 3.38 3 6.26 5 1.565 14 10.00 8 88.3 8 9.03 7 9.94 6 5.94 5 3.146 23 3.21 3 3.732 21 0.48 3 0.29 2 0.49 5 0.!5 i 0.35 i 0.77 13 0.34 6 0.64 6 0.46 1 0.82 2 0.84 26 0.57 2 mus 321 ± 4 8.01 7 109.3 9 1.5 2 7.35 4 7.47 ± 4 7.58 ± 4 99.6 8 102.7 8 105.2 6 1.1 i 2.0 i 1.2 5 ± complex bi 319 ± 3 ± bi 323 ± 3 mus 326 ± 2 hbl 320 ± 4 formation Gniessen, near Schweinheim diorite 2.674 21 3.54 8 formation Kufengrund, near U n t e r a f f e r b a c h Kufengrund, near U n t e r a f f e r b a c b SteinrUcken, near G l a t t b a c h amphibolite 0.199 4 0.260 ± 4 ± ± 275 ± 6 hbl R o t e r Grund, near Wenigh~sbach gneiss 315 ± 5 Rauenthal Rotgneiss Sp62-1166 (wt.%) Argon rad. atm. (nl/g)STP - Rottenberg Rauenthal Mdmbris Sp62-1112 - Feldkah] Rauenthal gneiss K Huckelheim S t e r n b e r g , between hbl F e l d k a h l and Wenighdsbach hbl Rosenberg, near Rauenthal hb] Rauenthal gneiss Sp83-Rh5 - Feldkahl fels biotite-hornblende HSrstein ZiegelhUtte Aschaffenburg amphibolite Sp83-Rh3 belt K-Ar date (Ma) Mineral granodiorite Stengerts, near Schweinheim Ameisenbrunnen, near Soden 0.739 8 I0.06 8 0.48 2 1.282 12 1.344 ± 12 17.93 14 18.19 15 !.42 17 0.35 6 ± complex hbl 328 ± 4 ± hb] 318 ± 4 C o n v e n t i o n a l K-Ar a n a l y s e s ( S e i d e l e t a l . , 1982). Argon by t o t a l f u s i o n i s o t o p e d i l u t i o n static m a s s - s p e c t r o m e t r i c a l a n a l y s i s in n a n n o l i t e r per gram a t s t a n d a r d t e m p e r a t u r e and p r e s s u r e , c o r r e c t ed f o r a v e r a g e b l a n k a n a l y s e s . K in d u p l i c a t e by f l a m e p h o t o m e t r y w i t h Li as i n t e r n a l standard. E r r o r e s t i m a t e s r e f e r to the i n t r a l a b o r a t o r y analytical p r e c i s i o n at a 95 % c o n f i d e n c e l e v e l . For the K a n a l y s e s t h e y a r e a r b i t r a r i l y e n h a n c e d by 0.002 wt,% to a c c o u n t f o r l o w l e v e l b i a s s e s . The IUGS-recommended c o n s t a n t s ( S t e i g e r and J ~ g e r , 1977) are used. Our date f o r the s t a n d a r d g l a u c o n i t e GL-O i s i% l e s s than the average v a l u e o f the c o m p i l a t i o n o f Odin (1982). Abbreviations: bi = b i o t i t e , hbl = h o r n b l e n d e , mus = m u s c o v i t e . Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 49 For comparison of precise data on samples from a more limited interval of Rb/Sr ratios, we decided to re-analyze six samples reported by Kreuzer et al. (1973), as well as three additional ones described, but not analyzed by these authors. Moreover, the two samples of Lippolt (1986) with high Rb/Sr ratios (S 423, S 425) were also re-analyzed. Three samples re-analyzed for Rb and Sr reveal major discrepancies compared with the former results: The samples Sp63-1237 and Sp72-105 which mainly define the isochron of Kreuzer et al. (1973) revealed markedly higher 8~Sr/86Sr ratios, the sample S 423 of Lippolt (1986) gave a 10~ lower 87Rb/86Sr ratio. Taking into account all 11 samples, our new results (Table 3) define a regression line conforming to an isochron date of 439 +_ 15 Ma, with an IR of 0,7038 _+ 0.0026 (Fig. 6). The scatter of the data points is significantly reduced. Yet the resulting M S W D = 4.9 is still far too large to assume a statistically sound isochron. AccordTable 3. Rb-Sr whole-rock data of orthogneisses from the Rotgneiss Complex, Spessart 87Rb 865r (ppm) (ppm) Sample No. 87Rb 86Sr 87Sr 86Sr Charges of 600 mg Sp63-1279 Sp61-1011 Sp62-i026 Sp63-1271 Sp64-1313 Sp62-i075 Sp63-1254 Sp72-I05 47.78 51.05 52.73 51.92 58.89 55.24 57.53 63.78 63.37 5.403 5.409 5.158 5.002 5.256 4.800 4.728 3.178 3.177 Sp63-1237 53.91 52.83 2.537 2.498 S 425 ditto at BGR Lippolt 63.59 63.76 1.162 1.143 S 423 ditto at BGR Lippolt 59.20 56.80 1.063 0.947 Charges of 100 mg, mean v a l u e s NBS 607 standard dev. of 8.74 9.33 10.11 10.26 11.08 11.38 12.03 19.84 19.72 19.78 21.01 20.91 20.96 (9) (9) (10) (i0) (11) (11) (12) (2.0) (20) (14) (21) (21) (15) 0.75881 0.76283 0.76806 0.76919 0.77702 0.77395 0.77877 0.82802 0.82878 0.82840 0.83891 0.84034 0.83962 (23) (23) (23) (23) (23) (23) (23) (25) (25) (38) (25) (25) (72) 54.10 55.41 54.76 55.07 59.50 (54) (19) (66) (55) (30) 1.03641 1.03810 1.03726 1.04865 1.04760 1.04812 (31) (4) (85) (31) (34) (83) f o r c o n t r o l of c o n t a m i n a t i o n und cross contam i n a t i o n 10 d e t e r m i n a t i o n s w i t h i n about 2 years. 148.16 49 6.046 24 24.21 13 1.20007 80 Rb and Sr d e t e r m i n e d by c o n v e n t i o n a l i s o t o p i c d i l u t i o n methods (Harre e t a l . , 1968)., Sr on a MAT-261 mass s p e c t r o m e t e r using a double c o l l e c t o r , Rb on a VG=Micromass MM 30 spectometer. R e p l i c a t e s on the NBS 607 standard and on samples lead to i n t r a l a b o r a t o r y standard d e v i a t i o n s of I% and 0.03% f o r the 87Rb/86Sr and 87Sr/86Sr r a t i o s , r e s p e c t i v e l y . In parentheses e r r o r s at 68% confidence o f i n t r a l a b o r a t o r y p r e c i s i o n . IUGS-recommended c o n s t a n t s ( S t e i g e r and J~ger, 1977). 50 S. Nasir et al. 0 10 20 50 40 .... 1.10 50 60 . . . . . . . . . . . . . . . . X = 1.4-2 x 1 0 - ~ ~ f 1.00 c~ CO 03 b,CO 0.90 /~o date = 439~ 15 Ma ../" 0.80 IR : 027048 ~ 0.0026 J MSWD = 4.9 7°:: ..... 0.70 0.75 i,,I,r,,' ......... '',rl,,'''' . . . . . . . . . ' . . . . . . . . . '',,r,11, @419 rl 133 o 0.72 1237I I425 "5 o.71 422 ¢ oO p-, CO. 0.70 t ...................................................... 0 10 20 30 87Rb/86Sr 40 50 12,t 60 Fig. 6. Isochron plots of the Rb-Sr analyses. Upper part: Conventional Nicolaysen (1961) diagram. The regression line is defined by the 11 analyses of Table 3 (crosses); the analyses of Lippolt (1986) are given for comparison (open circles). Lower part: Extended-scale presentation by subtracting the gain of radiogenic Sr since 414 Ma, the date originally derived by Lippolt (1986) from his five anlyses represented by open circles. Consequently, the 414 Ma regression line is horizontal in this plot (dashed line). Error bars in this plot represent 4-1 standard errors ing to the criterion of Wen& (1989) and Wen& and Carl (in press), with nine degrees of freedom one expected a M S W D in the range of 1 _+ 2x/2/(11 - 2), i.e. M S W D ~ 1 _+ 0.9. This means that - the rocks are derived from different batches of granitic m a g m a which intruded roughly at the same time and/or the Sr isotopes were not homogenized in the granitic m a g m a and/or - the rocks did not behave as closed systems to Rb and Sr during Variscan metamorphism. - The first possibility seemed to be indicated by the wide range of Rb/Sr ratios mentioned above which cluster in three different intervals. However, a significant scatter is observed in all three groups: If we regress only the 7 samples of the group with low Rb/Sr ratios, we end up with a similar date (439 +_ 40 Ma, IR 0.705 _+ 0.006) and an M S W D of 5.6. The same holds true, if we include the two samples with m e d i u m Rb/Sr ratios. Moreover, it should be noted that the wide spread in the Rb/Sr ratios is mainly due to a variation in the Sr, rather than the Rb contents Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 51 (Table 3). This may suggest a postmagmatic, e.g. metamorphic Sr loss, according to the third possibility. In the latter case, the date of 440 Ma would be a mininum estimate of the intrusion age, as one often observes losses radiogenic Sr roughly proportional to the Rb/Sr ratio, leading to "rotated isochrons" (e.g. Wendt et al., 1970). Discussion The oldest radiometric records so far available in the Spessart Crystalline Complex are the Rb-Sr whole-rock dates for the orthogneisses of the Rotgneiss complex. Despite the uncertainties resulting from the wide scatter of the data points around the regression line in the Nicolaysen diagram, an Ordovician to Silurian intrusion age of the granitic protolith can be inferred. According to their chemical characteristics the orthogneisses are derived from S-type granitoids, although the IR of 0.705 is remarkably low. The trace element characteristics are consistent with an emplacement into a post-collision geotectonic environment (Okrusch and Richter, 1986) according to the systematics evaluated by Pearce et al. (1984). This would conform to a phase of extensional tectonics at the end of the Caledonian orogeny (Weber, 1984). In the simplified, H 2 0 saturated granite system Qz-Or-Ab(-An) the data points plot close together in the vicinity of the melting minimum at low water vapour pressures ( < 1 kbar) indicating that the precursor granitoids intruded into a relatively shallow level, probably less than 4 km (Okrusch and Richter, 1986). Judging from the paleontological evidence recorded by Reitz (1987), part of the Geiselbach formation is of Silurian age whereas the underlying M6mbris formation is regarded as Cambro-Ordovician as inferred from lithostratigraphic correlations (Matthes, 1954; Bederke, 1957; Hirschmann and Okrusch, 1988). Hence sedimentation was still going on while the granite magma was being intruded in deeper levels of the M6mbris-Geiselbach sediment pile. If we assume a normal stratigraphic succession, the cover of the granitoids, consisting of the M/Smbris, and part of the Geiselbach formation, amounts to roughly 3 km (Hirschmann and Okrusch, 1988). Allowing for a reduction of the initial bed thickness by tectonic strain, this value should be increased approximately by a factor of 2, but still compares well with the shallow level of intrusion estimated on the basis of petrological criteria. The volcanic protoliths of the metabasites in the belt of Aschaffenburg-FeldkahlRottenberg, presumbably intruded or ejected after the intrusion of granitoids, reveal a calc-alkaline affinity (Hesselmann, 1982; Nasir, 1986; Nasir and Okrusch, in prep.) This would testify to an island-arc geotectonic environment as a result of renewed compressional tectonics at the beginning of the Variscan Orogeny (Weber, 1984). During the Variscan orogenetic processes, the Proterozoic and Lower-Paleozoic volcano-sedimentary sequence, the Silurian/Devonian granitoids, and the later calc-alkaline volcanites were metamorphosed at temperatures of about 600 °C and pressures of about 6 kbar (Okruseh, 1983, 1990) corresponding to a burial at a depth of about 20 k m . The K-Ar dates presented in Table 2 and by Lippolt (1986, Table 6) are related to the geologic history which started at or after the peak of the Variscan metamorphism and the syn- to posttectonic pegmatite emplacement. The dates show no systematic differences, neither for the three minerals, nor for the rock types, nor for the lithostratigraphic units. Considering this fact and maintaining the 52 S. Nasir et al. conventional estimates for the closing temperatures of the K-Ar systems, i.e. about 500 °C for hornblende, about 350 °C for muscovite, and about 300 °C for biotite (e.g. Hart, 1964, Gerlin9 et al., 1965; Purdy and Jiiger, 1976; Dodson, 1973), our dates suggest a rapid uplift and cooling, from 500 to 300 °C, of the Spessart Crystalline Complex. The age of uplift would be 327 +__2 Ma if we rely on the two hornblendes and the single muscovite with the oldest dates, or 321 + 1 Ma using the 11 hornblendes, muscovites and biotites of the main group. The 5 mica dates of Lippolt (1986) average to a similar date of 320 Ma. According to the Carboniferous timescale of Hess and Lippolt (1986, Fig. 4) the cooling took place at the Dinantian/ Namurian boundary or in the Namurian A. The two hornblendes and the single muscovite with the oldest dates averaging at 327 _ 2 Ma may be relics of an earlier stage in the metamorphic history. The assumed closing temperatures of hornblende, muscovite, and biotite are within the range of the greenschist facies. Therefore, one could assume that the age of about 320 Ma refers to the retrogressive overprint under greenschist-facies conditions which affected parts of the Spessart Crystalline Complex (Matthes, 1954). However, the dated minerals are virtually unaffected by retrograde alteration. Experience in polymetamorphic areas, (e.g. in the Tauern Window: Raith et al., 1978 ) shows that hornblendes are not completely reset by a metamorphic overprint, even under amphibolite facies conditions, unless the metamorphic assemblage has been fully re-equilibrated, especially under the influence of deformation. We therefore assume that our dates are closely related to the culmination of the amphibolite facies metamorphism which, as a consequence, would belong to the Sudetic phase of the Variscan orogeny. Nearly the same cooling age is indicated for the adjacent parts of the MidGerman Crystalline Rise, namely for the B611stein Odenwald in the south-west (Lippolt, 1986) and for the hidden basement of the Rh6n Volcanic Complex in the north-east (Schmidt et al., 1986). This age is also well established in the Saxothuringian of the Fichtelgebirge and the Moldanubian realm immediately to the south (e.g. Kreuzer et al., 1989). In contrast, the northern Bergstr/isser Odenwald, further to the south-west, yielded early Carboniferous hornblende dates around 342 Ma (Kreuzer and Harre, 1975, recalculated; Hellmann et al, 1982: Rittmann, 1984). Moreover, critical assemblages and migmatite phenomena in the Bergstr/isser Odenwald testify to metamorphic peak conditions differing from those in the Spessart crystalline complex, i.e. lower pressures of about 3 kbar and higher temperatures up to 700 °C (e.g. Okrusch, 1990). These differences indicate a diverging metamorphic history for different parts of the Mid-German Crystalline Rise. Acknowledgements Hans Joachim Lippolt (Heidelberg) kindly provided us with some of his samples for reanalysis; his altruistic cooperation is highly appreciated. We want to thank Onno Oncken (Wiirzburg) and John Richards (Canberra) for critical reading of the manuscript and valuable suggestions. S.N. is indebted to the Deutscher Akademischer Austauschdienst (Bonn) for financial support and to the Bundesanstalt fiir Geowissenschaften und Rohstoffe (Hannover) for use of their facilities. We thank Horst Klappert, Margot Metz, Lutz 7hieflwald and Detlef Uebersohn (Hannover) for technical assistence with the K-Ar analyses and Klaus-Peter Kelber (Wtirzburg) for part of the line drawings. Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 53 References Bederke E (1957) Alter und Metamorphose des kristallinen Grundgebirges im Spessart. Abh hess L-Amt Bodenforsch 18: 7-19, Wiesbaden Behr H-J, Engel W, Franke W, Giese P, Weber K (1984) The Variscan Belt in Central Europe: main structures, geodynamic implications, open questions. Tectonophysics 109:15-40 Heinriehs T (1987) Geological interpretation of DEKORP 2-S: A deep seismic reflection profile across the Saxothuringian and possible implications for the Late Variscan structural evolution of Central Europe. Tectonophysics 142:173-202 Braitseh 0 (1957a) Beitrag zur Kenntnis der kristallinen Gesteine des sfidlichen Spessarts und ihrer geologisch-tektonischen Geschichte. Abh hess L-Amt Bodenforsch 18: 21-72, Wiesbaden - - (1957b) Zur Petrographic und Tektonik des Biotitgneises im sfidlichen Vorspessart. Abh hess L-Amt Bodenforsch 18: 73-99, Wiesbaden Brandes T (1919) Die varistischen Zfige im geologischen Bau Mitteldeutschlands. N J Miner Geol Palfiont BB 43:190-250 Brinkmann R (1948) Die mitteldeutsche Schwelle. Geol Rdsch 36:56-66 Brooks C, Hart S, Wen& I (1972) Realistic use of two-error regression treatments as applied to Rubidium-Strontium data. Geophysics and Space 10:551-577 Biiekin 9 H (1892) Der nordwestliche Spessart. Abh K6nigl Preuss geolog Landesanstalt. Neue Folge, Heft 12, 274 pp, Berlin DEKORP RESEARCH GROUP (1985) First results and preliminary interpretation of deep-reflection seismic recordings along Profile DEKORP 2-S. J Geophys 57:137-163 Dodson M (1973) Closure temperature in cooling geochronological and petrological systems. Contrib Mineral Petrol 40:259-274 Franke W (1989) Tectonostratigraphic units in the Variscan belt of central Europe. Geol Soc Amer Spec Paper 230:67-89 Gabert G (1957) Zur Geologic und Tektonik des n6rdlichen kristallinen Vorspessarts. Abh hess L-Amt Bodenforsch 18: 101-133, Wiesbaden Gerlin9 EK, KoI'tsova TV, Petrov BV, Zul'fikerova Z K (1965) On the suitability of amphiboles for age determination by the K/Ar method. Geochemistry Intern 2:148-154 Giese P, Jfdicke H, Prodehl C, Weber K (1983) The crustal structure of the Hercynian mountain system--A model for crustal thickening by stacking. In: Martin H, Eder F W (eds) Intracontinental Fold Belts. Springer, Berlin Heidelberg New York Tokyo, pp 405-426 Harre W, Lenz H, Miiller G, Wen&, I (1964) Untersuchungen zur Altersbestimmung von Gesteinen nach der Rubidium/Strontium-Methode. Ber d Bundesanst Bodenforsch Hannover, 139 pp Hart SR (1964) The petrology and isotopic mineral-age relations of a contact zone in the Front Range, Colorado. J Geol 72:493-523 Heinrichs T (1986) Structure and development of the Saxothuringian zone. Third EGT Workshop, Bad Honnef, 14-16 April 1986, Proceedings, pp 135-140, Strasbourg Hellmann KN, Lippolt H J, Todt W (1982) Interpretation der Kalium-Argon-Alter eines Odenw/ilder Granodioritporphyrganges und seiner Nebengesteine. Aufschlul3 33: 155-164 Hess JC, Lippolt HJ (1986) 4°Ar/a9Ar ages of tonstein and tuff sanidines: New calibration point for the improvement of the upper Carboniferous time scale. Chemical Geology (Isotop Geosci Sect) 59:143-154 Hesselmann H (1982) Petrographische und geochemische Untersuchungen an Gesteinen aus dem mittleren Vorspessart. Unpubl Dipl Arbeit, Miner-Petrogr Inst TU Braunschweig, 97 pp Hirsehmann G, Okruseh M (1988) Spessart-Kristallin und Ruhlaer Kristallin als Bestandteile der Mitteldeutschen Kristallinzone--ein Vergleich. N J Geol Pal/iont Abh 177:1-39 - - 54 S. Nasir et al. Klemm G (1895) Beitr~ige zur Kennthnis des krystallinen Grundgebirges im Spessart mit besonderer Ber/icksichtigung der genetischen Verh/iltnisse. Abh Grossherzogl hess geol L-Anst 177: 165-257, Darmstadt Kreuzer H, Harre W (1975) K/Ar-Altersbestimmungen an Hornblenden und Biotiten des Kristallinen Odenwaldes. In: Amstutz GC, Meisl S, Nickel E (Ed) Mineralien und Gesteine im Odenwald. AufschluB, Sonderband 27 (Odenwald): 71-77 - - Lenz H, Harre W, Matthes S, Okrusch M, Richter P (1973) Zur Altersstellung der Rotgneise im Spessart, Rb/Sr-Gesamt gesteinsdatierungen. Geol Jb A9:69-88 - - Seidel E, Schfissler U, Okrusch M, Lenz K-L, Raschka H (1989) K-Ar geochronology of different tectonic units at the northwestern margin of the Bohemian Massif. Tectonophysics 157:149-178 Leake BE (1978) Nomenclature of amphiboles. Amer Mineral 63: 1023-1052. Lippolt H J (1986) Nachweis altpal/iozoischer Prim/Jr-Alter (Rb-Sr) und karbonischer Abk/ihlungsalter (K-Ar) der Muskovit-Biotit-Gneise des Spessart und der Biotit-Gneise des B611steiner Odenwald. Geol Rdsch 75:569-583 Matthes S (1954) Die Para-Gneise im mittleren kristallinen Vor-Spessart und ihre Metamorphose. Abh hess L-Amt Bodenforsch 8: 1-86, Wiesbaden Okrusch M (1965) Petrographische Untersuchung der Rotgneise im Spessart. Geologie 14: 1148-1200, Berlin - - (1977) The Spessart crystalline complex, North-West Bavaria: rock series, metamorphism and position within the Central German crystalline rise. In: La chaine varisque d'Europe moyenne et occidentale. Coil Intern CNRS, Rennes, 243:375-390 Nasir S (1986) Die Metabasite im mittleren kristallinen Vorspessart: Petrographie--Geochemie--Phasenpetrologie. Dr rer nat thesis Univ W/irzburg, 191 pp Okrusch M (submitted) Metabasites from the central Vor-Spessart, North-West Bavaria. Part 1. Geochemistry. Submitted to N Jb Min Abh Nicolaysen LO (1961) Graphic interpretation of discordant age measurements on metamorphic rocks. Anal New York Acad Sci 91:198-206 Odin GS (1982) Interlaboratory standards for dating purposes. In: Odin G S (ed) Numerical dating in stratigraphy. Wiley, New York, pp 123-150 Okrusch M (1963) Bestandsaufnahme und Deutung dioritartiger Gesteine im s/idlichen Vorspessart. Ein Beitrag zum Dioritproblem. Geologica Bavarica 51: 4-107, M/inchen - - (1983) The Spessart Crystalline Complex, Northwest Bavaria.--DMG SFMC Joint Meeting. Excursion E 4. Fortschr Mineral 61, Beiheft 2:135-169 (1990) Metamorphism in the Odenwald and Spessart crystalline mountains (Mid-German Crystalline Rise). In: Franke W (ed) Mid-German Crystalline Rise & Rheinisches Schiefergebirge. IGCP 233, Terranes in the Circum-Atlantic Paleozoic Orogens, Conference on Paleozoic Orogens in Central Europe, G6ttingen-Giessen 1990. Field Guide to Pre-Conference Excursion, pp 81-91 - - Richter P (1967) Petrographische, geochemische und mineralogische Untersuchungen zum Problem der Granitoide im mittleren Spessart-Kristallin. N J Min Abh 107:21-73 - - (1986) Orthogneisses of the Spessart crystalline complex, North-west Bavaria: Indicators of the geotectonic environment? Geol Rdsch 75:555-568 - - Streit R, Weinelt Wi (1967) Erl/iuterungen zur geologischen Karte von Bayern 1 : 25 000, Blatt 5920, Alzenau i Ufr, 336 pp, Miinchen Weinelt Wi (1965) Erl~iuterungen zur geologischen Karte von Bayern 1 : 25 000, Blatt 5921 Schrllkrippen, 327 pp, M/inchen Pearce JA, Harris NBW, Tindle AG (1984) Trace element discrimination for the tectonic interpretation of granitic rocks. J Petrol 25:956-983 Purdy JW, Jgtger E (1976) K-Ar ages on rock-forming minerals from the Central Alps. Mem Inst Geol Mineral Univ Padova, 30, 31 pp - - - - - - - - - - - - Geochronology of the Spessart Crystalline Complex, Mid-German Crystalline Rise 55 Raith M, Raase P, Kreuzer H, Miiller P (1978) The age of the Alpidic metamorphism in the Western Tauern Window, according to radiometric dating. In: Closs H, Roeder D, Schmidt K (eds) Alps, Apennines, Hellenides. Inter-Union Comm Geodynamics, Sei Rep 38: 140-148. Schweizerbart, Stuttgart Reitz E (1987) Palynologie in metamorph6n Serien: I Silurische Sporen aus einem granatfiJhrenden Glimmerschiefer des Vor-Spessart. N J Geol Pal Monatsh 1987:699-704 Rittmann KL (1984) Argon in Hornblende, Biotit und Muskovit bei der geologischen Abkiihlung--4°Ar/39Ar-Untersuchungen. Dr rer nat thesis Univ Heidelberg, 278 pp Schmidt F-P, Gebreyohannes Y, Sehliestedt M (1986) Das Grundgebirge der Rh6n. Z dtsch geol Ges 137:287-300 Seholtz H (1930) Das varistische Bewegungsbild. Fortschr Geol Pal 8, 25: 235-316, Berlin Smoler M (1987) Petrographische, geochemische und phasenpetrologische Untersuchungen an Metasedimenten des NW Spessart/Bayern. Dr rer nat thesis Univ Wfirzburg, 256 pp Steiger RH, Jiioer E (1977) Subcommission on geochronology. Convention and the use of decay constants in geo- and cosmochronology. Earth Planet Sci Letters 36:359-362 Streit R, Weinelt Wi (1971) Edfiuterungen zur geologischen Karte von Bayern 1:25 000, Blatt 6020 Aschaffenburg, 398 pp, Miinchen Th~rach H (1893) Ober die Gliederung des Urgebirges im Spessart. Geognost Jahresh 5, 1892: 1-160, Cassel Weber K (1984) Variscan events: Early Palaeozoic continental rift metamorphism and late Palaeozoic crustal shortening. In: Hutton DH, Sanderson DJ (eds) Variscan tectonics of the North Atlantic region. Geol Soc London Blackwell Scientific Publications, pp 3-22 Behr HJ (1983) Geodynamic interpretation of the mid-European Variscides. In: Martin H, Eder W (Eds) Intracontinental fold belts. Springer, New York, pp 427-472 Wendt I (1989) Comparative Rb-Sr and U-Pb geochronology of late- to post-tectonic plutons in the Winning River belt, northwestern Ontario, Canada--Comments. Chemical Geology (Isotope Geosci Sect ) 79:95 - - Lenz H, Harre W, Sehoell M (1970) Total rock and mineral ages of granites from the southern Schwarzwald, Germany. Eclogae geologicae Helv 63:365--370 Carl C (in press) The statistical distribution of the Mean Squared Weighted Deviation. Isotope Geosciences Weinel Wi (1962) Erl~uterungen zur Geologischen Karte von Bayern 1 : 25 000, Blatt 6021, Haibach, 246 pp, Mfinchen - - - - Authors' addresses: Dr. S. Nasir, Institute of Earth and Environmental Science, University of Yarmuk, Irbid, Jordan, Prof. Dr. M. Okrusch, Mineralogisches Institut der Universtfit Wiirzburg, Am Hubland, D-W-8700 W/irzburg, Federal Republic of Germany, Dr. H. Kreuzer and Dr. A. H6hndorf, Bundesanstalt f/Jr Geowissenschaften und Rohstoffe, Postfach 510153, D-W-3000 Hannover 51, Federal Republic of Germany.