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ous/Early Permian horst-basin formation, documented in the RCC, is due to dextral transtensional move- ments along the NW-trending Franconian fault system.
Int J Earth Sci (2000) 89 : 52±71

 Springer-Verlag 2000

ORIGINAL PAPER

A. Zeh ´ M. A. Cosca ´ H. Brätz ´ M. Okrusch M. Tichomirowa

Simultaneous horst-basin formation and magmatism during Late Variscan transtension: evidence from 40Ar/39Ar and 207Pb/206Pb geochronology in the Ruhla Crystalline Complex Received: 2 February 1999 / Accepted: 15 November 1999

Abstract The Ruhla Crystalline Complex (RCC) is a constituent of the NE-trending Mid-German Crystalline Rise (MGCR) formed during the Variscan Orogeny. Furthermore, it is part of the NW-trending Franconian Fault system, which caused an intensive block faulting from the Late Carboniferous/Early Permian onward. In this paper new 40Ar/39Ar±mica and 207Pb/ 206 Pb±zircon dates are presented and combined with existing P±T data and the sedimentary record. These data indicate that the RCC was faulted into three segments which underwent different exhumation histories during the Late Carboniferous/Early Permian. The eastern segment shows 40Ar/39Ar±biotite data of 336  4 and 323  3 Ma. Furthermore, it is intruded by the Thuringian Hauptgranite dated at 337  4 Ma by the 207Pb/206Pb single zircon method. At approximately 300 Ma rocks of the eastern segment were finally exposed and, subsequently, subsided as part of the Oberhof pull-apart basin, filled by Late Carboniferous/Early Permian molasse sediments and volcanic rocks (296±285 Ma; Goll 1996). A similar Late Carboniferous evolution is inferred for the western segment, since it is also overlain by Upper Carboniferous volcanic rocks. In contrast to the eastern and western segments, distinctly younger intrusion and cooling ages were recorded for the central segment of the RCC (40Ar/39Ar muscovite: 311  3 Ma; 40Ar/39Ar biotite: 293±288  3 Ma) that was intruded by the Trusetal Granite, the Ruhla Granite and Brotterode Diorite (207Pb/206Pb single zircon: 298  2, 295  3, 289  4 Ma, respectively). These young data are A. Zeh ()) ´ M. A. Cosca ´ H. Brätz ´ M. Okrusch M. Tichomirowa Mineralogisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Fax: +49-931-888462) e-mail: [email protected] M. A. Cosca UniversitØ de Lausanne, Institut de MinØralogie, BFSH-2, 1015 Lausanne, Switzerland

unique in the MGCR and testify that plutonic activity and cooling of basement rocks took place simultaneously with basin formation and volcanism in the eastern and western segments. Overlying Upper Permian (Zechstein) and Triassic sediments indicate final exposure of the central segment by approximately 260 Ma, as a part of the Ruhla-Schleusingen Horst. Combination of these results with P±T data from the contact aureole of the Trusetal granite indicate that the central segment was unroofed by at least 8.5 km during the Late Carboniferous. The Late Carboniferous/Early Permian horst-basin formation, documented in the RCC, is due to dextral transtensional movements along the NW-trending Franconian fault system. It may have been enhanced by mantle upwelling widespread in Central Europe during the Early Permian that also caused intensive magmatism in the Thuringian Forest region. Key words Central European Variscides ´ Mid-German Crystalline Rise ´ Ruhla Crystalline Complex ´ Geochronology ´ 40Ar/39Ar dating ´ 207 Pb/206Pb dating ´ P±T±t paths ´ Extension ´ exhumation ´ basin formation

Introduction The NE-trending fold-and-thrust belt of the Central European Variscides is transected by numerous NW trending faults, which were initially formed during the Late Carboniferous/Lower Permian, and repeatedly reactivated during the Mesozoic and Cenozoic. The most prominent of these faults are the Tornquist-Teisseyre Line, the Elbe Lineament and the Franconian Fault System, the formation of which were initiated by dextral translation of Africa relative to Europe. In certain areas, movements along these faults caused basin formation and volcanism (e.g. Franzke and Rauche 1991; Schröder 1987; Benek 1989). By contrast, processes simultaneously experienced by the

53

exhumed basement complexes themselves have not been considered thus far. In the northwestern part of the Thuringian Forest, crystalline basement of the Mid-German Crystalline Rise is transected by the Franconian Fault System. Movements along this fault system caused an intensive block faulting that led to the formation of deep basins, filled with Permo-Carboniferous molasse sediments and volcanites, in close vicinity to horst blocks of metamorphic basement rocks, truncated by several plutons. In order to constrain the timing of the Variscan and Late Variscan evolution in the northwestern Thuringian Forest, we present 40Ar/39Ar mica and 207 Pb/206Pb single zircon evaporation data on crystalline rocks of the Ruhla Crystalline Complex (RCC). This geochronological evidence is combined with P±T data of metamorphic rocks as well as stratigraphic observations on the volcano-sedimentary cover sequences.

Geological setting At the northwestern end of the Thuringian Forest horst block, basement rocks of the Mid-German Crystalline Rise are surrounded by several molasse basins (Oberhof, Eisenach, Meiningen basin; see Figs. 1, 2). Fig. 1 Geological sketch map of the main tectonic units in the central European Variscides Blank: sedimentary cover of Permian and younger; 1 Variscan plutonites; 2 Rhenohercynian Realm: Devonian and Carboniferous sedimentary and volcanic rocks with very low-grade metamorphism of Variscan age; 3 Northern Phyllite Zone: pre-Devonian and Devonian sedimentary and volcanic rocks with lowgrade metamorphism of Variscan age; 4 exposed and 5 covered parts of the MidGerman Crystalline Rise: Pre-Devonian sediments and volcanic rocks, Late Silurian/ Early Devonian plutonites metamorphosed mainly under amphibolite facies conditions during the Variscan Orogeny; granites, diorites and gabbros dominantly of VisØan/Namurian age; 6 exposed Saxothuringian Realm: Precambrian to Lower Carboniferous sedimentary, volcanic and plutonic rocks, with metamorphism ranging from greenschist to granulite facies conditions, with eclogite relics

These were formed in Late Carboniferous/Early Permian times (Lützner 1988) by dextral movements along the NW-trending Franconian Fault System which, in Thuringia, consists of the Gotha-ArnstadtSaalfeld Fault, the Creuzburg-Ilmenau Fault and the Stahlberg Fault (Figs. 1, 2, 4). Recent exposure of basement rocks and molasse sediments results from normal faulting along the Franconian Fault System, due to Late Cretaceous inversion tectonics (Ziegler 1989; Schröder 1987). These movements led to an uplift of the Thuringian Forest horst block of several 1000 m (S.N. Thomson and A. Zeh, submitted). The Variscan basement rocks of the Mid-German Crystalline Rise are well exposed in the RCC extending for ~ 100 km2. The RCC consists of four structural-metamorphic units: (a) the Truse Formation in the SE; (b) the Ruhla Formation in the W; (c) the Brotterode Formation in the NE; and (d) the Central Gneiss Unit (Zeh et al. 1995; Zeh 1996). These units are transected by the NW-trending Klinge and Stahlberg faults and the N-S trending Engestieg and Westthuringian faults (Figs. 3, 4). The Truse and Ruhla formations consist predominantly of metapelites, quartzites and amphibolites. Bodies of granitic orthogneiss of variable size are intercalated within the metasediments of the Ruhla Formation. Both formations preserve different stages

54 Fig. 2 Distribution of crystalline basement and molasse basins as well as structural relationships at the end of the Early Permian (Rotliegendes) in the Thuringian Forest and its surroundings (after Andreas et al. 1992)

of structural-metamorphic overprint (see Figs. 4, 9; Zeh 1996). The Truse Formation is a folded pile of metamorphic rocks with flat NE- or SW-plunging fold axes and stretching lineations, and a SE-dipping foliation (Fig. 4). These structures were formed under medium-pressure, amphibolite facies conditions culminating at approximately 630  30 C and 7  1 kbar, as indicated by cogenetic kyanite, staurolite and garnet (Zeh 1996). The assemblage phengite+K-feldspar+ quartz included in garnet cores provides evidence that the prograde P±T path went through a higher pressure stage, however, with poorly defined P±T conditions. Where the Westthuringian Fault transects the Truse Formation, the foliation dips steeply to the NE, and the fold axes and stretching lineations plunge mostly flat to the SE. In places, refolded fold axes and stretching lineations are observed. Polygonally recrystallized quartz and both recrystallized and brittly deformed plagioclase, as well as local ultracataclasites, indicate that deformation along the Westthuringian Fault occurred under ductile and brittle conditions.

Ductile structures result from Late Variscan movements along the Westthuringian Fault (Zeh 1996), whereas most of the brittle structures are related to post-Variscan reactivations in different stress fields (see Franzke and Rauche 1991). The Ruhla Formation is characterized by amphibolite facies rocks which experienced a penetrative greenschist facies overprint at conditions of approximately 400  30 C and 4.5  1 kbar (Zeh 1996). The initial peak metamorphic conditions were presumably similar to those of the Truse Formation. The lowgrade overprint was accompanied by the formation of a new foliation, which dips moderately to the WSW. The present orientation of the foliation planes, the ecc-cleavages, and the stretching lineations indicate top-to-the-SW-directed movements (Fig. 4). In contrast to the Truse and Ruhla formations, the Brotterode Formation and the Central Gneiss Unit are of distinctly higher metamorphic grade. They consist predominantly of leucocratic migmatitic gneisses, hornblende gneisses, amphibolites, metapelites and

55 Fig. 3 Geological map of the Ruhla Crystalline Complex. Inset: geological setting of the Thuringian Forest

rare marbles, with mineral assemblages of the upper amphibolite facies. Nevertheless, the inner structure of these two units differ significantly. The migmatitic gneisses of the Brotterode Formation show a steeply SE- or NW-dipping mylonitic foliation and a flat SEtrending stretching lineation (Fig. 4) defined by sillimanite, biotite and hornblende. These mineral lineations indicate that formation of these structures took place at high-grade amphibolite facies conditions of approximately 700  50 C and 5.4  1.1 kbar, estimated from garnet±sillimanite±K-feldspar gneisses (see Fig. 9; Zeh 1996). Present structural orientations indicate strike-slip movement. However, this kinematic interpretation must not be valid for the time when the structures were formed, because the crystalline units of the RCC were affected by block rotation of unknown magnitude until the Late Cretaceous (see below). Garnet±sillimanite±K-feldspar gneisses of the Central Gneiss Unit yield, within errors, the same metamorphic peak conditions of 700  50 C and 5.0  1.2 kbar as determined for the Brotterode Formation. In contrast, garnet-bearing assemblages are rare in the Central Gneiss Unit, whereas assemblages with cordierite are common. Locally, cordierite is observed to replace garnet, consistent with decompression at high temperatures. The P±T conditions estimated from cordierite±sillimanite±K-feldspar gneisses are 700  60 C at 4.2  1.5 kbar (see Fig. 9; Zeh 1996).

The Central Gneiss Unit is characterized by a dome structure with periclinal foliation and stretching lineations (Fig. 4). Wunderlich (1992) and Zeh (1996) postulated that these structures resulted from the updoming of the Steinbach augengneiss and unroofing of the overlying ortho- and paragneisses, followed by static crystallization of plagioclase and cordierite under lower-pressure/high-temperature conditions. In a zone situated on top of a marked, flat SW-dipping cataclastic horizon, orthogneisses of the Central Gneiss Unit subsequently underwent an intensive greenschist facies overprint (Fig. 4: profile). During this overprint the orthogneisses were intermingled with metapelites and quartzites of the Truse Formation and a new, subhorizontal foliation was formed. The deformation style in parts of the Central Gneiss Unit, overprinted under greenschist facies conditions, is similar to that observed in the Ruhla Formation (Zeh 1996). Finally, a flat, SW-dipping cataclastic to ultracataclastic zone, up to 10 m thick, was formed under brittle conditions.

Intrusions in the Ruhla Crystalline Complex The four structural-metamorphic units of the RCC were intruded by the Thuringian Hauptgranite, the Trusetal Granite, the Ruhla Granite, and the Brotterode Diorite (Fig. 3). Geochemically, all these intru-

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Fig. 4 Structural map and profile of the Ruhla Crystalline Complex

sions display type-I characteristics, with a relatively high amount of crustal contamination (Anthes 1998). With exception of the Thuringian Hauptgranite, these intrusions caused extensive thermal overprinting of the regionally metamorphosed units of the RCC (Zeh 1996). Univariant assemblages at the contact of the Trusetal Granite with the Truse Formation indicate pressure conditions between 3.7 and 2.3 kbar, and assemblages from the western contact aureole of the Ruhla Granite indicate maximum pressures of 2.3 kbar (Zeh 1996). Assuming a lithostatic pressure gradient of 270 bar/km, typical for the upper continental crust, the contact aureole of the Trusetal Granite was formed at depths between 8.5 and 14 km, whereas the intrusion of the Ruhla Granite took place at a level of 8.5 km or less. The intrusion depth of the Thuringian Hauptgranite exposed in the RCC is unknown. Petrological data from the contact aureole

of the Thuringian Hauptgranite in the Vesser area, situated approximately 30 km SE of the RCC (Fig. 1), indicate a maximum intrusion depth of approximately 10 km (Zeh et al. 1998). All plutons of the RCC display evidence of a more or less intensive, syn- to postintrusive deformation. The Brotterode Diorite, the Trusetal Granite and the Ruhla Granite are weakly deformed under ductile to brittle conditions, locally showing contact-parallel foliations at their margins. In contrast, the Thuringian Hauptgranite displays a marked foliation which, however, does not penetrate the complete granite body, but is restricted to a zone following the eastern flank of the RCC, parallel to the Westthuringian Fault. Within this zone, semi-ductily deformed K-feldspar and well-recrystallized quartz indicate that the granite deformation took place after the intrusion at temperatures between 500 and 300 C (S. van der Klauw, pers. commun.). Subvolcanic dikes of kersantite, dolerite and granite porphyry of Late Carboniferous to Early Permian age (Benek and Schust 1988; Mädler and Voigt 1994) intersect all regionally metamorphosed units as well as

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the Thuringian Hauptgranite, the Trusetal Granite, the Brotterode Diorite and, to a lesser extent, the Ruhla Granite (Figs. 3, 10). Some dikes intersecting the Ruhla and the Truse formations are well foliated and contain ductily deformed quartz (Mädler 1969; Benek and Schust 1988; Mädler and Voigt 1994). That indicates deformation of these dikes at temperatures above 300 C.

Sedimentary record It has been known since the initial mapping by Zimmermann (1930) that the age and the orientation of the bedding of the oldest sedimentary rocks varies with location within the RCC: 1. East of the Westthuringian Fault, rocks including the Brotterode Formation and the Thuringian Hauptgranite, are directly overlain by Upper Carboniferous (Stephanian) molasse sediments and related volcanic rocks, dipping up to 35 toward the NE (Fig. 4). Viewed within a wider regional context, the eastern segment forms the base of the Oberhof pull-apart basin (Lützner 1988), which is filled by approximately 4 km of Upper Carboniferous and Lower Permian (Rotliegend) volcanic rocks and sediments (Figs. 2, 5). According to Lützner (1988) and Schneider (1996), the sedimentation in the Oberhof Basin started at approximately 300 Ma with deposition of the Gehren Subgroup, and ceased at approximately 280 Ma with deposition of the Tambach Beds (Fig. 5). Biotites from volcanic rocks of the Gehren Subgroup yielded mean 40Ar/39Ar ages of 296  4 Ma and those from the Oberhof Formation 288  5 Ma (Goll 1996). In addition, 40Ar/39Ar laser ablation data on biotites from the Oberhof rhyolite yielded 285±286 Ma (  2; Friedrichs et al. 1997). 2. Between the Westthuringian and the Engestieg faults the crystalline basement ± comprising the Truse Formation, the Central Gneiss Unit, the eastern part of the Ruhla Formation, the Trusetal Granite, the Ruhla Granite and the Brotterode Diorite (Fig. 3) ± is overlain by Upper Permian (Zechstein) Kupferschiefer beds, limestones and evaporites and, locally, by Lower Triassic (Bunter) sandstones. At the southern flank of the RCC, between the Stahlberg and Klinge faults, Zechstein and Bunter sediments locally dip 30 to the south, whereas Zechstein sediments at the northern flank of the RCC dip with 5±10 to the northwest. This indicates that different parts of the central segment were affected by block rotations in different directions during the Mesozoic. In a wider context, the central segment is part of the NW-trending RuhlaSchleusingen Horst, which is also known from drill holes northwest and southeast of the Thuringian Forest (see Fig. 2; Andreas 1988, 1996; Andreas et al. 1992).

3. West of the Engestieg Fault, the Ruhla Formation is overlain by Upper Carboniferous volcanic rocks of the Ilmenau Formation (see Fig. 5; Andreas 1996) in the southwest and by up to 800 m of Lower Permian fanglomerates of the Eisenach Formation in the northwest. These fanglomerates, which contain crystalline boulders up to 0.5 m, were deposited in the Eisenach pull-apart basin, and may have the same age as the Tambach Beds in the Oberhof Basin (ca. 280 Ma; H. Lützner, pers. commun.). The fanglomerates dip 10±15 to the NW and were uplifted, probably during Cretaceous inversion, along the Engestieg Fault by several hundred metres against crystalline rocks of the central segment. The sedimentary cover clearly indicates that the central part of the RCC formed a horst structure during the Late Carboniferous and/or Early Permian, while the eastern and western segments subsided, forming basins. Furthermore, the sediments testify to partial relief inversion and block rotations, which result from post-Variscan wrench and Upper Cretaceous inversion tectonics.

Analytical methods 40

Ar/39Ar geochronology

Biotite and muscovite concentrates were obtained by magnetic separation of the 125- to 500-mm size fractions, from samples which show no or only minor chloritization. High purity was achieved by hand picking, discarding all grains with any sign of alteration or with inclusions of opaques, apatite and zircon. Sample locations are shown in Fig. 8 and coordinates are given in Table 1. The 40Ar/39Ar analyses were carried out at the Institut de MinØralogie, UniversitØ de Lausanne (Switzerland). Samples, together with standard minerals of known age, were irradiated for 20 MWH in the central thimble position of the Triga reactor in Denver, Colorado (Dalrymple et al. 1981). Production ratios for the Triga reactor were determined from analyses of irradiated salts. The standard minerals HDB1 with an age of 24.71 Ma (Hess and Lippolt 1994) and MMHB-1 with an age of 520.4 Ma (Samson and Alexander 1987) were used to correct for the neutron flux, which was determined with a precision of 0.5%. After the samples were incrementally heated in a low blank, double vacuum resistance furnace and purified using activated Zr/Ti/Al getters and a cold finger maintained at liquid nitrogen temperatures, the purified gas was analyzed statically in a Mass Analyzer Products 215±250 mass spectrometer. Blanks were subtracted from the sample signal. For mass 40, blank values ranged from 2±5 ” 10 ± 15 mol below 1350 C to 7±9 ” 10 ± 15 mol at 1650 C. Blank values for masses 36±39 were below 2 ” 10 ± 17 mol for all temperatures.

58 Fig. 5 Stratigraphy, lithology, volcanism and ages of the Upper Carboniferous and Permian in the Thuringian Forest; after Lützner et al. (1995) and Andreas (1996); inferred ages after Menning (1995) and Schneider (1996); geochronological ages after Goll (1996)

Eight scans per analysis were made over the mass range 40 to 36. Peak heights above backgrounds were corrected for mass discrimination, isotopic decay and interfering Ca-, K- and Cl-derived isotopes of argon. The 40Ar/39Ar results for muscovite and biotite with 2s errors are given in Figs. 6 and 8, and Table 2. With the exception of sample PT, all samples investigated produced relatively flat 40Ar/39Ar age spectra. Following the criteria given by Cosca et al. (1991), plateau

ages were calculated for samples with 40Ar/39Ar ratios of two or more consecutive steps overlapping within error at 2s, and comprising more than 50% of the 39 Ar released. In all cases the plateau ages are in agreement with the total fusion ages. Isochron plots (both 39Ar/40Ar vs 36Ar/40Ar and 40Ar/36Ar vs 39Ar/ 36 Ar) were generated for all samples and, 39Ar/40Ar vs 36 Ar/40Ar isochrons indicate trapped initial 40Ar/36Ar ratios of atmospheric argon. All samples yielded high

59 Table 1 Sample localities, lithologies and minerals analysed Sample

Locality

RW

HW

Unit

Lithology

Minerals analysed

P14

Kleinschmalkalden

360391

562645

Truse Formation

Staurolite±garnet gneiss

Biotite, Muscovite

A69

Schleifkothengrund

359683

563494

Central Gneiss Unit

Biotite±plagioclase gneiss

Biotite

A15

Lotzerödchen

359638

563354

Central Gneiss Unit

Augengneiss

Biotite

P16

Kugeliges Köpfchen

360289

563385

Brotterode Formation

Sillimanite±garnet gneiss

Biotite

PA

Unteres Alttal

360335

563388

Brotterode Formation Sillimanite±garnet gneiss

Biotite

P09

NW Trusetal

360039

562950

Truse Formation

Andalusite±biotite± fels

Biotite

PT

Summit Trockenberg

360511

563494

Thuringian Hauptgranite

Granite

Biotite, Zircon

TG-S

Drive path between Brotterode and Mommelstein

360162

563220

Trusetal Granite (Seimberg granite)

Granite

Zircon

TG-B

Quarry NW Trusetal

360038

562930

Trusetal Granite (bairodite)

Granite

Zircon

BD

Former cargo station in Brotterode

360054

563245

Brotterode Diorite

Diorite

Zircon

RG

S-Eselskopf

359675

563591

Ruhla Granite

Granite

Zircon

radiogenic argon concentrations, often preventing the calculation of statistically significant isochrons, or resulted in poorly constrained ages and trapped initial 40 Ar/36Ar ratios. None of the isochron calculations, however, yielded any evidence for trapped argon with non-atmospheric ratios (within 2s errors). 207

Pb/206Pb geochronology

Single zircons from the non-magnetic, 0.1- to 0.2-mm size fraction were analysed using a Finnigan MAT 262 mass spectrometer at the Institut für Mineralogie, TU Bergakademie Freiberg (Germany), employing the evaporation method of Kober (1986, 1987). In this method, one or more chemically untreated grains are embedded in a canoe-shaped Re double-filament arrangement. Radiogenic Pb is then evaporated in the mass spectrometer after heating the grain at temperatures around 1600 C. Kober (1986) has shown that the Pb components with the highest activation energy normally reside in the undamaged crystalline zircon phase that shows no post-crystallization Pb loss and, therefore, yields concordant 207Pb/206Pb ages. Pb components in zircon domains, which were damaged by radiation (metamict zones) have low activation energy and thus are removed during low-temperature evaporation. Because all investigated magmatic samples are virtually undeformed and the zircons with oscillatory zonation yielded reproducible ages within errors, the ages obtained are interpreted as the time of zircon crystallization. All 207Pb/206Pb ratios were corrected

for a mass bias of 0.36  0.01 amu, determined with two single-zircon standards. Measurement of the zircon standards 91500 (Wiedenbeck et al. 1995) and S-2-87 (Canadian Geological Survey) yielded 207/206Pb ages of 1066  1 Ma and 380  5 Ma, respectively, identical to the adopted ages of 1065.4  0.4 Ma and 381.5  4 Ma. Pb isotope ratios were corrected for initial common Pb (see Table 3) using the common Pb evolution model of Stacey and Kramers (1975). The results of the single zircon dating are summarized in Figs. 7 and 8, and Table 3. The uncertainties are based on the means of all ratios evaluated with 2s mean errors (Fig. 7; Table 3). All ages reported in this paper were calculated using the constants of Steiger and Jäger (1977).

Results of geochronology In general, the 40Ar/39Ar experiments result in broad, flat age spectra with most samples forming 40Ar/39Ar age plateaus adhering to the strict criteria mentioned previously. Four samples failed to meet these criteria, but three of them (samples PA, P16 A and A69; Fig. 6) yielded generally flat 40Ar/39Ar age spectra nearly forming age plateaus. In all cases the 40Ar/39Ar plateau ages and total fusion ages are identical within error. One sample (PT) yielded a hump-shaped 40Ar/ 39 Ar spectrum that could indicate 39Ar-recoil and redistribution into submicroscopically intergrown chlorite (e.g. Hess and Lippolt 1986; Lo and Onstott 1989). Depending on whether or not the recoiled 39Ar

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Fig. 6 40Ar/39Ar age spectra for micas from different metamorphic units of the RCC. Errors given for the ages and the filled boxes in the age spectra are 2s

escapes from the biotite and chlorite mixture before the mass spectrometric analysis, the total fusion age may or may not record a valid cooling age for closure below its temperature for argon retention. In any case, the total fusion age for this sample PT is considered a maximum age. For all of the other samples, the general flatness of the 40Ar/39Ar age spectra, the excellent agreement between the plateau ages and the total fusion ages, and the lack of evidence from the isochron diagrams for any non-atmospheric trapped component, indicates that all 40Ar/39Ar plateau and/or total fusion ages are good estimates for the time of biotite and muscovite cooling below their respective argon closure temperatures.

Thuringian Hauptgranite Six clear, inclusion-free, euhedral zircon grains with prismatic morphologies were evaporated from the Thuringian Hauptgranite (sample PT) and yielded identical 207Ar/206Pb ratios with a mean age of 337  4 Ma. This is the oldest zircon age reported for plutonic rocks of the RCC. The 40Ar/39Ar total fusion age of 334  3 Ma, considered as a maximum age, is obtained on a biotite separate from the same sample. This age is identical, within error, to a mean biotite 40 Ar/39Ar total fusion age of 332  4 Ma from other localities of the Thuringian Hauptgranite (Goll 1996), and with 40Ar/39Ar laser data of 329  3 Ma (Friedrichs et al. 1997), measured in the same samples, respectively. These results indicate that the Thuringian Hauptgranite cooled quickly from igneous temperatures to below ~ 300 C (Harrison et al. 1985). It should be noted that the distinctly higher closure temperatures of 450 C proposed by Villa (1998) for biotite, is only valid for ª... thermal-only volume diffusion in the absence of recrystallization ....º and not for

Fig. 7 Histograms showing the results of the 207Pb/206Pb single-grain zircon analyses. The older ages of sample TG-B and TG-S were obtained from round, multifaceted grains; the younger ages are from longprismatic zircons (for explanations see text) 61

62 Table 2 Analytical data for experiments T ( C)

Ca/K

36Ar/ 39Ar

40

Ar/39Ar incremental heating 2%s

T ( C)

Sample 1800 1825 1850 1875 1900 1925 1950 1975 1000 1025 1050 1075 1100 1300 Fuse

P16 biotite, wt. = 8.68 mg, J = 0.004809 0.5% 10.0937 0.00013 39.36 99.9 313  118 10.1386 0.00020 40.75 99.9 323  111 10.2534 0.00016 41.29 99.9 327  116 10.2194 0.00041 40.54 99.7 321  114 10.2982 0.00071 40.51 99.5 321  114 10.3469 0.00082 41.04 99.4 325  114 10.4957 0.00084 40.68 99.4 322  113 10.3327 0.00074 41.05 99.5 325  113 10.3494 0.00066 40.72 99.6 323  113 10.2607 0.00062 40.54 99.6 321  113 10.2785 0.00051 40.91 99.7 324  114 10.2444 0.00037 40.74 99.8 323  114 10.2612 0.00034 41.07 99.8 325  114 10.2843 0.00017 41.11 99.9 325  114 16.5502 0.00758 34.43 94.6 276  115 Total fusion age = 323 3 Ma

Sample 1600 1650 1700 1750 1800 1850 1875 1900 1925 1950 1975 1000 1050 1100 1350 1651

PA biotite; wt. = 14.20 mg, J = 0.004953 0.5% 10.4432 1.04520 10.31 13.2 190  164 10.2513 0.06444 20.33 51.6 173  113 10.1402 0.02358 23.57 77.2 199  112 10.0599 0.01807 37.97 87.7 311  114 10.0219 0.01011 40.85 93.2 332  113 10.0141 0.00365 40.77 97.4 332  113 10.0115 0.00146 41.06 99.0 334  114 10.0114 0.00092 41.75 99.4 339  113 10.0112 0.00074 41.91 99.5 340  113 10.0115 0.00068 41.69 99.5 339  113 10.0138 0.00077 41.44 99.5 337  113 10.0139 0.00078 41.77 99.5 339  113 10.0101 0.00062 41.34 99.6 336  114 10.0069 0.00037 41.12 99.7 334  113 10.0056 0.00038 42.05 99.7 341  114 10.0705 0.01540 39.06 89.6 319  113 Total fusion age = 336 4 Ma

Sample 1700 1750 1775 1800 1825 1850 1875 1900 1925 1950 1975 1000 1025 1050 1075 1100 Fuse

O9 biotite, wt. = 4.57 10.5814 0.00720 10.5149 0.00074 10.6750 0.00074 10.8916 0.00085 10.8164 0.00066 10.5987 0.00070 10.7210 0.00085 10.5062 0.00103 10.2740 0.00061 10.2520 0.00044 10.4751 0.00032 10.2090 0.00016 10.2468 0.00027 10.3786 0.00057 11.0461 0.00085 13.3579 0.00207 11.6227 0.00509

Sample 1650 1700 1750 1800 1850 1875 1900 1925 1950 1975 1000 1050 1100 1350 1651

Pt biotite; 10.2826 10.1699 10.1163 10.0253 10.0090 10.0077 10.0069 10.0068 10.0062 10.0076 10.0104 10.0116 10.0082 10.0405 10.0351

Sample 1600 1700 1800 1850 1900 1950 1000 1050 1100 1351 Fuse

A69 biotite, wt. = 3.69 mg, J = 0.004809 0.5% 10.7213 0.05947 35.12 66.7 282  118 10.1616 0.00220 37.13 98.3 296  115 10.1615 0.00026 36.02 99.8 288  113 10.3774 0.00064 35.34 99.5 283  115 10.6041 0.00053 36.36 99.6 291  115 10.3156 0.00038 36.08 99.7 289  115 10.1856 0.00019 35.97 99.9 288  113 10.1602 0.00026 36.30 99.8 290  113 10.2482 0.00027 35.80 99.8 287  113 10.5430 0.00132 33.93 98.9 273  116 13.1846 0.29295 27.56 24.2 225  116 Total fusion age = 288 3 Ma

Sample 1700 1800 1850 1900 1950 1975 1000 1050 1100 Fuse

P14 muscovite, wt. = 4.57 mg, J = 0.004809 0.5% 10.0642 0.01350 39.34 90.8 313  118 10.0475 0.00277 39.08 98.0 311  114 10.0478 0.00028 39.07 99.8 311  114 10.0166 0.00030 38.99 99.8 310  114 10.0180 0.00116 38.89 99.1 309  116 10.0451 0.00208 38.57 98.4 307  119 10.0043 0.00121 38.72 99.1 308  119 10.0000 0.00046 39.38 99.7 313  114 10.1355 0.00050 39.58 99.6 314  114 10.2881 0.00442 41.17 97.0 326  116 Total fusion age = 311 3 Ma Plateau age (700±1100 C) = 311 3 Ma

wt. = 8.45 0.04908 0.01127 0.00839 0.00364 0.00157 0.00113 0.00099 0.00091 0.00095 0.00107 0.00109 0.00083 0.00063 0.00487 0.00766

40Ar*/ 39ArK

%40Ar* Age 

Table 2 Continued

mg, J = 0.004950 0.5% 16.14 52.7 139  113 16.56 83.3 142  112 33.37 93.1 276  113 38.37 97.3 314  113 41.12 98.9 334  113 41.25 99.2 335  113 41.92 99.3 340  113 42.12 99.4 342  113 42.27 99.3 343  113 42.40 99.3 344  113 43.39 99.3 351  113 43.75 99.4 354  113 42.46 99.6 344  113 41.90 96.7 340  113 41.46 94.8 337  113 Total fusion age = 334 3 Ma

Ca/K

36Ar/ 39Ar

40Ar*/ 39ArK

%40Ar* Age 

2%s

mg, J = 0.004811 0.5% 36.06 94.5 289  117 36.26 99.5 290  114 36.49 99.5 292  113 36.29 99.4 290  113 36.55 99.6 292  113 36.66 99.5 293  113 36.77 99.4 294  113 36.67 99.2 293  113 37.00 99.5 296  116 36.47 99.7 292  114 36.63 99.8 293  116 36.13 99.9 289  115 36.48 99.8 292  114 36.23 99.6 290  114 36.16 99.4 289  113 35.92 98.7 288  113 34.06 97.0 274  114 Total fusion age = 291 3 Ma Plateau age (825±1025 C) = 293 3 Ma Sample A15 biotite, wt. = 8.35 mg, J = 0.004807 0.5% 1700 10.0710 0.02293 36.06 84.2 288  114 1800 10.0135 0.00117 35.96 99.0 288  113 1850 10.0120 0.00004 36.24 99.9 290  113 1900 10.1615 0.00038 36.12 99.7 289  114 1950 10.1753 0.00065 36.31 99.5 290  114 1000 10.0812 0.00039 36.31 99.7 290  113 1050 10.0790 0.00058 36.40 99.5 291  113 1100 10.1453 0.00047 36.05 99.6 288  114 1350 10.8566 0.00042 35.51 99.7 284  111 Fuse 13.0736 0.01476 43.77 91.2 345  130 Total fusion age = 289 3 Ma Plateau age (800±1100 C) = 290 3 Ma Sample P14 biotite, wt. = 3.60 mg, J = 0.004810 0.5% 1700 11.0045 0.01273 34.39 90.2 276  110 1750 10.7133 0.00081 35.59 99.4 285  116 1775 10.7408 0.00071 36.29 99.5 290  113 1800 10.9856 0.00065 36.59 99.6 292  114 1825 10.8321 0.00052 36.66 99.7 293  113 1850 10.4431 0.00068 36.75 99.5 294  113 1875 10.8080 0.00072 36.75 99.5 294  113 1900 10.6136 0.00054 37.18 99.6 297  115 1950 10.4235 0.00033 36.41 99.8 291  119 1000 10.4402 0.00038 37.01 99.7 296  116 1100 10.2832 0.00034 36.95 99.8 295  118 1150 12.4552 0.00232 36.13 98.4 289  113 1200 19.0962 0.00361 35.79 98.1 287  114 1351 15.7615 0.00817 32.02 94.7 258  115 Fuse 74.8855 0.20999 17.07 22.0 142  118 Total fusion age = 290 3 Ma Plateau age (800±1100 C) = 294 3 Ma

63 Table 3 Results of single zircon evaporation analyses Sample

Zircon morphology, type (Pupin 1980), marks

Scans

204 Pb/206Pb ratio  2s

207 Pb/206Pb ratio  2smean error

PT PT PT PT PT PT

Short prismatic, type S24 Short prismatic, type S18, brownish, muddy Prism fragment Short prismatic, type S19, twinned L, inclusion Short prismatic, type S24 Short prismatic fragment, type S24

187 170 180 173 190 190

0.00037  12.0 0.00049  11.3 0.00026  10.6 0.00020  10.5 0.00012  10.4 0.00018  11.2

0.05325  120 339.4  18.4 Ma 0.05316  116 335.6  16.7 Ma 0.05316  114 335.5  11.5 Ma 0.05307  115 331.6  12.0 Ma 0.05340  110 345.6  14.3 Ma 0.05318  119 336.3  18.2 Ma Mean age = 337.3  3.9 Ma

RG RG RG RG RG RG RG RG RG

Long prismatic L : W = 4, type S17, inclusion Short prismatic, type S19 Short prismatic L : W = 2, type S24, slightly rounded Short prismatic L : W = 2, type S19, inclusions Prismatic L : W = 3, type S19±24 Long prismatic L : W = 5, type S19 Short prismatic L : W = 1.5, flat shape Prismatic, type S17 Long prismatic, type S19

134 190 183 184 184 152 190 186 190

0.00140  12.7 0.00031  10.2 0.00003  10.2 0.00010  10.4 0.00012  10.4 0.00059  11.1 0.00013  10.5 0.00041  11.3 0.00020  10.3

0.05222  124 294.8  10.3 Ma 0.05230  117 298.3  12.9 Ma 0.05229  116 297.9  12.4 Ma 0.05225  112 296.1  15.1 Ma 0.05213  110 290.9  14.3 Ma 0.05202  115 286.3  16.7 Ma 0.05230  115 298.6  16.4 Ma 0.05228  115 297.6  16.7 Ma 0.05225  118 296.4  13.6 Ma Mean age = 295.2  3.1 Ma

BD BD BD BD BD BD

Prismatic, type S19 190 Prismatic, type S23, end slightly dissolved 186 Long prismatic, type S22, inclusion, slightly dissolved 190 Prismatic, type S22 185 Round shape, multifaceted 187 Round shape, multifaceted 189

0.00029  11.3 0.00026  10.8 0.00049  10.7 0.00041  10.6 0.00031  10.7 0.00026  10.5

0.05217  114 292.9  16.3 Ma 0.05223  112 295.3  15.4 Ma 0.05209  111 289.3  15.1 Ma 0.05202  118 286.1  13.3 Ma 0.05210  111 289.6  15.0 Ma 0.05193  111 282.4  14.8 Ma Mean age = 289.3  3.8 Ma

TG-S TG-S TG-S TG-S TG-S TG-S TG-S TG-S

Short prismatic, type S12±13 Long prismatic, type S19 Short prismatic-round shape Short prismatic, type S18 Long prismatic, type S13, one dark inclusion Long prismatic, type S18, 4 colourless inclusion Short prismatic, type S23 Short prismatic, type S13

152 190 184 180 134 171 153 168

0.00059  12.0 0.00068  12.2 0.00050  12.5 0.00049  11.3 0.00030  11.4 0.00065  10.4 0.00028  10.6 0.00016  10.4

0.05218  133 293.3  14.4 Ma 0.05229  121 298.1  19.3 Ma 0.05235  111 300.7  14.8 Ma 0.05226  117 297.0  13.1 Ma 0.05217  128 292.8  12.0 Ma 0.05232  114 299.5  11.8 Ma 0.05234  119 300.1  14.0 Ma 0.05234  118 300.0  13.3 Ma Mean age = 297.7  2.3 Ma

TG-S TG-S TG-S TG-S TG-S TG-S

Round Round Round Round Round Round

136 190 187 187 117 180

0.00840  15.9 0.00049  10.9 0.00058  10.9 0.00367  19 0.00631  14.7 0.00073  10.3

0.05248  145 306.4  19.8 Ma 0.05250  114 307.3  16.0 Ma 0.05264  118 313.4  13.5 Ma 0.05280  111 320.0  47.6 Ma 0.05260  141 311.5  17.8 Ma 0.05260  115 311.4  12.0 Ma Mean age = 311.7  4.5 Ma

TG-B TG-B TG-B TG-B TG-B

Short prismatic L : W = 2 Prismatic fragment, type S18 Prismatic L : W = 4, type S21±22 Short prismatic, type S13±14 Short prismatic, type S13

188 147 190 180 172

0.00021  10.7 0.00041  10.7 0.00010  10.3 0.00027  10.6 0.00042  10.9

0.05225  114 296.4  16.3 Ma 0.05222  117 295.2  13.0 Ma 0.05226  116 296.9  12.8 Ma 0.05216  113 292.3  15.6 Ma 0.05231  123 298.7  19.9 Ma Mean age = 295.9  2.1 Ma

TG-B TG-B TG-B TG-B

Round shape, multifaceted Round shape, multifaceted Prismatic-round shape, type S24, inclusion Round shape, multifaceted

187 134 190 190

0.00052  11.1 0.00312  15.1 0.00099  11.8 0.00049  10.6

0.05269  116 315.4  16.7 Ma 0.05257  124 310.3  10.2 Ma 0.05278  120 319.2  18.6 Ma 0.05244  110 304.8  14.5 Ma Mean age = 312.4  6.3 Ma

shape, shape, shape, shape, shape, shape,

multifaceted multifaceted multifaceted multifaceted multifaceted multifaceted

ª... fluid assisted recrystallization ...º, a case highly unlikely for natural rocks. Since all investigated rocks of the RCC, including the Thuringian Hauptgranite, provide evidence for deformation and recrystallisation under ductile, semiductile and locally under brittle conditions, the lower closure temperature given by Harrison et al. (1985) is preferred in this study.

207 Pb/206Pb age  2s mean error

Trusetal Granite Two samples from different localities of the Trusetal Granite were investigated; both contain two types of translucent zircons. The predominant variety is longprismatic, whereas the second one is round, nearly isometric with multifaceted surfaces. The second group of zircons either forms separate grains or cores of

64

from the ages obtained for the prismatic zircons of the Trusetal Granite. However, the absence of the round zircon population in the Ruhla Granite and a different bulk rock chemistry (see Werner 1974; Anthes 1998) indicate a different source and/or evolution of the granite magma. The zircon age of the Ruhla Granite is consistent with the laser 40Ar/39Ar biotite cooling ages of 295  6 Ma obtained by Friedrichs et al. (1997) for the Ruhla Granite. A 40K/40Ar biotite date of 310  5 Ma reported by Neumann (1974) is inconsistent with these recent age data. Brotterode Diorite

Fig. 8 Distribution of sample points and ages obtained by 40Ar/ 39 Ar mica and 207Pb/206Pb single-grain zircon analyses in the RCC. ms muscovite; bt biotite; zc-P prismatic zircon; zc-R round, multifaceted zircon. Note that the distribution of Early and Late Carboniferous intrusion and cooling ages is restricted to the areas east and west of the Westthuringian Fault, respectively

Zircons from the Brotterode Diorite (sample BD) show a wide diversity of shapes from euhedral-prismatic to round-multifaceted grains. The six zircon grains analysed (2 round, 4 prismatic) yielded a mean age of 289  4 Ma (Fig. 7; Table 3). This result conforms to field observations, showing that the Brotterode Diorite intruded the Trusetal Granite. Our data are younger than the 40K/40Ar biotite ages of 304  7 Ma and 340  30 Ma recorded by Goll (1996) and Neumann (1974), respectively, for the Brotterode Diorite.

Regional metamorphic rocks long-prismatic zircons as observed in cathodoluminescence images. Clear, euhedral, prismatic zircons from sample TG-S (eight grains) and TG-B (five grains) yield mean ages of 298  2 and 296  2 Ma, respectively. In contrast, six round, multifaceted zircons from sample TG-S and four from sample TG-B give identical, but higher, mean ages of 312  5 and 312  6 Ma, respectively (Fig. 7; Table 3). One prismatic zircon grain from sample TG-S showed a significant change in the 207Pb/206Pb ratio after the first 20 scans, indicating an older core. We interpret the prismatic zircons as recording the time of crystallization of the Trusetal Granite, whereas the older generation of round, multifaceted zircons formed prior to the intrusion. They may represent assimilated crustal material, e.g. high-grade metamorphic rocks of the lower crust. An 40Ar/39Ar plateau age of 291  3 Ma for a biotite, sampled from an undeformed hornfels of the contact aureole around the Trusetal Granite (P09), conforms to the age of the prismatic zircons, indicating rapid cooling after intrusion. The 40K/40Ar biotite date of 470  25 Ma given in Neumann (1974) is inconsistent with our data. Ruhla Granite Nine clear, euhedral zircons from the Ruhla Granite (sample RG) yielded a mean age of 295  3 Ma (Table 3, Fig. 7) that is indistinguishable, within error,

40

Ar/39Ar plateau and total fusion ages of ~ 290 Ma have been determined on biotites from the Truse Formation (P14 = 294  3 Ma) and the Central Gneiss Unit (A15 = 290  3 Ma; A69 = 288  3 Ma; Fig. 6). These dates are consistent with 207Pb/206Pb ages on zircons from the Trusetal and Ruhla Granites and the Brotterode Diorite of 298  2, 295  3 and 289  4 Ma, respectively, and with a weighted mean zircon fission track age of 272  7 Ma estimated for the central segment of the RCC (S.N. Thomson and A. Zeh, submitted). Much older 40Ar/39Ar ages are observed for biotites of samples P16 (total fusion age = 323  3 Ma) and PA (total fusion age = 336  4 Ma) from the Brotterode Formation, similar to the 40Ar/39Ar ages from the Thuringian Hauptgranite. These data are regarded as reliable cooling ages since, with exception of sample PA, the 40Ar/39Ar spectra are generally flat and the isochrone plots lack evidence of any non-atmospheric trapped argon. Moreover, these data agree with the 207 Pb/206Pb zircon age of the Thuringian Hauptgranite of 337  4 Ma, and with 40Ar/39Ar ages of 332  4 (Goll 1996) and 329  3 Ma (Friedrichs et al. 1997). Muscovite from the Truse Formation (sample P14) yields a plateau age of 311  3 Ma, significantly older than the plateau age of biotite from the same sample (294  3 Ma). Assuming an Ar closure temperature of 400  50 C in muscovite (Kirschner et al. 1996) and 300  50 C in biotite (Harrison et al. 1985), a mean linear cooling rate of approximately 6 C/Ma can be estimated for this sample.

65

Discussion The P±T data, summarized in Fig. 9, indicate that the RCC consists of two groups of metamorphic units, which underwent different prograde metamorphic evolutions and thermal peak conditions (see above). The Fig. 9a±d Pressure±temperature±time paths for the four metamorphic units of the RCC (modified after Zeh 1996; for explanations see text)

medium-pressure/medium-temperature group comprises the Truse- and Ruhla formations, the mediumpressure/high-temperature group the Brotterode Formation and the Central Gneiss Unit. After the metamorphic peak, both groups were tectonically juxtaposed (Zeh 1996). Subsequently, the unstable tectonic pile underwent extensional movements, retro-

66 Fig. 10 Geological profile through the RCC and its surroundings (location of the profile similar as in Fig. 4), with synoptic presentation of geochronological dates. RG Ruhla Granite; TG-Trusetal Granite; THG Thuringian Hauptgranite; BD Brotterode Diorite: 1) 207Pb/206Pb single zircon ages, this study; 2) 40Ar/35Ar biotite ages, this study; 3) 40Ar/39Ar-muscovite age, this study; 4) average biotite cooling ages after Goll (1996)

grade metamorphism, and thermal overprinting associated with the intrusion of the Trusetal and Ruhla Granites and the Brotterode Diorite. Our 40Ar/39Ar and 207Pb/206Pb data, in combination with the sedimentary record, clearly indicate differential exhumation histories within the RCC, preserved in three crustal segments (Figs. 8, 10, 11), which result from block faulting during Permo-Carboniferous times. These segments are separated by the steeply dipping, generally N-S-trending Engestieg and Westthuringian faults which cut through the different tectono-metamorphic units of the RCC (Fig. 3).

mation indicate approximately 18  3 km of unroofing between the metamorphic peak (335  2 Ma; Brätz et al. 1998) and deposition of the Stephanian sediments (Schneider 1996). The eastern segment subsided until at least 280 Ma, the presumed depositional age of the Lower Permian Tambach Beds (Schneider 1996; Lützner 1988; Andreas 1996). Judging from observations in surface outcrops and drill holes, the amount of subsidence reached at least 1.5 km (see Fig. 2; Voigt 1972; Andreas et al. 1992).

Eastern segment (Oberhof Basin)

The central segment is situated between the WestThuringian and the Engestieg faults. It comprises the Central Gneiss Unit, the Truse Formation and the western part of the Ruhla Formation. The Late Carboniferous exhumation of the central segment started with the intrusion of the Trusetal Granite at approximately 298  2 Ma. Petrological observations provide evidence that this intrusion took place at a depth between 8.5 and 14 km, whereas the Ruhla Granite intruded into a shallower level below 8.5 km (Zeh 1996 Fig. 9b±d). Since the 207Pb/206Pb single zircon ages of both granites are identical within error (298  2 and 295  3 Ma), rapid uplift of the central segment during the Late Carboniferous is inferred. Subsequently, the RCC was intruded by the Brotterode Diorite at 289  4 Ma, presumably at a shallower, but unspecified depth. Within error, the intrusion age of the Brotterode Diorite is identical to the last regional cooling below 300 C in the central segment, as indicated by 40Ar/39Ar dates of biotites from the country rocks ranging from 293 to 288 Ma (  3). The intrusion of the Trusetal and Ruhla

The segment east of the Westthuringian Fault comprises the Brotterode Formation which was intruded by the Thuringian Hauptgranite. The 207Pb/206Pb zircon age of 337  4 Ma and the 40Ar/39Ar biotite ages between 334  3 and 329  3 Ma (this study; Goll 1996; Friedrich et al. 1997) from this granite indicate a short time span between zircon crystallization and Ar closure in biotite, which would be in agreement with a shallow intrusion depth. Furthermore, a U/Pb monazite age of 335  2 Ma (Brätz et al. 1998) and 40Ar/ 39 Ar biotite ages between 336  4 and 323  3 Ma from the adjacent Brotterode Formation (Fig. 8) indicate relatively rapid cooling from approximately 700 C down to approximately 300 C. Approximately 300 Ma ago, exhumation of the eastern segment was completed and Stephanian and Lower Permian (Rotliegend) volcanic rocks and molasse sediments were deposited on top of it (Fig. 9a). The P±T data from the Brotterode For-

Central segment (Ruhla-Schleusingen Horst)

67 Fig. 11 Model for the LateVariscan evolution of the Thuringian Forest and its surroundings in Late Carboniferous to Upper Permian time. The timing of plutonic and volcanic events is demonstrated in relation to the uplift of the Ruhla-Schleusingen Horst and the concomitant subsidence of the Oberhof (OB) and Eisenach (EB) pull-apart basins

granites took place along steeply dipping, NNEoriented faults. The orientation of these faults is consistent with the overall east/west-directed extensional stress field established in Central Europe since Upper Carboniferous times (Arthaud and Matte 1977). The development of these NNE structures pre-date the formation of the N-S-trending Westthuringian and Engestieg faults (Figs. 9b±d).

Last of all, the central segment was affected by brittle deformation, accompanied by the formation of cataclastic and ultracataclastic rocks. The most spectacular example of this deformation is a cataclastic zone up to 10 m thick, which occurs below the strongly retrogressed and deformed upper part of the Central Gneiss Unit (Fig. 4). Presently, there is no evidence as to whether these structures were formed

68

during the Upper Carboniferous/Permian times or as a result of Cretaceous tectonics. Judging from the age of the sedimentary cover, exhumation of the central segment was completed by Late Permian (Zechstein) to Early Triassic (Bunter) times, between approximately 260 and 250 Ma (Menning 1995). Western segment (Eisenach Basin) The western segment comprises the southwestern part of the Ruhla Formation, west of the Engestieg Fault. Thus far, no geochronological data exist from this part of the RCC; thus, age constraints can be derived only from the cover sequences. In the SW, the Ruhla Formation is overlain by a thick pile of volcanic rocks of the Lower Permian Ilmenau Formation, for which an age of approximately 295 Ma has been assumed by Andreas (1996). In contrast, the crystalline basement in the NW is covered by an 800-m-thick fanglomeratic sequence of the Eisenach Formation. These fanglomerates fill the Eisenach pull-apart basin (Fig. 2), the formation of which presumably started approximately 280 Ma ago (Figs. 5, 11; H. Lützner, pers. commun.). Rapid uplift and erosion of the Ruhla-Schleusingen Horst during this time is evident from the large boulders of crystalline rocks in the Eisenach fanglomerates, which were derived from the central segment of the RCC (Zeh et al., in press). Regional correlations The geochronological data from the Brotterode Formation and the Thuringian Hauptgranite, both forming part of the eastern segment, are consistent with intrusion and cooling ages between 340 and 320 Ma, recorded from other crystalline complexes of the MidGerman Crystalline Rise. These age data testify to a period of widespread magmatic activity during the VisØan and Namurian in the Mid-German Crystalline Rise, immediately followed by regional cooling. This is evident from numerous 40K/40Ar and 40Ar/39Ar ages on amphiboles, muscovites and biotites, ranging between 340 and 320 Ma (Kreuzer and Harre 1975; Rittmann 1984; Lippolt 1986; Hess and Schmidt 1989; Nasir et al. 1991; Dombrowski et al. 1994; Neuroth 1997) and from 207Pb/206Pb and U/Pb data on zircons which yielded ages between approximately 335 and 325 Ma (Anthes and Reischmann 1996, 1997; Hammer et al. 1996; Reischmann and Anthes 1997). According to the geotectonic model of Oncken (1997), regional cooling and exhumation in the Mid-German Crystalline Rise during VisØan/Namurian times coincides with crustal shortening and thrusting in the Rheonhercynian fold-and-thrust belt and in the Saxothuringian retrowedge, which is exposed in the southeastern Thuringian Forest. In a wider context all these processes result from collision between the Rhenohercy-

nian (Eastern Avalonia) and the Saxothuringian terranes in a NW-directed compressional stress field (Oncken 1988). In contrast, the young data recorded for the central segment of the RCC are unique in the Mid-German Crystalline Rise. The only exception are 207Pb/206Pb single zircon dates of 311  17 and 300  2 Ma obtained from a granite near Delitzsch (Eidam et al. 1995; Anthes and Reischmann 1996). Furthermore, our geochronological data, together with the data of Goll (1996) and Friedrichs et al. (1997), indicate that volcanic activity in the Oberhof Basin (eastern segment) and pluton emplacement in the Ruhla-Schleusingen Horst (central segment) were synchronous. Therefore, the Trusetal and Ruhla Granites as well as the Brotterode Diorite in the RCC are interpreted as plutonic counterparts of the rhyolithic and andesitic volcanic rocks in the Oberhof and Eisenach basins. Our P±T±d and age data, as well as the sedimentary record in the RCC, show that the Ruhla-Schleusingen Horst was uplifted, by several kilometres, simultaneously with subsidence of the surrounding basins (Fig. 11). Presumably, these processes are not only a result of the dextral transtensional movements along the Franconian Fault System, but may have been enhanced by active upwelling of the mantle, which caused widespread magmatic activities throughout the late Hercynian Europe from southern Norway to the Alps. In central Germany Late Variscan magmatic activity is restricted mostly to the dominant NE-, NWand/or N-trending fault zones, e.g. the Halle fault zone, the Franconian fault sytem and to the vicinity of the Hunsrück fault. All these fault zones limit sedimentary basins of different size. In the Thuringian forest region, witnesses for Late Carboniferous/Early Permian mantle upwelling (Fig. 12) are intrusions of

Fig. 12 Schematic diagram illustrating the distribution of molasse basins and the position of the Franconian fault system in the Thuringian Forest and its surroundings (cf. Fig. 2). The regional stress field and the upwelling mantle during the Late Carboniferous and Early Permian are also indicated.

69

type-I granites and diorites, dikes of tholeiitic withinplate basalt (Obst 1993), lamprophyre, granite- and diorite-porphyry as well as the widespread rhyolithe and andesite volcanism. Moreover, it can be speculated that the combined effects of transtensional movements and mantle upwelling played an important role for the evolution of the prominent NE-trending intramontane molasse basins, formed by inversion of the Mid-German Crystalline Rise (Fig. 12). Subsidence of the Saale Basin in the NE started in VisØan times (e.g. Ellenberg et al. 1987; Steinbach 1997), whereas the evolution of the Saar-Werra Basin in the SW began somewhat later in the Westphalian (Schäfer 1989). Subsequently, both basins evolved toward each other and merged in the Thuringian Forest region, during the Stephanian. By the same time, dextral transtensive movements along the Franconian Fault system started, resulting in a dextral offset of the two NE-trending basins and formation of small pull-apart basins (Fig. 12). All the processes are consistent with an E-W-directed extensional stress field, which governed Central Europe during the Late Carboniferous/Early Permian times, due to the combined effects of gravitational collapse of the Variscides and the dextral translation of northern Africa relative to Europe (Arthaud and Matte 1977; Lorenz and Nicholls 1984; Menard and Molnar 1988; Eisbacher et al. 1989). Later, in Mesozoic times, the Late Permian horstbasin configuration was disturbed, due to wrench tectonics and normal faulting along the Franconian Fault System, related to Tethys rifting and Alpine foreland compression, respectively (Trümpy 1985; Schröder 1987; Franzke and Rauche 1991). Due to the last process, defined as Cretaceous inversion tectonics (Ziegler 1989), the RCC and parts of the surrounding molasse basins were elevated by several thousand metres as part of the Thuringian Forest Horst Block, as indicated by recent fission-track data (S.N. Thomson and A. Zeh, submitted). Cretaceous inversion along the Franconian Fault has also been demonstrated in the KTB target area, situated approximately 200 km SE of the RCC (Peterek et al. 1994; Zulauf and Duyster 1997).

Summary and conclusion New 40Ar/39Ar and 207Pb/206Pb data, as well as field observations, provide evidence for intensive block faulting within the northwestern Thuringian Forest during Late Carboniferous/Early Permian times, due to motions along the NW-trending Franconian Fault System in an E-W-trending extensional stress field. As a result, the Ruhla Crystalline Complex, forming part of the NE-trending Mid-German Crystalline Rise, was faulted into three segments which underwent different exhumation histories.

Geochronological data testify to igneous activity and regional cooling of the eastern segment between 337  4 and 323  3 Ma. Conformingly, the sedimentary record indicates final exposure at approximately 300 Ma. Volcanic rocks on top of the western segment point to a similar exhumation history. Subsequently, the eastern and western segments subsided as part of the Oberhof and Eisenach pull-apart basins, respectively, and were overlain by thick sequences of Upper Carboniferous to Lower Permian molasse beds and intercalated volcanic rocks. In contrast, geochronological dating for the central segment of the RCC yielded distinctly younger intrusion and cooling ages between 298  2 and 288  3 Ma, which are unique througout the Mid-German Crystalline Rise. Unconformably overlying Upper Permian (Zechstein) and Triassic (Bunter) sediments provide evidence that final exposure of the central segment, as part of the Ruhla Schleusingen Horst, took place in Late Permian times (ca. 250±260 Ma). The data set presented in this paper suggests that volcanic activity in the Oberhof and Eisenach molasse basins and pluton emplacement in the Ruhla-Schleusingen Horst were synchronous. Furthermore, the data indicate that final exhumation of basement rocks of the Ruhla Schleusingen Horst, by several kilometres, and subsidence of the Oberhof and Eisenach basins took place within the same time span. The simultaneous horst-basin formation and magmatic activities are attributed to dextral transtensional movement along the NW-trending Franconian Fault System, and may have been enhanced by active upwelling of the mantle throughout Europe, including the Thuringian Forest region. The dextral motions took place in an E-W-directed extensional stress field, which governed Central Europe during the Late Carboniferous/Early Permian times, due to the combined effects of the gravitational collapse of the Variscides and the dextral translation of northern Africa relative to Europe (Arthaud and Matte 1977; Lorenz and Nicholls 1984; Menard and Molnar 1988; Eisbacher et al. 1989). Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft, grant Ok 2/44-1,-2 and Ti 211/5-1, and the Swiss National Science Foundation is gratefully acknowledged. The paper benefitted from stimulating discussions with W. Schubert, H. Lützner, H. Rauche and S. van der Klauw, and from constructive reviews by N. Arnaud, N. J. Cook, W.-C. Dullo, K. Mezger, F. Neubauer and O. Oncken.

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