Evolution of Moldanubian rocks in Austria: review and synthesis

Abstract

A B S TR A CT The Moldanubian zone in Austria comprises three major lithological units. Despite general agreement that nappe tectonics contributed to its current structure, the number and position of tectonic boundaries, or continental pieces that were involved in its evolution, as well as the age, extent and position of oceanic sutures are disputed. Recent models ascribe the Moldanubian tectonostratigraphic structure to its oblique, N-to NE-directed collision with Moravia only.The rocks of the Moldanubian Bunte series and Gfö hl unit experienced a common, intensive overprint in the range 700-800°C and 8-11 kbar. Textural evidence suggests that this overprint was attained during nearly isothermal decompression, so the rocks experienced higher pressures prior to this overprint. These conditions constrain a continent-continent collision environment that contributed to the formation of the Moldanubian granulites. The estimated metamorphic temperatures are close to T max . During this Hercynian, high-T overprint, the minerals underwent extensive diffusion-controlled homogenization of elements. The early stages of retrogression of these units were characterized by isobaric cooling at c. 6 kbar in the range 650-500°C that is related to the oblique collision of the Moldanubian and Moravian zones. Cooling to c. 400°C is demonstrated by unstrained, diasporized corundum inclusions in garnet of common Moldanubian granulites.The available age data (including cooling ages) from metamorphic rocks show a very wide variation between 490 and 280 Ma that depends on sample characteristics and the dating method used. They demonstrate clearly, however, that the metamorphic overprint is Hercynian. The possibility that the large variation in ages reflects homogenization, resetting and closure of the isotopic systems attained at different, sample-and method-specific times is discussed. Age data varying between c. 370 and c. 346 Ma tentatively date different stages during the Hercynian, high-T decompression. The majority of zircon and monazite U/Pb ages as well as the hornblende and muscovite Ar/Ar cooling ages cluster between c. 345 and c. 326 Ma and date the effective closure conditions and the onset of rapid, nearly isobaric cooling. The continentcontinent collision that formed the granulites pre-dates c. 370 Ma. The intra-Moldanubian nappe-stacking pre-dates thrusting of the Moldanubian zone over the Moravian zone. The range c. 340-335 Ma is the lower limit for completion of tectonic activity in the Moldanubian zone.The Moldanubian series are post-tectonically intruded by granitoids of the Southern Bohemian Pluton. Recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the Hercynian high-grade overprint.The Moldanubian Monotone series in Austria is separated from the other Moldanubian units by a conspicuous tectonic horizon. It also differs from them by its characteristic high-T , low-P overprint, which is best demonstrated by a wides...

Magmatic fabrics and emplacement of the cone-sheet-bearing Knížecí Stolec durbachitic pluton (Moldanubian Unit, Bohemian Massif): implications for mid-crustal reworking of granulitic lower crust in the Central European Variscides

“…In general, the Moldanubian Unit has undergone polyphase metamorphism and has a complex deformational history. Its overall structure resulted from stacking of several major lithotectonic units at~350-330 Ma (e.g., Vrá na 1979Vrá na , 1988Vrá na et al 1995;Petrakakis 1997), followed by HT-LP metamorphism and anatexis dated at 337-323 Ma (Friedl et al 1993;Gerdes et al 2000;Kalt et al 2000), and later (~Permian) wrench tectonics (Brandmayr et al 1995;Edel et al 2003). However, the exact time constraints for these tectonic events may vary within the orogenic root and are still open to discussion.…”

“…The Drosendorf Unit is structurally the lowermost unit and consists of metasedimentary sequences Paleozoic; Krö ner 1988; Drá bek and Stein 2003) dominated by sillimanite-biotite (±cordie-rite) paragneisses and migmatites (referred to as the Monotonous Group) or paragneisses with abundant bodies of metaquartzites, marbles, calc-silicate rocks, graphite-bearing schists and amphibolites (altogether referred to as the Varied Group). Estimated P-T conditions of regional metamorphism of the Drosendorf Unit range from 630-720°C and 0.3-0.6 GPa (Petrakakis 1997;Vrá na et al 1995;Linner 1996).…”

“…The high-grade Gfö hl Unit, the protolith ages of which range from Precambrian to Lower Paleozoic (Friedl 1993;Petrakakis 1997;Schulmann et al 2005), is structurally the uppermost unit and is composed of orthogneisses, felsic migmatites, granulites, eclogites and peridotites. Estimations of P-T conditions of peak metamorphism in the Gfö hl Unit are~950-1,050°C and 1.4-2.0 GPa (in crustal peridotites) dated at~351 ± 6 (Carswell and O'Brien 1993;Wendt et al 1994;Medaris et al 2005) and~345 ± 5 (Van Breemen et al 1982) followed by retrograde metamorphism (T~600-800°C and~0.6-0.8 GPa; Owen and Dostal 1996) at 337-333 Ma (Friedl et al 1993;Wendt et al 1994;Gebauer and Friedl 1994).…”

The Rand Granite in the southern Schwarzwald and its geodynamic significance in the Variscan belt of SW Germany

“…Common Pb isotopic compositions of Stacey and Kramers (1975) are used for the correction. Four single zircons were dated for each sample Westphal and Keller (1995) 334-332: postcollisional and post-thrusting granites: Crustal anatexis Schaltegger (2000) 335-330: HT-LP-metamorphism: collision, rise of astenospheric diapir Kalt et al (1994a) 335: "LP collisional event" in the BM 340: "MP collisional event" in the BM Friedl et al(1993) 340-335: lower time limit for thrusting in the moldanubian BM Petrakakis (1997) Gruler et al (1999) Rand Granite is a heterogeneous plutonic complex, which is partly derived from a mantle source and partly from the continental crust similar to the Moldanubian basement rocks of the Schwarzwald (Liew and Hofmann 1988). Similar Sr-Nd isotopic variations as shown in Fig.…”

High-pressure/temperature metamorphism in the St. Leonhard Granulite Massif, Austria: evidence from intermediate pyroxene-bearing granulites

“…6.5 kbar, 725 C. On the other hand, Petrakakis and Jawecki (1995) examined garnet+orthopyroxene-bearing granulites and proposed peak metamorphism at 10.7 kbar and 770 C, followed by decompression and isobaric cooling in the range 5±6 kbar and 500±600 C. The discrepancy between estimates for peak P±T conditions and the derived P±T paths has profound consequences for the tectono-metamorphic interpretation of the Moldanubian zone. The lower P±T estimates advocated by Petrakakis and Jawecki (1995) were used by Petrakakis (1997) to suggest that a similarity in metamorphic conditions between the granulite massifs and rocks from the Drosendorf unit, tectonically underlying the granulite-bearing Gföhl unit, indicates that they experienced a common metamorphic evolution, albeit with different protolith environments. Conversely, a significant difference in the metamorphic evolution of the Drosendorf unit and the overlying granulite-bearing Gföhl unit, as suggested by Carswell and O'Brien (1993), implies deep burial and high-P±T metamorphism of the granulite precursors followed by their exhumation and juxtaposition onto the lower-grade Drosendorf unit at a mid-crustal level.…”