Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge)

Transcription

1 Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) Karsten Reiter TU Bergakademie Freiberg, Germany Abstract. The granulite massif is an exotic element of the Saxo-Thuringian zone. The rocks display various stages of a metamorphic evolution. The core of the massif is composed of felsic and mafic granulites and rare mafic to ultramafic rock units, which have been subducted during the Variscan orogeny. The metamorphism culminated at 340 my ago at a depth of km and a temperature of over 1000 C. A fast uplift (8-19 mm/y) brought the rocks to their final positions. This process was accompanied by granite intrusions. The inner mantle of the massif consists of mica schists and gneisses and is interpreted as ductile shear zone during the north-west orientated uplift. The outer schist mantle presents a lowergrade metamorphic metasedimentary succesion of Lower Paleozoic age. Introduction / Geological setting The Granulitgebirge (Granulite Mountains) is located to the north-west of the Erzgebirge (Bohemian Massif), separated by the Suberzgebirge-basin. The Granulitgebirge displays an elliptical dome-like structure with an extention of 50 km in length and 20 km in width, striking in SW-NE direction parallel to the general Variscan structures (Fig.1). The core is composed of high-pressure granulites, intersected by some granite intrusions and pods of mafic rocks. A mantle of schist surrounds the granulite core. The mantle is subdivided in an inner and outer schist mantle. Compared to the granulites, the rocks of the schist mantle are more resistant to weathering and, therefore, form a wall-like hilly belt.

2 2 Karsten Reiter The exotic connection of a high-grade metamorphic core with apparently lower-grade mantle schist did attract the interest of many generations of geologists. Fig. 1 Geolgical map of the Saxonian Granulite Massif (RÖTZLER & ROMER, 2001) The Granulite core Granulite The Saxonian Granulit Massif is the locus typicus of granulite rock. It is indirectly the eponym of the granulite facies. ESKOLA visited the area before he established this term. The notation Weißstein (white stone) (ENGELBRECHT, 1802) is based on A.G.WERNER and, subsequently, has been redefined as granulite (WEISS, 1803). The felsic and mafic granulite has a foliation and a pronounced layered fabric (KRONER, 1995). The leucocratic granulite consists of garnet, kyanite, rutile, al-

3 Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) 3 kalifeldspar, plagioclase and quartz. Biotite appeared during retrogression. The mafic granulites are made up by garnet, clinopyroxene and orthopyroxene. Amphiboles and biotite are common retrograde phases (REINHARDT & KLEEMANN, 1994). As protoliths of the acid granulites are discussed either a Lower-Paleozoic acidic magmas, with ages of 499±2 my (Pb-Pb) (RÖTZLER et al., 2004), or a volcano-sedimentary sequence (REINHARDT & KLEEMANN, 1994) of Ordovician age, 470 my (Sm-Nd) (QUADT, 1993). The mafic granulites are interpreted as former olivine-tholeiites or MORB-like protoliths, which could represent volcanic deposits of an active plate boundary. The Variscan granulites are subdivided into two groups. The Saxonian granulite is derived from the early compressive stage (PIN & VIELZEUF, 1983) in context with a subduction belt. The protoliths have been subducted during the Variscan collision, however by a classical NW-SE directed convergence or SW-NE convergence (KRONER et al., 2007). As result of the collision of the northern Old Red Continent with Gondwana and the closure of the Rheic Ocean, a stack of crustal units was amalgamated, with the emplacement of upper crustal segments in direct vicinity of the lower crust. The granulite metamorphic record shows a peak pressure at about 23 kbar, with a minimum temperature of C, which can be calculated from reintegrated hypersolvus K-feldspar in felsic granulites. U-Pb-titanite dating yielded an age of the peak metamorphism at 340.7±0.8 my (U-Pb) (RÖTZLER et al., 2004). A similar development affected the mafic granulites. Garnet-clinopyroxenegranulites underwent minimum temperatures of C, at pressures of 22 kbar, recorded by the clinopyroxene-garnet thermometer (RÖTZLER, 1992). The combination of P-T-path and age data demonstrates that the exhumation of the granulites to a middle-to upper-crustal level proceeded first at a fast average rate of 9-18 mm/y, a cooling of C/my and at subsequently slowed down rates at <2 mm/y and slow cooling (6 K/my) (RÖTZLER & ROMER, 2001). The apparent closure temperature of monazite ( C) was dated at 315 my (U-

4 4 Karsten Reiter Pb) at pressures of 3 kbar (BAUMANN, 1997). In consequence, the granulite was transported from a crustal depth of km to 10 km during 25 my. Post-peak decompression reactions and influx of H 2 O in the leucogranulites produced biotite and sillimanite at the expense of garnet and K-feldspar, and sillimanite pseudomorphs after kyanite (REINHARDT & KLEEMANN, 1994). First granulite pebbles appeared in Upper Carboniferous sediments (Westphalian D or B/C) of the Erzgebirge Basin (PIETZSCH, 1963). The rocks, thus, were eroded at least since ~310 my. A younger metamorphic event is not observed in this area. Mafic rocks within the granulite core In addition to the mafic granulites, layered with the felsic granulite, other, often serpentinisized basic to ultra-basic rocks like eclogite, pyroxenite, flasergabbro and amphibolites occur. These rocks appear commonly as lenses or boudins. In a rock formerly described as an eclogite, an assemblage with clinopyroxene and garnet consists of garnet with a high pyrope content. The clinopyroxene is diopside without jadeite. It is interpreted as a diopside-grossularite skarn assemblage (MATHE & SOBOLEV, 1969) which crystallized under high pressures together with the surrounding granulite. The eclogite from Gilsberg was heated to C at a pressure of 40 kbar determined by Fe 2+ /Mg distribution coefficient of lamella-like garnet exsolutions in orthopyroxene (REICHE & BAUTSCH, 1985). Appearance of meta-gabbro with websteritic mode and protocataclastic structure recorded temperatures between 825 and 1280 C at 5.8 kbar. This rock was interpreted as a quenched cumulate (SOELLNER et al., 1990). Garnet pyroxene layers embedded in serpentinized garnet peridotite originally consisted of megacrystals. These megacrystals were either Al-rich clinopyroxenes formed at ~17 kbar and 1400 C or majoritic garnets with a minimum pressure of 145 kbar. This corresponds to a depth of more than 400 km (MASSONNE & BAUTSCH, 2002), a part of the upper portion of the transition zone in the mantle. It is interpreted as a restite or cumulate which was uplifted within a rising mantle plume. The tectonic

5 Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) 5 mixing of the granulite with peridotite and eclogite is correlated with the collision stage which led to major crustal thickening in the central zone of the orogen (REINHARDT & KLEEMANN, 1994). Granite The biotites (or muscovite)-bearing, amphibole-free monzogranites, have low contents of mafic minerals (GOTTESMANN & GERSTENBERGER, 1987). Three types of granite veins or bodies can be distinguished (KRONER, 1995): 1. granitization developping from fissure surfaces, 2. granite without sharp contact to the granulite, partly bounded by assimilation of large amounts of granulite xenoliths, 3. large granitic bodies partly with distinct intrusion contacts (e.g. Mittweidaer Granit). The granitic intrusions occur within the granulite complex and the schist mantle. The foliation-parallel granitic sills in the schist mantle were partly transformed into orthogneiss (REINHARDT & KLEEMANN, 1994). The shearing had continued during or after the intrusion. In the final stage of exhumation (2-3 kbar), the monzogranite intruded at 333 my ago (NASDALA et al., 1996) (KRÖNER et al., 1998). Oxygen, deuterium and nitrogen isotopes as well as REE (MÜLLER et al., 1987) indicate that the granulite was the source rock of the granitic melt, generated by influx of H 2 O (REINHARDT & KLEEMANN, 1994).

6 6 Karsten Reiter The inner schist mantle A wide variety of metamorphic rocks occurs together in this small roof position of the granulite. For a better understanding of the structural development (see below), this small cover is subdivided into an internal and an outer domain (KRONER, 1995). Internal domain of the inner schist mantle Cordierite and garnet bearing gneisses are the most common rock types. They are described as a transition area from granulite to modified granulite to garnet gneiss and then to cordierite gneiss (KRONER, 1995). It varies from garnet free cordierite-sillimanite-k-feldspar-biotite gneiss to garnet cordierite sillimanite fels with garnet contents of up to 45 % (REINHARDT & KLEEMANN, 1994). The metamorphic grade drops from the sillimanite-k-feldspar zone in contact to the granulite core, to greenschist facies within 2 km. The low pressure overprint of the high-grade assemblage at 350 C and 4.5 kbar is interpreted as a stage of exhumation, without a genetical context to the (outer) schist (REINHARDT & KLEEMANN, 1994). A large amount of the cordierite gneiss is marked by migmatisation. Augengranulites are the product of ultra-mylonitic restyling. There are more variations of reworked granulite like ultra mylonite and biotite gneiss (KRONER, 1995). Consequently, the internal domain consists only of reworked granulite. Other authors described it as high-grade meta pelites (RÖTZLER & ROMER, 2001). External domain of the inner schist mantle This lithological assemblage consists of flasergabbro, bronzite serpentinite, orthogneiss and phyllonitic mica schists (KRONER, 1995). The mica schist has a continuous transition to low metamorphic metapelites. The mafic constituents are interpreted as a former ophiolithic assemblage (WERNER, 1981).

7 Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) 7 Internal and external domain of the inner schist mantle The observed metamorphic overprint can be characterized as retrograde for the granulite (Internal domain of the inner schist mantle) and prograde for the schist (External domain of the inner schist mantle). The granulite complex was considered as the primary heat source for the metamorphism of the inner schist mantle (REINHARDT & KLEEMANN, 1994). Detritus derived from the schist mantle is reported from sediments of the uppermost Visean in the Borna-Hainichen-basin. The outer schist mantle These low metamorphic schists (phyllites) consist of pelitic to psammitic metasediments. Biostratigraphic data provide a Lower-Paleozoic age (KRONER, 1995). The Cambrian to lower Carboniferous succession corresponds to the Saxo- Thuringian allochthon (KRONER & HAHN, 2003). A post-sedimentary low-grade metamorphic event at 321 ± 7 my or 316 ± 7 my (K-Ar) affected the Ordovician layers and was interpreted as a stage of exhumation (MARHEINE, 1997). Younger Sediments The granulite massif exhibits a hiatus between the latest Paleozoic and the Upper Cenozoic. Neogene deposits with a low thickness, constituted of gravel, sand, clay and lignite are widely distributed. Quaternary sediments are fluvial terraces, flood plain gravels, loess loam, loess marl and warves and correlate with the last three glaciations events (WOLF, 1991).

8 8 Karsten Reiter Models of structural evolution Since the late 19. Century, the granulite has been described as a metamorphic rock. Traditional models preferred a diapir-like uplift (PIETZSCH, 1963). Other processes, like polyphase vertical movements or north-vergent crust stacking were discussed. The granulite massif should be regarded as a tectonic window which exposes lower granulitic crust from below. The major extensional detachment, the contact is a low angel normal fault (or shear zone) (REINHARDT & KLEEMANN, 1994). The most up to date interpretation is a metamorphic core complex model (FRANKE, 1991; KRONER, 1995). The emplacement of the granulite core from mantle depths into the schist cover happened in the course of the Variscan extensional collapse. During the fast uplift, the hot granulite caused a LP-HT metamorphism in the roof area. The north-west vectored movement in the finale position enabled the deformation of the ductile shear zone (KRONER & HAHN, 2003) which is represented by the inner schist mantle. The P-T-time-path is shown in Fig.2. Fig. 2 P-T-t-path for the Saxonian granulites derived from the Tirschheim core and some other observations (RÖTZLER et al., 2004)

9 Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) 9 References BAUMANN, N., 1997, U-Pb-, Pb-Pb- und Sm-Nd-Altersbestimmungen vom Sächsischen Granulitgebirge und deren Implikationen: Freiberg, TU Bergakademie Freiberg. ENGELBRECHT, C.A., 1802, Kurze Beschreibung des Weißsteins, einer im geognostischen System bis jetzt unbekannt gewesenen Gebirgsart: Leipzig, Schr. der Linneischen Sozietät. FRANKE, W., 1991, The Saxonian Granulite - a metamorphic core complex? Terra Abstr., v. 3, p GOTTESMANN, B., and GERSTENBERGER, H., 1987, Petrographic studies of granite gneisses and granites from the Saechsisches Granulitgebirge: ZFI - Mitteilungen, v. 133, p KRÖNER, A., JAECKEL, P., REISCHMANN, T., and KRONER, U., 1998, Further evidence for an early Carboniferous (~340 Ma) age of highgrade metamorphism in the Saxonian Granulite Complex, Geologische Rundschu, Volume 86: Berlin, p KRONER, U., 1995, Postkollisionale Extension am Nordrand der Böhmischen Masse - die Exhumierung des Sächsischen Granulitgebirges: Freiberger Forschungshefte, Reihe C 457, p KRONER, U., and HAHN, T., 2003, Sedimentation, Deformation und Metamorphose im Saxothuringikum während der varistischen Orogenese: Die komplexe Entwicklung von Nord-Gondwana während kontinentaler Subduktion und schiefer Kollision, Das Saxothuringikum: Dresden, Ulf LINNEMANN, p KRONER, U., HAHN, T., ROMER, R.L., and LINNEMANN, U., 2007, The Variscan orogeny in the Saxo-Thuringian zone - Heterogenous overprint of Cadomian/Paleozoic Peri-Gondwana crust: Geological Society of America Special Paper, v. 423, p MARHEINE, D., 1997, Zeitmarken im variszischen Kollisionsbereich des Rhenoherzynikums - Saxothuringikums zwischen Harz und Sächsischen Granulitmasiv - Ergebnisse von K/Ar-Altersbestimmungen, Universität Göttingen. MASSONNE, H.-J., and BAUTSCH, H.-J., 2002, An unusual garnet pyroxenite from the Granulitgebirge, Germany: Origin in the transition zone (>400 km depths) or in a shallower upper mantle region? International Geology Review, v. 44, p MATHE, G., and SOBOLEV, N.V., 1969, Der Kalksilikat-'Eklogit' von Waldheim im sächsischen Granulitgebirge. The calc-silicate 'eclogite' of Waldheim in the granulite complex of Saxony: Geologie, v. 18, p MÜLLER, A., STIEHL, G., BÖTTGER, T., BOTHE, H.K., GEBHARDT, O., GEISLER, M., HANDEL, D., NITZSCHE, H.M., SCHMÄDICKE, E., and GERSTENBERGER, H., 1987, Geochemical, stable isotope and petrographic investigations of granulites, pyriclasites and metagranulitic

10 10 Karsten Reiter rocks of the Saechsisches Granulitgebirge: ZFI - Mitteilungen, v. 133, p NASDALA, L., GRUNER, T., NEMCHIN, A.A., PIDGEON, R.T., and TICHOMIROWA, M., 1996, New SHRIMP ion microprobe measurement on zirkons from Saxonian magmatic and metamorphic rocks, Ppoceedings of the Freiberg Isotope Colloquium: Freiberg, University of Mining and Technology, p PIETZSCH, K., 1963, Geologie von Sachsen: Berlin, VEB Deutscher Verlag der Wissenschaften. PIN, C., and VIELZEUF, D., 1983, Granulites and related rocks in variscan median Europe: a dualistic interpretation, Tectonophysics, Volume 93, p QUADT, A., 1993, The Saxonian Granulitmassif: New aspects from geochronological studies, Geologische Rundschau, Volume 82: Berlin, p REICHE, M., and BAUTSCH, J.H., 1985, Electron microscopical study of garnet exsolution in Orthopyroxene, Phys Chem Minerals, Volume 12, p REINHARDT, J., and KLEEMANN, U., 1994, Extensional unroofing of granulitic lower crust and related low-pressure, high-temperature metamorphism in the Saxonian Granulite Massif, Germany, Tectonophysics, Volume 238, p RÖTZLER, J., 1992, Zur Petrogenese im Saechsischen Granulitgebirge; Die pyroxenfreien Granulite und die Metapelite: Geotektonische Forschungen, v. 77, p RÖTZLER, J., and ROMER, R.L., 2001, P-T-t Evolution of Ultrahigh- Temperature Granulites from the Saxon Granulite Massif, Germany Part I & II, Journal of Perology, Volume 42, p RÖTZLER, J., ROMER, R.L., BUDZINSKI, H., and OBERHÄNSLI, R., 2004, Ultrahigh-temperature high-pressure granulites from Trischheim, Saxon Granulite Massif, Germany:P-T-t path and geotectonic implications, European Journal of Mineralogy, Volume 16, p SOELLNER, R., KOPP, J., ADAM, K., and Anonymous, 1990, Geothermometrische und geobarometrische Untersuchungen an Pyroxenen aus websteritischen Gesteinen des Saechsischen Granulit Gebirges, Berichte der Deutschen Mineralogischen Gesellschaft, Volume 1990, p WEISS, C.S., 1803, Über die Gebirgsart des Sächsischen Erzgebirges, welche unter dem Namen Weiss-Stein neuerlich bekannt gemacht worden ist: Berlin, Neue Schr. Ges. naturforsch. Freunde. WERNER, C.D., 1981, Sächsisches Granulitegebirge - Saxon Granulite Massif. Ophiolites and initiales of northern border of the Bohemian Massif. Guide bock of excursions: Potsdam-Freiberg.