Resolving the relationship between high P–T rocks and gneisses in collisional terranes: an example from the Gföhl gneiss–granulite association in the Moldanubian Zone, Austria
Introduction
A common feature of deeply eroded collisional orogens is that, within allochthonous nappe sheets, high-pressure rocks, such as granulites and eclogites, can often be found intimately associated with gneissose rocks in which predominantly lower pressure mineral assemblages are found (e.g. Cuthbert and Carswell, 1990, O'Brien and Carswell, 1993, Carswell et al., 2000). This situation can be viewed as a result of two possible end-member scenarios. The first is that, following peak metamorphic equilibration at a deep crustal level, the high-pressure rocks were subsequently tectonically juxtaposed with the lower pressure gneissose matrix, at a shallower crustal position, as a result of exhumation processes. The second scenario is that both the high-pressure rocks and enclosing gneisses experienced high-pressure conditions; however, the gneisses either failed to react at the high-pressure conditions for kinetic reasons or, alternatively, high-pressure mineral assemblages developed, but were later pervasively overprinted or even obliterated as a result of deformation and changes in pressure and temperature conditions during the passage of the rocks back to the surface (e.g. Austrheim, 1987, Carswell et al., 2000).
Clearly, to deduce the true metamorphic history of areas where one of the two above situations could be invoked is a prerequisite for understanding the large-scale tectonometamorphic evolution of such regions. This study presents a petrological and geothermobarometric investigation of one such area within the Moldanubian Zone, the crystalline core of the Variscan orogen, exposed in the Bohemian Massif of central Europe (see Fig. 1a).
Section snippets
Geological setting
Within the parts of the Bohemian Massif exposed in Austria and the Czech Republic (Fig. 1b), high-pressure–high-temperature (HP–HT) granulites are found in individual massifs at the highest structural levels of the Gföhl Unit, the uppermost of the series of nappe units which constitute the Moldanubian Zone (e.g. Fuchs, 1986, Franke, 1989, Weber and Duyster, 1990). The nappe units below the Gföhl Unit (Fig. 1c), the Variegated and Monotonous Units (together sometimes termed the Drosendorf Unit),
Field setting
The majority of mapped granulite layers within the Gföhl gneiss have been recorded from its southernmost extent in Lower Austria, close to the River Danube (see Fig. 1, Fig. 2). They consist predominantly of leucocratic garnet+kyanite-bearing rocks, although rarely garnet+pyroxene-bearing rocks can also be found Fuchs and Scharbert, 1979, Matura, 1984. The samples described in this study were obtained from a large outcrop at the side of the road, which runs parallel to the Weitenbach river, ca.
Granulite (sample M.500)
Anhedral garnet porphyroclasts range in size from 200 up to 1.7 mm (see Fig. 3a), and contain inclusions of plagioclase, rutile, quartz and clinopyroxene, the latter often being partially replaced by amphibole. The garnets are chemically zoned. Fig. 4 shows a typical example in which a broad, relatively homogenous core region (ca. 500 μm wide) has high Ca and Mg contents and low Mn. At the rims (ca. 120 μm wide), both Ca and Mg drop as Fe and Mn rise. Typical compositions (see Table 1) are:
Granulite (sample M.500)
Both the petrographic and compositional information indicate a lack of widespread equilibrium related to a single metamorphic event. The preserved inclusion suite and porphyroclasts indicate a former high-P granulite-facies assemblage of garnet+clinopyroxene+plagioclase+quartz (e.g. De Waard, 1965, Green and Ringwood, 1967). Chemically, the homogenous interior garnet compositions can be related to the effects of volume diffusion at high temperatures and are thus considered to represent peak
Geothermobarometry
To evaluate peak metamorphic conditions in the granulite (sample M.500), the garnet–clinopyroxene Fe–Mg exchange thermometer (calibration of Powell, 1985) can be combined with the GADS geobarometer (calibration of Eckert et al., 1991; note for all calculations Fe2+/Fe3+ estimated from charge balance).
Temperatures are calculated using the homogenous garnet core compositions combined with either matrix or inclusion clinopyroxene compositions. Minimum temperatures of 900 °C are provided by using
Discussion
The close coherence between the calculated maximum P–T estimates for the pyroxene granulite and enclosing felsic gneiss suggests that the two lithological types share a common history. The derived P–T paths for both samples shown on Fig. 10 indicate that they not only underwent equivalent high-P metamorphism, but also had an identical decompression and cooling history. It is, therefore, suggested that the granulite layers enclosed within the Gföhl gneiss do not represent exotic, tectonically
Conclusions
Petrographic and thermobarometric data indicate that the association of felsic gneisses and intercalated granulites within the basal part of the Austrian Gföhl Unit shows strong indications for being a coherent HP–HT body. The widespread lack of assemblages characteristic of HP–HT equilibration in felsic rocks is considered to be a function of deformation and metamorphic retrogression, combined with the localised effects of anatexis. The P–T estimates presented here, combined with those
Acknowledgements
Financial support to R.A.C from the Austrian Science Foundation, Project No. P12248GEO is gratefully acknowledged. P. Williams (Sheffield) and C. Bertoldi (Salzburg) are thanked for their technical assistance. D.A. Carswell, F. Finger and G. Friedl have over several years provided fruitful discussions and access to their Gföhl gneiss samples. L.G. Medaris, Jr. and an anonymous reviewer are thanked for their helpful comments, which improved the clarity of the manuscript.
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