Elsevier

Lithos

Volume 58, Issues 1–2, August 2001, Pages 33-54
Lithos

Resolving the relationship between high PT rocks and gneisses in collisional terranes: an example from the Gföhl gneiss–granulite association in the Moldanubian Zone, Austria

https://doi.org/10.1016/S0024-4937(01)00049-4Get rights and content

Abstract

The increasing recognition of high PT metamorphic conditions in crustal rocks has lead to a major reappraisal of the processes inherent to continental collision. Within high-grade regions in orogenic belts, a particular problem is that bodies of high PT rocks are often found preserved within an apparently lower grade gneissose matrix. Resolving the true extent of high PT conditions in such situations requires a detailed examination of the mineral assemblage and compositional evolution in spatially related lithologies, so that realistic geothermobarometric comparisons can be made.

Within the Gföhl Unit in Austria, an integral part of the Moldanubian Zone of the Variscan orogen of Europe, granulites containing the high-P assemblage garnet+clinopyroxene+plagioclase+quartz are found intercalated within the voluminous Gföhl gneiss. Calculated PT conditions for the granulites indicate peak metamorphic equilibration at 14–16 kbar, 950–1050 °C. Post-peak, close to isothermal decompression to a medium-P orthopyroxene-bearing overprint at 7–8 kbar, 800–870 °C, was followed by cooling at <6 kbar, 800–500 °C. In the host felsic gneiss, an early high-P assemblage of garnet+kyanite+ternary feldspar+quartz can be recognised in rarely preserved, less deformed samples. Calculated peak PT conditions for this assemblage are 14–16 kbar, >950 °C, whilst temperature estimates for a widespread biotite+sillimanite retrograde overprint range from 700 to 500 °C, at a minimum pressure of ca. 4 kbar. The coherence in metamorphic conditions and derived PT paths from both rock types indicates that the granulite–gneiss association represents a coherent high PT unit, with the felsic gneisses being a retrogressive product of former leucocratic granulites. The lack of preservation of high PT assemblages in many parts of the gneiss is considered to be a function of pervasive deformation and associated fluid influx, together with the effects of syn-deformational migmatisation along basal parts of the unit during consolidation of the Moldanubian nappe pile.

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 PT estimates for the pyroxene granulite and enclosing felsic gneiss suggests that the two lithological types share a common history. The derived PT 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 PT 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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