Variscan high P-T metamorphism and uplift history in the Moldanubian Zone of the Bohemian Massif in Lower Austria
Carswell, Dennis A.
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 323 - 342
published: Apr 18, 1991
manuscript accepted: Jul 20, 1990
manuscript received: Oct 27, 1989
ArtNo. ESP147050302007, Price: 29.00 €
Abstract The uppermost thrust sheet of the Moldanubian Zone in Lower Austria (the Granulite Complexes within the Gfôhl Unit) comprises a unusual association of metaperidotite bodies (with lenses of metabasites) and relatively homogeneous quartzo-feldspathic granulites. Whole-rock compositions indicate an upper-mantle origin for the former, whilst the granulites are interpreted to have been mainly derived from acid-intermediate igneous rocks differentiated along a calc-alkaline trend. A six-stage evolutionary history has been defined for the metaperidotite bodies with early Variscan (350-370 Ma) garnetiferous assemblages which equilibrated at 1050±20°C and 31 ±3 kbar, variably replaced by later, lower P-T assemblages. The latter have been used to establish a P-T-time uplift path for these rocks, making due allowance for difficulties in recognition of true chemical equilibrium between mineral phases for thermobarometric assessment because of the presence of multiple mineral growth generations and of appreciable intragranular compositional zoning. Rarely preserved exsolution features suggest that the high-pressure garnetiferous peridotites originally formed from higher-temperature protolith assemblages (with more highly aluminous pyroxenes) which probably crystallised on the peridotite solidus in the spinel stability field. The enclosing acidic granulites also partly retain relatively high-pressure assemblages, most typically of garnet +kyanite + mesoperthite + quartz. However, the lack of evidence for stability of true eclogites in scarce associated metabasic rocks is taken to indicate that the mantle-derived garnetiferous peridotite bodies were only juxtaposed with the crustal granulites later (at 1000°C) assemblages predating garnet formation in the metaperidotite bodies, and the subsequent spatial association with calc-alkaline meta-igneous granulites, are considered to reflect protolith formation close to a continental plate margin (possibly a major back-arc basin) during an important episode of lithospheric extension which must have preceded the main compressive phase of the Variscan orogeny. The latter is thought to have been responsible for the tectonic intercalation of crustal and mantle rocks, the establishment of a thick crustal thrust/nappe stack, and the stabilisation (and subsequent exhumation) of the high-pressure rocks. This study emphasises the important distinction between the Moldanubian-Zone early Variscan high-pressure granulites, associated with garnet-bearing metaperidotite bodies, and the common lower-pressure Variscan granulites observed elsewhere. The latter record late Variscan (ca. 320 Ma) conditions at lower structural levels in the crustal nappe pile and are in contrast depleted in L1L elements due to anatexis and granitic melt removal.