GARNET-BEARING ULTRAMAFIC ROCKS FROM THE ERZGEBIRGE, AND THEIR RELATION TO OTHER SETTINGS IN THE BOHEMIAN MASSIF

Garnet-bearing ultramafic rocks (GBU) enclosed in high-grade gneisses are known from several parts of the Bohemian Massif. One of these is t he high-pressure(HP)-unit 1 in the Erzgebirge Crystalline Complex, whi ch is the subject of the present study. Hitherto, two different models have been put forward to explain the stabilisation of garnet in mantl e-derived ultramafic rocks from the Bohemian Massif and their emplacem ent into the crust. (1) Garnetiferous assemblages were formed in the u ltramafics before they came in contact with their crustal host rocks. (2) Garnet was formed in the ultramafics at the expense of spinel due to cooling caused by their tectonic emplacement in the crust. The PT-e volution revealed by the investigated GBU from the Erzgebirge Crystall ine Complex, however, requires a third model. The reconstruction of th e PT-paths for the Erzgebirge GBU is based on both conventional thermo barometry and phase relations. Thermodynamic calculations allowed the construction of a PT-phase diagram for the system Na2O-CaO-MgO-Al2O3-S iO2H2O, which is the first quantitative petrogenetic grid in this mode l system relevant to ultramafic HP-rocks. The grid shows the uni-, di- , and tri-variant assemblages stable in peridotitic rocks at different PT-conditions, providing a tool to constrain PT-paths from the succes sion of mineral assemblages observed in a rock. The PT-path obtained f or the Erzgebirge GBU suggests that the garnet-bearing assemblages for med by HP-meta-morphism of spinel peridotite which was emplaced into t he crust prior to or during the HP-compressional stage. This model is supported by peak PT-conditions around 900 degrees C and 30-35 kbar re corded by the ultramafic rocks, which are very similar to those attain ed in the eclogites sensu stricto which occur in the same tectonic uni t (HP-unit 1) and for which an in situ metamorphism has been inferred (Schmadicke et al. 1992). On the other hand, the other two high-pressu re units in the Erzgebirge, HP-units 2 and 3, which also contain eclog ites sensu stricto but lack peridotites, record lower peak PT-conditio ns of 650-750 degrees C/24-26 kbar and 600-650 degrees C/20-24 kbar, r espectively. Postulating an in situ HP-metamorphism for the garnet per idotites as result of continental collision during the Variscan orogen y, a crustal thickness of 90-110 km would be required. A comparison of the distribution of eclogites sensu stricto and mantle-derived rocks from the Bohemian Massif together with their reported PT-conditions re veals a correlation between peak PT-conditions in eclogites sensu stri cto and the prevailing assemblages in the ultramafic rocks in the same unit. Furthermore, the Erzgebirge Crystalline Complex and the Sniezni k Complex on one hand as well as the Granulitgebirge and the Sowie Gor y on the other hand are thought to be genetically linked. The garnet p eridotites from the Granulitgebrige, the Sowie Gory and the Gfohl unit seem to have experienced peak PT-conditions not recorded by their cou ntry rocks with non-eclogitic basic interlayers, inferring a formation of HP-assemblages in the ultramafics prior to their emplacement into the crust.