The P-T evolution of ultra high temperature garnet-bearing ultramafic rocks from the Saxonian Granulitgebirge Core Complex, Bohemian Massif

Garnet-bearing ultramafic rocks (GBUR) enclosed in granulite or high-grade gneiss are rare, yet typical constituents of alpine-type collisional orogens. The Bohemian Massif of the European Variscides is exceptional for the occurrence of a large variety of mantle-derived rocks, including GBUR (garnet peridotite and garnet pyroxenite). GBUR occur in several metamorphic units belonging to both the Saxothuringian and the Moldanubian zones of the Bohemian Massif. The northernmost outcrops of GBUR in the Bohemian Massif are situated in the Saxonian Granulitgebirge Core Complex in the Saxothuringian zone and are the subject of this study. Thermobarometric results and exsolution textures imply that the Granulitgebirge GBUR belong to the ultra high temperature group of peridotites. They experienced a decompression-cooling path being constrained by the following four stages: (i) similar to 1300-1400 degrees C and 32 kbar, (ii) 1000-1050 degrees C and 26 kbar, (iii) 900-940 degrees C and 22 kbar, and (iv) 860 degrees C and 12-13 kbar. Occasional layers of garnet pyroxenite within GBUR lenses are interpreted as high pressure cumulates that crystallized at 32-36 kbar by cooling below 1400 degrees C. The GBUR were most probably derived from upwelling asthenosphere and came in contact with crustal granulite at similar to 60 km depth. Slab break-off is suggested here as the most probable cause for: (i) asthenosphere upwelling and cooling of the latter as well as (ii) ultra high temperature granulite facies metamorphism of the crustal host rocks. The Granulitgebirge-type peridotite is very similar to the Mohelno-type peridotite from the Gfohl unit, Moldanubian zone, in the southern part of the Bohemian Massif. In contrast, peridotite from the adjacent Erzgebirge (also within the Saxothuringian zone) is derived from the subcontinental mantle and much resembles the Nove Dvory-type peridotite from the Gfohl unit (Moldanubian zone). The fact that the Saxothuringian and Moldanubian zones host the same types of mantle rocks (asthenospheric and lithospheric) of the same metamorphic ages suggests that the classic distinction into the Saxothuringian and Moldanubian zones cannot be supported, at least as far as high-grade units hosting GBUR are concerned.