HIGH PRESSURE WATER-FLUXED MELTING OF LOWER FELSIC CRUST IN CONTINENTAL SUBDUCTION: EXAMPLE FROM EGER CRYSTALLINE COMPLEX, BOHEMIAN MASSIF

The Eger crystalline unit in Bohemian Massif is dominated by high-grade anatectic rocks as gneisses, migmatites, granites and granulites. They represent felsic orogenic lower crust that was rapidly exhumed at rate of1.1-2.5 mm/year during the Variscan orogeny at around 340 Ma along the Saxothuringian suture zone. Peak metamorphic conditions for these anatectic gneisses and granulites were estimated at ca. 740-845°C and 14-16 kbar. The gneisses and granulites reveal different degrees of migmatization from stromatic migmatites, inhomogeneous diatexites to isotropic granitic gneisses and granites. Field relationships suggest that these rocks are all derived from the same protolith and represent a continuous sequence. In this study, we aim to understand timing, conditions, type and consequences of the partial melting in a subduction setting. Based on mineral assemblage and composition we suggest these rocks underwent extensive hydration and subsequent anatexis at high pressures. Locally, the migmatites contains up to 25% of phengite that remained stable during partial melting. Thus we conclude that these rocks underwent water-fluxed melting at these high pressures, and the weakening associated with partial melting has assisted the rapid ascent of these lower crustal rocks. No further melting occured during decompression in mid-crustal levels. Moreover, the resulting melt caused brittle failure of such rocks in the presence of melt has not been previously described and we postulate that it was triggered by high melt pressures coupled with the volume increase of the melting reaction together with increased pore pressure due to pervasive melt migration.