CORDIERITE-BEARING MIGMATITES FROM THE VELKA-FATRA MTS, WESTERN CARPATHIANS - GEOTHERMOBAROMETRY AND IMPLICATIONS FOR VARISCAN DECOMPRESSION

Cordierite-bearing migmatites are found in the pre-Mesozoic crystallin e basement of the Velka Fatra Mts., Central Western Carpathians as xen oliths and blocks in the granitoid pluton. Cordierite with Fe/Fe + Mg = 0.4 occurs in the assemblages with sillimanite, biotite, plagioclase , quartz, muscovite and ilmenite. Relies of spessartine-rich garnet ar e only sporadically preserved, replaced by cordierite porphyroblasts, therefore the origin of cordierite is related mainly to the breakdown of garnet, according to the reaction 2 garnet + 4 sillimanite + 5 quar tz = 3 cordierite. On the basis of mineral composition, conventional a s well as internally consistent geothermobarometry (TWEEQU method), co rdierite originated at pressure conditions below 4-5 kbar and a temper ature near 600 degrees C, as a consequence of nearly isothermal decomp ression. The decompressional P-T path of cordierite-bearing migmatites was associated with the rapid (ca. 2 mm/yr) emplacement of the granit oid pluton during late Variscan (342-338 Ma) time, which was most prob ably facilitated by tectonic extension.