P-T PATH DETERMINATIONS IN THE TORMES GNEISSIC DOME, NW IBERIAN MASSIF, SPAIN

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of ex tensional deformation (D2). The syn-D2 P-T path of the Lower Unit of t he TGD was deduced from the analysis of reaction textures related to s uperimposed fabrics developed during exhumation, analysis of mineral z oning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the TWEEQU thermobarometric techni que, from 6.4-8.1 kbar at 735-750 degrees C (upper structural levels) and 7.2 kbar at 770 degrees C (lower structural levels) to 3.3-3.9 kba r and 645-680 degrees C. This evolution is consistent with the observe d sequence of melting reactions and the generation of garnet- and cord ierite-bearing anatectic granitoids. The later part of the syn-D2 P-T path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segm ent of the P-T path is recorded by assemblages with And + Bt + Ms and Ms + Chl + Ab related to the later mylonitic S2 fabrics, which indicat e retrogression to low-amphibolite and greenschist facies conditions.