VERY-HIGH PRESSURE METAMORPHISM OF THE BROSSASCO CORONITE METAGRANITE, SOUTHERN DORA-MAIRA-MASSIF, WESTERN ALPS

The Brossasco metagranite, Dora Maira Massif, Western Alps, underwent (early?) Alpine very-high pressure metamorphism and (meso/late?) Alpine amphibolite to greenschist-facies retrogressions. The Brossasco metagranite is part of the Brossasco Isasca unit of the southern Dora Maira Massif, in which pyrope + talc + coesite assemblage and kyanite eclogite indicate temperatures greater than 700-degrees-C and pressures in excess of 2.8 GPa. The Brossasco metagranite consists of small, relatively undeformed volumes of porphyritic granite separated by anastomosing mylonitic shear zones. The undeformed volumes grade into augen gneiss. The augen gneiss lacks any evidence of eclogite facies metamorphism. The granite magmatic assemblage consisted of K-feldspar phenocrysts, plagioclase, quartz and biotite. The eclogitic assemblage is jadeite, garnet, phengite, zoisite, rutile and possibly coesite. The complex retrograde metamorphic evolution produced a peculiar mineral association with an igneous appearance. Late metamorphic plagioclase and biotite replaced the eclogitic minerals developed after magmatic plagioclase and biotite. Chemical compositions and zoning of jadeite, biotite, phengite and garnet are investigated in detail. Zoning of the metamorphic phases, despite the relatively high metamorphic temperature, indicates a low rate of inter- and intra-crystalline diffusion. Phengite, which developed at the expense of the igneous K-feldspar, shows a progressive decrease in Tschermak substitution with time. The zoning is interpreted as evidence for progressively decreasing activity of H2O in a closed system.