35 MA OLD ULTRAHIGH-PRESSURE METAMORPHISM AND EVIDENCE FOR VERY RAPIDEXHUMATION IN THE DORA-MAIRA MASSIF, WESTERN ALPS

Ion microprobe (SHRIMP) data on zircons from various rock types of an ultrahigh-pressure (UHP) metamorphic whiteschist-type pyrope quartzite lens of the Dora Maira Massif (DMM) consistently show domains giving a Late Eocene age of 35.4 +/- 1.0 Ma which is taken as the age of UHP metamorphism. These domains partially replace the older oscillatory zo ning pattern of the zircons formed during primary magmatic crystalliza tion at about 275 Ma. Zircons of a metagranitic country rock next to t he UHP metamorphic lens have these primary features best preserved. Al l zircons measured also yield intermediate 'ages' between 275 and 35 M a with a statistical concentration between 260 and 210 Ma. Thus the un iformity of the initial zircon population both in the lens and the cou ntry rock evidences a common protolith, that is a granite intruded dur ing the Late Herynian. While the intermediate ages are at least partly due to incomplete resetting of the zircons during UHP metamorphism, t hose in the 260-210 Ma range may be related to rifting episodes in the Permotriassic. The Mg-rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite, perhaps by fluids de rived from evaporitic sediments dating as early as the Permotriassic a s well. The more pervasive resetting of zircon ages during UHP metamor phism in the pyrope quartzite lenses is explained by the ubiquity of f luids and/or melts produced during subduction by a series of dehydrati on reactions that occurred only in the more hydrous Mg-rich protoliths and not in their drier granitic neighborhood. Fission track ages dete rmined partly on the same zircon samples yielding 29.9 +/- 1.4 Ma mark the time when the UHPM unit had reached about 290 degrees C at a shal low location within the crust. Thus exhumation over a vertical distanc e of about 120 km must have occurred within about 5-6 Ma indicating an average uplift rate of about 20-24 km/Ma and an average cooling rate of about 85-100 degrees C/Ma. The radiometric data obtained do not len d any support to an Eo-Alpine Cretaceous subduction event so that deep subduction and immediately following exhumation all took place during Early to Mid Tertiary time. This scenario seems to apply to large por tions of the Western and Central Alps as well calling for drastic geod ynamic reinterpretations of these parts of the Alps. (C) 1997 Elsevier Science B.V.