Preliminary report on the excess argon bearing K-feldspar from metagranite in the Brossasco-Isasca UHP Unit of Dora-Maira Massif, Italy

Abstract

The metagranite, that still preserves the igneous structure, is composed mainly of K-feldspar, plagioclase, quartz, biotite, white mica and accessory ilmenite. Quartz, K-feldspar, plagioclase and biotite preserve the original igneous shape, but are either re-equilibrated or replaced by new phases. Quartz occurs as fine-grained granoblastic aggregate, statically derived from inversion of coesite. Plagioclase is now composed of a fine- grained mineral aggregate including albite, zoisite, phengite, titanite and apatite. Biotite is surrounded and partly replaced by fine-grained white mica. K-Ar analyses of K-feldspar from the metagranite were carried out, giving 42.6 ± 0.9 Ma. This age, significantly older than the SHRIMP zircon U-Pb age (35.4 ± 1.0 Ma) previously estimated for the metamorphic peak of the Brossasco-Isasca UHP unit, is apparent due to inherited excess argon (~ 3.4%) from the host lithologies. This inherited excess argon is interpreted as related to the fact that K-feldspar has trapped the excess argon wave generated by the argon release from micas (that have large amount of radiogenic argon) during exhumation and cooling of the host lithologies.