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.