The ultra-high pressure coronitic and pseudomorphous reactions in a metagranodiorite from the Brossasco-Isasca Unit, Dora-Maira Massif, western Italian Alps: a petrographic study and equilibrium thermodynamic modelling

Metagranodiorite samples from the Brossasco-Isasca Unit, Dora-Maira Massif, western Alps, show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra-high pressure (UHP) metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K-feldspar, biotite and minor phases. During UHP metamorphism, the plagioclase (site P) was replaced by zoisite, jadeite, quartz, K-feldspar and kyanite, and coronitic reactions developed between biotite and adjacent minerals. At the original igneous biotite-quartz contact (site A), a single corona of poorly zoned garnet is developed, whereas at the biotite-K-feldspar (site B) and biotite-plagioclase (site C) contacts, composite coronas are formed. Integration of results from petrographic observations, calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions, which reached as high as 24 kbar and 650 degreesC. Accurate microanalysis by energy-dispersive spectroscopy (EDS) and statistical analysis of the data allowed us to identify, for the first time in a natural Na-pyroxene of metagranitoid rocks, the end-member Ca-Eskola.