Coesite in subducted continental crust: P-T history deduced from an elastic model

Coesite has been found in a metasedimentary rock in a continental unit of the Western Alps (Dora Maira massif). The rock preserves the high-pressure, low-temperature assemblage of nearly pure pyrope-talc-phengite-kyanite in a quartz matrix; coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz. Textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field. A simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix. Combined with petrological data the model constrains the crystallisation conditions of the pyrope-coesite rock (P > 28kbar, 650 < T < 830°C) and possible uplift paths. The paths derived suggest a mechanism of rapid decompression without any temperature increase, which must be related to tectonic processes.

This finding has an important bearing on the significance of P-T conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones.