New Constraints on the P–T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)

Abstract

The metamorphic evolution of the garnet peridotite body of Alpe Arami, Central Alps, is a matter of current controversy. In this paper, the inter- and intragrain distribution of major and trace elements obtained by electron and ion probe microanalyses is used to better constrain the P–T evolution of this peridotite. Using the compositions of homogeneous porphyroclast cores, peak metamorphic conditions of 1180 ± 40°C and 5·9 ± 0·3 GPa are estimated, based on consistent results from the application of several independent thermometers (Fe–Mg exchange between garnet, pyroxenes and olivine, Ni exchange between garnet and olivine, Co and Ni exchange between orthopyroxene and clinopyroxene), the Al-in-orthopyroxene barometer and the Ca–Cr systematics of garnet. Orthopyroxene and clinopyroxene porphyroclasts are, however, not in equilibrium with respect to some elements with low diffusivities, such as Ca, Ti, Cr, V and Sc. This disequilibrium appears to be the main cause for the lower P–T values suggested by some of the previous workers. On the other hand, there is no evidence for an ultradeep (>200 km) origin of the Alpe Arami body as postulated recently. Chemical zonation profiles across mineral grains suggest that during retrograde evolution a near-isothermal decompression was followed by accelerated cooling.