PYROXENE MINERAL CHEMISTRY OF A MICROGABBRO DYKE FROM VILA NOVA DE FOZ COA (NORTHERN PORTUGAL)

Abstract

This work is a mineral chemistry study of the pyroxene crystals composing a microgabbro dyke found in the Vila Nova de Foz Coa (VNFC) region of northern Portugal. Several pyroxene geothermometers and geobarometers have also been employed to infer the likeliest thermodynamic conditions of crystallization. Following the IMA 1988 report for the nomenclature of pyroxene minerals, the VNFC microgabbro is rich in Ca-Mg-Fe species. The studied clinopyroxenes are aluminian, chromian, and/or ferroan diopside and augite, whereas the existing orthopyroxenes are ferroan enstatite. The pyroxene compositional data suggest that the analyzed clinopyroxenes were generated from an OIB-like alkaline melt, at low to intermediate pressures, and high fO2 conditions, associated with a non-orogenic and presumably riftrelated setting. Furthermore, the clinopyroxene grains of the VNFC microgabbro probably crystallized under pressure conditions ranging from 5 to 13 kbar and temperatures between 1130 to 1187�C. The variations concerning the pressure estimates may be related to the isothermal decompression phenomenon, typically associated with dyke-type intrusions. Based on these estimates and assuming crustal densities of 2700 to 2890 kg/m3, the likeliest depths at which clinopyroxene formed vary between 49 to 16 km, with most grains crystallized at depths ranging from 49 to 40 km.