Anticlockwise P-T Path of Granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain)

Abstract

The study of mafic and aluminous granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain) reveals an anticlockwise P–T path that we interpret as related to the tectonothermal activity in a magmatic arc, probably an island arc. The P–T path was obtained after a detailed study of the textural relationships and mineral assemblage succession in the aluminous granulites, and comparing these with an appropriate petrogenetic grid. Additional thermobarometry was also performed. The granulites are highly heterogeneous, with distinct compositional domains that may alternate even at thin-section scale. Garnets are generally idiomorphic to subidiomorphic, and in certain domains of the aluminous granulites they show overgrowths forming xenomorphic coronas around a more or less idiomorphic core. Both types of garnets show significant Ca enrichment at the crystal rims, which, together with the other mineralogical and textural characteristics, is compatible with a pressure increase with low T variation. P–T estimations indicate a peak of T > 800°C and P ∼ 9·5 kbar, attained after a significant increase in pressure that took place at high temperatures (in the sillimanite field). We suggest that this kind of trajectory, probably anticlockwise, is compatible only with a terrane heated by an intense magmatic activity after or during tectonic crustal thickening (magmatic injection at the base and/or into the crust), which is characteristic of magmatic arcs.