Podiform chromitite-bearing ultrabasic rocks from the Bragança Massif, northern Portugal: fragments of island arc mantle?

Abstract

AbstractThe Upper Allochthonous Thrust Complex (UATC) of the Bragança massif in northern Portugal contains a set of ultrabasic rocks interthrust with granulites. The ultrabasic rocks have refractory silicate mineral and whole rock compositions which indicate an origin as depleted mantle. Phase relationships of harzburgite samples suggest that they formed in equilibrium with high-Mg picritic melts created through a high degree of mantle partial melt extraction. Chromite in small podiform deposits has 100 Cr/(Cr + Al) ratios of 62–85, which are consistent with crystallization from such melts. Most of the chromite composition parameters are similar to those of ophiolite deposits except for the high ferric iron contents (2.77–8.95 wt% Fe2O3). Such enrichment is a feature of chromite from island arc magmas. It is suggested that the extensive partial melt extraction and chromite mineralization in the ultrabasic rocks occurred in the upper few kilometres of island arc mantle. The ultrabasic rocks were tectonically emplaced into a granulite and eclogite-bearing arc-continent collision complex during the Early Ordovician and subsequently, in the mid-Devonian emplaced over the Central-Iberian terrane.