PODIFORM CHROMITITE-BEARING ULTRABASIC ROCKS FROM THE BRAGANCA MASSIF, NORTHERN PORTUGAL - FRAGMENTS OF ISLAND-ARC MANTLE

The Upper Allochthonous Thrust Complex (UATC) of the Braganca massif i n northern Portugal contains a set of ultrabasic rocks interthrust wit h granulites. The ultrabasic rocks have refractory silicate mineral an d whole rock compositions which indicate an origin as depleted mantle. Phase relationships of harzburgite samples suggest that they formed i n equilibrium with high-Mg picritic melts created through a high degre e of mantle partial melt extraction. Chromite in small podiform deposi ts has 100 Cr/(Cr+A1) ratios of 62-85, which are consistent with cryst allization from such melts. Most of the chromite composition parameter s are similar to those of ophiolite deposits except for the high ferri c iron contents (2.77-8.95 wt% Fe2O3). Such enrichment is a feature of chromite from island are magmas. It is suggested that the extensive p artial melt extraction and chromite mineralization in the ultrabasic r ocks occurred in the upper few kilometres of island are mantle. The ul trabasic rocks were tectonically emplaced into a granulite and eclogit e-bearing are-continent collision complex during the Early Ordovician and subsequently, in the mid-Devonian emplaced over the Central-Iberia n terrane.