Caracterización petrológica y geoquímica de las rocas ultramáficas del macizo de Herbeira, complejo de Cabo Ortegal (NO de España)

Abstract

The uppermost unit in Cabo Ortegal complex is composed of ultramafic rocks that crop out in three massifs: Herbeira, Limo and Uzal. Herbeira massif shows two different areas separated by a NNE-SSW fault zone. The western area is formed by a layered complex of dunites and pyroxenites which lies on top of a basal harzburgite. Amphibole-bearing peridotites and hornblendites; with aluminous spinels (Cr#<0.4), are the main lithologies in the eastern area. Hornblende is invariably present in all types of ultramafic rocks in the Herbeira massif, but the eastern peridotites mostly contain more than 15% of amphibole. Almost all ultramafic units show light rare earth elements (LREE) enriched patterns, however the basal harzburgite shows LREE-depleted patterns with negative slope while the rocks from layered complex are LREE enriched. Both ultramafic rocks from Herbeiia present marked significant Nb and Zr negative anomalies. The mineral chemistry, the REE abundances, geological and petrological data for Herbeira suggest a open-system melting to explain the formation of ultramafic rocks. The appropriate geodynamic scenario is a mantle wedge above a supra-subduction zone where the influx of an exotic fluid/melt generated in the upper parts of the subducting slab produces melting and melt extraction in the overlying mantle wedge.