Petrografía, mineralogía e isótopos estables en oficalcitas del entorno de Ziga (Región Vasco Cantábrica)

Resum

Cretaceous extension during the opening of the Bay of Biscay resulted in the creation of deep basins between Iberia and Eurasia, with exhumation of mantle rocks cropping out at present in several localities along the Pyrenean-Cantabrian belt. The westernmost outcrop is located near the village of Ziga (Navarra) and was traditionally associated to the Leiza Fault. In this work we present a petrographic, geochemical and isotopic study of these peridotites with the aim of gaining insight into their alteration and emplacement mechanisms. Peridotites crop out as metric-sized blocks along with fragments of other rock types (granulites, marbles, ophites, sandstones) in the junction between the Leiza and the Pamplona faults. The ultramafic rocks are highly serpentinized lherzolites with veins of calcite (ophicalcites). To check if these veins can be related to low-temperature alteration of serpentinized ultramafic rocks in contact with seawater, we analysed their carbon and oxygen isotopic signature. For comparison, we also analysed some ophicalcites from the Col d’Urdach peridotites (French Western Pyrenees) whose submarine exposure is well documented by field evidence. Results exhibit an isotopic signature typical of marine carbonates for the Col d’Urdach samples, while Ziga ophicalcites reflect the presence of a deeper carbon source.