Magmatic Evolution and Tectonic Setting of the Iberian Pyrite Belt Volcanism

Abstract

The Iberian Pyrite Belt, which extends from Portugal to Spain in southwest Iberia, constitutes one of the world's largest reservoirs of massive sulphide deposits. Volcanic-hosted massive sulphide mineralization occurs at several stratigraphic horizons within an Early Carboniferous volcano-sedimentary package formed of turbiditic siliciclastic deposits and basaltic, intermediate and silicic volcanic rocks. Volcanic rocks do not show significant temporal or spatial variations in the stratigraphic sequence of the Iberian Pyrite Belt and mainly occur as shallow intrusions into wet marine sediments with some minor lavas, hydroclastic rocks and volcanogenic sediments. A geochemical study, including major, trace and rare earth elements, and Sr and Nd isotopes, of the least altered volcanic rocks has been carried out to determine the primary magmatic affinity and tectonic setting of the Iberian Pyrite Belt volcanism. Most of the basaltic rocks are continental tholeiites, but a few samples show an alkaline affinity. The origin of the basaltic rocks and their diversity of compositions are explained by a single mixing model between E- and N-MORB (mid-ocean ridge basalt) and assimilation of crustal material. Calc-alkaline intermediate and silicic rocks include basaltic andesites, andesites, dacites and thyolites. Volumetrically, dacites and thyolites are the most abundant. Intermediate and silicic rocks are not related by fractional crystallization, nor is there a relationship between the basaltic and calc-alkaline rocks by the same process. We suggest that in the Iberian Pyrite Belt silicic calc-alkaline magmas were generated on a large scale by the invasion of continental crust by mafic magmas generated in the underlying upper mantle. The diversity of compositions shown by dacites and thyolites can mainly be explained either by differences in the composition of the source rocks or by different degrees of partial melting of upper-crust rocks. Andesites, however, formed by mixing between basaltic magmas and upper-crust material. The new geochemical data agree with previously published tectonostratigraphic data which suggest that the Iberian Pyrite Belt volcanism formed on the South Portuguese plate owing to strike-slip tectonics. This local extensional tectonic setting was related to transtension as a result of oblique continental collision that followed the subduction of the South Portuguese plate beneath the Ossa Morena plate. This tectonically driven magmatism does not have a modern analogue, but it is not inconsistent with the proposed geodynamic evolution of the studied area. This model gives insights into the petrology and geochemistry of strike-slip settings in the continental part of sub-ducting plates, a region usually poorly constrained from a petrological point of view.