Constraints of mantle and crustal sources and interaction during orogenesis: A zircon SHRIMP U-Th-Pb and O isotope study of the ‘calc-alkaline’ Brovales pluton, Ossa-Morena Zone, Iberian Variscan Belt

Abstract

Magmas generated by melting of the continental crust and the mantle during collapse of collisional orogenic belts are compositionally heterogeneous and preserve evidence for mixing of the two magma sources. Deciphering the components of hybrid magmas is the key for assessing the relative contributions of mantle and crustal sources during orogenesis. Variscan plutons in the Ossa-Morena Zone are unusual, and particularly useful, because they comprise numerous mafic-intermediate-felsic intrusions generated in an intra-orogenic extension-related event, suggesting a complex interplay between mantle and crustal processes. In particular, the Brovales pluton represents a hybrid ‘calc-alkaline’ rock suite generated by the magmatic interaction between mantle- and crustal-derived magmas. The new SHRIMP U-Th-Pb zircon age of the Brovales pluton presented here, 340 ± 2 Ma, is unrelated to the subduction setting. By contrast, the new age links the pluton with the OMZ Variscan intra-orogenic episode of extension-transtension associated with a period of high temperature and low pressure metamorphism. In this scenario, the mid-crustal intraplating of mantle-derived magmas apparently caused heat transfer provoking crustal fusion and allowed the interaction of compositionally contrasted but rheologically similar magmas. The interaction of the mantle and crustal magmas in Brovales pluton is reflected in its whole- rock, petrographic, mineral chemistry and O isotopic compositions in zircons that preserve the compositional changes from alkaline mantle-derived to hybrid ‘calc-alkaline’ magmas. Identifying the components of hybrid ‘calc-alkaline’ intra-orogenic extension-related magmatism is the first step for a better understanding the pro- cesses that control recycling and crustal growth.