Los metagranitos de las Unidades Basales de los complejos alóctonos de Galicia (NO del Macizo Ibérico)Evolución geoquímica y contexto geodinámico

Abstract

The Basal Units of the allochthonous complexes of Galicia include para-derived metamorphic rocks alternating with abundant ortho-derived rocks with protolith ages ranging between 500 and 470 Ma. According to their whole -rock chemical and mineralogical composition, the ortho-derived rocks are divided in two major groups: one consisting of granitic gneisses s.l. (tonalitic, granodioritic, and granitic) with a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types, and a second group including alkaline and peralkaline terms, with ferroan alkalic and peralkaline to metaluminous composition, some of the most differentiated terms reaching also a peraluminous composition. The granitic gneisses s.l. show negative Nb and Ta anomalies, typical for rocks formed in an active margin, whereas the alkaline and peralkaline rocks are rich in incompatible trace elements and show a high content of Th, Ga and Nb, as expected for crustal extension-related magmas.This geochemical evolution probably reflects the switch from a convergent geodynamic setting featured by magmatic arcs to an intracontinental rifting.