Combined zircon UPb and LuHf isotopes study of magmatism and high-P metamorphism of the basal allochthonous units in the SW Iberian Massif (Ossa-Morena complex)

Abstract

UPb and LuHf isotope ratios measured in zircon crystals from magmatic and metamorphic rocks provide crucial information to understand the origin and tectonothermal evolution of allochthonous terranes, and can contribute to paleogeographic reconstructions and to the understanding of orogenic processes. We investigate the age of magmatic and tectonothermal events, and the provenance of a high-pressure unit from the Ossa-Morena Complex (SW Iberian Massif). Zircons for LA-ICP-MS analyses were separated form garnet-bearing amphibolites (retroeclogites) and quartzo-feldspathic gneisses from the Central Unit (Coimbra-Córdoba shear zone). Recent geological correlations propose that the high-pressure units cropping out in the Ossa Morena Complex of the SW Iberian Massif are equivalent to the Basal Allochthonous Units of NW Iberian Massif, and one of the objectives of the study is to check the validity of the proposed correlation.

The new obtained ages presented in this study indicate that the high-pressure metamorphism of the Central Unit developed in the Variscan cycle. The metamorphic zircons reveal two distinct events: a younger one well constrained by an age of 339 ± 4 Ma and an older one of 377 ± 19 Ma. The former coincide with the age of the high-grade and post-high-P metamorphism described in other areas of the Ossa-Morena Complex, whereas the second one is interpreted as the age of the high-P metamorphism. The igneous ages of metabasites and metagranitoids (c.585–615 Ma and c.480–577 Ma, respectively) reveal a diachronism between magmatic events in Allochthonous Units from the SW and the NW Iberian Massif, suggesting that both domains were located in different parts of the North-Gondwanan margin. The magmatic ages of the metagranitoids resemble the evolution described for the lower paleozoic rifting in the Ossa-Morena Complex. The relatively juvenile character of the basic and acid Cadomian magmatism in the Central Unit, revealed by the new LuHf data, indicate limited mixing of mantle melts with crustal components, which is compatible with a generation in a continental magmatic arc. These similarities in the metamorphic and magmatic evolution support the correlation of the Basal Allochthonous Units from the SW and NW domains of the Iberian Massif.