Mantle-like Sr-Nd isotope composition of Fe-K subalkaline granites : the Peneda-Gerês Variscan massif (NW Iberian Peninsula)

Abstract

The Peneda-Gerês massif is one of the most representative NW Iberian late- to post-orogenic Variscan granitic plutons. It resulted essentially from the subsynchronous emplacement, at 290-296 Ma, of two granitic magmas of Fe-K subalkaline affinity, with primitive isotopic composition: Sri = 0.703-0.707 and eNdi = -1.5 to -2.4. An origin by mantle input followed by mantle-crust interactions is proposed, implying the contribution of a less enriched mantle component than that involved in the genesis of syn-orogenic hybrid granitoids of Mg-K subalkaline affinity. A less voluminous aluminopotassic and isotopically more evolved magma (Sri = 0.708-0.709 and eNdi = -3.5 to –3.9) with little or no mantle input was also generated, suggesting the involvement of lower crust materials. Therefore, this study suggests an input of juvenile magmas in late-Variscan times, the mantle-like isotope signature of Fe-K granitic magmatism being clearly related to a geodynamic setting of extensional processes, large-scale uplift and thinning.