Distinct sources for syntectonic Variscan granitoids: insights from the Aguiar da Beira region, Central Portugal

Abstract

The Variscan syntectonic granitoid plutons from the Aguiar da Beira region (central Portugal) were emplaced into metasediments of Late Proterozoic-Early Cambrian age during the last Variscan ductile tectonic event (D3), which is related to dextral and sinistral shearing. The older intrusion, with a U-Pb ID-TIMS zircon age of 321.8±2.0Ma, consists of porphyritic biotite granodiorite-granite with transitional I-S type geochemical signature, relatively low 87Sr/86Sr322 ratios (0.7070-0.7074), εNd322 values of -3.9 to -4.6 and whole-rock and zircon δ18O values of 10.6‰ and 8.0‰, respectively. By contrast, the younger intrusion is an S-type muscovite-biotite leucogranite, emplaced at 317.0±1.1Ma, showing more radiogenic 87Sr/86Sr317=0.7104-0.7146, lower εNd317 values of -7.7 to -8.7 and higher δ18O-wr=11.3‰ and δ18O-zr=9.5‰. The combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite-granite, involving the anatexis of lower crustal metaigneous protoliths, and possible hybridization with mantle-derived magmas. A shallower origin, at mid crustal levels, from pure crustal derivation, through moderate degrees of partial melting of Proterozoic-Cambrian metasediments is proposed instead for the muscovite-biotite leucogranite. © 2014 Elsevier B.V.