Syntectonic Variscan magmatism in the Aguiar da Beira region (Iberian Massif, Portugal)

Abstract

The Aguiar da Beira region (Portugal) is located in the core of the Iberian Massif, more precisely in the Central-Iberian Zone, which is dominantly composed by abundant volumes of plutonic rocks, emplaced into Late Proterozoic – Early Cambrian and Palaeozoic metasediments, mainly during or slightly after the third deformation phase of the Variscan Orogeny (D3). A considerable amount of these granitoids are syntectonic, intruded during the peak of this deformation event (D3). In particular, at the Aguiar da Beira region, there are two syntectonic granitoids that represent two distinct magmatic series: a medium- to coarse-grained porphyritic biotite granodiorite-granite (322 Ma), which belongs to the early granodiorite series, and a medium-grained muscovite-biotite granite (317 Ma) that is part of the two-mica peraluminous leucogranites suite. The petrographical, geochemical (whole-rock and mineral compositions) and isotopical (Sr-Nd, 18O-wr and 18O-zr) study of the two intrusions revealed their remarkably different character, and allowed to conclude that they correspond to two independent magma pulses, derived from distinct sources and petrogenetic processes. The biotite granodiorite-granite is a weakly peraluminous intrusion, characterized by intermediate to felsic SiO2 contents (66 – 68%), high Ba, Sr and REE, and high Al and Mg biotite contents, typical of the calc-alkaline associations. The Sr-Nd initial ratios are homogeneous and overlap the lower crustal felsic metaigneous granulites signatures (Villaseca et al. 1999) which might suggest an origin by the anatexis of lower felsic metaigneous rocks, and is further supported by 18O-wr and 18O-zr data. However, the data, allied to the presence of microgranular enclaves present in this intrusion also raises the hypothesis of an origin by mixing of lower crustal derived magmas and mantle melts. Based on the available data these seem to be the two genetic scenarios that can best explain the geochemical signature of the biotite granodiorite-granite, not being possible to opt for any of the options. By contrast, the muscovite-biotite granite has an entirely distinct geochemical signature, typical of S-type granites: a highly evolved and strongly peraluminous character (SiO2 = 72 - 74%; CaO = 0.3-0.6%; A/CNK = 1.18 - 1.36, low Mg, Ti, Ba, Sr, REE, HFSE contents, and high Al2O3/TiO2 e Rb/Sr ratios), high (87Sr/86Sr)317 (0.71037 - 0.71459), low Ndi (-7.7 to -8.7), and high 18O (18O-wr = 11.33 ‰; 18O-zr = 9.5 ± 0.2‰). The data suggest that this magma was derived from the partial melting of metasedimentary middle crustal protoliths, which has been successfully modeled. The observed major and trace element composition variation suggests an evolution controlled by fractionation crystallization of a mineral association composed by plagioclase + biotite + apatite + zircon ± monazite ± ilmenite.