The importance of sequential partial melting and fractional crystallization in the generation of syn-D3 Variscan two-mica granites from the Carrazeda de Ansiães area, northern Portugal

Abstract

In the Carrazeda de Ansiães region, northern Portugal, a mesozonal granitic suite intruded Precambrian to Ordovician metasedimentary rocks during the late kinematic stages of the Variscan orogeny. In this multiphase granitic complex, consisting of ten granite types, the youngest group of two-mica granites (G7–G10) was emplaced between 318 ± 1 Ma and 316.2 ± 0.7 Ma, as determined by ID-TIMS U–Pb on zircon and monazite. Granite types G7–G9 were affected by the third phase of deformation (D3) before they were completely crystallized, as indicated by their internal NW–SE magmatic foliation concordant with the regional structures. The granite type G10 shows some distinctive textural features, showing a strong brittle deformation, probably due to its preferential emplacement in late NNE-SSW fault zones. Granites G7–G9 have equal or higher amounts of muscovite than biotite and contain surmicaceous enclaves, xenoliths, “schlieren”, and, more rarely, microgranular enclaves. The muscovite-dominant granite G10 does not contain enclaves. These Variscan granites are peraluminous, with ASI ranging between 1.22 and 1.39 and normative corundum of 2.79–4.39%, having the characteristics of S-type granites. In fact, the enrichment in LREE relatively to HREE, the negative Eu anomalies, and similar mean values of (87Sr/86Sr)i, εNdt and δ18O for G7 (0.7156 ± 0.0005; − 8.5; 11.49 ‰) and G8 (0.7155 ± 0.0007; − 8.4; 11.39 ‰) show that these two granite types resulted from sequential partial melting of the same metasedimentary material, where granite G8 would have derived from a higher degree of partial melting than G7. Granites G8–G10 and their minerals show a fractionation trend that is confirmed by modeling of major and trace elements. The subparallel REE patterns and the decreasing REE contents within the differentiation series, the Rb–Sr isochron for G8, G9 and G10 (315.5 ± 5.4 Ma; MSWD = 1.3) and the relatively uniform εNdt and δ18O data suggest that fractional crystallization was the main mechanism, which would have lasted less than 1 Ma. The tin-bearing granites G7 and G10 have ≥ 20 ppm Sn, but the main quartz veins containing cassiterite and wolframite cut granite G10, which contains 31 ppm Sn. Fractional crystallization was responsible for the increase in Sn content in granites from the G8–G10 series and their micas.