The role of fractional crystallization in the genesis of early syn-D3, tin-mineralized Variscan two-mica granites from the Carrazeda de Ansiães area, northern Portugal

Abstract

A multiphase granitic complex (G1–G6) in the Carrazeda de Ansiães area, Central Iberian Zone, was emplaced during syn-kinematic stages of the Variscan orogeny, between 329.9±0.8Ma and 318.9±1.9Ma, as determined by ID-TIMS U–Pb on zircon and monazite. The complex intruded the metasedimentary Douro Group in a NW trending antiform formed during the D1 and D3 deformation phases, but was itself affected by ductile and brittle deformation. The granites have equal or higher amounts of muscovite than biotite, and contain surmicaceous enclaves and more rarely xenoliths, “schlieren” and microgranular enclaves. They are S-type, mainly alkaline-calcic and peraluminous with ASI ranging between 1.21 and 1.45 and normative corundum of 2.63–4.77%. They are enriched in LREE relatively to HREE, with negative Eu anomalies, (87Sr/86Sr)i=0.7097–0.7159, εNd=−6.2 to −9.1, and δ18O=10.96–11.81‰. Geochemical and isotopic characteristics show that granites G1, G2, G4 and G5 correspond to distinct pulses of magma formed by partial melting of heterogeneous metasedimentary material. Geochemical fractionation trends recorded by whole rocks and minerals permit to distinguish and model two fractional crystallization series G2–G3 and G5–G6, the latter also affected by crustal assimilation during fractionation. Granites G1, G2, G3 and G6 have ≥ 20ppm Sn and G5 16ppm Sn; they are also associated with mineralized Sn- and W-bearing quartz veins. Fractional crystallization was the responsible process for the increase in Sn content in granites from the series and their micas.