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

摘要

A multiphase granitic complex (G1-G6) in the Carrazeda de Ansiaes area, Centra! Iberian Zone, was emplaced during syn-kinematic stages of the Variscan orogeny, between 329.9 ±0.8 Ma and 318.9 ± 1.9 Ma, 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, (~(87)Sr/ ~(86)Sr), = 0.7097-0.7159, sNd = - 6.2 to - 9.1, and δ~(18)O = 10.96-11.81‰. Geochemical and isotopic characteristics show that granites Gl, 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 Gl, G2, G3 and G6 have ≥ 20 ppm Sn and G5 16 ppm 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.