Geochemistry of the Villisman granite, associated aplite-pegmatites and surrounding Li-pegmatites, Sierra de Ancasti, Argentina

摘要

The Villism ' an pegmatite group belonging to the Ancasti district of the Pampean Pegmatite Province is geologically formed by banded schists of the Ancasti Formation which are intruded by the homonymous granitic stock and Li-pegmatites. It is located in the northern sector of the Sierra de Ancasti, Catamarca province, Argentina. The Villisman stock-VGis a small, sub-rounded in shape two-mica-syenogranite. The VG hosts felsic aplitic and pegmatitic dyke-FDVG with muscovite, black-tourmaline (schorl) +/- garnet as main accessory minerals. The Lipegmatites contain giant crystals of spodumene and are classified as of the Rare-Element class (REL) and REL-Li subclass pegmatites and belong to the LCT (Li-Cs-Ta) pegmatite family. They are more or less symmetrically zoned with outward zones composed of felsic aplite and coarse-grained to very coarse-grained granite-OZLiP-. The minerals of OZLiP are quartz, feldspars (secondary albite 'cleavelandite' predominant over K-feldspar), muscovite and scarce black-tourmaline (schorl), garnet and F-apatite. The VG, FDVG and OZLiP have high and similar SiO2 concentrations (72-78 wt %). They are peraluminous (ASI(VG) 1.10-1.16; ASI(FDVG) 1.23-1.77, ASI(OZLiP) 1.00-1.14). The VG has subalkaline-calc alkaline character with K2O Na2O. Ferromagnesian elements (included Ti) and CaO have low concentrations in VG and the FeOt/FeOt + MgO ratio is low (0.80-0.82) classifying it as magnesian granite. High Rb/Sr (FDVG similar to 16; OZLiP similar to 19) and Rb/Ba (FDVG similar to 41; OZLiP similar to 7) and low K/Rb (FDVG similar to 107; OZLiP similar to 56) ratios clearly indicate that aplites and pegmatites are geochemically more evolved than the Villism ' an Granite (Rb/Sr-VG similar to 3; Rb/Ba-VG similar to 0.8; K/Rb-VG similar to 161). The Zr-saturation temperature in the granites, as expected, is higher than in the dykes and the outward zones of the Li-pegmatites. The VG is a post-orogenic granite developed in the easternmost sector of the Ordovician Famatinian active arc. All geological, mineralogical and geochemical features of the VG are comparable to typical S-type granites, probably originated from partial melting of the metasedimentary rocks of the Ancasti Formation. FDVG could have been formed by fractional crystallization from the VG parental-magma.