Trace element composition of quartz from alkaline granites – A factor supporting genetic considerations: Case study of the Pitinga Sn–Nb–Ta–Th-cryolite deposit

Abstract

The Madeira pluton, bearing the world-class tin-cryolite deposit of Pitinga, and the closely associated Europa pluton lie in the central part of the Amazon craton, Brazil. The Madeira pluton consists of four principal rock types: early metaluminous porphyritic amphibole-biotite granite with rapakivi texture, metaluminous biotite granite, metaluminous hypersolvus porphyritic granite, and peralkaline albite granite, while the Europa pluton consists of peralkaline riebeckite-biotite alkali feldspar granite. Texture and chemical composition of quartz from all mentioned rock types was evaluated in order: (i) to estimate the contents of trace elements in quartz from peralkaline granites, (ii) to define major differences in the uptake of trace elements into quartz in peraluminous vs. peralkaline conditions, (iii) to better constrain the evolution of the composite Madeira pluton. The contents of Li, Be, B, Na, Al, P, K, Sc, Ti, Mn, Fe, Ga, Ge, Rb, Sr, Zr, Nb, Sn, Sb, Ce, Yb, Hf, Ta, Th, and U in quartz were analyzed using laser-ablation ICP-MS. Quartz from the studied metaluminous and peralkaline granites of the Madeira and Europa plutons is poor in Al (<150 ppm Al) and Li (mostly <20 ppm Li) but enriched in Ge (max. 10 ppm), Sc (max. 6 ppm), and Ga (max. 13 ppm). As suggested by the TitaniQ thermobarometer, the Madeira biotite granite crystallized between ca. 700 and 650 °C at a nearly constant pressure of 3 kbar, while the hypersolvus granite crystallized between 800 and 700 °C at 15–20 to 2 kbar. Textural and chemical data on quartz are in agreement with previous geochronological data: the Madeira pluton comprises two older pulses of less evolved, mostly metaluminous biotite-hornblende and biotite granites, and two younger pulses of mutually mingled metaluminous hypersolvus granite and rare metal-enriched peralkaline albite granite. Differences in texture and composition of quartz do not support direct relations between the hypersolvus granite and the albite granite via magmatic fractionation, but exposure to similar metasomatic processes after their mingling is highly probable. Typical chemical features of quartz from peralkaline granites are relative high contents of Ti and Ge accompanied by low contents of Al and Li, forming an array along the Ti–Ge join in the Ti–Al–Ge ternary diagram.