Volatile accumulation for the mineralization of Li-Be pegmatites in the northeastern Pamir, Western Kunlun, China

Li-Be mineralization in the northeastern Pamir is the most westerly portion of the newly defined Western Kunlun-Songpan-Ganze rare metal belt in China. This paper discusses the geochemistry and isotopic U-Pb-Sr-Nd-Hf systems of two Triassic granites and Li-Be pegmatites, trying to identify their genetic relationship. Our study shows that although the biotite monzogranites, the muscovite granites, and the Li-Be pegmatites have similar zircon and coltan U-Pb ages of 210 Ma, continuous crystallization could not completely account for their compositional and Sr-Nd-Hf isotopic diversity. In comparison with the biotite monzogranites, the muscovite granites exhibit strongly fractionated characteristics, with higher levels of SiO2, K2O, P2O5, Li, Be, Cs, Rb, and Ta, and are sourced from a partial melt of meta-pelite rocks, showing a genetical relationship with the Li-Be pegmatites. A model of volatile accumulation is an attempt to interpret some characteristics of the strongly fractionated muscovite granites and Li-Be pegmatites, which show dramatically elevated contents of P2O5 of 0.20-0.23% to Li-Be pegmatites of 0.4-2.64%, respectively. P2O5 might be an agency of the accumulation of fluxing compositions also including B, F, and H2O. Succeeding pegmatite-forming fluid exsolution and selective removal of Ca-Na-K to precipitate feldspar minerals far away from the parent granites should be accountable for some strongly fractionated characteristics in the residual melt, e.g. the enrichment of Rb, loss of Sr and Ba, and elevated negative Eu anomaly. Furthermore, this selective extractant of alkalis metal triggered by the accumulation of flux notably enhanced the contents of Li and Be in the remaining melts by three times, which might be also an important mechanism for the rare metal concentration in the Li-Be pegmatites by thousands of times.