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.帕米尔东北部的锂-铍（Li–Be）矿化带是中国新厘定的西昆仑-松潘-甘孜稀有金属带（Western Kunlun–Songpan–Ganze rare metal belt）的最西端部分。本文针对两处三叠纪（Triassic）花岗岩及锂-铍伟晶岩开展地球化学与U-Pb、Sr-Nd-Hf同位素体系研究，旨在厘清二者的成因联系。研究结果表明，尽管黑云母二长花岗岩（biotite monzogranites）、白云母花岗岩（muscovite granites）及锂-铍伟晶岩的锆石与铌钽铁矿（coltan）U-Pb年龄均为210 Ma，但仅靠持续结晶作用无法完全解释其组分与Sr-Nd-Hf同位素组成的分异。相较于黑云母二长花岗岩，白云母花岗岩具有显著的分异特征：SiO₂、K₂O、P₂O₅、Li、Be、Cs、Rb及Ta含量更高，其源区为变泥质岩（meta-pelite rocks）的部分熔融产物，与锂-铍伟晶岩存在成因联系。挥发性组分聚集模型被用于解释强分异白云母花岗岩与锂-铍伟晶岩的部分特征：二者的P₂O₅含量分别显著升高至0.20%~0.23%与0.4%~2.64%。P₂O₅或许是包括B、F及H₂O在内的助熔组分聚集的媒介。后续形成伟晶岩的流体出溶作用，以及从母花岗岩远端选择性析出Ca-Na-K以形成长石矿物的过程，可解释残余熔体的部分强分异特征：例如Rb富集、Sr与Ba亏损，以及负铕异常加剧。此外，由助熔组分聚集引发的碱金属选择性萃取作用，可使残余熔体中的Li与Be含量提升3倍，这或许也是锂-铍伟晶岩中稀有金属富集数千倍的关键机制之一。