Geochronological and geochemical constraints on magmatic evolution and mineralization of the northeast Ke’eryin pluton and the newly discovered Jiada pegmatite-type lithium deposit, Western China

Abstract

The Ke’eryin ore field in the Songpan-Garze Fold Belt (SGFB), western China, is an important area of pegmatite-type lithium accumulation. However, the petrogenesis of the pegmatites and their genetic relationships with the neighboring granitic pluton are still a topic of debate. In this work, LA-MC-ICP-MS zircon and columbite-tantalite U-Pb dating, whole-rock and mica geochemical analysis, and lithium isotopic analysis of the Ke’eryin pluton and pegmatites in the newly discovered Jiada deposit were performed. Zircon U-Pb dating of biotite K-feldspar granite (BG), fine-grained two-mica monzogranite (FTG), and medium-grained two-mica monzogranite (MTG) of the Ke’eryin composite pluton yielded crystallization ages of 219.5 ± 0.7 Ma, 212.8 ± 1.1 Ma, and 209.1 ± 0.7 Ma, respectively. Columbite-tantalite U-Pb dating of albite-spodumene type pegmatite (ASP) from the Jiada deposit yielded crystallization ages of 204.7 ± 1.0 Ma. The results show that the lithium mineralization in the Jiada deposit was temporally closest to the MTG emplacement. Combined with the regional magmatic evolution sequence, it is suggested that the Ke’eryin composite pluton and the Jiada deposit were emplaced during the period of transition from tectonic compression to extension under the effect of lithospheric delamination during post-collision. Geochemically, the compositional variation in the major elements of mica minerals indicate that BG, FTG, and MTG of the Ke’eryin composite pluton and the AP (albite type pegmatite) and ASP in the Jiada deposit were formed via the same continuous magmatic evolution process. The increasing Li, Rb, Cs, Nb, Ta concentrations and decreasing K/Rb, K/Cs, Nb/Ta ratios in mica minerals show that the degree of magmatic differentiation increased in the order BG-FTG-MTG-AP-ASP, indicating that MTG could be the parent rock of the Jiada pegmatites. The increase in the δ7Li values in muscovites from MTG through AP to ASP provides new evidence to support the fact that the fractional crystallization played a significant role in the formation of pegmatites in the Jiada deposit. Alkali metal elements (e.g., K, Rb, Cs) in the muscovites of MTG, AP, and ASP followed the Rayleigh fractionation law, indicating that a Rayleigh-type fractionation process occurred. Rayleigh fractional crystallization modelling of these alkali metal elements showed that the MTG could produce a magma melt of ASP through extreme fractional crystallization (∼98–99 % fractionation). Taken together with associated geological evidence, the aforementioned features indicate that the favorable tectonic setting and the prolonged and extreme fractional crystallization of the granitic melt jointly contributed to the formation of the Jiada pegmatite-type lithium deposit.