Petrogenesis of granite aplite in the Lijiagou lithium polymetallic ore district in western Sichuan: constraints from geochemistry, zircon U-Pb geochronology and Hf isotope

The Super-large Lijiagou lithium polymetallic deposit is a newly discovered deposit in the southeastern margin of Songpan-ganzi Fold Orogenic Belt and has widespread outcrops of granite aplite veins. whole-rock geochemical analysis, zircon U-Pb dating and zircon Hf isotopic analysis for granite aplite are performed to explore its petrogenesis and geological significance. Zircons yielded weighted mean U-Pb age of 200±2 Ma for granite aplite, indicating that it was emplaced in the Late Triassic to Early Jurassic. The rocks exhibit high SiO₂ concentration (73.27%~75.14%), high Al₂O₃ concentration (14.8%~15.25%), high total alkali concentration (4.86%~8.08%) and low CaO concentration (0.45%~1.45%) with A/CNK ratios >1.1, which are defined as median-K to high-K calc-alkaline strongly peraluminous S-type granites. Trace elements of the granite aplite show positive anomalies for Rb, Th, U, K and relative negative anomalies for Ba, Sr, P, Ti. The granite aplite display enrichment of LREE, depletion of HREE, and weak negative Eu anomalies (δEu=0.68~0.81). zircon ε_Hf(t) values range from －6.36 to －3.39 with t_DM2 varying from 1424 Ma to 1616 Ma. The study shows that the granite aplite veins were formed in the post-collision orogenic stage after the closure of the Songpan-Ganzi ocean (a branch of the Paleo-Tethys Ocean), and the source rocks are the ancient crustal meta-greywacke. Combined with the previous research results, it is considered that the lithium mineralized granitic pegmatites and the granite aplites are the products of partial melting of different source rocks (meta-argillite vs meta-graywacke) in the same source area.