Source of the Zhawulong granitic pegmatite-type lithium deposit in the Songpan-Ganzê orogenic belt, Western Sichuan, China: Constrants from Sr-Nd-Hf isotopes and petrochemistry

Abstract

The Zhawulong ore field hosts lots of rare-element pegmatite dikes and is an important producing area for lithium resources in the Songpan–Ganzê orogenic belt (SGOB), east Tibet. This paper aims to evaluate the sources and ore-forming processes for granite and pegmatite in the Zhawulong ore field, as well as the petrogenetic links between these rocks, using petrography, whole-rock major and trace-element analysis, and Sr–Nd–Hf isotope analysis. In this ore field, pegmatites are common as dike swarms that emanate from the external and/or internal contact zone of the Zhawulong granitic pluton and strata of the Triassic Xikang group, showing close spatial relationships. Measured values of K/Rb vs. Zr, Hf, and Cs contents indicate geochemical affinities between granite and pegmatite, which suggests that the pegmatite was generated by processes involving high fractional crystallization from the residual parental magma of the host granite. Additionally, the granite have high (87Sr/86Sr)i values (0.711064 to 0.716105), negative εNd(t) values (−9.6 to −9.1) and εHf(t) values (−32.6 to −2.0), and ancient Nd and Hf model ages (1.2–3.1 Ga), which indicate a continental crustal source. Metasedimentary rocks of the Xikang group are rich in Li (~95 ppm), Rb (~124 ppm), Nb (~15 ppm), and Cs (~17 ppm), and their rare-earth element (REE) patterns show moderate enrichment in light REEs and a negative Eu anomaly (Eu/Eu* = ~0.64), revealing characteristics similar to those of granite. The coeval nature and close spatial relationships between the Zhawulong granite and other syn-orogenic S-type granites indicate that the formation of this granite was related to crustal melting. The petrology and geochemical signatures of the Zhawulong rare-metal-bearing granite–pegmatite imply that it was generated by the partial melting of metasedimentary rocks during the Late Triassic orogeny assembly and was further enriched in Li through processes involving high fractional crystallization.