Geochemical and geochronological constraints on the petrogenesis of granitoids from the Ke’eryin area

Abstract

Three granitic plutons with varying degrees of pegmatite development were selected from the Ke’eryin area to investigate the metallogenic characteristics of granitoids related to pegmatite-type rare metal mineralization in the Songpan-Garze Fold Belt (SGFB). Comparative studies, including chronological, geochemical, and isotopic analyses, were conducted on the parental rock (i.e., the Ke’eryin pluton) of lithium-bearing pegmatite in the Ke’eryin lithium ore field, SGFB, China. LA-ICP-MS zircon U-Pb dating revealed crystallization ages of 211.4±1.5 Ma, 209.1±0.9 Ma, and 204.0±1.5 Ma for the Siyuewu (SYW), Nahe (NH), and Elangshan (ELS) plutons, respectively. These findings suggest that all three granitoids were formed in the same tectonic environment as the Ke’eryin pluton. Geochemically, the plutons are enriched in large-ion lithophile elements (LILE) such as Rb, U, and Th, and light rare earth elements (LREE), while showing depletion in high-field-strength elements (HFSE) such as Nb, Ta, and Ti, and heavy rare earth elements (HREE). The plutons also exhibit low A/CNK and Zr + Nb + Ce + Y values, high [La/Yb]N and [Gd/Yb]N ratios, and low concentrations of Cr, Ni, Y, Yb, Sr, P₂O₅,and Zr, with a negative correlation between P₂O₅ and SiO₂ concentrations. These characteristics indicate that the plutons can be classified as I-type granites, originating from the garnet-plagioclase stability field in the lower crust under granulite facies metamorphic conditions, which contrasts significantly with the Ke’eryin pluton. Zircon εHf(t) and corresponding T_DMc values were determined to be −2.77 – −8.15 and 1.42–1.76 Ga for the SYW pluton; 1.81 – −3.26 (with only one spot >0) and 1.13–1.45 Ga for the NH pluton; and −12.81 – 1.62 (with most spots ranging from 0.18 to 1.62) and 1.14–2.05 Ga (with only one spot >1.44 Ga) for the ELS pluton. These results suggest that the three plutons were derived from the partial melting of the Paleoproterozoic to Neoproterozoic crust. The I_Sr and εNd(t) values of the ELS pluton samples (I_Sr = 0.706688–0.707127, εNd(t) = −3.99 – −5.08, T_DM2 = 1.31–1.40 Ga)closely correspond to those of the Western Yangtze Block. The SYW pluton samples (I_Sr = 0.709392–0.712291, εNd(t) = −6.98 – −9.24, T_DM2 = 1.56–1.74 Ga) partially overlap with the isotopic range of the Triassic Xikang Group. Meanwhile, the NH pluton samples (I_Sr = 0.707948–0.708368, εNd(t) = −6.90 – −7.45, T_DM2 = 1.55–1.59 Ga) exhibit isotopic values intermediate between those of the ELS and SYW plutons. Importantly, the Sr-Nd-Hf isotopic characteristics are consistent with pegmatite occurrences, suggesting that the assimilation of surrounding Triassic Xikang Group rocks may have contributed to the diverse isotopic compositions and the development of pegmatites. Based on these findings and the regional geological context, it is proposed that granitoids capable of hosting pegmatite-type rare metal deposits in the SGFB were likely emplaced during the tectonic transition or extensional phase of the post-collisional stage of the Indosinian orogeny. The source rocks for these granitoids are likely the high-maturity metasedimentary rocks of theTriassic Xikang Group.