Late Triassic Biotite Monzogranite from the Western Litang Area, Yidun Terrane, SW China: Petrogenesis and Tectonic Implications

Abstract

The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo‐Tethys in western China. We present new zircon U–Pb, whole‐rock geochemistry, and Sr–Nd–Pb–Hf isotopic data for the Litang biotite monzogranites, Yidun terrane. The biotite monzogranites have a zircon U‐Pb age of 206.1±1.0 Ma (MSWD=1.9, n=30), which indicates Late Triassic magmatism. The biotite monzogranites display I‐type affinity, high Na2O (3.38–3.60wt%) contents, medium SiO2(67.12–69.13wt%), and low P2O5 contents (0.10–0.12wt%). They are enriched in Rb, Th, and Ba and depleted in Nb and Ta, with negative Eu anomalies (Eu/Eu∗=0.74–0.81). They have evolved Sr‐Nd‐Pb‐Hf isotopic composition, i.e., (87Sr/86Sr)i=0.714225 to 0.714763, negative εNd(t) values of –2.0 to –2.6 with two‐stage Nd model ages ranging from 1.01 to 1.05 Ga, negative εHf(t) values of –3.4 to –4.1 with two‐stage Hf model ages of 1.85 to 1.88 Ga, suggesting a matured crustal sources. Their low Al2O3/TiO2 ratios and medium Cao/Na2O ratios, medium Mg# and SiO2 contents, low [molar Al2O3/(MgO+FeOT)] values, and high [molar Cao/(MgO+FeOT)] values indicate that the Litang biotite monzogranite was formed by partial melting of metabasaltic rocks. Based on the previous studies, we propose that the Litang biotite monzogranite derived from the westward subduction and closure of the Ganzi–Litang ocean during the Late Triassic. The mantle wedge‐derived mafic melts provided sufficient heat for partial melting of ancient metabasalt protolith within the middle‐lower crust.