Geochronology, geochemistry, and isotopic composition of the early Neoproterozoic granitoids in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China: Petrogenesis and tectonic implications

Abstract

The early Neoproterozoic granitoids exposed in the Bikou Terrane preserve vital information for understanding the Tonian tectonic evolution along the northwestern margin of the Yangtze Block, South China. However, their accurate forming age, petrogenesis, and tectonic attributes remain ambiguous. This study reports new results on zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Baiquesi and Bahaihe quartz monzonite, Shilingou monzogranite, and Maliupu granodiorite from the Bikou Terrane. Detailed zircon U–Pb dating results reveal that the quartz monzonite and monzogranite were formed at ca. 860 Ma, whereas the granodiorite was emplaced at ca. 825 Ma. The quartz monzonite and monzogranite samples exhibit certain linear correlations in the Harker diagram, congruent distribution patterns of rare earth elements, and consistent isotopic compositions, indicating that they shared the same magma source, whereas the monzogranite was formed by differentiation crystallization of primitive quartz monzonite melt. The quartz monzonite samples are calc-alkaline in composition and have high ratios of Sr/Y and (La/Yb)N, coinciding well with the adakitic rocks. The relatively high contents of MgO, Ni, Cr, and Sr, low contents of Yb and Y, low ratios of La/Ce, Rb/Sr, and Th/La, and positive values of εHf(t) (+4.8–+6.7) and εNd(t) (+1.7–+2.1) indicate that they were formed by the interaction between slab-derived melt and mantle wedge. This mechanism may be triggered by a slab rollback process in an arc-related setting. The Maliupu granodiorite samples are classified as calc-alkaline I-type granitoids, and have low concentrations of MgO, Cr, Ni, and Y, and low ratios of Sr/Y and (La/Yb)N. They are featured by highly evolved εHf(t) values from −15.0 to −10.9 and εNd(t) values from −11.8 to −11.9, with Hf and Nd model ages ranging between 2.63 and 2.20 Ga, implying they were derived from partial melting of the ancient lower mafic crust and were formed in a subduction-related setting. Therefore, the new data, combined with those from previous studies in the Bikou Terrane, provide an essential link in the evidence chain for the hypothesis that the northwestern periphery of the Yangtze Block was in a persistent subduction-related tectonic regime in the early Neoproterozoic.