Chronology, petrogenesis and tectonic setting of the Neoproterozoic Tongchang dioritic pluton at the northwestern margin of the Yangtze Block: Constraints from geochemistry and zircon U–Pb–Hf isotopic systematics
Abstract The Neoproterozoic Tongchang dioritic pluton is situated at the northwestern margin of the Yangtze Block and dominated by quartz diorite and granodiorite. Laser Ablation–ICPMS zircon U–Pb isotopic dating shows that the Tongchang dioritic pluton was emplaced at three Neoproterozoic magmatic episodes of ~ 879 Ma, ~ 840 Ma and ~ 824 Ma, and the copper deposits formed following the second magmatic episode, revealing important magmatic episodes during the rarely reported “magmatic gap” between older arc magmatism (950–895 Ma) and younger plume-related magmatism (740–825 Ma) along the western margin of the Yangtze Block. The Tongchang dioritic pluton is characterized by high alkali (Na2O + K2O = 6.26–8.75 wt.%), Cr (186–561 ppm) and Ni (76–292 ppm), medium to high Mg# (44.71–64.21), and possesses metaluminous to weakly peraluminous features with ASI (aluminous saturation index) of 0.70 to 1.06. The chondrite-normalized REE patterns display the moderately enriched LREE ((La/Sm)N = 2.41–4.55) and weakly differentiated HREE ((Gd/Yb)N = 0.91–1.40) and negative to positive Eu anomalies (EuN/EuN⁎ = 0.68–1.86). The primitive mantle-normalized multi-element diagrams show enrichment in Ba, Th and K but depletion in Nb, Ta and Ti. The in situ zircon Lu–Hf isotopic data show that the Tongchang dioritic pluton has significantly variable Hf isotopic compositions with the eHf(t) values ranging from − 12.1 to + 3.3 and the Hf-depleted mantle model ages of 1.36 to 1.86 Ga. Accounting the previous whole-rock Rb–Sr and Sm–Nd isotopic data, we conclude that the Tongchang dioritic pluton was produced by magma mixing, and the end-members of mixing magmas were derived from the depleted mantle, which was metasomatized by fluids fluxed from the subducted oceanic slabs and the lower continental crust, respectively. The above whole-rock geochemical data and petrogenetic studies suggest that the Tongchang dioritic pluton formed in a subduction-related arc setting.