Discovery of Neoproterozoic highly fractionated syenogranite in the southwestern part of the Erguna Massif in NE China and its geological implication

Abstract

ABSTRACT Earlier studies of the Neoproterozoic magmatism and tectonic evolution in Erguna Massif of NE China are spatially confined to the central and northern parts of the area, but the southwestern part of the massif is poorly studied. This contribution documents new zircon U-Pb geochronology, Lu-Hf isotopic systematics, and whole-rock geochemistry from Neoproterozoic granites in the southwestern part of the area. The aim of this study is to determine the petrogenesis of the granites and further elucidate on the geodynamic setting of Neoproterozoic magmatism in the massif and their geological implications. Syenogranite samples from the region yield LA-ICP-MS U-Pb zircon weighted mean ages of 853 ± 5 Ma and 824 ± 3 Ma, and both dates are evidence for Neoproterozoic magmatism in the southwestern part of the massif. Both syenogranite samples are peraluminous (A/CNK = 1.06–1.43), calc-alkaline with high-K values, and assay 76.06–76.52 wt% SiO2, 1.98–3.20 wt% Na2O, and 4.63–4.77% wt% K2O. The syenogranites’ Chondrite-normalized REE pattern display a ‘gull-wing’ shape, with low (La/Yb)N ratios of 2.28–4.29 and significant negative Eu anomalies of δEu = 0.09–0.19. The syenogranites are characterized by depletion in Ba, Nb, Ta, Sr, P, and Ti and enrichment in Rb, Th, U and K compared to the N-MORB and non-fractionated granites elsewhere in the massif. These geochemical characteristics show that the Neoproterozoic syenogranite plutons in SW Erguna Massif are highly fractionated. Zircons from the granites also have εHf(t) values of +7.68 to −1.48 with corresponding two-stage model ages of ca. 1828 to 1258 Ma, indicating that they formed from the partial melting of a Palaeo- and Mesoproterozoic igneous source in the middle crust. In combination with previously published geological data, it is proposed that the Neoproterozoic granites in the southwestern study area were emplaced in an extensional environment before or during the initial stages of the break-up of the Rodinia.