Insights into the origin of coexisting A1- and A2-type granites: Implications from zircon Hf-O isotopes of the Huayuangong intrusion in the Lower Yangtze River Belt, eastern China

Abstract

The origin of A-type granites has been the subject of great debate, especially the enigmatic synchronous A1- and A2-type granites. Cretaceous (~125 Ma) A1- and A2-type granites are common throughout the Lower Yangtze River Belt (LYRB), eastern China. However, their genesis still remains unclear. In this study, in-situ zircon Oisotopic data and chemical compositions of the Huayuangong (HYG) A-type granites in Anhui province, provide new insights into the origin and evolution of A-type granites in the LYRB, as well as the genetic link for synchronous A1- and A2-type granites. The HYG granites include syenogranite (75.9 wt%–76.6 wt% SiO2) and quartz syenite (66.1 wt%–66.9 wt% SiO2). Both are metaluminous and belong to ferroan series. They are characterized by high alkalis (K2O + Na2O = 8.36 wt%–8.55 wt% and 11.7 wt%–11.9 wt%), high field strength elements (Zr + Nb + Ce + Y = 909 ppm–1269 ppm and 1092 ppm–1329 ppm) and high Ga/Al ratios (10,000 ∗ Ga/Al = 4.91–4.96 and 2.64–2.68). The zircon saturation thermometer results indicate high magmatic temperatures (896–964 °C and 860–882 °C). All those geochemical features show an A-type granite affinity. They can be further classified into A1-and A2-type granites, corresponding to reduced and oxidized A-type granites, respectively. Additionally, the in-situ zircon O-Hf isotope compositions are also distinctly different, with δ18O = 4.7‰–6.0‰ and εHf(t) = −1.5 to −3.6 for A1-type granites, and δ18O = 7.0‰–7.8‰ and εHf(t) = −3.3 to −6.9 for A2-type granites. The geochemical signatures and newly discovered δ18O and εHf(t)values of the two A-type granite subgroups, indicate that they were derived from different source components and under disparate physicochemical conditions (e.g.,temperature, redox state and water contents). Lithospheric mantle-like isotopic data from zircons of A1-type granites suggest fractional crystallization of reduced, anhydrous basaltic magmas resulting in the formation of A1-type granites. In contrast, A2-type granites with relatively high δ18O and negative εHf values were generated from partial melting of the lithospheric mantle which was metasomatized by slab-derived melts/fluids. The coexisting A1- and A2-type granites were formed under the extensional setting where lithospheric thinning and asthenosphere upwelling occurred.