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

The origin of A-type granites has been the subject of great debate, especially the enigmatic synchronous A(1)- and A(2)-type granites. Cretaceous (similar to 125 Ma) A(1)- and A(2)-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 A(1)- and A(2)-type granites. The HYG granites include syenogranite (75.9 wt%-76.6 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 degrees C and 860-882 degrees C). All those geochemical features show an A-type granite affinity. They can be further classified into A(1)- and A(2)-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 delta O-18 = 4.7 parts per thousand-6.0 parts per thousand and epsilon(Hf)(t) = -1.5 to -3.6 for A(1)-type granites, and delta O-18 = 7.0 parts per thousand-7.8 parts per thousand and epsilon(Hf)(t) = 3.3 to 6.9 for A(2)-type granites. The geochemical signatures and newly discovered delta O-18 and epsilon(Hf)(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 Artype granites suggest fractional crystallization of reduced, anhydrous basaltic magmas resulting in the formation of A(1)-type granites. In contrast, A2-type granites with relatively high delta O-18 and negative epsilon Hf values were generated from partial melting of the lithospheric mantle which was metasomatized by slab-derived melts/fluids. The coexisting A(1)- and A(2)-type granites were formed under the extensional setting where lithospheric thinning and asthenosphere upwelling occurred. (C) 2018 Elsevier B.V. All rights reserved.