Constraints from zircon Hf-O-Li isotopic compositions on the genesis of slightly low-δ18O alkaline granites in the Taohuadao area, Zhejiang Province, SE China

Abstract

The origins of the Mesozoic low-δ18O A-type granites in eastern China are quite controversial. A new “low-δ18O” granite, the Taohuadao alkaline (A1-type) granite from the Zhoushan archipelago, offshore from Zhejiang, is reported in this study. Integrated in situ analyses of Hf-O-Li isotopes in zircon are used to provide insights on the petrogenesis and evolution of this granite, and low-δ18O granites in general. Three types of zircon have been recognized in Taohuadao samples: zircon with sector/oscillatary zoning (Type 1), disturbed zoning (Type 2) and extremely high-U (Type 3) grains. Data are carefully screened for radiation damage and post-magmatic alteration and only Type 1 zircon represents a magmatic origin. In Type 1 zircons, Type 1a with normal mantle δ18Ozrn value of 5.23‰ has a positive δ7Li value (2.0±3.3‰), while Type 1b has a δ18O value of ∼4.81‰ and highly variable negative δ7Li values (−31.9‰ to −0.3‰). In contrast to the O-Li isotopes, zircon is quite homogeneous in terms of Hf isotopes (εHf(t)=−3.5±1.7), suggesting that the Taohuadao A1-type granites were derived from a simple source. Our results show that the magmatic zircon δ18Ozrn values are negatively correlated with Hf content but positively correlated with Zr/Hf ratio in zircon, which indicates that fractional crystallization plays an important role in producing the slightly low δ18O signature. The anhydrous nature of the A1-type granites suppressed the crystallization of biotite and hornblende, while low oxygen fugacity is responsible for insufficient precipitation of magnetite, both of which drive the later differentiates towards slight depletion in 18O. Therefore, we suggest a ferrogabbro-type fractional crystallization may have played a major role in producing the chemical and oxygen isotopic variations of the Taohuadao A1-type granite and accounts for ∼0.5‰ 18O-depletion. The extremely depleted δ7Li in Type 1b zircons is most likely a result of diffusion-driven fractionation. Mantle-like O-Li isotopes and very low Li abundance in zircon suggest a predominantly mantle origin with little, if any, involvement of continental crustal materials in the granite. Thus the Taohuadao A1-type granite was most likely produced by intensive fractionation of reduced, anhydrous basaltic melts, rather than by mixing of mantle- and crustally-derived melts as previously thought.