Origin of the Low δ18O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton

The origin of low delta O-18 signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed. It is uncertain whether the O-18-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement. In this paper, zircon oxygen isotopes in the similar to 130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed. The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images. Their delta O-18 values also vary in different types and show negative correlation with U and Th contents and accompanying cumulative alpha-decay doses, implying that their delta O-18 values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage. The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs. It is inferred that only the least-influenced zircons, with slightly elevated delta O-18 values than normal mantle, have preserved the magmatic oxygen isotopes. In combination with other evidences, it is proposed that the A-type granites are lower-crustal-derived, unnecessarily invoking a high-temperature hydrothermal altered source. The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.