Morphology and geochemistry of zircon: a case study on zircon from the microgranitoid enclaves

There are three types of zircon (i.e. Zircon A, Zircon B and Zircon C) in the microgranitoid enclaves from the Qingtian granite. Zircon A is of the smallest I_pr, I_py and I_el values with the largest range of variations; Zircon C is of the largest I_pr, I_py and I_el values with the smallest range of variations; and Zircon B is intermediate among the three types. The microprobe analysis of zircon demonstrates that the contents of trace elements (Hf, U, Y, Th) increase progressively with larger and larger variation from Zircon A through Zircon B to Zircon C. These characters snggest that the three types of zircon in the enclaves may have formed successively during the cooling mess of enclave magma, corresponding to different sites along with the intrusion of enclave magma. Because of positive correlations of the UO₂/HfO₂ratio with I_pr, the Th0₂/Y₂0₃ ratio with I_py, and the UO₂/(ThO₂+Y₂0₃) ratio with I_el, it is suggested that the variation in zircon typology is caused by selective substitution of trace elements on different surfaces of zircon. On the basis of enclave petrology and zircon typomorphism, it is believed that the enclave magma may be generated from partial melting of residual materials in deep regions where the granitic magma has been extracted. Project supported by the National Natural Science Foundation of China (Grant No. 49632080).