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 suggest that the three types of zircon in the enclaves may have formed successively during the cooling process of enclave magma, corresponding to different sites along with the intrusion of enclave magma. Because of positive correlations of the UO₂/HfO₂ ratio with I(pr), the ThO₂/Y₂O₃ ratio with I(py), and the UO₂/(ThO₂ + Y₂O₃) 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.