Petrogenesis of Himalayan Leucogranites: A Perspective from Zircon Trace Elements

Magmatic zircon trace element compositions and their variation trends provide valuable insights into the nature and evolutionary processes of magmatic rocks. The Himalayan orogen contains widespread leucogranites. Despite extensive studies on these granites, the features and petrogenetic implications of trace element composition of zircons from the leucogranites remain poorly constrained. In this study, we present a comprehensive dataset comprising new cathodoluminescence (CL) images, U-Pb ages, and trace element compositions of zircons from the Himalayan leucogranites, and compare them to the previously reported trace element data of zircon from I-type granites. Our results show that zircons from the Himalayan leucogranites have high Hf, U, Y, P, Th, Sc, and heavy rare earth element contents (HREE), and low Nb, Ta, Ti, and light rare earth element contents (LREE), and can be divided into two types. Type I (low-U) zircons exhibit well-developed oscillatory zoning, and the U concentrations are mostly <5000 ppm. Type II (high-U) zircons display mottled or spongy textures and possess elevated U contents that are mostly >5000 ppm. Zircons from the Himalayan leucogranites have higher contents of U, Hf, Nb, Ta, and elevated U/Yb ratios, but lower Th/U, Eu/Eu*, Ce/Ce*, LREE/HREE, and Ce/U values than those from I-type granitic zircons. Furthermore, zircons in the Himalayan leucogranites have gradually decreasing Th, Ti, Th/U, Eu/Eu*, and Ce/Ce*, and increasing U, Nb, Ta, and (Yb/Gd)N with increasing Hf. These geochemical features suggest the magmas involved in the genesis of leucogranites originated from the partial melting of metasedimentary sources under relatively reduced conditions, and underwent a high degree of magmatic fractionation.