Rare earth element enrichment in the ion-adsorption deposits associated granites at Mesozoic extensional tectonic setting in South China

Abstract

Ion-adsorption type rare earth element mineralization occurs in weathered profile of three Mesozoic granites in the Bachi area, Guangdong Province, South China. In this study, we investigate the REE enrichment processes in these granites using geochemical and geochronological data. The three granites yield zircon U-Pb ages of ca. 189 Ma, 153 Ma and 94 Ma, respectively, and could be divided into Early Jurassic, Late Jurassic and Cretaceous granites. The Early Jurassic and Cretaceous granites show similar geochemical characteristics, i.e. high K2O and low CaO, Fe2O3, and MgO contents, low K2O/Na2O ratios, and depletion of Eu, Sr, Ba, Ti, and P. They are characterized by high HFSE (high field strength elements) contents and enriched isotopic compositions with εNd(t) values ranging from −5.4 to-2.7, and εHf(t) values ranging from −4.5 to 1.7. The Late Jurassic granite also shows Eu, Sr, Ba, Ti, and P depletion and high HFSE contents. It has, however, higher CaO, Fe2O3, MgO and lower K2O contents than the Early Jurassic and Cretaceous granites. The Late Jurassic granite also shows more enriched isotopic compositions with εNd(t) values ranging from −9.0 to −8.9, and εHf(t) values ranging from −11.1 to −6.1. In addition, the Late Jurassic granite comprises, in contrast to the Early Jurassic and Cretaceous granites, abundant inherited/captured zircons cores and accessory REE-rich minerals of allanite, titanite, apatite and fluorite. These Mesozoic granites have the geochemical affinity of A-type granite. The Zr saturation temperature is >820 °C for the Early Jurassic and Cretaceous granites, and ~900 °C for the Late Jurassic granite. The Early Jurassic and Cretaceous granites were derived from partial melting of felsic igneous rocks under high-temperature, low-pressure, and anhydrous conditions. The felsic igneous source could provide relatively high REE contents for the melts, and the high-temperature, low-pressure, and anhydrous conditions resulted in residual plagioclase and melting of REE-rich minerals in the source area, which favour to REE enrichment in the melt. The Late Jurassic granite, on the other hand, was derived from a more mafic source and associate with fast crustal magma generation as indicated by its abundant presence of inherited/captured zircon cores in the high-temperature condition. The fast magma generation of the Late Jurassic granite prevented the separation of REE-rich minerals from the melt resulting in REE enrichment. Further, the high-temperature, low-pressure, and anhydrous physicochemical conditions and fast magma generation, which favour to REE enrichment in the granites, are tightly associated with large-scale Mesozoic extension in South China.