Geochemistry and U-Th-Pb Geochronology of Monazite in the Suzhou A-type Granite Pluton: Implications for Nb-Ta Mineralization

The Suzhou granitic pluton is the first identified Nb-Ta-rich granite in China. To reveal the genetic link between the sequence of magmatic and hydrothermal evolution and Nb-Ta mineralization in different intrusive phases of the Suzhou granite, whole-rock geochemistry, geochemistry and U-Th-Pb dating of monazite was analyzed. The unique geochemical characteristics show that the Suzhou pluton can be discriminated as an A-type granite. LA-ICP-MS U-Th-Pb dating of monazite in both the medium- and coarse-grained biotite granite (MBG) and the fine-grained biotite granite (FBG) indicates that the granite formed between 124 and 127 Ma. Based on geochemical characteristics and mineral textures, the MBG (Mnz-Ia) and FBG (Mnz-Ib) monazites are classified as magmatic monazites; another monazite (Mnz-II) from the MBG formed during a magmatic-hydrothermal transitional stage. Nb-Ta in the Suzhou pluton gradually concentrated during fractional crystallization and alteration of Ti-rich minerals and biotite. Ultimately, with the involvement of F-Li-rich fluid, Nb-Ta mineralization occurred during the magmatic–hydrothermal transition. The Suzhou pluton is considered part of a 600-km- and NE–SW-trending Nb-rich A-type granite belt together with other Early Cretaceous A-type granites in the Jiangnan Orogen that offers prospects of a new target for Nb-Ta prospecting.