Petrogenesis of Triassic granite from the Jintan pluton in Central Jiangxi Province, South China: implication for uranium enrichment

Abstract Numerous Triassic biotite granites and two-mica granites crop out in the interior of South China, and some of them possess high U contents, which have been regarded as the sources for later hydrothermal mineralization. Their petrogenesis is therefore crucial for constraining the possible origins of the U enrichment. Here we report new LA-ICPMS zircon U Pb ages, mineral geochemistry of biotite and muscovite, whole rock geochemical results and Sr Nd and zircon Hf isotope data from the Jintan pluton in Central Jiangxi Province, South China. LA-ICPMS zircon U Pb dating indicates that both biotite granite (BG) and two-mica granite (TMG) in the Jintan pluton crystallized at ~220 Ma. The TMG have higher U contents (7.85 to 48.90 ppm, average18.44 ppm) than theBG (4.99 to 17.72 ppm, average 8.64 ppm). Both BG and TMG show negative whole-rock eNd(t)and zircon eHf(t)values and contain some inherited zircons. The TMG are strongly peraluminous (A/CNK = 1.13–1.33), contain abundant primary muscovite, and display S-type affinity on plots of Yvs Rb and Th vs Rb, suggesting that they are S-type granites. The BG also display S-type granite affinities on plots of Yvs Rb and Th vs Rb. The suites display similar Sr Nd isotope compositions (BG initial 87Sr/86Sr values = 0.711389 to 0.714225 and eNd(t) = −9.91 to −9.16, TMG initial 87Sr/86Sr values = 0.711832 and eNd(t) = −10.02) and are spatially associated, suggesting that the BG should also be classified as S-type granites. The TMG have higher zircon eHf(t)values (−6.4 to −1.1) than the BG (−8.7 to −3.7), indicating the TMG and BG might be derived from similar sediments but possibly with some distinct characteristics in their sources. The BG exhibit linear covariations in chemical compositions with relatively high total REE and light REE contents and MgO contents, while the TMG displays broader compositional variations but with relatively low total REE, light REE and MgO contents. Biotite geochemistry indicates the TMG formed in a more reduced magmatic system than the BG. The temperatures estimated by zircon saturation thermometry indicate the BG had distinctly higher magmatic temperatures than the TMG. The TMG display relatively high Al2O3/TiO2 ratios and low CaO/Na2O ratios than the BG but have higher Sr/Y and La/Yb ratios. The geochemical and petrological data suggest the BG were derived from clay-poor psammite sources at deeper levels with higher temperatures and higher oxygen fugacity, and underwent an extensive fractional crystallization, while the TMG was derived from clay-rich pelitic sources at higher levels and lower temperatures and oxygen fugacity with only limited fractional crystallization. We conclude that the combination of U-rich sources, physical-chemical conditions such as low partial melting temperature or low degrees of partial melting, a reduced environment and low REE and LREE contents of magmas controlled the U enrichment in TMG.