LA-ICPMS in situ UPb dating of skarn garnet from the Tongling ore district of eastern China

Accurate dating of metallogenesis is important for understanding the origin of mineral deposits including skarn. This study presents LA-ICPMS UPb dating and trace element analysis of garnets from three skarn deposits in the Tongling district, namely Xiaotongguanshan, Guanyinchong, and Baoshantao. All six skarn garnets give UPb ages of 141–146 Ma, which are consistent with the zircon UPb ages of the corresponding pluton. Besides, the garnet and zircon UPb ages of Baoshantao skarn are consistent within error. In particular, the garnet UPb age of the Xiaotongguanshan skarn suggests that it is the first stage of the Xiaotongguanshan magmatism that is directly linked with the formation of skarn. The correlation of garnet U concentrations with REE and/or andradite endmembers indicates that U entered garnet structure instead of being adsorbed by crystal surface during growth or captured in mineral or fluid inclusions. The U concentrations in Baoshantao skarn garnet correlate positively with LREE, which indicates that U is incorporated into garnet in the form of U4+ and occupies the X site following this equation: [U4+]VIII + 2[Al, Fe3+]IV - [Ca2+]VIII + 2[Si4+]IV. In the garnet of the Xiaotongguanshan skarn, U positively correlates with Ti, Zr, and Hf, which mainly occupy the octahedral Y site, indicating that U substitutes the Y site in the form of U6+. The substitution mechanism can be expressed as [U6+]VI + 3[Al, Fe3+]IV - [Al, Fe3+]VI + 3[Si4+]IV. The U contents in the garnet of the Guanyinchong skarn show good correlations with X2+ (REE and andradite) and Y3+ (Ti, Zr, and Hf), which indicates that both substitutions worked. The varied oxygen fugacity in different skarns might be responsible for different substitution mechanisms of U.