SHRIMP zircon dating of the Zijinshan pluton in southwestern Fujian and its implications

Abstract：The rock- and ore-forming ages of the Zijinshan pluton were precisely redefined based on SHRIMP zircon dating. CL images of zircons in the Zijinshan biotite granite show that there are three zircon morphologies. The results of dating indicate that the three types of zircon have different ages and different geological implications. The first type of zircon crystal is perfect and has core-rim structure. The core is rounded. This type of zircon is considered inherited zircons dated at ~1000 Ma. The second type of zircon has well-developed but incomplete crystal morphology. It is half-baked with indistinct growth zoning or no growth zoning, showing the features of magmatic zircons. The age of the zircons is 168±4 Ma, which represents the crystallization age of the pluton, indicating an Early Mesozoic Middle Jurassic age. The third type of zircon is perfect and has clear growth zoning. It has undergone late-stage reworking. Its age is 119±15 Ma, suggesting an Early Cretaceous age. There is no report of large-scale mineralization at 168±4 Ma in the study area; so the main part of the Zijinshan pluton is not a mineralized pluton, only showing weak tin mineralization. The third group of SHRIMP U-Pb ages is considered as the record of large-scale Cu-Au mineralization. It is possibly just this stage of hydrothermal processes that commenced the prologue to the large-scale, long-continued, multistage magmatic hydrothermal processes and thus laid a foundation for the copper-gold mineralization in the area.