Geodynamic setting of the Zijinshan porphyry-epithermal Cu-Au-Mo-Ag ore system, SW Fujian Province, China: Constrains from the geochronology and geochemistry of the igneous rocks

摘要

Zijinshan is a large porphyry-epithermal Cu-Au-Mo-Ag ore system located in the Zijinshan mineral field (ZMF) of southwestern Fujian Province, China. Although it is commonly accepted that the early Cretaceous magmatism and the metallogenesis of the mineral field are closely related, the tectonic setting for the ore-forming event(s) has been controversial and regarded as either extensional or subduction-related. New U-Pb zircon geochronology, Sr-Nd-Pb isotopic systematics, and geochemical data presented here from granites and volcanic rocks in the mineral field help to clarify this uncertainty. LA-MC-ICP-MS U-Pb zircon analyses yield weighted mean ages of between ca. 165 and 157 for the monzogranite, ca. 112 Ma for granodiorite, and between ca. 111l and 102 Ma for nine samples of volcanic units in the study area. These dates, integrated with previous geochronological data, indicate that there were two magmatic events in the area during the Middle to Late Jurassic and the Early Cretaceous. Major and trace element geochemistry indicates that these rocks are high-K, calc-alkaline granites, are enriched in LREE and Th, U, Ta, Nd, Sm and Yb, and depleted in Ba, K, Sr, P, Ti and Y. These features are characteristic of volcanic-arc granites or active-continental margin granites. The Middle to Late Jurassic monzogranitic plu-tons in the region have initial ~(87)Sr/~(86)Sr ratios of 0.7096 to 0.7173, εNd_T values of -10.1 to - 7.6,~(206)Pb/~(204)Pb isotope ratios of 18.51-18.86, ~(207)Pb/~(204)Pb isotope ratios of 15.64-15.73, and ~(208)Pb/~(204)Pb isotope ratios of 38.76-39.18. The Early Cretaceous granodiorite and volcanic rocks are distinctly different with initial ~(87)Sr/ ~(86)Sr ratios of 0.7055-0.7116, εNd_T values of -8 to 0.5, ~(206)Pb/~(204)Pb ratios ranging between 18.49 and 19.77, ~(207)Pb/~(204)Pb ratios of 15.63-15.71, and ~(208)Pb/~(204)Pb ratios of 38.71-40.62. These characteristics suggest that the source for the Middle to Late Jurassic monzogranitic plutons is a partially melted Mesoproterozoic substrate, with a minor component from Paleozoic material, whereas the Early Cretaceous granodiorite and volcanic rocks may represent mixing of crustal and mantle-derived melts. It is therefore suggested that the Middle to Late Jurassic monzogranitic plutons, and the Early Cretaceous granodiorite and volcanic rocks in the ZMF are the result of an active continental-margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. Given that the mineralization and the early Cretaceous granodiorite and volcanic rocks in the area are genetically related, the Zijinshan porphyry-epithermal ore system formed in the subduction-related tectonic setting.