New insights into the petrogenesis of volcanic rocks in the Shanghang Basin in the Fujian Province, China

Abstract

The Mesozoic Shanghang Basin in southeastern China consists of Early Cretaceous mottled coarse-grained clastic and volcanic rocks, and Late Cretaceous clastic rocks. The volcanic rocks are intermediate-mafic to felsic and spatially close to the famous Zijinshan Mineral Field. In order to better understand the timing, petrogenesis and tectonic setting of these volcanic rocks and the relationship between magmatism and metallogeny in the mineral field, U–Pb zircon geochronological, geochemical and Sr–Nd–Pb–Hf isotopic studies were completed on the volcanic rocks. Fifteen LA-MC-ICP-MS U–Pb zircon analyses of the volcanic rocks yield weighted mean ages of between 105 and 99Ma. Major and trace element geochemistry shows that these rocks are mostly high-K to shoshonitic, enriched in LREE and Th, U, and depleted in Ba, Nb, Sr, P and Ti. The volcanic rocks have 87Sr/86Sri ratios of between 0.70732 and 0.70977, 206Pb/204Pb isotope ratios of 18.57–19.77, 207Pb/204Pb isotope ratios of 15.64–15.71, 208Pb/204Pb isotope ratios of 38.87–40.62, 176Hf/177Hf ratios of 0.282589–0.282823, εNdT values of −7.5 to −4.7, and εHf(t) values of −4.2 to 4. Such characteristics, with similarities to coeval volcanic and intrusive rocks adjacent to the basin, suggest that the parent magma of the Cretaceous volcanic rocks in the basin and their contemporaneous intrusives originated from crustal melts with a juvenile component. Petrogenetically, fractional crystallization with minor wall-rock assimilation is the controlling process in deriving a wide range of more evolved rocks. With reference to the ore-forming events and isotopic features of ore-related intrusions in the Zijinshan Mineral Field, we propose that volcanism in the Shanghang Basin and coeval magmatism in the mineral field are related to the formation of the regional porphyry and epithermal Cu–Au–Mo–Ag deposits in an extension tectonic setting related to the subduction of the Paleo-Pacific Plate.