Zircon U–Pb geochronology and geochemistry of Early–Middle Jurassic intrusions in the Daheishan ore district, NE China: Petrogenesis and implications for Mo mineralization

Abstract

Two types of Jurassic intrusive rocks have been recognized in the Daheishan ore district, northeastern China, according to new SIMS zircon U–Pb ages, whole-rock major and trace element abundances, and Sr–Nd–Pb isotopic compositions. The first type is represented by biotite granite and monzogranite, with zircon U–Pb dating yielding ages from 182 to 175 Ma. These rocks are high-K calc-alkaline series, and span from metaluminous to weakly peraluminous. They exhibit enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), with significant negative Eu anomalies, low (La/Yb)N and Sr/Y ratios. They have low (87Sr/86Sr)i ratios of 0.70202–0.70474, positive εNd(t) values of 1.7–2.3, and Nd model ages (TDM2) of 826–771 Ma as well as (206Pb/204Pb)i values of 18.400–18.494. These arc-like geochemical characteristics suggest that the Early Jurassic intrusions were formed by the partial melting of a mixture of dominated juvenile component and pre-existing crust in a subduction zone setting. The second type is represented by diorite and granodiorite, with zircon U–Pb ages of 172–170 Ma. These Middle Jurassic intrusive rocks are high-K calc-alkaline and mainly metaluminous, with high Mg# (>29) and elevated (La/Yb)N and Sr/Y ratios. They display similar whole-rock Sr–Nd–Pb isotopic compositions ((87Sr/86Sr)i = 0.70432–0.70480, εNd(t) = 1.7–2.3, (206Pb/204Pb)i = 18.3016–18.4531) to the Early Jurassic granites. Their adakitic geochemical compositions suggest that they formed at pressures greater than 15 kbar by the partial melting of thickened lower crust with a juvenile component. We further propose that the tectonic setting of the Zhangguangcai Range in the Early Jurassic might be a back-arc related to the subduction of the Paleo-Pacific Plate, and that subduction stopped temporarily in the Middle Jurassic due to the presence of over-thickened continental crust. The prolonged Jurassic adakitic magmatism and concomitant fluid activities may have played important roles in the formation of the porphyry Mo deposits in the Daheishan ore district.