Geochemical characteristics and magma fertility for the Jurassic arc rocks in the Gangdese belt, Tibet

Abstract

Jurassic arc magmatic rocks are widely distributed along the Gangdese belt, southern Tibet, however, only one Jurassic arc-related porphyry Cu deposit (the Xietongmen deposit) has been found. The Jurassic arc magma fertility needs to be further constrained in order to understand the rarity of arc-related porphyry Cu deposits in this belt. This paper seeks to combine new data on the Jurassic granitoid rocks with all published data on the Jurassic magmatic rocks in the Gangdese belt to comprehensively explore their geochemical variation and magma fertility. Our new data show that the Jurassic granitoid rocks have zircon U-Pb ages of 171.6 ± 1.1 Ma to 195.3 ± 1.3 Ma. These Jurassic granitoids are calc-alkaline series with SiO2 contents of 59.4 to 70.7 wt%, Mg# values of 35 to 53, and A/CNK values of 0.72–1.25 (mainly < 1.1). They show enrichment in LREE and LILEs, depletion in HREE and HFSEs, and have depleted Sr-Nd-Hf isotopic compositions. These characteristics suggest that they were probably formed by the interaction of mantle-wedge-derived basaltic magmas with juvenile lower crust. The compiled Jurassic magmatic rocks in the Gangdese belt generally contain amphibole and a mineral assemblage of titanite + magnetite + quartz. These rocks have high whole-rock Sr/Y (>20), V/Sc ratios (>8) and high zircon EuN/EuN* (>0.3), Ce4+/Ce3+ ratios (>300), and low titanium-in-zircon temperatures (<750 °C), except for some localized areas that deviate from the main trend. These features indicate that nearly all Jurassic arc magmas in the Gangdese belt are water-rich and oxidized, which are fertile for porphyry Cu mineralization. The rarity of the Jurassic subduction-related porphyry Cu deposits may result from subsequent erosion or insufficient exploration.