Across-arc geochemical variation in the Jurassic magmatic zone, Southern Tibet: Implication for continental arc-related porphyry Cu[sbnd]Au mineralization

Abstract

The Jurassic Gangdese arc hosts the giant Xietongmen (Xiongcun) porphyry Cu[sbnd]Au magmatic-hydrothermal centre. In order to understand the tectonomagmatic framework of the arc and its controls on the origin and temporal-spatial distribution of porphyry mineralization we conducted extensive major and trace, and isotopic analysis on a suite of samples collected regionally. Geochemical variations were observed across the Jurassic arc from south to north, controlled by northward subduction of the Neo-Tethyan oceanic plate and overlying Lhasa lithosphere. In the frontal arc (latitude 29°–29.55°N), the magmas had high Ba/La and Ba/Th ratios, high εNd(t) (+ 3 to + 7) values, and their zircons have high εHf(t) (+ 10 to + 18) values, and heterogeneous O isotopic compositions (δ18O = + 3.6 to + 6.6‰). The low titanium-in-zircon temperature and zircon saturation temperature (both average = ~ 700 °C) suggest that a greater flux of dehydration fluids from the subducting slab triggered partial melting of the mantle wedge in the southern arc. High zircon Eu/Eu* and Ce/Nd ratios (> 0.4 and > 20, respectively) indicate highly oxidized, fertile magmas, from which the Xietongmen porphyry Cu[sbnd]Au deposit formed. In contrast, the interior arc (north of latitude 29.55°), close to the ancient Tibetan basement, shows low Ba/La and Ba/Th ratios, low εNd(t) (− 0.1 to + 2.5) and zircon εHf(t) (+ 2 to + 12) values, and less heterogeneous zircon O isotopic compositions with higher δ18O values. Higher titanium-in-zircon and zircon saturation temperatures (mostly > 750 °C) suggest that less water was involved during the partial melting of mantle wedge in this region. Low zircon Eu/Eu* and Ce/Nd ratios (mostly < 0.4 and < 20, respectively) point towards less oxidized magmatic conditions. We conclude that across-arc geochemical and fluid-flux variations controlled the formation and spatial distribution of Jurassic porphyry deposits in the Gangdese belt.