Heavy magnesium isotopes in the Gangdese Magmatic Belt: Implications for magmatism in the Mesozoic subduction system of southern Tibet

Abstract

The Gangdese magmatic belt of southern Tibet preserves important records of the Mesozoic Neo-Tethyan orogeny. Here we investigate a newly identified Late Cretaceous hornblende gabbro pluton using zircon U–Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf-O-Mg isotopes with a view to evaluate the magma source characteristics and fluid evolution. The geochronological data and Sr-Nd-Hf isotopes suggest that the Late Cretaceous gabbro crystallized in the Cenomanian stage of the Late Cretaceous (ca. 98 Ma), from magma generated by partial melting of the depleted mantle. We report the first set of Mg isotopic data for the Late Cretaceous and Late Triassic arc-type gabbroid rocks in the Gangdese magmatic belt, where both rocks show homogeneous heavy Mg isotopic compositions with δ26Mg values ranging from 0.01 to 0.12‰ and − 0.09 to 0.09‰, respectively. Their δ26Mg values are higher than the average mantle values (−0.25 ± 0.07‰), suggesting that 26Mg-rich fluids were involved in their mantle sources. Based on Sr-Nd-Hf-O isotopic data and published information on Mg isotopic reservoir, we argue that talc-rich serpentinite fluids played an important role in producing the heavy Mg isotopes in these arc-related intrusions. Integrating data from previous studies and this study, we propose that the northward subduction of the Neo-Tethys oceanic lithosphere and formation of arc-type magmas provide a viable model to account for the heavy Mg isotopic compositions through fluid-rock interaction within the subduction channel. This further indicates that the northward subduction of the Neo-Tethys oceanic lithosphere commenced in the Late Triassic or earlier rather than in the Early Cretaceous (ca. 145 Ma).