Magmatism of the Shuteen Complex and Carboniferous subduction of the Gurvansaikhan terrane, South Mongolia

Abstract

The Carboniferous Shuteen Complex, a volcano-plutonic ring complex associated with Cu–Au porphyry mineralization, is located in the Gurvansaikhan island arc terrane of South Mongolia. This paper presents new data on the petrography, major and trace element chemistry, and Sr–Nd isotopic chemistry of the Shuteen Complex. We discuss the relationship between volcanic and plutonic rocks of the complex, and consider their similarity to high-Al 2O 3 trondhjemite–tonalite–granodiorite and adakites. We also consider the origin, magma source, and dynamic processes of the Shuteen Complex; propose a petrogenetic model; and investigate the composition of the subducting slab and the features of arc volcanism at the time. We assess some of the magmatic processes likely to have occurred within the Shuteen Complex, such as Carboniferous slab subduction and partial melting, and examine their influence on magma composition. The Shuteen Complex is geochemically similar to adakite-type rocks. The complex is silica-saturated (SiO 2 ⩾ 56%), rich in Al 2O 3 (⩾15%), MgO (<6%), Y (⩽18 ppm), and Yb (⩽1.9 ppm), depleted in heavy rare earth elements, rich in Sr (>400 ppm), and depleted in high field strength elements. It also has a high Sr/Y value, and ( 87Sr/ 86Sr) I < 0.7040. The Shuteen Pluton yields a well-defined isochron age of 321 ± 9 Ma, whereas the age of the Shuteen andesites is 336 ± 24 Ma. The Shuteen Complex formed within an island arc setting, and partial melting was the dominant process during petrogenesis. The primary Shuteen magma had an adakitic composition and was probably derived from the partial melting of subducting oceanic crust, possibly with minor local interaction with mantle material. The results of quantitative modelling of mass balance and partial melt equilibrium for the magma source indicate that the subducting slab contained oceanic basalt and a minor component of oceanic sediment, which together with a restite eclogite phase formed the source of the Shuteen magma. The conclusive results of this study provide new insights into the magmatic evolution of the Shuteen Complex.