Geochronology, geochemistry and zircon Hf isotope constraints on petrogenesis and tectonic setting of Early Permian volcanic rocks from Sonid Youqi area, Solonker Zone

Abstract

ABSTRACT The mineral composition, whole-rock geochemistry, Sr–Nd isotopes, and in situ zircon U–Pb and Lu–Hf isotopes for mafic and intermediate-felsic volcanic rocks in the Sonid Youqi area were determined to unravel their petrogenesis and the Early Permian tectonic setting of the Solonker Zone, Central Asian Orogenic Belt. Zircon U–Pb ages suggested that these rocks formed in the Early Permian. The mafic volcanic rocks were depleted in Ba, Th, and Nb, enriched in Rb, U, and Sr, and had variable zircon εHf(t) values, with constant Th/Yb and variable Ba/La ratios, indicating that the mafic magma originated from the fluid-metasomatized mantle. The intermediate-felsic volcanic rocks were depleted in Ba, Th, Nb, Ta, and Ti and enriched in Sr, Zr, and Hf. Geochemical modelling and mineralogical observations suggested that magma mixing dominated the magmatic process. These features, along with variable SiO2 and MgO contents and zircon εHf(t) values of +9.40 to +13.47, suggested that these rocks originated from the mixing of melts from the mantle and juvenile crust. Early Permian volcanic rocks from the Solonker Zone had depleted Sr–Nd isotopic compositions, similar to those from the Japan Sea back-arc basin basalts. Their geochemical characteristics differed from those rocks formed during the initial subduction. In combination with previous studies, we suggest that the generation of these Early Permian rocks from the Solonker Zone occurred in an extensional tectonic setting that induced by the slab break-off of Hegenshan Ocean.