The genesis of early Carboniferous adakitic rocks at the southern margin of the Alxa Block, North China

Abstract

Adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting. Subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma. We studied adakitic rocks from the Taohuala Mountain at the southern margin of the Alxa Block, North China. These rocks are characterized by high Sr concentrations (300–450ppm), high Sr/Y (20–75ppm) and (La/Yb)N (25–67ppm) ratios, and low Y (<18ppm) and Yb (<1.9ppm) concentrations, which are typical of adakite. The distribution of these data on a Sr/Y versus (La/Yb)N discrimination diagram, combined with their high (87Sr/86Sr)i ratios (0.7113–0.7131) and low εNd(t) (−15.8 to −16.8) and εHf(t) (−18 to −10) values of zircon, indicates that the adakitic rocks formed by partial melting of thickened continental crust. U–Pb dating of zircons using LA–ICP–MS yields an early Carboniferous age of 330±5Ma. The ages and spatial distribution of magmatic rocks indicate that the Paleo-Asian oceanic crust subducted towards the Alxa Block in the late Paleozoic. Subsequently, northward slab rollback occurred during the Carboniferous. Therefore, we propose that the adakitic rocks from the Taohuala Mountain formed by partial melting of previously thickened lower continental crust, induced by the upwelling of asthenospheric mantle during slab rollback.