Petrology and geochemistry of the early Mesozoic pyroxene andesites in the Maixiu Area, West Qinling, China: Products of subduction or syn-collision?

Abstract

The Qinling–Dabie–Sulu Orogen is dotted with Mesozoic igneous rocks and its exact tectonic origin is still controversial, especially the precise timing of initial collision between the North China Block (NCB) and the Yangtze Block (YB) and the subsequent closure of the Paleo-Tethys Ocean in Qinling, China. This paper presents geochronological and geochemical data for pyroxene andesites in Maixiu area, West Qinling. Laser fusion 40Ar/39Ar dating for matrix glass yields an isochron age of 234±3Ma. The Maixiu pyroxene andesites (MPAs) display a hyalopilitic texture, and the predominant phenocryst phases are plagioclase, orthopyroxene and clinopyroxene. Orthopyroxene generally displays delicately normal zoning, whereas some clinopyroxene grains exhibit reverse zonings. Textural relations indicate that magma mixing plays a key role for the genesis of the MPAs. The MPAs, with 53.75–57.29wt.% SiO2, 0.6–0.82wt.% TiO2 and 48–72 Mg#, are characterized by high magnesium contents in some samples. The MPAs display enriched light rare earth elements (LREEs) and relatively high (La/Yb)N ratios (5–9). Clinopyroxene phenocrysts are depleted in some HFSE (e.g., Nb, Zr, Hf, and Ti) and some LILE (i.e., Ba, K and Sr), and are enriched in some other HFSE (e.g., Th and U), REE (e.g., Nd and Sm) and some other LILE (e.g., Rb and Pb). The MPAs have uniformly low εNd(t) values (−7.74 to −9.27) and high (87Sr/86Sr)t ratios (0.70788 to 0.71225), implying a continental rather than oceanic type magma source. Based on data for clinopyroxene phenocrysts, we estimate a temperature range of 956 to 1087°C with the mean value of 1032±39°C (1σ), and a pressure range from 5.9 to 13.6kbar with an average of 9.8±1.9kbar (1σ). We conclude that the petrogenesis of the MPAs in West Qinling Orogen may have involved magma mixing between melts derived from the sedimentary cover of the northward-subducting A'nyemaqen–Mianlue oceanic slab and peridotite-derived basaltic melts from the overriding mantle wedge during the initial collision stage between the NCB and the YB.