Early Palaeozoic subduction-accretion in East Junggar (NW China): Insights from age, geochemical, and Sr-Nd-Hf isotopic data of andesitic rocks in the northern Yemaquan Arc

Abstract

Abstract East Junggar is a key element of the southwestern Central Asian Orogenic Belt and the subject of debate relating to the initiation of accretion during the early Palaeozoic. We present zircon U Pb ages, geochemistry and Sr–Nd–Hf isotopic compositions of early Palaeozoic andesitic rocks from the Kayishekuola area, northern East Junggar, NW China. The volcanic rocks consist of andesites and basaltic andesites, which yielded zircon U Pb ages of 466–422\xa0Ma. Andesites with ages of ca. 466\xa0Ma were probably part of the earliest magmatic arc within northern East Junggar. These andesitic rocks are medium-K and calc-alkaline, with high SiO2 (55.02–58.59\xa0wt%), Na2O\xa0+\xa0K2O (3.86–5.63\xa0wt%), and Al2O3 (16.04–17.01\xa0wt%) contents. They are enriched in the large-ion-lithophile elements and depleted in Nb, Ta, and Ti, consistent with an arc origin related to subduction of the Paleo-Asian Ocean. They have positive whole-rock eNd(t) and zircon eHf(t) values (3.8–4.3 and 9.7–15.3, respectively). These features are consistent with production of primary magmas by partial melting of depleted mantle modified by subduction. Based on these data, we infer that the Yemaquan arc formed from island arc-related rocks from ca. 466\xa0Ma and records advancing subduction and associated crustal growth during the early Palaeozoic, and retreating subduction during the Early Devonian.