Early Paleozoic subduction and accretion of the East Junggar terrane: Constraints from geochronology and Sr-Nd-Hf isotopes on andesitic rocks in the Kayishekuola area, NW China

Abstract:

East Junggar, a crucial segment of the southwestern Central Asian Orogenic Belt, is mainly made up of ophiolitic mélanges and island arcs (Xiao et al. 2004; Zeng et al. 2015). It is generally supposed that East Junggar is the primarily Paleozoic Orogenic product. On account of the poor outcrops, however, the researches of its Early Paleozoic dynamics of continental accretion and tectonic evolution have been barred. Here we present new zircon U-Pb-Hf isotope, and Sr–Nd isotopic analyses for the Early Paleozoic andesitic samples from the Kayishekuola area, the northern East Junggar, NW China, in order to rebuild the tectonic model and discuss the possible mechanism for the lateral crustal growth of East Junggar in Early Paleozoic. These andesitic samples were newly magmatic zircon U-Pb dated to be Ordovician- Silurian (467.4–421.9 Ma), and show positive whole-rock ε_Nd(t) and zircon ε_Hf(t) values. By Combining data from this and former works, we suggest that the comparatively more positive isotopic compositions at ca. 420 Ma infer a rising mantle contribution with time. The mantle contribution may be subjected to episodes of overlying plate crustal rift and back-arc basin development during trench retreat (Collins et al., 2011). The retreating subduction system may lead to tectonic switching from the Late Silurian to Early Devonian in East Junggar and horizontal crustal growth, as well as influence and change the original Early Paleozoic tectonic evolution history of East Junggar.