Early Paleozoic arc magmatism in the Kalamaili orogenic belt, Northern Xinjiang, NW China: Implications for the tectonic evolution of the East Junggar terrane

Abstract

The Kalamaili orogenic belt in northwestern China preserves a record of the magmatic and tectonic evolution of the Paleo-Asian Ocean and is thus an important region for reconstructing the Paleozoic evolution of the East Junggar terrane and the Central Asian Orogenic Belt. This study presents detailed field investigations, new SHRIMP U-Pb ages, whole-rock geochemistry, and in situ zircon Hf isotopic data for the early Paleozoic igneous rocks exposed in the Kalamaili orogenic belt. These igneous rocks are predominantly felsic plutons and mafic–intermediate volcanic rocks but include minor mafic–intermediate dikes. The igneous rocks have zircon U-Pb ages ranging from 463 Ma to 433 Ma and unconformably overlain by Late Silurian and Early Devonian sedimentary rocks. These early Paleozoic igneous rocks exhibit signatures similar to those of arc-related magmatic rocks such as enrichment in large-ion lithophile elements and depletion of high field strength elements. They have high positive εHf(t) values of +10.7 to +19.0, suggesting a juvenile source and considerable continental growth in the East Junggar terrane during the early Paleozoic. The geochemical signatures of the mafic–intermediate rocks suggest that they originated from the partial melting of mantle wedge material that had been metasomatized by slab-derived fluids; the felsic plutons were derived from the partial melting of juvenile crust. This early Paleozoic arc magmatic event, together with other evidence, indicates that the Kalamaili Ocean formed in the early Paleozoic, was subducted during Middle Ordovician–Early Silurian, and closed before Middle Silurian. Stratigraphic correlations reveal that the East Junggar terrane might not have experienced full extension to form a broad ocean during the late Paleozoic.