Source characteristics and provenance of metasedimentary rocks from the Kangxiwa Group in the Western Kunlun Orogenic Belt, NW China: Implications for tectonic setting and crustal growth

Abstract

The West Kunlun Orogenic Belt is a result of the closure of Proto– and Paleo–Tethys Ocean and subsequent continental collision. In order to better understand the tectonic evolution of this orogenic belt, whole-rock geochemistry of major and trace elements, detrital zircon U–Pb geochronology and Lu–Hf isotopic composition were determined for metasediments of the Kangxiwa Group (KG) in the South Kunlun Terrane (SKT). These rocks have low Chemical Index of Alteration and high Index of Compositional Variability values, indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lacks alumina-rich minerals. The rocks are all enriched in light rare earth element and display relatively flat heavy rare earth element patterns. Their low contents of ferromagnesian (e.g. Cr, Co and Ni) and high field strength elements (e.g. Zr, Hf, Nb, Ta and Y) are similar to those of graywackes from active continental margins. Detrital zircons from three samples of the KG metasediments can be divided into igneous and metamorphic grains. Zircon U–Pb dating of the igneous grains shows similar age spectra dominated by Early Paleozoic to late Paleozoic ages. The Precambrian igneous grains yield Neoproterozoic 206Pb/238U ages and a few early Mesoproterozoic to Paleoproterozoic 207Pb/206Pb ages. The youngest igneous zircons define the maximum depositional age at ~307Ma for the KG metasediments. The U–Pb dating of all the metamorphic grains yield 206Pb/238U ages at ~245Ma, suggesting the metasediment rocks were deposited prior to the early Triassic. These results indicate that the KG metasediments were deposited on an active continental margin during the time between late Carboniferous and late Permian. Lu–Hf isotopic analyses of the igneous detrital zircons from the three samples show large variations of εHf(t) values mostly between −21 and +12. Combined with their U–Pb ages, two respective crustal growth events are identified in the SKT at the Early Paleozoic and the late Carboniferous.