Silurian post-orogenic volcanic rocks of the South Kunlun Belt, NW China: links to the Proto- to Paleo-Tethyan tectonic transition

Abstract

ABSTRACT Newly discovered early Palaeozoic intraplate volcanic rocks within the southern Kunlun Orogen provide an opportunity to examine the middle to late Silurian tectonic setting of this area. This study presents new whole-rock geochemical and zircon U–Pb geochronological and Lu–Hf isotope data for the Silurian volcanic rocks of the Wanbaogou area, South Kunlun Belt. These volcanic rocks are split into trachytic and basaltic groups based on field observations and petrographic and geochemical data. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating of zircons from four samples from this area yields similar ages of 428.2 ± 3.4, 426.4 ± 4.2, 426.1 ± 3.2, and 424.5 ± 3.2 Ma, indicating this volcanic complex formed during the middle to late Silurian. The basalts in this area have high Nb/La ratios, generally positive εHf(t) values and hump-shaped ocean island basalt (OIB)-like intraplate compositions that are free of negative Nb, Ta, and Ti anomalies, indicating the magmas that formed these units underwent at most only slight crustal contamination. In comparison, the trachytes have incompatible trace element concentrations that are higher than the concentrations found in typical arc-type magmas. The melting of the underplated young basaltic crust is a more likely explanation for the formation of the trachytes. These igneous rocks are all of within-plate affinity, were derived either from the asthenospheric mantle or from a mantle plume, and underwent some crustal contamination prior to their emplacement. Combining these data with the results of previous research on the Kunlun high to ultrahigh-pressure (UHP) belt and contemporaneous volcanic and intrusive rocks elsewhere in this region suggests that the Silurian volcanic rocks of the Wanbaogou area formed in a post-orogenic setting as a response to the final closure of the Proto-Tethys Ocean and the transition from Proto- to Paleo-Tethyan tectonic regimes within the East Kunlun Orogen. The geology of this region indicates that middle–late Silurian post-orogenic extension was not just limited to the Wanbaogou area but probably extended across the entire EKO.