Cessation of collisional tectonism and rapid crustal uplift recorded by 430–420 Ma igneous rocks in the South Kunlun belt, northwest China

Abstract

ABSTRACT The Wanbaogou granitic plutons are exposed along with coeval volcanic rocks, and provide an opportunity to investigate Silurian tectonism in the South Kunlun belt, northwest China. This study presents new whole-rock geochemical, and zircon U–Pb geochronological and Lu–Hf isotope data for Silurian intrusive rocks of the Wanbaogou area. We analysed four samples: a porphyritic monzogranite and dioritic dike from the Wanbaogou pluton and a granodiorite and monzogranite from the Wudaogou pluton. Zircon U–Pb dating yielded similar ages of 423.8 ± 1.9, 422.9 ± 1.9, 424.9 ± 2.0, and 425.3 ± 2.6 Ma, respectively. indicating that the intrusions were coeval. The granitic samples are mostly transitional between the high-K and shoshonite series. The Wanbaogou monzogranite is metaluminous and has features typical of A-type granites in terms of trace element geochemistry, whereas the Wudaogou granitoids are metaluminous to weakly peraluminous I-type granites. The Wanbaogou dikes are intermediate to mafic in composition and belong to the low- to medium-K series. The four intrusive samples have εHf(t) values of −10.62 to +0.99 (average = −2.52), similar to the εHf(t) values of intraplate volcanic rocks (−6.35 to +2.89; average = −1.82) and, combined with the geochemical data, indicate the 430–420 Ma magmatism could have been generated by crustal melting with input from mantle-derived materials, followed by crustal contamination. Data for the Wanbaogou and Wudaogou plutons, dikes, and volcanic rocks from the same area suggest rapid orogenic uplift at 430–420 Ma. Our data and the results of previous research on the Kunlun high- to ultra-high-pressure metamorphic belt and coeval volcanic and intrusive rocks elsewhere in the region suggest that collisional tectonism ceased abruptly at 430–420 Ma. Slab break-off may be responsible for this termination of tectonism and rapid uplift.