Sm–Nd geochronology and geochemistry of a Neoproterozoic gabbro in the Kuluketage block, north-western China

Abstract

In this article, we report whole-rock and mineral Sm–Nd isotopic and whole-rock elemental and Sr–Nd isotopic data of Xingdi No. 1 mafic–ultramafic intrusion in the western Kuluketage block, north-eastern Tarim. Xingdi No. 1 mafic–ultramafic intrusion is the largest in the Xingdi mafic–ultramafic belt, with an exposed area of ca. 20 km2. It intruded into the Palaeoproterozoic basement. Gabbro is the major rock type and there is minor olivine pyroxenite. Sm–Nd geochronometry of the gabbro gives an isochron age of 761.2 ± 31.2 million years, identical to the intrusive age of Xingdi No. 2 pluton (760 ± 6 million years). The gabbro is systematically enriched in large ion lithosphile elements and light rare earth elements and depleted in high field strength elements and heavy rare earth elements. The studied rocks are characterized by low whole-rock and mineral ϵNd(t) values (−7.8 to −7.1) and elevated (87Sr/86Sr) i values (0.7066–0.7073). These geochemical characteristics, together with the presence of abundant hornblende, biotite, bladed biotite enclosed in amphibole, and crescent-shaped Palaeoproterozoic wall-rock xenoliths in the intrusion, are key features of magma mixing in the source or assimilation during its emplacement. The rocks have a Zr/Y ratio of 3.81–13, which falls in the within-plate basalt area. As Xingdi No. 1 and No. 2 plutons formed at the same period and display similar geochemical characteristics, we propose that they formed within the same tectonic setting and were derived from the same source, but No. 1 pluton experienced a higher extent of evolution and contamination. Previous studies have shown that the Neoproterozoic tectonic and magmatic events in Kuluketage comprise syn-collisional granite around TC (ca. 1.0–0.9 Ga), post-collisional K-rich granite and alkaline mafic–ultramafic intrusions (ca. 830–800 Ma), and rifting-related mafic–ultramafic plutons, dikes, and bimodal volcanic rocks (ca. 774–744 Ma).