Petrogenesis and Tectonic Setting of the Late Carboniferous Igneous Rocks in the Baluntai Region of the Chinese Western Tianshan

The Paleozoic tectonic evolution of the Tianshan orogenic belt, which is situated between the western Central Asian Orogenic Belt and the Tarim Craton, is still in heated debate. The widespread Paleozoic igneous rocks in the Tianshan orogen are a powerful tool to reveal the subduction and accretion processes as well as the tectonic switching. In this paper, detailed geochronological and geochemical studies were presented for seven suites of Paleozoic igneous rocks in the Baluntai domain of the Chinese western Tianshan. Zircon LA-ICP-MS U-Pb dating reveals that their crystallization ages are in the range of ca. 319-307 Ma, i.e., Late Carboniferous. Geochemical characteristics indicate that the Late Carboniferous mafic igneous rocks were mainly derived from asthenosphere mantle or subduction-modified lithospheric mantle; while the coeval granitoid rocks are generally metaluminous to weak peraluminous medium- and high-K calc-alkaline I-type granites that were mainly originated from partial melting of the lower crustal meta-basic rocks. In combination with the tempo-spatial distribution of the Paleozoic igneous rocks and published results of the metamorphic, ophiolitic and sedimentary rocks in the western Tianshan, we propose that the ca. 320-310 Ma igneous rocks were generated during slab breakoff, marking the tectonic transition from continental collision to post-collision settings. By inference, the subsequent 310-307 Ma magmatism was formed in a post-collisional extensional setting caused by lithospheric delamination.