Late Paleozoic tectonic transition of the East Tianshan: Constraints from geochronology and geochemistry of the igneous rocks in the Hongshannan-Tianmudong area

The East Tianshan, as an important part of the Central Asian Orogenic Belt (CAOB), is characterized by its extensive Late Paleozoic magmatism and metallogeny, and thus serves as a critical area for understanding the tectonic evolution and metallogeny of the southern CAOB. Nevertheless, the time and process of tectonic transition from subduction to collision of the East Tianshan remain debate. In this contribution, detailed field investigation, petrographic observation, geochronological and geochemical analysis have been conducted to constrain the ages, genesis and tectonic settings of the Late Paleozoic igneous rocks in the Hongshannan-Tianmudong area. Petrological and zircon U-Pb dating results indicate that the Early Carboniferous volcanic rocks are mainly composed of andesite (328. 8 +/- 2. 0Ma) and dacite, while the intrusive rocks are gabbro-diorites (328.7 +/- 1. 8Ma); the Early Permian intrusive rocks are biotite monzogranite (290. 3 +/- 2. 1Ma) and quartz diorite (290. 0 1. 6Ma). The Early Carboniferous igneous rocks are rich in amphibole and plagioclase, belonging to the calc-alkaline, meta-aluminous series. They are enriched in large ion lithophile elements such as Rb, Ba, Th, U and Pb, and depleted in high field strength elements (e. g., Nb, Ta and Ti), and exhibit relatively depleted Sr, Nd and Hf isotopic compositions, i. e., they are resemble to the island arc magmatic rocks derived from partial melting of metasomatic mantle wedge. Compared with the Early Carboniferous igneous rocks, the Early Permian intrusive rocks are composed of plenty of biotite and alkali feldspar, which are enriched in SiO2, Na2O and K2O, and depleted in Al2O3, MgO, Fe2O3T and CaO, and Sr, Nd and Hf isotopic compositions, indicating that their magmas were originated from partial melting of a thickened juvenile crust under the post-collisional setting. Hence the Early Carboniferous and the Early Permian magmatic rocks show significant petrological and geochemical differences, which is indicative of a tectonic setting transition from the Early Carboniferous oceanic subduction to Early Permian post-collision orogeny.