Geochronology, Geochemistry and Geological Significance of the Late Carboniferous Bimodal Volcanic Rocks in the Balikun Area, Eastern Tianshan

The bimodal volcanic rocks distributed in the Balikun area are important to study on the timing of oceanic closure and the tectonic transition process of the Central Asian orogenic belt (CAOB), which are critical to deciphering the geological history of CAOB. In this paper, data on major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age of zircons are reported for Carboniferous volcanic rocks in the Balikun area, in order to investigate their sources, petrogenesis and implications for the Carboniferous evolution of study area. The main results are obtained as follows: (1) Late Carboniferous volcanic rocks from Erdaogou Formation, mainly basalts, trachybasalts, basaltic-trachyandesite, trachyte rhyolitic and volcaniclastics, are bimodal volcanic rocks. The zircon U-Pb age of the Erdaogou basaltic-trachyandesite and trachybasalt and rhyolite yield a early Late Carboniferous age (312±4 Ma, 312±3 Ma and 308±3 Ma respectively). (2) The basaltic volcanic rocks and the acidic volcanic rocks from Erdaogou Formation show the same magma source. The Erdaogou basaltic volcanic rocks were derived from mantle which were modified by slab fluids at an rifting stage. The magma was derived by partial melting (5%-25%) of asthenosphere mantle. The acid rocks were generated from the direct fractional crystallization of basaltic magma. (3) Two episodes of Late Paleozoic bimodal suites have been identified and it is proposed that the essentially bimodal character of the complex reflects the features of back-arc rifting magmatism in Balikun area. © 2018, Editorial Department of Earth Science. All right reserved.