Carboniferous arc-related volcanism in SW Bogda Mountain, Northwest China, and its implications for regional tectonics

Abstract

The Upper Carboniferous of Bogda Mountain is a critical area for understanding final oceanic closure of the Junggar Ocean in the Central Asian Orogenic Belt (CAOB). The 4 km thick Liushugou and Qijiaojing formations, comprising tuffs and lesser lavas that accumulated in a subsiding marine basin, were studied to understand the architecture and petrogenesis of the volcanic succession. Tuffs, lavas and hypabyssal intrusions are mostly of calc-alkaline composition, ranging from basalt through andesite and dacite to rhyolite, interbedded with rare high-K rocks of basaltic trachyandesite to trachyte composition. Clinopyroxene and FeTi oxides are the main fractionating minerals in the calc-alkaline rocks. Plagioclase has altered to albite by sea-floor metasomatism. The rocks are enriched in LILE and show relative depletion in Ta and Nb, suggesting an arc-related origin. The tuffs and lavas pass northward into dominantly trachytic rocks, suggesting a northward subducting, intra-oceanic arc to back-arc system, with polarity contrary to previous literature, that was active from the time of the Kalamaili arc collision (~350 Ma) to shortly after the North Tianshan–Junggar collision (~315 Ma). Following that collision, West Junggar experienced sinistral strike-slip deformation, and related crustal scale faults in Bogda Mountain allowed the rise of basaltic magma, forming pillow lavas and hyaloclastites in a deepening basin. Finally, in the latest Carboniferous, the subduction-related volcanism ended and the Bogda basin was then onlapped by turbidites. The recognition of northward subduction in the Bogda segment of the CAOB has regional implications for final closure of the Junggar Ocean. Comparison with published data from Kalamaili, the Junggar Basin, West Junggar and Arbassy demonstrates three tectonic phases with different volcanism, sedimentation and deformation in different parts of the amalgamating orogen.