Carboniferous Magmatic Cyclicity of the NW Junggar Arc, Central Asian Orogenic Belt: Insights From Seismicity, Zircon U-Pb Ages and Geochemistry

Abstract

The Central Asian Orogenic Belt (CAOB) is Earth's largest Phanerozoic accretionary orogen, yet the tempo and mechanisms of arc magmatism remain incompletely constrained. Carboniferous island-arc volcanism in the Zhongguai Uplift, northwestern Junggar Basin, is investigated to establish magmatic cyclicity during closure of the Paleo-Asian Ocean. Integrated 3D seismic interpretation and well-log correlation, together with zircon U-Pb geochronology, whole-rock major–trace element geochemistry and Sr-Nd-Pb isotopes, subdivide the Carboniferous volcanic succession into three volcanic cycles. Cycle I (ca. 335–322 Ma; Early Carboniferous) is dominated by intermediate to mafic lavas and volcanic breccias and exhibits typical subduction-arc characteristics. Cycle II (ca. 321–317 Ma; Late Carboniferous) comprises a felsic-dominated interval (dacite–tonalite) and is interpreted to relate to slab-window processes. Cycle III (ca. 313–303 Ma; latest Carboniferous–earliest Permian) forms a thick bimodal assemblage from andesite to rhyolite, consistent with arc–continent collision and subsequent tectonic reworking. Geochemical–isotopic signatures evolve from a fluid-modified depleted mantle source in Cycle I, to highly fractionated and slab-influenced melts in Cycle II, and to mixed mantle–crust magmas with locally developed A-type affinities in Cycle III. The cycle-based stratigraphic and geochemical framework documents a transition from oceanic subduction to collision along the West Junggar margin, refines the timing of Paleo-Asian Ocean closure and highlights that Carboniferous arc magmatism occurred in discrete pulses linked to tectonic events. This integrated approach provides constraints on episodic arc magmatism and associated continental crustal growth in accretionary orogens.