Tectonic setting and evolution of the Kangguer Ocean: New constraints from structural and geochemical analyses of the late Paleozoic Kangguer mélange belt in Eastern Tianshan, NW China

Abstract

Deciphering the lithological and structural characteristics of mélanges in orogenic systems is crucial for understanding their tectonic setting, subduction polarity, and the evolution of closed oceanic basins. Here, we report new structural, geochronological, and geochemical data from the Kangguer mélange belt, situated between the North Tianshan Belt and the Central Tianshan Block in Eastern Tianshan, NW China, to elucidate the evolution of this previously proposed tectonic boundary. Detailed field mapping and structural anatomy reveal that the Kangguer mélange belt consists of accreted and dismembered ocean plate stratigraphy. Zircon U-Pb ages of basalts, gabbros, forearc siltstones, and sandstone matrices indicate the mélanges formed at ca. 312–288 Ma. Combined with maximum depositional ages of retro-foreland sandstones and crystallization ages of syn-accretionary and post-kinematic intrusions, the timing of tectonic stacking and terminal accretion is constrained at ca. 286–280 Ma. Geochemically, the massive basalts and gabbros display both normal mid-ocean ridge basalt (N-MORB) and island-arc tholeiite (IAT) affinities, suggesting they formed in the subduction zone-distal spreading centers of a back-arc basin. In contrast, the pillow basalts and their underlying gabbros exhibit enriched MORB-like signatures, likely representing accreted seamounts that are ascribed to plume-ridge interaction. These data, together with structural and paleogeographical evidence, indicate that the Kangguer mélange formed in a back-arc setting during the Early Carboniferous to Early Permian (ca. 336–288 Ma), rather than representing a mid-ocean-ridge-type oceanic lithosphere sustaining into the Early Triassic. Kinematic indicators, spatial distribution of subduction-related magmatism, and provenance analyses support a model of bidirectional subduction-accretion within the Kangguer back-arc basin. Integrating our new results with previously published data, we propose that the formation of the Kangguer back-arc basin was likely triggered by the north-dipping subduction of the South Tianshan Ocean, and the mélange records bidirectional subduction-accretion processes and a terminal arc-continental collision at ca. 286–280 Ma.