The Late Carboniferous to Early Permian is a critical period of the Chinese Tianshan, witnessing the tectonic transition from subduction to post-collisional extension during the final amalgamation of the Central Asian Orogenic Belt (CAOB). The late Carboniferous Mozbaysay mafic–ultramafic complex in the Qijiaojing–Balikun area, eastern North Tianshan, provides important clues for revealing the nature and timing of this tectonic transition. The Mozbaysay complex comprises mainly hornblende gabbros and lherzolites. LA-ICP-MS U-Pb zircon ages of hornblende gabbro yielded a weighted mean age of 306 ± 1.9 Ma for this complex. These mafic–ultramafic rocks have high contents of MgO (up to 30 wt.%), Cr (up to 2493 ppm), and Ni (up to 1041 ppm), but low contents of SiO₂ (40.34–47.70 wt.%). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Th/Yb and Ba/Nb ratios imply the mantle sources could have been metasomatized by slab–mantle interaction with aqueous fluids from dehydration of the subducted slab. Thus, these mafic–ultramafic rocks were most likely produced by partial melting of the asthenospheric and lithospheric mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous mafic–ultramafic rocks was triggered by the decompression-induced influx of asthenospheric heat and melting through a slab window during post-collisional slab breakoff. Combined with geological data, the petrogenetic links of the Late Carboniferous mafic–ultramafic rocks in eastern North Tianshan to slab breakoff suggest that the tectonic transition from convergence to post-collision most likely initiated in situ at ca. 306 Ma and lasted to ca. 300 Ma.

The Early Paleozoic magmatic rocks in the North Qilian Orogenic Belt provide crucial insights into the deep crustal structure and tectonic evolution of the Proto-Tethys Ocean. This study presents geochronological, geochemical and Lu–Hf isotopic data for three representative plutons. The Pandaoshi (415 Ma) and Nantouhe (414 Ma) plutons are I-type granite with high SiO ₂ contents (71.65–74.21 wt%, 73.09–74.24 wt%) and Mg ^# values (38.2–44.6, 42.3–51.3), and negative Eu anomalies (Eu/Eu* = 0.54–0.71, 0.55–0.71). The Pandaoshi pluton has evolved ε _Hf ( t ) values (−6.8 to + 8.9), indicating that it formed from partial melting of ancient crust mixed with mantle-derived material. The Nantouhe pluton has positive ε _Hf ( t ) values (+4.9 to +8.9), suggesting that it formed through partial melting of juvenile crustal material with addition of mantle-derived materials. The Guanshanhe pluton (411–412 Ma) is A ₂ -type granite with high SiO ₂ contents (72.33–75.22 wt%) and Mg ^# values (33.8–45.6). It exhibits negative ε _Hf ( t ) values (−25.6 to −20.2), indicating that it formed from partial melting of the ancient crust. The three plutons were formed in a post-collisional orogenic phase. We propose the subduction of the Wushan–Shangdan Ocean, forming the Hongtubao back-arc limited ocean basin, and collision between the North Qinling Orogenic Belt and the North Qilian terrane owing to the closure of the Hongtubao Ocean.

Volcanic rocks of the Qi’eshan Group, which are widely distributed in the Dananhu arc of the Eastern Tianshan, NW China, have long been debated in terms of their formation age and tectonic setting. In this study, we conducted an integrated study of U-Pb apatite geochronology, whole-rock major and trace element geochemistry, in situ major element analyses of clinopyroxene, and “Rhyolite-MELTS” thermodynamic modeling on the basalts from the Qi’eshan Group. Geochronological data show that the weighted mean of ²⁰⁶Pb/²³⁸U ages of apatite is 329 ± 10 Ma. The basalts belong to the tholeiitic series and are characterized by enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and enrichment of light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs) with weak negative Eu anomalies. They were derived by partial melting of garnet-spinel lherzolite in a depleted mantle source metasomatized by subduction-related fluids, followed by fractional crystallization of spinel, olivine, and clinopyroxene. Clinopyroxene is dominated by augite, characterized by high Mg and Ca contents and low Al and Na contents. Machine-learning-based thermobarometry indicates that clinopyroxene crystallized at temperatures of 1027–1033 °C and pressures of 1.1–1.6 kbar. “Rhyolite-MELTS” isobaric crystallization simulations suggest that mantle-derived magma, with an initial water content of 4 wt.% and oxygen fugacity of FMQ, can generate melts compositionally similar to the volcanic rocks of the Qi’eshan Group through fractional crystallization at a pressure of 1.5 kbar. Combined with previous studies, we propose that the Qi’eshan Group basalts formed in an extensional arc setting related to southward rollback of the northward-subducting Kanguer oceanic slab, which caused asthenosphere upwelling and lithospheric extension, thereby promoting partial melting of the subduction-metasomatized mantle. Our data provide new insights into the Carboniferous rollback of the Kanguer oceanic slab in the northern part of the Eastern Tianshan.