Geodynamic evolution of the Paleo-Asian Ocean: Sr–Nd–Pb–Hf isotopic evidence from Paleozoic ophiolites and arc igneous rocks in the Central Tianshan, Northwest China

Orogenic belts worldwide record the geodynamic processes associated with the opening and closure of ancient oceans, and their study is fundamental to understanding global plate tectonics, mantle evolution, and continental growth. Unraveling the temporal evolution of oceanic lithosphere and related mantle domains in different orogens provides key insights into how regional tectonic processes reflect, and contribute to, global geodynamic systems. The Tianshan Orogenic Belt, as part of the Central Asian Orogenic Belt, was characterized by extensive Paleozoic magmatism linked to the evolution of the Paleo-Asian Ocean. However, its tectonic evolution and mantle characteristics remain unclear. To address this gap, we conducted a detailed field, geochronological, whole-rock and mineral geochemical, and Sr–Nd–Hf–Pb isotopic study of a newly identified ophiolite and dioritic pluton in Bingdaban, northern Central Tianshan. The diorite (462–449 Ma) has arc-like geochemical features and slightly depleted isotopic signatures. These characteristics indicate formation by 10 %–20 % partial melting of spinel lherzolite in the lithospheric mantle, with chemical modification by melts derived from the southward-subducting North Tianshan oceanic lithosphere. The ophiolitic mafic rocks yielded zircon U–Pb ages of 433–432 and 273 Ma, and have mid-ocean ridge basalt-like trace element and isotopic compositions, suggesting formation by 5 %–20 % partial melting of depleted spinel lherzolite, likely in a mid-ocean ridge setting. These results indicate that the North Tianshan Ocean existed until at least the late Permian. Moreover, Nd–Pb isotope data reveal a temporal transition from Pacific Ocean– to Indian Ocean–type mantle characteristics from the early to late Paleozoic, identifying the Tianshan Orogenic Belt represents a transitional zone between the Central Asian Orogenic Belt and Tethyan domain. Our findings indicate that tectonically-induced asthenospheric flow permits interactions between Pacific- and Indian-type mantle domains and contributes to evolving mantle compositions. More broadly, the documented Pacific- to Indian Ocean–type mantle transition in the Paleo-Asian Ocean provides a valuable analogue for understanding global-scale changes in mantle domains and their role in shaping the evolution of ancient oceans and orogenic systems.