Origin of the coeval Nb-enriched and arc mafic igneous rocks: Implications for Paleozoic tectonic evolution of the Southwestern Tianshan

Constraining the origin of the Nb-enriched mafic igneous rocks and associated rock assemblages is critical for understanding the tectonic evolution and geodynamic processes in the subduction zone. In this study, we focus on Early Devonian normal arc gabbros and Nb-enriched gabbros from the Central Tianshan Block (CTB), along with Early Carboniferous Nb-enriched basalts from the southern margin of Yili Block. The Early Devonian arc gabbros display pronounced Nb and Ta troughs, relatively depleted Sr–Nd–Hf isotopes, and low zircon δ18O value. In contrast, the Nb-enriched gabbros show both arc- and OIB-like trace element features, more enriched Sr–Nd–Hf isotopes, higher zircon δ18O value, and elevated Nb and Nb/La ratio. These rocks are coeval with N-MORB, E-MORB and OIB in the South Tianshan Belt and Nb-enriched mafic igneous rocks in the northern Tarim, suggesting their formation during back-arc opening of the South Tianshan Ocean, triggered by the southward subduction of Terskey Ocean beneath the Central Tianshan-Northern Tarim Craton. In such a circumstance, the normal arc gabbros represent mantle melts modified by aqueous solutions derived from subducting slab, while the Nb-enriched gabbros were generated by partial melting of mantle metasomatized by subducting oceanic slab-derived melts with rutile breakdown. The Early Carboniferous Nb-enriched basalts also display arc-like trace element signatures and depleted Sr–Nd–Hf isotope features, as well as high Nb and Nb/La. These basalts, coexisting with adakitic granite porphyry and sanukitic high-Mg andesites. These suggest their formation was primarily related to interactions between a mantle wedge and adakitic melts derived from basaltic oceanic crust, with or without sediment melts, in a back-arc setting developed during the northward subduction of South Tianshan Ocean. Thus, this study provides crucial insights into the origin of Nb-enriched mafic rocks and tectonic transition from Ordovician-Early Devonian subduction of the Terskey Ocean to Late Devonian-Late Carboniferous subduction of the South Tianshan Ocean.