The South Tianshan, which extends for 2,500 km from Uzbekistan in the west to northwestern China in the east, is a continuous tectonic unit situated on the southwestern margin of the Altaids or Central Asian Orogenic Belt (CAOB) (Figure 1a) (

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The Xiaotian-Mozitan Shear Zone (XMSZ) is the boundary between the North Huaiyang Tectonic Belt and the Dabie High-grade Metamorphic Complex (DHMC). It trends WNW-ESE and extends more than 170 km from Qingshan Town in Anhui's Jinzhai County in the west, via Mozitan Town in Huoshan County, to Xiaotian Town in Shucheng County, truncated by the Tan-Lu Fault in Tongcheng County in the east. This shear zone plays a key role in understanding the evolution of the Dabie Orogenic Belt (DOB) because of its close correlation with the DOB

The Altaids, also known as the Central Asian Orogenic Belt, lie between the Siberian and Baltic cratons to the north, and the Tarim and North China cratons to the south (Figure 1a). They are the largest accretionary orogenic collage on the planet, recording the world's highest rate of continental growth in the Phanerozoic (Jahn et al., 2000;Şengör et al., 1993;Wilhem et al., 2012;Windley et al., 2007). The Altaids comprise extensive outcrops of ophiolites, blueschists, eclogites, and schist-gneiss complexes, interpreted to be the remnants of island arcs, seamounts and oceanic plateaus, and microcontinents accreted during progressive subduction and consumption of the Paleo-Asian Ocean (

A-type granite is an important geodynamic indicator because it requires a high melting temperature that is commonly driven by extensional events. Here we report geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of newly identified A-type granitic rocks from the South Tianshan in the southern Altaids. Zircon LA-ICP-MS ages indicate that the granitoids were emplaced at ca. 298–272 Ma. Geochemically, they are metaluminous to slightly peraluminous (A/CNK = 0.95–1.10), and belong to the high-K calc-alkaline to shoshonitic series. They are characterized by relatively high zircon saturation temperatures (824–875°C), K2O + Na2O contents (7.31–9.36%), high field strength elements (HFSE; Zr + Nb + Ce + Y = 365–802 ppm), and Ga/Al ratios (2.8–4.2), which all point to an A-type affinity. In addition, they have slightly enriched Hf isotope compositions (εHf(t) = −10.9 to + 0.6), and corresponding Mesoproterozoic (1,272–1759 Ma) crustal model ages, suggesting they were probably generated by partial melting of mature crust that contained minor mantle-derived magmatic material. The granitoids have distinctive subduction-related trace element signatures, with deep Nb and Ta troughs, elevated large ion lithosphere elements (LILEs), and flat HFSEs patterns, very similar to arc-derived granites in the Lachlan accretionary orogen. Integration of these new sedimentological, structural and geochronological results with relevant published information provides a new data-archive, which indicates that neither the Tarim mantle plume nor post-collisional extension can explain the genesis of these A-type granitoids. Instead, we propose a new more pertinent and robust model according to which they formed due to high temperature gradient in a subduction-related extensional setting probably triggered by southward rollback of the South Tianshan oceanic lithosphere, which caused upwelling of asthenospheric mantle combined with an increased temperature that led to large-scale crustal melting. This process gave rise to a broad magmatic arc in the southern active margin of the Yili-Central Tianshan. Our new data shed light on the retreating accretionary orogenesis of the southern Altaids in the Permian.

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