Early Permian Syn-Subduction Extension in the South Tianshan (NW China): Insights From A-Type Granitoids in the Southern Altaids

Nijiati Abuduxun,Jingmin Gan,Brian F Windley,He Yang,Wenjiao Xiao,Peng Huang,Xijun Liu,Miao Sang

doi:10.3389/feart.2021.831677

Abstract

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.

Highlights

The Altaids (Şengör et al, 1993) (Figure 1A) is the younger part of the Central Asian Orogenic Belt (1.0 Ga250 Ma) (Windley et al, 2007; Xiao et al, 2015), one of the largest accretionary orogens on the planet, which contains a record of the most intense period of accretionary growth in the PaleozoicMesozoic (Şengör et al, 1993; Windley et al, 2007; Xiao et al, 2020)
Permian magmatic rocks are widespread in the western Tianshan (Figure 1B; Supplementary Table S1), petrogenesis of which has long been an issue of hot debate that has hampered a better understanding of the latest stages of evolution of the southern Altaids
By integration of current and previously published data, we propose that upper-plate extension triggered by slab rollback was the geodynamic process that was responsible for the spatially and temporally-related Early Permian granitoids in the South Tianshan

Summary

Introduction

The Altaids (ca. 600–250 Ma) (Şengör et al, 1993) (Figure 1A) is the younger part of the Central Asian Orogenic Belt (1.0 Ga250 Ma) (Windley et al, 2007; Xiao et al, 2015), one of the largest accretionary orogens on the planet, which contains a record of the most intense period of accretionary growth in the PaleozoicMesozoic (Şengör et al, 1993; Windley et al, 2007; Xiao et al, 2020). 600–250 Ma) (Şengör et al, 1993) (Figure 1A) is the younger part of the Central Asian Orogenic Belt (1.0 Ga250 Ma) (Windley et al, 2007; Xiao et al, 2015), one of the largest accretionary orogens on the planet, which contains a record of the most intense period of accretionary growth in the PaleozoicMesozoic (Şengör et al, 1993; Windley et al, 2007; Xiao et al, 2020) It grew southwards from the Siberian Craton by the successive accretion of multiple arcs, accretionary complexes and micro-continents (e.g., Windley et al, 2007; Xiao et al, 2009; Safonova et al, 2017; Yakubchuk, 2017; Li et al, 2018), followed by its final amalgamation with the Tarim and North China Cratons along the South Tianshan and Solonker sutures (Xiao et al, 2003, 2014).

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