Tectonic framework of Eastern Tianshan in the Early Carboniferous: constraints from alkalic intrusive rocks

Paleo-reconstructing the tectonic framework of an accretionary orogen is inherently difficult. The spatio-temporal occurrence and petrogenetic process of magmatic rocks originating from accretionary prism can provide insights into understanding the evolution of accretionary orogenesis. The North Tianshan Belt of the Eastern Tianshan consists of the Dananhu and Yamansu arcs and the in-between Kanggur ductile shear zone. However, it is still debated whether the arc-basin system in the Eastern Tianshan was generated by the bipolar subduction of the Kanggur Ocean or the southward subduction of the Junggar Ocean. Herein, we synthesized new and published geochronological and geochemical data of Carboniferous alkalic intrusive rocks in the Kanggur ductile shear zone to reconstruct the tectonic framework of the Eastern Tianshan. The suite of alkalic intrusive rocks is characterized by porphyritic texture, and phenocrysts consist of K-feldspar, quartz and albite. These alkalic rocks (K 2 O = 4.91–11.30 wt%) are enriched in large ion lithophile elements (Rb = 201 ± 66 ppm, Ba = 1683 ± 823 ppm) and possess high Rb/Sr ratios (10.75 ± 8.35) and K/Na ratios (36.0 ± 15.8). Combined with relatively evolved Sr–Nd–Hf–O isotopic compositions (εNd ( t ) = + 1.8 ± 1.2, εHf ( t ) = + 6.6 ± 1.5, δ 18 O = 6.3 ± 0.3‰) and intimate geochemical affiliation with the Kushui Formation accretionary complexes, we suggest an origin from anatexis of accretionary complexes for this suite of alkalic rocks. Relatively high zircon saturation temperatures (761 ± 23 ℃) of these alkalic rocks indicated geothermal disturbance, which is caused by corner flow of asthenosphere resulting from slab rollback. According to the geochemical variation of arc-related granitoid rocks in the Eastern Tianshan and N-trending Kanggur accretionary complexes, we suggested that long-lived bipolar accretion-subduction of the Kanggur Ocean gave rise to the accretionary prism of the Dananhu and Yamansu arcs.