Late Carboniferous adakitic granodiorites in the Qiongkusitai area, western Tianshan, NW China: Implications for partial melting of lower crust in the southern Central Asian Orogenic Belt

• 314 Ma Qiongkusitai granodiorites in the western Tianshan.
• The Late Carboniferous granodiorites display adakitic signature.
• Their parental magmas include both the Mesoproterozoic basement and juvenile material.
• Asthenospheric upwelling triggered by slab roll-back.

Carboniferous granitic rocks are wide spread in the western Tianshan, but their tectonic contexts remain controversial. We present zircon U–Pb age, major element, trace element, and in situ zircon Hf isotopic data for the Qiongkusitai granodiorites from the southern part of the Yili block in the western Tianshan. The granodiorites were emplaced in the Late Carboniferous (ca. 314 Ma) and are characterized by high SiO2 (62.1–65.7 wt.%), high Sr (376–485 ppm), low Y (13.7–17.7 ppm) and low Yb (1.34–1.81 ppm) with Sr/Y ratios of 23–34. Such characteristics imply adakitic affinities. Their low concentrations of Cr (3.85–6.75 ppm), Co (7.63–10.7 ppm) and Ni (2.94–6.80 ppm) suggest little, if any, interaction with a mantle source. Compared to the thickened lower crustal-derived adakitic rocks, the relatively low Nb/Ta ratios of the Qiongkusitai adakitic granodiorites indicate that they were probably generated by partial melting of amphibolite facies rocks, but not eclogite facies rocks. The Qiongkusitai adakitic granodiorites have positive zircon εHf(t) values of +0.7 to +5.1 and TDM2 of 1.28–1.68 Ga, and they are interpreted as being derived from remelting of amphibolite facies Mesoproterozoic basement with an input of juvenile material. Late Carboniferous magmatism in the western Tianshan was possibly triggered by asthenospheric upwelling as a result of the roll-back of the subducted southern Tianshan oceanic lithosphere.

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