Petrogenesis and tectonic setting of Late Devonian I-type granitic plutons in the Kekesala area, Chinese western Tianshan: implication for tectonic evolution of the North Tianshan Ocean

Abstract

ABSTRACT Massive Late Palaeozoic magmatic rocks are well developed along the northern margin of the Chinese western Tianshan. Previous studies mainly focused on the Carboniferous to Permian magmatism, and the Devonian magmatic rocks were rarely reported. Thus, this study presents new zircon U-Pb dating, whole-rock geochemistry and Lu-Hf isotopic data for the Late Devonian granitic plutons from the Kekesala area of the Chinese western Tianshan, aiming to elucidate their ages, petrogenesis, and tectonic settings. The Kekesala granitic plutons comprise granodiorites, monzogranites and syenitic granites. Zircon U-Pb dating results show they were emplaced in Late Devonian (ca. 372 Ma). Geochemically, the granodiorites and monzogranites are characterized by high-K calc-alkaline I-type series, and have low Ni and Cr, and weakly positive εHf(t) values, interpreted to be predominantly derived from partial melting of a lower crust with minor addition of mantle-derived materials; The syenitic granites with medium-K calc-alkaline features, exhibit high Sr and low Y and Yb, likely originated from partial melting of a subducted slab involving some mantle peridotites. The Kekesala I-type granitic plutons provide robust evidence for the existence of a subduction regime for the northern part of the Chinese western Tianshan in the Late Devonian. Based on the studied results, together with available data, we conclude that initial subduction of the North Tianshan Ocean began at least in the Middle Ordovician and terminated at the beginning of the Late Carboniferous rather than in the Middle Silurian.