Petrogenesis and tectonic implications of the late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain, western Tianshan

Abstract

The widespread late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain provide crucial constraints on the tectonic evolution of the western Tianshan. Here, we perform detailed petrological investigations as well as zircon U-Pb chronological, whole-rock geochemical and Sr-Nd isotopic analyses on these magmatic rocks from two geological sections along the Duku road. Magmatic rocks in the section I with zircon SHRIMP U-Pb ages of 306.8 Ma and 306.4 Ma are composed of medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte, while those in the section II consist of shoshonitic trachy-andesite, trachyte with a U-Pb age of 308.1 Ma, and monzonite with a U-Pb age of 309.6 Ma. All these magmatic rocks are characterized by strong enrichments in large iron lithophile elements with depletions of Nb, Ta and Ti, indicating the origination from subduction-modified lithospheric mantle. The εNd(t) values of the rock samples collected from the section I (2.80–5.45) and section II (3.34–5.37) are generally higher than those of the Devonian to early Carboniferous arc-type magmatic rocks in the Yili-central Tianshan, suggesting that depleted asthenosphere might also be involved in their generation. Based on these geochemical data and petrological observations, we suggest that the early-stage (308.1–309.6 Ma) shoshonitic monzonite, trachy-andesite and trachyte in the section II were generated by mixing between mafic magmas and trachytic melts, while the late-stage (306.4–306.8 Ma) medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte in the section I were produced by partial melting of depleted asthenospheric and metasomatized lithospheric mantle, followed by the processes of fractional crystallization and crustal contamination. Taking into account the available regional geological data, the subduction of south Tianshan ocean was probably ceased at ∼310 Ma, and these calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain formed in a post-collisional setting subsequent to slab break-off.