U-Pb ages and Hf-O isotopes of zircons from Late Paleozoic mafic-ultramafic units in the southern Central Asian Orogenic Belt: Tectonic implications and evidence for an Early-Permian mantle plume

Abstract

U-Pb age and Hf-O isotope data of zircons from the mafic-ultramafic complexes and their related rhyolites and granites in the Eastern Tianshan and Beishan regions were obtained by SIMS and LA-ICPMS. The Hf-O isotopic composition and Hf model age of the zircons are closely related to their tectonic setting and formation age. The zircons from the mafic-ultramafic complexes in the Bogeda-Haerlike and Jueluotage belts have higher εHf(t) (+8-+17) and lower δ18O (+4‰-+6‰), whereas most of the zircons in the gabbros, granites and rhyolite from the Middle Tianshan Massif and Beishan Rift display lower εHf(t) (0-+8) and higher ε18O (+5‰-+8‰). The positive εHf(t) and relatively lower δ18O values suggest that these mafic-ultramafic complexes were derived from depleted mantle which was subjected to subduction-related modification processes. The Hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the Central Asian Orogenic Belt during the Paleozoic. The Hf model ages of the analyzed zircons, together with the regional geology suggest that the Beishan area had a northward subduction, possibly from ca. 900Ma to ca. 400Ma, whereas the Eastern Tianshan had a south-directed subduction most likely from ca. 600Ma to ca. 310Ma. Additionally, zircons with ca. 280Ma U-Pb ages display wider and more scattered εHf(t) and δ18O variations than the relatively older and younger ones, which further support the Early Permian mantle plume model.