Petrology, Sr–Nd–Hf isotopic geochemistry and zircon chronology of the Late Palaeozoic volcanic rocks in the southwestern Tianshan Mountains, Xinjiang, NW China
The Late Palaeozoic volcanic rocks, mainly consisting of basalt, trachyte, trachy-andesite, andesite and rhyolite, widely distributed in the southwestern Tianshan Mountains, have been proven to be formed during Late Devonian to Late Carboniferous time (>361–313 Ma) based on zircon sensitive high-resolution ion microprobe dating. The geochemistry demonstrates that the studied volcanic rocks represent a continental arc formed during the subduction of the Palaeo-southern Tianshan Ocean. The εHf(T) values of zircons in these volcanic rocks vary from +1.4 to +15.6 with weighted average values of +9.5 (Late Devonian), +8.9 (Early Carboniferous) and +10.3 (Late Carboniferous), suggesting a depleted mantle origin. However, the Late Devonian basaltic samples have negative εNd(T) values (from −5.16 to −3.07) and high initial 87Sr/86Sr ratios (0.7073–0.7098), whereas the Early Carboniferous volcanic rocks mostly have positive εNd(T) values (from −0.18 to +3.07) with low initial 87Sr/86Sr ratios (0.7044–0.7067), and the Late Carboniferous volcanic rocks are characterized by high εNd(T) values (+2.79 to +5.89) and low initial 87Sr/86Sr ratios (0.7032–0.7054). The assimilation–fractional crystallization (AFC) model is used to explain the isotope characteristics of the Late Devonian volcanic rocks in the southwestern Tianshan Mountains. Calculation shows that the Late Devonian samples could be formed by the AFC process between depleted mantle and continental crust. The Carboniferous basaltic rocks originated by partial melting of the mantle wedge.