Geochronology and geochemistry of the Chuanwulu complex in the South Tianshan, western Xinjiang, NW China: Implications for petrogenesis and Phanerozoic continental growth

Abstract

The South Tianshan Collisional Belt (STCB), formed by the collision between the Tarim and Central Tianshan blocks, is pivotal for understanding the complex and prolonged tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Chuanwulu complex is located in the eastern part of the STCB, Xinjiang province, NW China. It consists of a gabbro–diorite unit (unit I) and a monzonite–syenite unit (unit II), which are both cut by some granite dykes (unit III). Based on LA-ICP-MS U–Pb zircon dating, the units I and II were emplaced at 287.8±4.3Ma and 286.4±2.5Ma, respectively. The SiO2 contents of the samples from the complex display a wide range from 50.52wt.% to 70.64wt.%, and most samples are of alkaline affinities. The magma mixing process during the formation of the complex is suggested by considerable petrographic and geochemical evidence such as mafic microgranular enclaves (MEEs) that occur within unit II, disequilibrium textures, linear negative correlations between SiO2 and some major elements, and a wide range of (87Sr/86Sr)t ratios and δ18OSMOW values. Unit I is characterized by relatively low SiO2 contents (50.52–55.05wt.%), high MgO contents (3.29–5.43wt.% with Mg#=0.50–0.56), high Sr contents (1646–3101ppm), pronounced negative Nb–Ta anomalies and light rare earth element (LREE) enrichment. In combination with their isotopic compositions [(87Sr/86Sr)t=0.70543–0.70751, εNd (t)=−2.3 to −1.8, δ18OSMOW=5.7–9.4‰, εHf (t)=0.2–4.9], these features indicate that they are derived from an incompatible element-enriched lithospheric mantle. The SiO2 contents of units II and III range from 58.88wt.% to 65.91wt.% and 67.10wt.% to 70.64wt.%, respectively. They exhibit pronounced positive Zr–Hf anomalies and negative P and Ti anomalies, and relatively elevated Sr and O isotopic compositions [(87Sr/86Sr)t=0.70543–0.70751, δ18OSMOW=8.9–9.1‰]. In addition, samples of unit III have high Sr (600ppm to 1201ppm) contents and Sr/Yb (373 to 1905) and low Y/Yb (8 to 12) ratios. The felsic rocks (units II and III) have a “C-type” adakite-like geochemical signature and are comparable with those of rocks derived from an ancient garnet-bearing amphibolite facies lower crust. Our study, complied with other geological evidence, indicate that the collision between the Central Tianshan and Tarim blocks and the final amalgamation of the CAOB should have occurred during Late Carboniferous. On the regional scale, we propose that the recycling of ancient lithosphere is the predominant mechanism which is able to account for Phanerozoic continental evolution in STCB, differing from other tectonic units in CAOB.