Petrogenesis of Late Paleozoic volcanics from the Zhaheba depression, East Junggar: Insights into collisional event in an accretionary orogen of Central Asia

Abstract

The Carboniferous to Permian period marks an important transition from accretion to collision in the East Junggar terrane, NW China. Field and Well Fuqian-1 well core data from the Zhaheba depression provide a better window for understanding the magmatic process during this period and its implications for the continental growth of Central Asia. Field investigations reveal structural features characterized by NE–SW compression, with lithology composed of basic and intermediate-felsic volcanic rocks and pyroclastic rocks. The core samples from Well Fuqian-1 show dominantly basalt, basaltic andesite, dacite and minor pyroclastic rocks. We report zircon SHRIMP U–Pb ages of 276.0–279.8Ma from rhyolites and zircon LA–ICP-MS U–Pb ages of 315.4–317.4Ma from dacite and basaltic andesite. Our data suggest that the volcanic rocks from surface exposures and the well cores of Well Fuqian-1 formed in the Early Permian and the Late Carboniferous, respectively. The Late Carboniferous mafic rocks have geochemical characteristics similar to the intercalated Early Permian felsic rocks. The mafic rocks show low initial 87Sr/86Sr (0.703162–0.703783) and high εNd(t) (5.5–7.5), enrichment in LREE and some LILEs (such as Rb and Th) as well as HFSEs (such as Zr, Y), and depletion in Nb, Ta and Ti. Furthermore, they also display lower Sm/Th (1.6–8.4) and higher Th/Y (0.03–0.12) ratios than those of MORB, and variable Th/Zr (0.004–0.016), Ba/Th (61–839), Ba/La (6.13–48.77) and Ba/Nb (10–101) ratios. The geochemical data suggest that these rocks were derived dominantly from a 5–10% partial melting of a mainly garnet-rich with minor spinel-bearing Iherzolite mantle source metasomatized by slab-derived fluids. The felsic rocks are rich in silica (SiO2=57.43–78.07%) and alkalis (K2O+Na2O=5.33–9.28%) and possess high TFe2O3 (0.70–6.95%) contents and Ga/Al ratios, and low CaO (0.18–5.11%) and MgO (0.13–2.02%) contents. They represent typical high–medium-K calc-alkaline A-type rhyolite with enrichment of LREE and HFSE (Zr, Hf) and depletion of Ba, Sr, Eu, Nb and Ta. These rocks display high εNd(t) (6.9–7.3) and εHf(t) (10.3–14.6) values and young crustal model ages (348–557Ma). Integrating regional geological data from the East Junggar terrane and the available data from the proximal ophiolite belt, we propose that an arc–arc or arc–continent post-collisional setting in the Zhaheba area commenced at ca. 317Ma. The Late Carboniferous–Early Permian felsic and mafic magmas were derived from the partial melting of the juvenile mafic lower crust and subcontinental lithospheric mantle metasomatized by pre-Carboniferous subduction-related fluids through asthenospheric upwelling. This post-collisional event suggests that vertical accretion played an important role not only in the Late Carboniferous–Early Permian continental growth of the East Junggar, but also in the final evolutionary phase of the accretionary orogen.