Reconstructing multiple arc-basin systems in the Altai–Junggar area (NW China): Implications for the architecture and evolution of the western Central Asian Orogenic Belt

Abstract

The Altai–Junggar area in northwestern China is a critical region to gain insights on the tectonic framework and geological evolution of the western Central Asian Orogenic Belt (CAOB). In this study, we report results from integrated geological, geochemical and geophysical investigations on the Wulungu Depression of the Junggar Basin to determine the basement nature of the basin and understand its amalgamation history with the Chinese Altai, within the broad tectonic evolution of the Altai–Junggar area. Based on borehole and seismic data, the Wulungu Depression is subdivided into two NW-trending tectonic units (Suosuoquan Sag and Hongyan High) by southward-vergent thrust faults. The Suosuoquan Sag consists of the Middle–Late Devonian basaltic andesite, andesite, dacite, tuff, tuffaceous sandstone and tuffite, and the overlying Early Carboniferous volcano-sedimentary sequence with lava flows and shallow marine sediments from a proximal juvenile provenance (zircon εHf(t)=6.0–14.9), compared to the Late Carboniferous andesite and rhyolite in the Hongyan High. Zircon SIMS U–Pb ages for dacites and andesites indicate that these volcanics in the Suosuoquan Sag and Hongyan High erupted at 376.3Ma and 313.4Ma, respectively. The Middle–Late Devonian basaltic andesites from well LC1 are calc-alkaline and exhibit primitive magma-like MgO contents (7.9–8.6%) and Mg# values (66–68), with low initial 87Sr/86Sr (0.703269–0.704808) and positive εNd(t) values (6.6–7.6), and relatively high Zr abundance (98.2–116.0ppm) and Zr/Y ratios (5.1–5.4), enrichment in LREEs and LILEs (e.g., Th and U) and depletion in Nb, Ta and Ti, suggesting that they were probably derived from a metasomatized depleted mantle in a retro-arc extensional setting. The well LC1 andesitic tuffs, well L8 dacites, well WL1 dacitic tuffs and well L5 andesites belong to calc-alkaline and metaluminous to peraluminous (A/CNK=0.8–1.7) series, and display low Mg# values (35–46) and variably positive εNd(t) (4.5–8.5) and εHf(t) (10.2–16.8) values, as well as young isotopic model ages. These Devonian–Carboniferous intermediate–felsic volcanics are interpreted as the products of partial melting of a juvenile lower crust with some contributions from mantle components in an evolved island arc setting from immature to mature island arc. The basin filling pattern and the distribution of arc volcanics and their zircon Hf model ages with the eruptive time suggest that the Wulungu Depression represents an island arc-basin system with the development of a Carboniferous retro-arc basin. In combination with previous work, we propose that the northern Junggar area comprises three arc-basin belts from south to north: the Darbut–Luliang–Karamaili, Wulungu–Yemaquan, and Saur–Fuhai–Dulate. Such tectonic subdivisions are consistent with the regional gravity and magnetic anomaly data. The recognition of the Wulungu arc-basin system demonstrates that the Junggar Basin is likely underlain by juvenile continental crust rather than ancient Precambrian basement, and also implies that the CAOB was built by amalgamation of multiple linear arcs and accretionary complexes.