Geochemistry and geochronology of Carboniferous magmatic rocks in the Sawur Mountains, northern West Junggar, NW China: implications for accretionary orogeny

Abstract

The accretionary orogeny of the Central Asian Orogenic Belt (CAOB) has been a popular topic over the past decades due to its complex tectonic process. The Carboniferous magmatism in the Sawur Mountains of the northern West Junggar plays an important role in understanding the accretionary process of West Junggar. Detailed geological mapping has revealed that the Carboniferous Heishantou Formation in the Sawur Mountains consists mainly of pillow lavas, basaltic andesites, dacites, tuffs and sedimentary rocks, which were intruded by the Lasite intrusion. The pillow lavas display low SiO2 and TiO2 as well as high MgO and FeOt contents and enrichment in light rare earth elements (LREEs) and Nb–Ta, suggesting derivation from an enriched lithospheric mantle in an oceanic island setting. With the oceanic crust subduction, they were scraped off and accreted in a fore-arc setting. Basaltic andesites exhibit high SiO2 and Al2O3 contents, low Mg#, Zr, and Nb contents as well as Nb/La ratios, and a clear depletion of Nb–Ta, indicating derivation from a mantle wedge affected by the inputs of slab-derived fluids in an island arc setting. Dacites possess calc-alkaline and adakitic geochemical features with high Sr and Ba contents, high Th/Yb ratios, enrichment in LREEs, low Nb/La ratios, a depletion of Nb–Ta and positive eHf(t) values (10.3–15.0). Accordingly, they are interpreted as being derived from the melting of subducted oceanic crust mixed with minor sediments in an island arc setting. Magmatic zircons from dacite yield a weighted 206Pb/238U mean age of 326.8 ± 1.6 Ma, constraining the crystallization age of the dacite as well as the timing of the oceanic crust subduction. The Lasite intrusion that intruded the Heishantou Formation is K-feldspar granite, exhibiting pronounced I-type affinity and a high potassic calc-alkaline series. This granite shows low Na2O and Mg# (36.9–40.3) values; low Zr and Y contents as well as 10,000 × Ga/Al ratios; high LREE, K and Pb contents; and a depletion of Nb and Ta. Altogether, the low (87Sr/86Sr)i (0.7037–0.7043) and positive eNd(t) (6.22–6.36) and eHf(t) (10.1–15.5) values suggest that the granites were derived from the partial melting of a juvenile lower crust affected by mantle materials in a subduction setting. The zircon weighted 206Pb/238U mean age of 316.6 ± 2.8 Ma represents the intrusive age of the granite and related subduction during the late Carboniferous. All of the above geochemistry and geochronology of the magmatic rocks constrain the subduction–accretion tectonics in the Sawur Mountains of the northern West Junggar during ~ 327–317 Ma and confirm the subduction of the Paleo-Asian Ocean (PAO) lasted until the late Carboniferous in this region.