LAUSR

ABSTRACT The structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin offer some new constraints on the tectonic evolution of the Junggar Ocean. In this study, we conducted structural analysis of seismic profiles and report new geochronological and geochemical data for the late Carboniferous borehole volcanics (basalts, andesites, dacites, and rhyolites) in the Zhongguai arc. Seismic data suggest that the upper Carboniferous lies between the lower Carboniferous and middle Permian compressional tectonostratigraphic strata. Zircon secondary ion mass spectrometry U–Pb and whole-rock Ar–Ar dating results indicate that they erupted at 309–305 Ma. The andesites are low-Fe calc-alkaline, and characterized by relatively high Mg# (3.24–6.00%), Cr (41.2–70.4 ppm), and Ni (23.3–35.6 ppm) contents, and low TiO2 contents (0.69–0.83%) and TFe2O3/MgO ratios (1.48–2.08), similar to those typical of magnesian andesites. They were likely produced by reaction between slab-derived melts and mantle wedge. The basalts show tholeiitic to calc-alkaline affinity, high εNd(t) values (7.7–8.0) and Nb and Zr contents, variable enrichment in Th, U, and light rare earth elements (LREEs), and significant depletion in Nb and Ta. The felsic rocks display arc-like geochemical features with alkaline grantite, and positive εNd(t) values (5.6–7.1). The basalts are interpreted as resulting from partial melting of a metasomatized mantle by slab-derived melt and fluids with involvement of asthenospheric mantle in an extensional setting, whereas the felsic rocks were derived from juvenile lower crust. The formation of these late Carboniferous rocks is probably related to ridge subduction. Given the widespread presence of such rock association in the area from southern West Junggar terrane to Zhongguai arc and initial southeastward-vergent thrust structures in the Zhongguai arc, we suggest that the southern West Junggar terrane and Zhongguai arc might have been amalgamated as a unified overriding plate above a northwestward ridge subduction zone of the Junggar Ocean at 309–302 Ma. The occurrences of middle Permian terrestrial sediments and thrusting reactivation in the Zhongguai arc are possibly indicative of the final welding of the Zhongguai arc and other units of the Junggar basement.