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.
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.
The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high εNd(t) values (5.3–5.6) and initial 87Sr/86Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted εNd(t) (5.6–6.4) and εHf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.
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