Structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin, Northwest China: implications for tectonic evolution of the Junggar Ocean

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 TiO 2 contents (0.69-0.83%) and TFe 2 O 3 /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 ε Nd (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 ε Nd (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. ARTICLE HISTORY

Late Paleozoic Exhumation of the West Junggar Mountains, NW China

“…Dextral strike-slip motion along this new structure ( Figure 3) accommodated the counterclockwise rotation of the West Junggar Mountains, which was coeval with SE vergent thrusting in the West Junggar Mountains (Buslov, 2011). Seismic profiles in the Junggar Basin show that at this time the West Junggar Mountains were thrust over basement rocks of the Junggar Basin ( Figure 6) (Li, He, et al, 2017). 40 Ar/ 39 Ar dating of major subvertical ductile shear zones across central Asia produces ages ranging from 290-245 Ma, indicating widespread deformation during the Permian and into the Triassic (e.g.…”

“…Dextral strike‐slip motion along this new structure (Figure ) accommodated the counterclockwise rotation of the West Junggar Mountains, which was coeval with SE vergent thrusting in the West Junggar Mountains (Buslov, ). Seismic profiles in the Junggar Basin show that at this time the West Junggar Mountains were thrust over basement rocks of the Junggar Basin (Figure ) (Li, He, et al, ).…”

“…Direct evidence for Mesozoic fault activity within the West Junggar Mountains is scarce. Evidence from seismic profiles in the western Junggar Basin for reverse faulting suggests some activity during the Jurassic, although this is far more pronounced in the northeastern and southern parts of the Junggar basin than in the west (Li, He, et al, ; Yang et al, ). Cenozoic deformation and reactivation of faults in the West Junggar Mountains has been weak compared to that experienced in the Tianshan (Avouac et al, ).…”

“…Low-temperature thermochronological studies of the Chinese Altai (Yuan et al, 2006) and the Irtysh shear zone in NE Kazakhstan Seismic profiles of the western part of the Junggar Basin are interpreted by Yang et al (2015), Li, He, et al (2017) to show major reverse fault activity in the Permian, followed by gentle folding in the Late Triassic-Early Jurassic. Yang et al (2015) found evidence for only minor and localized Early Jurassic reverse faulting in the basin adjacent to the West Junggar Mountains.…”

Tectonics, magmatism, and mineralization of Circum–Balkash–Junggar area in the Central Asian Orogenic Belt

“…Li et al (2017d) conducted structural analysis of the seismic profiles and report new geochronological and geochemical data for the late Carboniferous borehole volcanics (basalts, andesites, dacites, and rhyolites) in the Zhongguai arc. They conclude 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.…”