Paleozoic accretionary orogenesis of the Chinese Altai, Central Asian orogenic belt : constraints from sedimentation and magmatic activities

Abstract

The Chinese Altai is a critical element of the Central Asian Orogenic Belt (CAOB), the largest ancient accretionary orogen in the world. High-grade metamorphic complex, thick sedimentary strata, extensive volcanic and intrusive rocks are developed in the Chinese Altai, making it an ideal place to probe into the mechanism and evolutionary pattern of the CAOB. Despite extensive research, the evolutionary stage from generation to final amalgamation/collision of the Chinese Altai is not yet well illustrated. This Ph.D. research mainly attempts to solve the controversies over the basement nature and the tectonic environments in the history of sedimentation and magmatism in the Chinese Altai. High-precision U-Pb zircon geochronology suggests that the high-grade gneisses in the southern Chinese Altai represent meta-equivalents of either early Paleozoic sedimentary rocks or Devonian granitoids, but not Precambrian basement rocks. Integrated detrital zircon age and Hf isotopic data show that age spectra and εHf(t) values of the early Paleozoic sediments are consistent with those of magmatic rocks from the Lake Zone and Mongolian microcontinents in the east. The early Paleozoic strata that prevailed in the entire region extending from western Mongolia to Chinese Altai thus represents a giant accretionary wedge. Chemical analysis for the orthogneiss reveals that their protoliths represent a Devonian arc assemblage (ca. 411–398 Ma) formed by partial melting of the juvenile lower crustal rocks. The petrogenetic study on the coeval enclave-bearing calc-alkaline rocks (ca. 395 Ma) recognizes the mixing process between magmas from the juvenile lower crust and the mantle. The meta-basaltic suites from Altay Formation are dated at ca. 380 Ma, showing depleted isotopic compositions and chemical compositions intermediate between MORB and IAB. They display back-arc-basin basalt affinities and were erupted with melting of the upwelling asthenosphere. The ca. 354-365 Ma A-type granites in the southern margin are either metaluminous or peralkaline in composition. The metaluminous rocks have a source dominated by juvenile lower crustal rocks, while the peralkaline rocks were interpreted to have crustal sources with components of OIB-type alkaline rocks, which were previously derived from the enriched asthenosphere. Studies on the granitoids and volcanic rocks above depict an extreme Devonian lithospheric extension with mantle upwelling in the Chinese Altai. The Permian mafic complex from the southern Chinese Altai (ca. 283–275 Ma) was generated by the interaction of partial melts of the sub-continental lithospheric mantle and the depleted asthenosphere. The coeval granites were derived from juvenile crustal sources with depleted isotopic compositions. Therefore, it is considered that Permian magmatic activities in this area were formed by upwelling and melting of the mantle, along with heating and anatexis of crustal rocks under the mechanism of post-collision extension. Data from the Chinese Altai and adjacent terranes in western Mongolia reveal a continuous evolution of an accretionary orogenic belt in the protracted subduction. The alternating compression and extension with concomitant sedimentation, magmatism, and metamorphism, together with the arc/continent collision and superimposed deformation, finally determined the accretionary orogens.