Provenance of the early Cretaceous sediments in the southern Alxa Area: Implications for tectonic evolution of the south Central Asian Orogenic Belt

Abstract

The Central Asian orogenic belt is an accretionary orogenic belt that lies between the Siberian block and the Tarim and North China blocks. Since the central south of the Central Asian Orogenic Belt has been covered by desert and the Meso-Cenozoic Bayingobi Basin, its spatial configuration, evolution, closure time, and the Paleo-Asian ocean location have not been well described. Through zircon UPb dating, in-situ Hf isotope and rock geochemistry of the Lower Cretaceous Bayingobi Formation sandstones, the provenance of the Bayingobi Formation, and the central south boundary of the Central Asian orogenic belt were determined, the tectonic evolution process was discussed, and the tectonic evolution models were established. The provenance of the Bayingobi Formation mainly originated from Permian to Early Triassic granodiorite, diorite and potassium feldspar granite that cover a large area of the Zongnaishan-Shalazhashan tectonic belt. In the early Permian, this tectonic belt was mainly a continental island arc but gradually transitioned to a continental active margin, accompanied by intense tectonic magmatic activity. With the boundary as the Quagan Qulu ophiolite belt, the north is the Central Asian Orogenic Belt that mainly exhibits juvenile crustal materials, while the south is the Alxa block, which has an affinity with the North China block, and mainly exhibits older crustal materials. From the late Carboniferous to the early Permian, the Paleo-Asian Ocean plate was compressed, subducted along the Enger Us suture, and magmatic distributed along the Nuru-Langshan tectonic belt; from the late early Permian to late Permian, the Paleo-Asian Ocean plate rolled back, changing the subduction zone stress state from compression to extension. During this period, there were large-scale developments of magmatic rocks in the Zongnaishan-Shalazhashan tectonic belt. From early Early Triassic to Middle Triassic period, the Siberian active margin collided with the Alxa block, the Paleo-Asian Ocean was closed, and the overlying Alxa plate experienced lithospheric extension and thinning, accompanied by asthenospheric upwelling, partial melting and the addition of new crustal material. The Meso-Cenozoic basin sediments along the original suture belt are characterized by an enrichment of large ion lithosphere elements, a loss of high field strength elements, LREE enrichment and loss of HREE.