PALEOZOIC TECTONIC EVOLUTION OF THE ALXA TECTONIC BELT (NW CHINA), SOUTHERN CENTRAL ASIAN OROGENIC BELT

The Alxa Tectonic Belt (ATB) is a key region for understanding the subduction history of the Paleo-Asian Ocean and the accretionary processes of the southern Central Asian Orogenic Belt. However, there has been little consensus on the architecture and timing of major orogenic processes of the ATB. This paper addresses the Paleozoic tectonic evolution of the ATB based on new field and laboratory data.

The Nuru-Langshan arc is is manifested by the ca. 460-400 Ma subduction-related granitoids and voluminous late Carboniferous-Permian granitoids. They generally have enriched whole-rock Nd and Hf-in-zircon isotopic compositions, indicating the involvement of ancient crustal basement in the generation of the magma. The Shalazhashan arc is mainly composed of late Carboniferous-Permian granitoids with subordinate volcanic and sedimentary rocks. The volcanic rocks are calc-alkaline dacite-rhyolite and show “block-in-matrix” structures in the field. Zircons in magmatic rocks reveal predominately positive ε_Hf(t)values, indicating mixing between mantle-derived magma and Precambrian basement during their genesis.The diagnostic zircon U-Pb-Hf isotopic characteristics and ubiquitous intermediate-felsic volcanic detritus in the Permian volcano-sedimentary rocks indicate rapid sediments accumulation sourced from a proximal magmatic terrane in a supra-subduction zone environment. The Zhusileng-Hangwula arc is mainly composed of Devonian to Permian granitoids, volcanic rocks and sedimentary rocks generated in an active margin setting. New detrital zircon U-Pb data from Late Devonian to late Permian sandstones revealed continuous magmatism from Ordovician to late Permian, with major age peaks at 437 Ma, 362 Ma, 302 Ma, and 277 Ma. The Yagan arc is mainly composed of Ordovician basalt, rhyolite and phyllite, Devonian rhyolitic and andesitic volcanics, and Permian volcanic-clastic rocks.

During the early Paleozoic, there were probably two subduction zones within the ATB, generating the Nuru-Langshan arc in the south and the Yagan arc in the north. From Carboniferous, the southward subduction zone migrated northward, potentially due to slab rollback. During the Permian, both the Engger Us Ocean and the Quagan Qulu Ocean subducted bidirectional leading to the formation of multiple subduction-accretion system and archipelagic paleogeography. The final stage subduction event took place in the late Permian to earliest Triassic as manifested by the Engger Us accretionary complex and the Permian arc-related basins.