The Nature of Precambrian Basement of South Gobi Microcontinent in Central Asian Orogenic Belt

Precambrian rocks preserved within the orogenic belt are interpreted as the Precambrian basement of their respective tectonic units. Their rock assemblages, formation ages, detrital zircon age spectra, and zircon Lu-Hf isotopic composition are commonly used to explore tectonic affinities with potential provenances. This issue is critically important, as it is integral to understanding the architecture and evolution of orogenic belt, as well as the amalgamation and fragmentation of supercontinents.

The tectonic affinities of ancient blocks and microcontinents within the Central Asian Orogenic Belt (CAOB) have aroused many controversies in recent years, hindering a comprehensive understanding of the tectonics of the CAOB. The South Gobi microcontinent (SGM), a significant component of the middle segment of the southern CAOB, has long been controversial because many questions remain regarding its existence and nature. This study focuses on the Chinese part of the SGM—the Zhusileng-Hangwula tectonic zone (ZHTZ) in northwestern China—to investigate the nature of its Precambrian basement. Systematic field-based zircon U-Pb-Hf isotopic and whole-rock elemental analyses were conducted on  plutonic and metasedimentary rocks.

The intrusions previously thought to be from Paleozoic time are now known to contain crystallization ages of 1462 Ma, 1365 Ma, and 884 Ma; a large number of ca. 1.4 Ga xenocrystal zircons; and numerous metamorphic zircons with ages of 939 Ma. Additionally, a two-mica quartz schist from the Beishan Group constrains the maximum depositional age to 1130 Ma. When integrated with previous studies, these data indicate that the Precambrian basement of the SGM, represented by the basement of the ZHTZ, underwent tectonothermal events at ca. 1.4 Ga and ca. 0.9 Ga, and deposited extensive late Mesoproterozoic–early Neoproterozoic siliciclastic-rich successions.

Based on these features, the SGM basement likely originated from northeastern Laurentia, possibly as a part of the Valhalla orogen. Furthermore, the weak tectonic affinity between the southern CAOB and adjacent ancient blocks or cratons suggests that some parts of the southern CAOB might not have been derived from the accretionary evolution of the flanking cratons, but rather from the accretion of microcontinents that originated from Laurentia.