Mesoproterozoic (∼1.4 Ga) magmatism in northern Alxa, China: Implications for tectonic affinity of microcontinents in the southern Central Asian Orogenic Belt

Abstract

The origin and paleogeographic reconstruction of Precambrian microcontinents in the Central Asian Orogenic Belt (CAOB) are crucial for understanding continental crustal evolution, mechanisms of accretionary orogeny, and assemblage and breakup of the supercontinent during the Earth’s early history. The existence and basement nature of the South Gobi microcontinent within the central segment of the southern CAOB remain controversial. This study represents the first work to unveil the presence of Mesoproterozoic (∼1.4 Ga) intrusive rocks in the Zhusileng-Hangwula tectonic zone that is the central segment of the southern CAOB, and is distributed along the border areas between China and Mongolia. It offers pivotal evidence for the existence of the South Gobi microcontinent.

Three alkali feldspar granites samples collected from the Zhusileng-Hangwula tectonic zone yielded crystallization ages ranging from 1371 ± 17 Ma to 1420 ± 29 Ma, while detrital zircons from a two-mica quartz schist xenolith yielded similar ²⁰⁶Pb/²³⁸U ages clustered around ∼1.4 Ga. The ∼1.4 Ga zircons display depleted Hf isotopic signatures (εHf(t) = 3.2–6.0) and relatively young two-stage Hf model ages (1.9–1.7 Ga), suggesting rapid reworking of juvenile mantle-derived materials. These findings indicate that the Zhusileng-Hangwula tectonic zone constitutes a microcontinental fragment with Mesoproterozoic basement, rather than a Paleozoic accretionary complex as previously postulated. Combining the contemporaneous magmatism data from the microcontinents in the southern CAOB, the 1.48–1.32 Ga arc-related magmatism exhibits a coherent trend of geochemical evolution, potentially indicating a geodynamic transition from a convergent to an extensional environment. Moreover, the temporal, tectonic evolution, and petrological similarities imply a tectonic affinity between these microcontinents and Fennoscandia, and they may have originated from an accretionary belt along the edge of Nuna during the Mesoproterozoic.