Neoproterozoic Granitic Gneiss from the South Altyn Tagh, NW China: Constraints on Rodinia Assembly and Gondwana Formation

The assembly and breakup of the Rodinia supercontinent and the subsequent formation of the Gondwana megacontinent represent a pivotal stage in Earth’s history, involving the transition of plate tectonic regime and the evolution of Earth’s habitability. Located to the southeast of the Tarim Craton, the South Altyn Tagh hosts extensive Neoproterozoic granitic gneiss. Previous studies on these rocks proposed multiple interpretations regarding their petrogenesis and tectonic setting, leading to a significant debate about the paleogeographic position of the South Altyn Tagh within the Rodinia supercontinent. In addition, the correlation between the early Paleozoic metamorphic ages recorded in the Neoproterozoic granitic gneiss and the amalgamation of the Gondwana remains underexplored.

This study focuses on the Xiaoluke (Xgn), Yuemaqige (Ygn), and Wengulu (Wgn) Neoproterozoic granitic gneiss in the Bashiwake area of the South Altyn Tagh. To constrain the crystallization and metamorphic ages, petrogenesis, and tectonic setting of these rocks, a petrological, geochronological, and geochemical data is presented. Zircon U-Pb dating yields the protolith crystallization ages of 924±8 Ma and 924±9 Ma for the Wgn and Ygn granitic gneiss, respectively, and 902±6 Ma for the Xgn samples. Combined zircon and titanite U-Pb dating constrained the metamorphic ages of the Xgn granitic gneiss to 554-459 Ma. The granitic gneiss rocks in this study are composed primarily of granodioritic and monzogranitic gneiss. They are characterized by high SiO₂ contents (68.2-76.1 wt.%), FeO^T/MgO ratios (1.5-6.2), and K₂O/Na₂O ratios (0.98-3.4), classified as ferroan and high-K calc-alkaline to shoshonitic granitoids. Furthermore, they mostly display enrichments in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), accompanied by distinctly negative Eu anomalies (Eu/Eu*=0.3-0.7). Specifically, the ca. 924 Ma granitic gneiss exhibits Al₂O₃/(CaO+Na₂O+K₂O) (A/CNK) ratios of 1.01-1.22, relatively low Zr + Nb + Ce + Y values (181-341 ppm), and negative zircon ε_Hf(t) values (-13.4 to 0.2), akin to peraluminous S-type granites derived from ancient crust related to collision. Comparatively, the ca. 902-895 Ma samples display variable A/CNK ratios (0.91-1.07), higher Zr + Nb + Ce + Y values (377-1083 ppm), and positive zircon ε_Hf(t) values (3.3 to 6.7), similar to A-type granites originating from juvenile materials in a post-collisional setting.

Therefore, we propose that the South Altyn Tagh experienced a transition from a syn-collisional setting at ca. 924 Ma to a post-collisional setting at ca. 902-895 Ma, in response to the final amalgamation of the Rodinia supercontinent. The subsequent metamorphic events occurring between 554 Ma and 460 Ma were probably related to the formation of the Gondwana megacontinent.

This study was financially supported by the National Natural Science Foundation of China (grant 42272249).