2.08 Ga andesitic gneisses in the southwestern Yangtze Block: Unveiling new insights into its geological evolution

Abstract

The early geological history of the Yangtze Block has been difficult to unravel due to limited exposure of Archean to Paleoproterozoic rocks. Through detailed geological investigations and mapping in the southwestern (SW) Yangtze Block, this study identified Paleoproterozoic andesitic and granitic gneisses, as well as associated sedimentary rocks, within the Xinanchang (XAC) Complex. Zircon U-Pb dating yields a concordant age of 2078 ± 11 Ma for the andesitic gneiss, and a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1835 ± 12 Ma for the granitic gneiss. The zircon grains from these gneisses exhibit oscillatory zoning and Th/U ratios of 0.40–0.94, corroborating their magmatic origins. Additionally, U-Pb age spectra of concordant zircon grains from the XAC strata range from 1.80 to 2.69 Ga. Together with the intrusive relationship between the 1.84 Ga granitic gneiss and the strata, this implies a depositional age of ca. 1.84 Ga for the XAC strata. The andesitic gneiss exhibits enrichment in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), but shows depletion in Nb, Ta, and Ti. These geochemical signatures are consistent with those of contemporaneous andesitic magmatic rocks in the northern Yangtze Block, suggesting that they were derived from subduction-related sources. In contrast, the granitic gneiss in the XAC Complex shows A-type granite affinities, similar to contemporaneous granitoids widely found across the Yangtze Block. This indicates that these granitoids were formed in a post-orogenic extensional tectonic setting. The northern and SW Yangtze blocks both show records of a transition from subduction to post-orogenic extensional environments between 2.10 Ga and 1.85 Ga, suggesting that the proto-Yangtze Block likely aggregated before 2.10 Ga.