Geochemical Characteristics of Felsic Volcanic Rocks in Active Continental Margin above Ridge Subduction: A Case Study of the Neoproterozoic Suxiong Formation in the Western Yangtze Block

An N-S trending Neoproterozoic magmatic belt is preserved along the western margin of the Yangtze Block, which is characterized by predominant felsic and minor mafic-ultramafic rocks. However, four geodynamic models have been proposed to elucidate the formation of these rocks. In this paper, we present geochronological and geochemical data for dacite crystal tuff, rhyolitic welded tuff, and rhyolite from the Suxiong Formation. These findings, combined with the previously published Sm-Nd isotopic data, aim to provide robust constraints on the genesis of these felsic rocks and offer additional support for the geodynamic model of ridge subduction occurring beneath the western margin of theYangtze Block. Our study reveals that a fresh sample of dacite crystal tuff has a weighted average age of 813 ± 20 Ma. Major elemental data indicate that the samples have predominately peraluminous compositions. In addition, the rhyolites belong to the High-K Calc-alkaline Series and the dacite crystal tuffs are affiliated with the Calc-alkaline Series. They show seagull-shaped chondrite-normalized Rare Earth Elements (REE) patterns with negative Eu anomalies. Multi-element spider diagrams suggest they are enriched in Light Rare Earth Elements (LREE) and Large-Ion Lithophile Elements (LILE) and display Nb-Ta depletion. Additional geological, mineralogical, and geochemical clues suggest these rocks were derived from partial melting of associated Kangding Tonalite-Trondhjemite-Granodiorite (TTG) suites under low-pressure and high-temperature condition. This conclusion is also supported by the Sm-Nd isotopic compositions obtained from previous studies. The rocks have an A-type affinity and belong to A2-type group. On Rb-Hf-Ta triangular plot, they fall in the «volcanic arc» field and are thought to form at an «active continental margin». Moreover, these rocks are comparable in trace-element geochemistry to the rhyolites and dacites of Coast Range in western California, southern Alaska. In conclusion, we proposed that the 813 Ma felsic volcanic rocks of the Suxiong Formation were formed above a slab window driven by ridge subduction. This idea offers additional evidence for supporting a geodynamic model of ridge subduction existing along the western margin of the Yangtze Block during the Neoproterozoic era.