Dating Constraints of Tuffs from the Upper Triassic Xujiahe Formation in Western Sichuan on Coeval Magmatic Activity

Key words: 
• Western Sichuan Foreland Basin /
• Xujiahe Formation /
• Tuff /
• zircon U-Pb dating /
• geochemical characteristics

Abstract: The Late Triassic period in the Western Sichuan Foreland Basin represents a critical phase of the Indosinian orogeny. The sedimentary strata formed during this period not only preserve the geological records of the basin's tectonic evolution but also contain key information about volcanic activities during that time. Analyzing the material sources of tuff in the strata can provide important evidence for revealing the tectonic-magmatic activity patterns of the surrounding orogenic belts during the Indosinian Orogeny. This study selected the tuff of the Xujia Formation in the Jiguan Mountain section of the Western Sichuan Foreland Basin as the research object. Through a combination of petrological analysis, geochemical characteristic analysis, and zircon U-Pb isotopic dating techniques, the sedimentary age of this tuff layer and the source of its magma were revealed. Zircon U-Pb dating yielded ages of 213.5 Ma and 208.5 Ma, corresponding to the Late Triassic Norian-Rhaetian stage. The rare earth element (REE) patterns of the tuffaceous rocks exhibit a right-leaning distribution characterized by enriched light rare earth elements (LREEs) and relatively flat heavy rare earth elements (HREEs), with pronounced negative Eu anomalies. Primitive mantle-normalized trace element spider diagrams show enrichment in large-ion lithophile elements (LILEs, e.g., Rb, Ba) and depletion in high-field-strength elements (HFSEs, e.g., Ta), consistent with geochemical signatures of continental lower crust. Zircon Hf isotopic data indicate that the magmas originated from a mixture of crustal contamination and depleted mantle sources. Building upon this foundation, and in consideration of the regional distribution and formation age of Late Triassic igneous rocks in the periphery of the western Sichuan foreland basin, integrated with geochemical characteristics of magmatic rocks, it is concluded that the volcanic material of tuff shares a common magmatic origin with the Late Triassic andesite of the Songpan-Ganzi terrane. This magmatic activity is closely associated with the melting of continental crust under the context of bidirectional plate subduction.