Geochemistry, zircon U-Pb age, and tectonic implications of the Late Cambrian volcanic rocks in the Zhaqiaohe area, western segment of the Lajishan structural belt

As a suture zone where the central Qilian and southern Qilian blocks converge within the Qilian orogenic belt, the Lajishan tectonic belt has long been subject to controversies regarding the formation age and tectonic setting of its Early Paleozoic volcanic rock series. This paper conducts petrological, geochemical, and chronological studies on the intermediate-basic volcanic rocks in the Zhaqiaohe area to clarify their formation age, tectonic background, and relationship with the evolution of the Proto-Tethys Ocean. LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 496±5 Ma for the andesites, providing isotopic chronological data for the formation age of volcanic rocks in this region. Geochemical analyses indicate that the volcanic rocks belong to the medium to high-k calc-alkaline series, characterized by strongly enriched light rare earth elements ((La/Yb)N = 15.55~26.60) and weak negative Eu anomalies (δEu = 0.89~1.03). The trace elements show significant enrichment in Rb, Ba and Th, along with obvious depletion in Nb, Ta and Ti. Tectonic setting discrimination diagrams consistently suggest that these rocks formed in an island arc environment. Based on a comprehensive study of regional tectonic evolution, it is proposed that the Late Cambrian volcanic rocks in Zhaqiaohe record the products of intra-oceanic subduction within the southern Qilian Ocean, forming in an intra-oceanic arc setting. Combined with the ages of contemporaneous volcanic rocks (523~506 Ma) in the central and eastern segments of the Lajishan belt and the age of ophiolites (~524 Ma), this study constrains the time range of intra-oceanic subduction activities in the Lajishan tectonic belt to 523~496 Ma. This further confirms the existence of the southern Qilian paleo-ocean basin, improves the understanding of intra-oceanic subduction evolution of the Proto-Tethys Ocean, and provides an important geological basis for reconstructing the Early Paleozoic plate tectonic framework of the Qilian region.