LA- ICP- MS U- Pb ages and geological significance of Upper Jurassic Tiaojishan Formation in Sihai Basin, central Yanshan Mountains

Volcanic rocks of the Tiaojishan Formation are extensively distributed in the Yanshan Mountains and are crucial for understanding the large- scale teconic—magmatic activity during Yanshan movement. Although the age of the Tiaojishan Formation has been discussed by previous studies, most have focused on the eastern and western parts of the Yanshan Mountains. Consequently, the timing and petrogenesis of the Tiaojishan Formation in the central Yanshan, particularly the Sihai Basin, remain poorly constrained. A bimodal volcanic rock assemblage has been identified within the Tiaojishan Formation section at Shuiquanzi Village, Sihai Basin, central Yanshan Mountains. To obtain the ages and geological significance of the volcanic rocks of the Tiaojishan Formation, a geological section was measured through the formation, and systematic samples were collected for isotope geochronological analysis. Methods:Based on detailed field geological survey, Petrological and LA- ICP- MS zircon U- Pb geochronological analysis are employed to reveal the formation age and environment of the Tiaojishan Formation. Results:Vesiclar amygdaloidal basalt and andesitic basaltic sed- volcanic rocks were discovered for the first time within the Tiaojishan Formation in the Shuiquanzi Village, Sihai Basin, Central Yanshan Mountains. These mafic rocks, together with previously discovered rhyolite, constitute a bimodal volcanic suite, indicating deposition in a rift basin. Petrological and LA- ICP- MS zircon U- Pb geochronological analysis were conducted on andesite—basalt (sampled from the lower part ) and rhyolite volcanic breccia (sampled from the middle part). The results indicate that andesite—basalt crystallized at 159. 8±3. 7 Ma, and rhyolite volcanic breccia at 159. 0±1. 5 Ma. Combined with previous dating results, thses ages demonstrate that the Tiaojishan Formation in the Yanshan Mountains was primarily formed in the early Late Jurassic. These new ages fill a critical gap in the central Yanshan Mountains and align with the westward younging trend of Tiaojishan volcanism, extending from eastern Liaoning (170~153 Ma) to the northwestern Hebei (157~146 Ma). Conclusions: The Sihai Basin represents a NE—SW- trending rift basin formed during the early to middle Yanshan Movement (pre- 160 Ma). The bimodal volcanism and precise geochronology support a tectonic model where Paleo- Pacific slab subduction triggered westward- migrating mantle melting and crustal extension. These findings refine the spatiotemporal framework of Jurassic magmatism and enhance understanding of subduction- related lithospheric thinning in East Asia.