The Cenozoic alkaline intrusions in Pihekou area, Central Jilin: Petrogenesis, geochronology and geological implication

Cenozoic intrusive rocks are rarely exposed in Northeast China, making the Oligocene granite porphyry and syenite porphyry in the Pihekou area of central Jilin Province particularly significant for deciphering the region's Cenozoic tectonic evolution. The petrogeochemical study and LA-ICP-MS zircon U-Pb geochronological dating show that the Pianshishan pluton is aged of 29.0±0.4 Ma, consisting with the age range of other alkaline bodies(27.3-31 Ma) in the study area. Together, they constitute the Pihekou alkaline complex, interpreted as the product of a single Oligocene alkaline magmatic intrusive-eruptive event. The alkaline intrusions exhibit coherent geochemical features, including very high total rare earth element(REE) contents, strong negative Eu anomalies, and significant depletions in Sr, Ba, P and Ti. These characteristics indicate an alkaline A-type granite affinity and suggest that the magmas experienced considerable fractional crystallization. The rocks form a comagmatic alkaline assemblage, comprising felsic granite porphyry and intermediate aegirine-augite syenite porphyry(trachyte), which are temporally, spatially and genetically consistent. The petrogeochemical characteristics further reveal that these intrusions were formed in a continental rift setting through fractional crystallization of mantle-derived alkaline basaltic magma, with the source being either an enriched mantle or a metasomatized depleted mantle.