Geochronology And Geochemistry Of Volcanic Rocks From The Daheshen Formation In Central Jilin Province

We present geochronological, geochemical, whole-rock Sr–Nd and zircon Hf-isotopicdata for late Paleozoic volcanic rocks from the Daheshen Formation in central JilinProvince, northeastern China, constrain the petrogenesis of the volcanic rocks and latePaleozoic tectonic evolution of the northern margin of the Northern China Craton, which isregarded as the eastern segment of the Central Asian Orogenic Belt (CAOB). Lithologically,the Daheshen Formation is composed mainly of rhyolite, rhyolitic tuff, dacite and andesite,with minor basalt. The zircons from three rhyolites, two dacites, one rhyolitic tuff and onebasalt are euhedral-subhedral, display oscillatory zoning and have high Th/U ratios(0.50–2.28), implying a magmatic origin. LA–ICP–MS zircon U–Pb age data indicate thatthe volcanic rocks from the Daheshen Formation formed at the Late Carboniferous(302–299Ma). Geochemically, late Paleozoic volcanic rocks have SiO2=52.13–81.77wt.%and K2O=0.86–6.88wt.%, belonging to mid-K to high-K calc-alkaline series. Theserocks are characterized by enrichment in large ion lithophile elements (LILEs) and lightrare earth elements (LREEs), and depletion in high field strength elements (HFSEs, such asNb, Ta, and Ti) and heavy rare earth elements (HREEs), with affinities to igneous rocksforming in an active continental margin setting. All volcanic rocks have depleted Ndisotopic compositions (εNd(t)=+2.4–+2.5for the basalts and+5.8–+7.1for the andesitesand dacites, respectively). In situ Hf isotopic results of zircon from the rhyolites show thatthey have εHf(t)=1.1to+10.6. All these geochemical features indicate that the andesites,dacites, and rhyolites likely originated from the partial melting of Meso-Neoproterozoic accreted lower crust (Hf and Nd model ages (TDM2) of1384–662Ma and1061–800Ma,respectively). In contrast, the basalts were derived from the partial fusion of a depletedlithospheric mantle that had subsequently been metasomatized by subducted slab-derivedfluids. These data, along with the regional geological investigations, suggest that thegeneration of late Paleozoic volcanic rocks from the Daheshen Formation was related tosouthward subduction of the Paleo-Asian oceanic plate beneath the northern margin of theNorth China Craton. This also indicates that the Paleo-Asian Ocean may have not closedbefore the Late Carboniferous.