Geochronology, geochemistry, and its geological significance of the Damaoqi Permian volcanic sequences on the northern margin of the North China Block

摘要

The Suji volcanic sequences are situated on the boundary between the northern margin of the North China Block (NCB) and southern margin of the Inner Mongolia-Daxinganling Orogenic Belt (IMDOB). The sequences mainly consist of dacites and rhyolites. Zircons from the Suji rhyolite and dacite yielded weighted mean ~(206)Pb/~(238)U ages of 270.2 ± 2.7 Ma and 268.0 ± 3.8 Ma. These ages constrain the volcanic sequences erupted at the late Early Permian. The SiO_2 content of the volcanic sequences ranges from 55.2 wt% to 78.8 wt%. These rocks exhibit high alkali contents, enrichment of large ion lithophile elements and light rare earth elements, and depletion in high field strength elements, with (~(87)Sr/~(86)Sr)_i ratios of 0.70601-0.71427, ε_(Nd) (t) of -9.5 to -11.8. The high (~(87)Sr/~(86)Sr)_i and negative ε_(Nd) (t) values are indicative of a significant contribution of crustal-derived rocks in the petrogenesis of the Suji volcanic rocks. The rhyolitic rocks have high ratios of Rb/Sr, K/Rb and 10000 × Ga/Al, and fractionated REE patterns with strong negative Eu anomalies. These geochemical features show a typical A_2-type affinity, suggesting the late Early Permian Suji volcanic sequences formed in a post-collisional extensional setting. The distinct geochemistry features of the Suji volcanic sequences, combined with many previous studies from neighbouring regions, demonstrate that the final collision orogenesis in southern IMDOB took place as early as 270 Ma.