Geochronology and Geochemistry of Northern Baiyunshan Granites in the Beishan Area,Inner Mongolia,China and Their Geological Significance

Baiyunshan orthoclase granite is located in the north of the Hongliuhe-Baiyunshan-Xichangjing ophiolitic mélange zone, there has been long controversial about the ending time of the early Paleozoic orogenic in Beishan area.In this paper, LA-ICP-MS zircon U-Pb chronology and geochemistry were studied in Baiyunshan orthoclase granite. The results show that the Baiyunshan orthoclase granite has the characteristics of high silicon, high alkali, low Al₂O₃, poor CaO and MgO, showing the characteristics of metaluminous-over aluminum and alkaline granite; high ΣREE, LREE slightly enriched,LREE fractionation significant HREE fractionation obviously, obvious negative anomalies of Eu,strongly enriched in Rb, Th, U, K and other large LILE, strongly depleted in Ba, Sr, P, Ti and weak losses of Ta, Nb.10 000Ga/A1 higher, 3.64~3.96, zircon saturation temperature is higher,808~881℃, Exhibited the feature of A-type granites.By LA-ICP-MS zircon U-Pb dating,a weighted mean ²⁰⁶Pb/²³⁸U age of (377.3 ±2.1) Ma (MSWD=0.034), is considered to be the formation age of the rock, that is to say Northern Baiyunshan granites were formed in late Devonian.Based on Characteristics of out crops,geochronology, geochemistry and geological setting, the authors hold that the Northern Baiyunshan granitic pluton might be a post-orogenic or late stage orogenic pluton,which formed in the tectonic setting of transformation from collision to extension in late Devonian.The newly developed Devonian A-type granite of the Baiyun Mountain in this work is of great significance to the time of the early Paleozoic orogenic activity.