Late Ordovician collision and orogen in middle Tianshan: Evidences of geochemical analyses and geochronology on metamorphosed granitoid rocks
中天山晚奥陶世碰撞造山:来自变质花岗岩地球化学及年代学证据

A series of metamorphosed and deformed granitoid rocks occurred in Baoertu area, middle Tianshan of Northwest China. After comprehensive study of petrology, mineralogy and geochemistry, such granitoid rocks are identified as typical calc-alkaline granitoids with following characters: high levels of Hf and Th, lower content of Li , Nb and Ta, the medium REE total, medium negative abnormity of Eu, and V-type REE distribution pattern. All of them indicate that the rocks are Caledon-I type and syn-collision granitoids. Ages of zircon were determined by LA-ICP-MS. Geochronology of U-Pb shows three ages, including 1207±87Ma, 455.6±1.8Ma and 433±19Ma. The first age of 1207±87Ma represents that of clastic zircon, reveals old basement of Proterozoic existed in Baoertu area. The second age of 455.6±1.8Ma indicates Baoertu pluton was intruded in Late Ordovician, which represents an important collision events while northern Tianshan ocean subducted southward beneath the middle Tianshan landmass. In Early Silurian about 433±19Ma, Baoertu pluton was re-built by the metamorphism which indicated a metamorphism had occurred in the area of middle Tianshan, Northwest China. Such results are significant to reveal the tectonic evolution in the area of middle Tianshan, Northwest China.