LA-MC-ICP-MS zircons U-Pb dating and Hf isotopic compositions of the Paleoproterozoic amphibolite in Bayan Ul area, Inner Mongolia

The Bayan Ul Formation was formed in Paleoproterozoic, but the precise dating data of the amphibolite are very insufficient. In this paper, the authors present U-Pb ages and Lu-Hf isotopic compositions of amphibolite and associated granite in the Bayan Ul Formation by mean of LA-MC-ICP-MS. Combined with data of the granite that intruded into the amphibolite, the authors hold that the protolith of amphibolite was formed at 2300Ma. Based on regional geological setting, it is suggested that the amphibolite's protolith was the recycled product of the Diebusige rock group and expereinced two metamorphic episodes that occurred in 1988~2239Ma and 1892~1940Ma respectively. The period of 1892～1940Ma might represent the formation age of granite and former metamorphic facies (granulite) of amphibolite, which indicated the collision between the Yinshan and Ordos blocks. The 1813~1877 Ma might signify the metamorphic age of granite and retrograde metamorphism from granulite to amphibolite, which indicated the collision time between the west and east blocks in the North China Plate. The age of 1813~1877Ma was recorded only in the granite and not in the amphibolite, which implies that the amphibolite in the dry system could not generate partial melting again after repeated partial melting.