The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are closely associated spatially and temporally, with a gradual variation between them. In order to understand timing of the high-grade metamorphism, we carried out SHRIMP U-Pb dating of zircons of the rocks. Zircons from the granulites and charnockitic gneisses are similar in structure and age. Zircon cores show magmatic zoning and have ages of 2507-2545 Ma. The ages are interpreted as the forming time of protolith of the granulites and charnockitic gneisses, indicating that a strong magmatism existed at that time in the Yinshan Block. The zircon mantles and rims show homogeneous structures and record a strong granulite facies metamorphism event around 2500 Ma, with a time interval between the metamorphism and magamatism being less than 50 Ma. These suggest that the Western Block was similar to the Eastern Block in tectono-thermal timing at the end of the Neoarchean.
The NNE-trending ductile shear zones in Yiwulü Mountain area were formed in relation to two successive extensional events.The Yiwulü High Temperature Extensional Ductile Shear Zone and the Waziyu Low Temperature Extensional Ductile Shear Zone were related to ductile deformation at higher temperatures and brittle-ductile deformation at lower temperatures,respectively.Both deformations were accompanied by large scale volcanic eruptions and magmatic intrusions.Based on structural analysis of macroscopic and microscopic deformations,and quartz lattice preferred orientations,we show that the early Yiwulü High Temperature Extensional Ductile Shear Zone was resulted from a NE-SW extension at amphibolite facies in the middle crust,whereas the Waziyu Low Temperature Extensional Ductile Shear Zone was due mainly to a NWW-SEE extension at greenschist facies in the upper crust.The SHRIMP zircon age of a syn-tectonic granitic dike emplaced at the late stage of high temperature extension is 155±2 Ma,indicating that the early extensional event took place in the Middle-Late Jurassic.40Ar-39Ar age of muscovite from tectonic schists in the low temperature extensional ductile shear zone is 131.6±1.0 Ma,suggesting that the late extension occurred in the Early Cretaceous.Subsequent overall uplifting succeeded the late extension.The new discovery of the Middle-Late Jurassic NNE-trending extensional ductile shear zone provides evidence constraining the switch of tectonic regimes and Middle Jurassic thinning of lithosphere in the eastern North China Craton.
LI GangLIU ZhengHongLIU JunLaiLI YongFeiXU ZhongYuanDONG XiaoJie
