The Disintegration Of Tuolai Complex-group In The West Segment Of Central Qilian Block And Its Geological Implication

For a long time, various views on the age and attribute of Qilian Block have been presented. However, we found the supposed Paleoproterozoic Tuolai Complex-Group actually comprises complex geologic bodies with diverse rock types, different metamorphic degrees, formation ages and nature, through regional geological investigation at 1:50000 scale in areas such as Shangrimuceer. Based on large scale geologic mapping, outcrops measuring and zircon dating, the Tuolai Complex-Group is divided into three parts: Tuolai Complex-Group, Neoproterozoic intrusive rocks and Paleozoic deformed intrusive rocks. The Tuolai Complex-Group included two rock formations that lower rock group and upper rock group. Neoproterozoic intrusive rocks are dominated by medium-coarse grained biotite monzonitic granite gneiss, intrude into Tuolai Complex-Group. Zircon U-Pb LA-MC-ICP-MS dating was conducted on samples of gneiss, quartzite, two-mica quartz schist with sillimanite and garnet and medium-coarse grained biotite monzonitic granite gneiss to constrain the depositional time of Tuolai Complex-Group. The results showed some characteristics. There is only one zircon showing the youngest age of 1058 Ma in lower rock group. There is only one zircon showing the youngest age of 1149 Ma. The youngest age assembly from two-mica quartz schist with sillimanite and garnet is ca.1178 Ma, implying the Tuo Lai Complex-Group is younger than ca.1178 Ma. The medium-coarse grained biotite monzonitic granite gneiss probably intruded into Tuolai Complex-Group at 896.7 ±3.6Ma. Therefore, the age of Tuolai Complex-Group can be constrained at 896.7 ~1178Ma. The three detrital samples possess similar age spectra. It is noteworthy that the zircons contain a small number of early-mid Paleoproterozoic zircons, suggesting the early-mid Paleoproterozoic magmatic activity barely happened. There are amount of late Paleoproterozoic and Mesoproterozoic detrital zircons, peaked at ca.1700 Ma, ca.1200 Ma and ca.1400 Ma, suggesting multi-phase intense magmatic activities occurred in Paleoproterozoic and Mesoproterozoic in the provenance. The detrital zircon age distribution characteristic is comparable to the Mesoproterozoic and Neoproterozoic detrital zircons from Northern Craton and Yangtze Craton, suggesting the Qilian Block has a closer affinity with North China Craton, and may witness the Columbia supercontinent aggregation and discrete. And the Tuolai Complex-Group was intrusioned by Jinningian(grenvillian) biotite monzonitic granite at 897 ± 4Ma, indicating that the Neoproterozoic Era of mid Qilian Block are likely to be involved in the Rodinia supercontinent aggregation. In the early Ordovician magma intrusion activity in the region, belongs to the early Paleozoic records about Qilian ocean subduction and closed.