The Technique Of ～(39)Ar-～(40)Ar Dating And Isotopic Geochronology Study Of Tibetan Plateau

1. To improve the classic 40Ar/39 Ar dating techniqueThe 40Ar/39Ar dating technique is developed from the conventional K-Ar method. Several studies display that geological events sometimes result in excess Ar component within the minerals and often result in radiogenic Ar loss. In the two cases, the wrong ages obtained from the classical K-Ar method are higher than the true age (excess Ar present) or lower than the true age (radiogenic Ar loss). However, in the 40Ar/39Ar stepwise incremental heating method, the calculation technique of plateau age can avoid this error. Therefore, this method is especially useful for dating rocks that have undergone complex thermo-tectonic history in polymetamorphic areas. By improving the technique of sample preparing, sample irradiation in the nuclear reactor and correction of blanks, our laboratory has ability to produce more and more high precision ages of 40Ar/39Ar.2. Fundamental of laser 40Ar/39Ar dating method:Our Laboratory has successfully set up the laser microprobe 40Ar/39Ar geological dating method. Our works include adjustment of the high-gain electron multiplier in mass spectrometer and correction of mass discrimination, adjustment of the Laser, test for absorption ability of minerals to laser, designing and manufacturing high-vacuum sample chamber, sample preparation and irradiation, research on the variability of J values on the surface of rock chips, measuring atmospheric argon, determination and correction of blanks and ages, etc.The laser microprobe technique is particularly effective for some geological samples. It can be used, for example, for research of distribution of the components in extremely small samples that are very difficult to be separated and purified and those which contain excess argon. If samples contain different generation of mineral fragments of same mineralogical species, then it is possible to analyze the different generation of minerals and receive information about their ages and migration of Ar isotopes during mineral grains were fully or partly recrystallized.3. Isotope geochronology study in the central part of the Tibetan plateauThe central part of the Tibetan Plateau is consists of the Kunlun Terrane, Bayanhar Terrane, Qiantang Terrane and Lhasa Terrane . On the north of it is the Qaidam Basin. This area was controlled by four large-scale faults: South Kunlun suture, Jinsshajiang suture, Bangonghu Suture and Yaluzangbu suture.This area has undergone polymetamorphism and deformation and at least three periods of magmatism. Continuous activities of magmatism, metamorphism were mainly controlled by collision of the Terranes. Combining the isotope geochronological results with the fieldwork observations, we could draw some conclusions as follows:The Kunlun-Bayan Har terranes experienced the subduction of oceanic crust and collision somewhat earlier than the Bayan Har-Qiangtang terranes, with only a very short interval in between and even the collisions overlapped partially in time.In the northern part, the oceanic crust occurring between the Bayan Har and Kunlun terranes started subduction from south to north at about 243 Ma, and did not stop until the development of post-orogenic POG-type granite in the Bayan Har terrane at 220 Ma, thus illustrating the orogeny in the form of subduction and collision kept active for over 20 Ma. But it remains to be identified whether the Bayan Har terrane collided with the Kunlun immediately after the termination of the subduction. Although the layers uplifted nearly 2 km prior to the emplacement of POG type granite in Xidatan and Xiaonanchuan, it can not be confirmed that there occurred large scale collision within the Kunlun terrane, since no continent-continent collision-related granites (CCG type) were found in the East Kunlun rock belt.In the Jinshajiang suture zone, it lasted only a span of 9 Ma from the start of subduction (marked by the eruption of andesitic magmas and intrusion of intermediate rock of IAG type at about 227 Ma) to the end of subduction, con...