A PRELIMINARY STUDY ON THE PETROLOGY AND PETROCHEMISTRY OF THE QUARTERNARY VOLCANIC ROCKS OF NORTHERN TIBET AUTONOMOUS REGION
Deng Wanming (Institute of Geology, Academia Sinica)
The northern Tibet and the adjoining Kunlunshan region are those of the geologically less-studied areas on the earth. Since the Cenozoic (especially Quaternary) time there have occurred rather strong volcanic activities in the northern Qiangtang plateau, and in consequence a east-west zone of volcanism was formed between the northern margin of the northern Tibet block and the Kunlunshan. Because of the high elevation, inclement climate and difficult terrain of the area no systematic geologic survey has been done here, notwithstanding there are some records on volcanies made by individual investigators in the past. This paper describes three areas of volcanies between 34°30' and 36°30'N, and at about 87°E, which are Bamaoqiongzong, Yongbohu and Qiangbaqian. They are all characterized by lava covers and lava flows and belong to the Hawaii type in respect of eruption form. The volcanics are believed to be of Quaternary age since the top horizon of the underlying rocks is assigned to Pliocene-Pleistocene. Geomorphologitally, these volcanic covers are usually dissected into table mountains, flat-topped and conical isolated hills. The microscopic investigation shows that the majority of the rocks are of vitrophyric structure. The mineral and chemical compositions indicate a marked regional difference: The Bamaoqiongzong volcanic rocks situated inside the northern Tibet block commonly contain aegyrite-augite, titanaugite, leucite, noselite, analcite and alkali-feldspar. The rock type assemblage appears to be rather complicated being represented by such peralkaline rocks as noselite phonolite, pseudoleucite porphyry and analcitite. The Yongbohu and Qiangbaqian volcanics located on the northern margin of the block seem to be more monotonous, being the normal series of augite dacite, trachyandesite and augite andesite. Chemically this type of volcanic rocks greatly differs from the former as well. The respective characteristic features of the alkaline and calc-alkaline rock series of this region are discussed and correlated with the simultaneous volcanics distributed in the eastern Asia. The difference between the volcanic series inside northern Tibet block and that of northern margin is not considered as an accidental phenomenon. The author attempts to explain the origin of said volcanics and the regularities of their distribution from the viewpoint of plate tectonics and attributes the formation of the volcanics to the compression and collision of the blocks of Talimu and northern Tibet, and the subsequent southward downthrusting of the former. This geological process has been going on since the Himalayan movements. This pattern may be an important supplement to the prevailing explanations for the formation of Qinghai-Tibet plateau as a result of the collision and subduetion of the Indian plate under the Eurasian plate.