The Cenozoic High-k Calc-alkaline Volcanic Rock Series In The Northern Tibetan Plateau: Implications Of Crust-mantle Interaction

The formation and evolution of the Tibetan Plateau, which occupies a very important position in contemporary geological research field, has gone through a very long geological time and the last 50 Ma is thought as the most important period with specific geological significances, during which the Cenozoic high-k calc-alkaline volcanic rocks occurred in the Northern Tibetan Plateau.The main part of the Cenozoic high-k calc-alkaline volcanic rock series distributes at the south margin of the prime back of the Kunlun mountain and at the Northern part of the Qiangtang mini-plate. As the respond in the shallow position of such deep geological events as the subduction and collision of the plates and the formation and evolution of the Tibetan Plateau, the Cenozoic high-k calc-alkaline volcanic rocks are thought as the important window and geological probe for researching the material composition of the lithosphere, the structure and interaction of the crust-mantle in the plateau and the mechanism of the plateau uplift. The rocks have been followed with great interest by geologists for a long time.Although some publications on geology, petrology and architectonics in Qinghai Tibetan Plateau have been involved, most of writings concentrate on researching field in Eastern and Southern Tibet. Due to such rough conditions as extremely cold, less oxygen, bad transport in this area with the elevation more than 5000 m, systematic study and regional geologic survey on the widely distributed Cenozoic high-k calc-alkaline volcanic rocks in Northern Tibetan Plateau have not been reported. The writer has worked for more than 5 years and got magnanimous first-hand field geological data in this area. In this dissertation, the volcanism study is put into the general framework of the formation and evolution of the whole plateau and the volcanism is thought as an important geologic event. Based on the systematic study of regional geology, petrology, petrography, mineralogy, isotopic geochronology and geochemistry, the following conclusions about the Cenozoic high-k calc-alkaline volcanic rocks in the Northern Tibetan Plateau are obtained.1. The distribution, occurrence, eruption type and strata sequence of the Cenozoic high-k calc-alkaline volcanic rocks were cleared through investigation. The main parts of the rocks distribute in the range of E 87~91, N3340' -35, with an outcrop area about 6000 km2. The high-k calc-alkaline rocks are mainly seen at Znerkenwula mountain region, Duogecuoren-Taiping lake region, Cuoni-Luanqing mountain, Yaojinla region and Meiriqiecuo region. In the Zuerkenwula region, the area of the volcavic rocks is the biggest (about 2500 km2) and the thickness is from 10 to 424 m. The volcanic rocks in this region are with gentle inclinations,exposed mainly as lava plateau and lava sheet. Typical continental central eruption can be found and two facies, explosion and extrusion-overflow facies can be mainly distinguished. Upwards, 4 lithological segments(volcanic breccia segment, olive basaltic trachyandesite segment, latite seqment and trachyte segment) can be divided in the volcanic sequence, among with the trachyte and latite segments can be commonly seen in the Cenozoic high-k calc-alkaline volcanic rock area in the Northern Tibetan Plateau.2. The K-Ar, Ar-Ar isotopic chronology measurement result shows that the ages of the high-k calc-alkaline rocks are from 45 to 35 Ma. The Ar-Ar isotopic age exactly prescribes the limit to the volcanic eruption at about 40 Ma and it can be regarded as synchronous volcanic movement. It not only provides with new ages of the Cenozoic high-k calc-alkaline volcanic rocks and makes corrections of the theories as many researchers believe that the high-k calc-alkaline volcanic rocks mainly occurred after the Miocene and during the Eocene there was no obvious volcanic movements but provides isotopic chronology binding to time limit of the formation of enriched EII mantle and the thickening, thinning and uplifting of the lithosphere in the Northern Tibetan Plateau. The Eocene hi...