Abstract:

Abstract:  Two kinds of Cenozoic potassicultrapotassic volcanic rocks of QiangtangMangkangDianxi and Gangdise are distributed in QinghaiTibet Plateau. The peak time of QiangtangMangkangDianxi ultrapotassic magmatism is 4030 Ma, and the main rocks have the characteristics of high content of MgO and low content of CaO and Al2O3 as typeI ultrapotassic rocks, while the 3024 Ma magmatism of middle and west Qiangtang area show the characteristics of typeⅢ potassicultrapotassic volcanics, which have low content of SiO2, high content of CaO and Al2O3, and low MgO/CaO ratio. The Cenozoic ultrapotassic volcanics of Gangdise also possess the characteristics of typeI ultrapotassic rocks in high content of MgO and low content of CaO and Al2O3, with forming time between 2512 Ma. Through analysis of the experimental data about ultrapotassic rocks, typeI ultrapotassic rocks came from the source region of phlogopite harzburgite that had undergone metasomatism of fluid (melt) enriched in silica and potassium; while the source region of typeⅢ potassicultrapotassic volcanics is carbonated peridotite mantle bearing potassic amphibole. The NangqianDianxi typeI ultrapotassic volcanic belt is strictly controlled by Honghe strikeslip fault zone in space. typeI ultrapotassic magmatism occurred during 4028 Ma, and the 16 Ma magmatism is translated into potassic basalt with OIB characteristics. The transition of source region from lithospheric mantle to asthenosphere indicates the thinning of lithospheric mantle and upwelling of asthenosphere caused by large scale strikeslip fault, which is the major controlling factor for decomposition melting of phlogopite in the metasomatized lithospheric mantle to generate the typeI ultrapotassic magmatism. The magmatism of 3534 Ma in middle Qiangtang area is the eruption of sodic alkaline basalt mainly from asthenosphere, and the 3024 Ma typeⅢ potassicultrapotassic magma was originated from lithospheric mantle. The transition of magmatic source indicates the activities of CO2rich fluid caused by the upwelling of asthenosphere is the major controlling factor in causing ancient metasomatized lithospheric mantle to warm up and melt to generate typeⅢ potassicultrapotassic volcanics. The upwelling mechanism of asthenosphere may be related to the breakingoff of subducted slab or delamination of lithospheric mantle. 

Key words:

Key words: Cenozoic; potassicultrapotassic volcanics; source characteristics; origin of magma; QinghaiTibet Plateau