Geochronology, Petrogenesis And Geological Significance Of The Cenozoic Potassic Magmatic Rocks In Sanjiang Area, Western Yunnan

The western Yunnan is situated to southeast of Tibet Plateau, where Cenozoic potassic lava is extensively distributed along the Ailaoshan-Red River fault zone and its vicinity. The researching area that are Dali, Jianchuan, Yaoan, Heqing, Fengqing in western Yunnan. This paper mainly deals with the geochronology, mineralogy, elemental and isotope geochemistry of the Cenozoic potassic lava including alkali-rich porphyry and mafic volcanic rocks in this area. The research are used to explore the temporal and spatial distribution, the nature of the source regions and the petrogenesis of the Cenozoic potassic lava.The study for the geochronology of the potassic lava in western Yunnan show that the U-Pb age range of the alkali-rich porphyries are40.53Ma-34.17Ma, and the age range of the mafic volcanic rocks are34.07Ma-4.9Ma. Combined with the previous studies, it suggest that the peak timing of magmatism are40Ma-30Ma, and its diagenesis timing later than40Ma which all in the time frame of the post-collision magmatism of the Tibetan Plateau. It is believed that the eruption of the potassic lava was a magmatic response to India-Eurasia collision.The study of the mineralogy for the Cenozoic potassic lava show that the pyroxene in the potassic lava are diopside and augite, and the mica in the alkali-rich porphyries are Mg-biotites, and mica are phlogopites in the mafic volcanic rocks, and feldspar in the alkali-rich porphyries are orthoclases, and in the mafic volcanic rocks are labradorites and bytownites. The Fo values are highly in the mafic volcanic rocks, and the olivines are chrysolites, respectively.The Cenozoic alkali-rich porphyries in western Yunnan belongs to the shoshonitic series, and the mafic volcanic rocks belongs to the potassic series, respectively. The alkali-rich porphyries in western Yunnan are characterized by the low medium value of Mg#, which is lower than that for the mafic volcanic rocks. All samples are enriched in LREE and LILE, and depleted in HFSE. The isotopes geochemistry showed that the source region have EM â…¡ enriched mantle characteristic and Dupal anomaly. The Cenozoic alkali-rich porphyries have similar geochemical characterisitics of adakitic rocks. The research showed that the Cenozoic potassic magma was not primary magma, but maybe the evolved magma which have undergone differentiation to various degrees. The origin of the alkali-rich porphyries maybe a partial melting of the thickened lower crust, and the source region of the mafic volcanic rocks maybe the lithospheric mantle metasomatism by the fluid which derived from the ancient subduction zone. Due to the collision between India and Eurasia, the crust and lithosphere of the western Yunnan substantially shorten and thicken. The delamination of the thickened lower crust and lithospheric caused the upwelling of the asthenosphere, and the role of large-scale shear-extensional in Ailaoshan-Red River fault zone are the important keys to the partial melting of the source region and the formation of potassic magmatism.