Geochemistry And Petrogenesis Of The Cenozoic Alkali-rich Porphyry In Yaoan Aera, Yunnan Province

The alkali-rich porphyries in Yaoan, Yunnan, which is located in the interaction between the western margin of Yangtze terrane and the east of the Jinshajiang-Ailaoshan fault belt, belong to the Jinshajiang-Ailaoshan alkali-rich instrusive rock belt. This paper regarding Alkali-rich porphyries as the research subject, mainly discuss its source nature, genetic mechanism and tectonic setting, through the testing and analysis of petrology, geochemistry, geochronology of zircon U-Pb and Sr-Nd isotope, so as to provide basis for the discussion of the deep dynamic process of alkali rich magmatic rocks, in the west of Yunnan province.(1)The U-Pb age of the alkali-rich porphyries is 33 Ma, which belongs to the late of Eocene Paleogene period and the alkali-rich porphyries are the product of post-collision magmatism of Tibetan Plateau.(2)The alkali-rich porphyries occur as stocks, dykes and dikes and belong to the potassic basaltic series. Rocks are rich of alkaline and K, low in Ti, and enrich in LREE and LILE(Rb, Sr, Ba, Th) and deplete in HREE and HFSE(especially Ti, Nb, Ta).(3)Isotope 87 Sr/ 86 Sr initial ratio is 0.709197~0.710028, and 143Nd/144 Nd initial ratio is 0.511976~0.512054 and ?Nd value is-12.5~-11.0. These geochemistry and isotopes geochemistry indicate that the source region have EMⅡenriched mantle Characteristic. The formation of the enriched mantle may correlate with the mixing and recycling between lower crust and oceanic sediments of Jinshajiang which belongs to Paleo-Tethys.(4)The alkali-rich porphyries may formed by low degree partial melting of the thickened lower crust, in post-collision tectonic background. Diagenesis has not undergo the crustal contamination and the isotopic composition is the inherent characteristic of the magma source region.(5)During the period of India-Eurasia collision(65-41Ma), the southeastern margin of the Tibetan Plateau in lateral compression background, lithosphere squeeze and contract, and the crust begins to appear large-scale of thrusting, then the crustal substantially shortens and thickens.During the period of India-Eurasia post-collision(40-26Ma), the stress field of the three-river area begins to change from transgression to torsion. The thickened lower crust and lithosphere cause the upwelling of the asthenosphere, in Jinshajiang-Ailaoshan suture zone. 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.