The Origin And Evolvement Of Cenozoic Vocanic Magma In Tengchong Area

Western Yunnan region located in the southeast margin of Qinghai-Tibet plateau, is part of the eastern tethys tectonic belt. Tengchong block is located in the west of Nujiang suture zonefracture, is mainly composed of high grade metamorphic complex, lowly metamorphased strata, the Paleozoic intrusive rocks and Paleogene volcanic-sedimentary sequence. Others main study has been focused on tectonic evolution, magmatism and mineralization of Tengchong block in Study of Cenozoic volcanism in the area is still lintensive debating. According to previous research, Cenozoic volcanic rocks are distributed in plot in Ma?an Shan area of the Tengchong block(near Lotus Township) with complete volcanic eruption series. The dissertation, on the basis of investigation and research in the field, dedicate study of petrology, element geochemistry, Sr-Nd isotopic compositons and mineral chemistry on the composition analysis the Cenozoic volcanic series in the Ma?an Mountain time samples system the, to deepen the understanding of Tengchong Cenozoic volcanism and their origin and evolution.33 samples of MF1-MF6 eruption period of Cenozoic volcanic rocks were collected. The volcanic rocks are consisted of basalt, basaltic-andesite and andesite, rhyolite and dacite. The Sr-Nd-Pb isotopic and chemical compostions of plagioclase and olivine phenocrysts are analyzed.And "phenocryst" mineral composition analysis and the results show that different eruption stages of volcanism in Tengchong region are characterized by high potassium calc-alkaline series, which are enriched in large ion lithophile elements(Pb, Th, Ba, Nd) and relative depleted in high field strength elements.The volcanic rocks from MF2, MF4 and MF6 eruption stages showed obvious Eu negative anomaly and depletion of Ti and P. Profile analyses of different plagioclase phenocrysts by electronic probe show that their compositions are basic, which are obviously characterized by the structure of ring. Profile analyses of olivine phenocrysts showed that:Mg# numbersvary from equilibrium between their edges and melts to unequilibrium centres.The melt phases show that the source area of olivine existence. Olivine and feldspar, even if they share the same eruption stage, show different nature, i.e., plagioclase?s reverse bands and olivine?s normal bands, which show that different formation types and mechanisms of them. Plagioclases originated from multistage magma chamber, or captured surrounding rocks. Olivines may be products of original magma source area.The Tengchong volcanic rocks are characterized by isotopic ratios of high 87 Sr /86 Sr and low ratio 143 Nd /144 Nd. Pb isotopic compositons are abnormal. Accordingly, we speculate that the source area of the Tengchong Cenozoic volcanic rocks is EMⅡmantle.