Studies Of The Geochemistry And Petrogenesis On Ma’anshan Trachyandesite Tengchong,Yunnan

In this paper, We have studied on petrology, mineral chemistry, major element, trace element and Sr-Nd isotopic compositions of the volcanic rocks in Maanshan, Tengchong volcanic field, Yunnan. And conbined with previous data, investigate the petrogenesis of Maanshan, Dayingshan and Heikongshan volcanic,we obtained following results:(1) The Maanshan volcanic field belonging to high-K calc-alkaline series, w Si O2 is among 56-59%, is intermediate volcanic rocks, REE distribution pattern showing they have similar LREE patterns negative Eu anomalies(0.68-0.75). trace element spider diagrams showing they enriched in LILE K、Rb、Th and LREE La, Ce, Nd, depleted in HFSE Nb, Ta, Ti, P, Yb and Sr,similar with island arc volcanic rocks.(2) The EMPA result of mineral shows the pyroxenes of phenocrysts and groundmass is clinopyroxene, plagioclase is middle-basic, which are obviously characterized by the structure of reverse zoning.And part of plagioclase have high K composition, showing that the magmatism is from basic to acid, The olvine show the normal ring, and the rock exist xenocrysts of mantle olivine.(3) Quartz phenocryst surrounded by pyroxenes(mainly) reaction rim texture in different morphology generally exist in the sample,showing that in the late stage of magmatic evolution there inject new silicon unsaturated magma,react to form pyroxenes.However the reverse zoning of plagioclase and basic plagioclase also show the inject magma is basic.(4) Comprehensive analysis the Maanshan, Dayingshan and Heikongshan volcanic rock, indicate the have cognate magma evolution characteristics, in accordance with the evolution sequence of basalt, basaltic andesite,andesite, rhyolite.La/Sm-La showing that the partial melting is the major magmatism in mag magma source.While in Harker diagram the Mg O, TFe2O3, Ti O2, Al2O3 are negatively correlated with Si O2, K2 O is positive correlated with Si O2,showing obviously fractional crystallization.Negative Eu anomalies and depletion of Sr show fractional crystallization of plagioclase, while the lower the content of compatible elements, Ni, Co, Cr and Mg O negative correlation proved fractional crystallization of olivine and pyroxene.The Sr-Nd isotopic characteristics of the overall display enriched mantle, 87Sr/86 Sr value 0.707510~0.707616, εNd value-7.37~-6.15 trachyandesite source area belongs EMII type enriched mantle, and basaltic andesite and crude dolerite has mainland OIB characteristics.