Geochemistry And Petrogenesis Of Heikongshan Holocene Volcanic Rocks In Tengchong, Yunnan

Tengchong block is in the west of Sanjiang region,located along the southeastern margin of the Tibetan Plateau.There are 68 volcanoes in Tengchong Volcanic field.Among them,Heikongshan,Maanshan and Dayingshan are all Holocene volcanoes.It is worth studying to find the relationship between Tengchong and Sanjiang region in tectonic structure,and further between Tengchong and Tibetan plateau.This paper focus on the characteristics of petrology,major elements,trace element and Sr-Nd isotopic compositions of the volcanic rocks in Heikongshan.Combined with previous data of Heikongshan,Maanshan and Dayingshan,this paper obtains the following results:?1?With the contents of SiO₂ are among 52.6%-59.1%,most samples of the Heikongshan volcanic rocks are trachyandesites or basaltic trachyandesites.Enriched in LILE such as K,Rb,Ba,Pb,depleted in HFSE such as Nb,Ta,Ti,P.?2?All the volcanic rocks came from the same source,and then went through the process of fractional crystallization.SiO₂ has a good correlation with both major and trace elements.The contents of major elements which induce TiO₂,Al₂O₃,TFe₂O₃,MgO,CaO,MnO are negatively correlated with the contents of SiO₂,together with trace elements Sr,and Cr.While the contents of K₂O and Rb,Ba,Th U are positively correlated with the contents of SiO₂.Negative anomalies of Ba,Sr and n Eu indicate fractional crystallization of plagioclase.Depleted in P and Ti indicate fractional crystallization of apatite and Titanium oxides,respectively.?3?The value of La/Sm has a weak positive correlation with content of La,indicating partial melting happens during magmatic evolution.The Sr-Nd isotopic characteristics(¹⁴³Nd^/144Nd between 0.512167-0.512626,⁸⁷Sr/⁸⁶Sr between0.706850-0.708860)of the overall display enriched mantle.Huge changes of values of Th/Ta and Rb/Nb show that may be caused by subduction of crust.?4?Above all,magma of Heikongshan,Maanshan and Dayingshan have the same source,while Dayingshan has the highest degree of evolution.Tengchong Volcanic rocks are formed in the tetonic environment of continental arc.