皮尔斯比率图解在腾冲新生代火山岩研究中的应用

The Application of Pearce Ratio Diagram to the Study of cenozoic Volcanics from Tengchong Area, Yunnan Province

腾冲新生代火山岩属钙碱性系列，可分为四个喷发期，构成两个岩浆旋回：Ｎ＿２－Ｑ＿１为玄武岩—英安岩；Ｑ＿３—Ｑ＿４为玄武岩—安山岩。应用皮尔斯比率图解对其进行研究表明：分离结晶作用是岩石化学成分变异的主要因素；第一旋回的玄武岩浆可能起源于上地幔的部分熔融，经（Ｐｌ＋Ｃｐｘ±Ｏ１±Ｍｔ）矿物组合的分离结晶而形成英安岩浆。英安岩浆在形成或演化过程中受到了地壳物质的轻度混染。该旋回具有张性构造环境中双峰式火成岩套的特点。第二旋回的玄武岩浆可能起源于早期俯冲洋壳的部分熔融，经（Ｍｔ＋Ｐｌ＋Ｃｐｘ±Ｏｐｘ）矿物组合的分离结晶而形成安山岩浆，为安山岩成因的分离结晶模式提供了一个良好的实例。该旋回具有典型的弧火山岩的特点。

The Cenozoic volcanics in Tengchong area are of calc-alkaline series and might bedivided into four eruption stages(N_2,Q_1,Q_3 and Q_4),which make up two magmaticcycles composed of basalt→dacite(N_2→Q_1)and basalt→andesite(Q_3→Q_4)respectively. studies by means of Pearce ratio diagram showthat fractional crystallization seems to bethe major cause for variation in petrochemical composition. Basalts of the first cyclemight have originated from partial melting of the upper mantle,The fractional crystal-lization of the(Pl+Cpx±O1±Mt) assemblage was the main factor responsible for theformation of dacite and the assimilation of small amounts of crustal materials.This cycleshows features of bimodal igneous suite in a tensional tectonic environment.Basalts inthe second cycle might have resulted from partial melting of the early subducted oceaniccrust, with the fractional crystallization of the(Mt+Pl+Cpx±Opx) assemblage beingthe main factor for the formation of andesite.This cycle has characteristics of islandarc volcanics,being real post-collision arc volcanics.