滇西碧罗雪山地区忙怀组火山岩地球化学特征

滇西碧罗雪山一带,澜沧江岛弧带上发育了中三叠纪忙怀组火山岩,岩石类型主要是流纹岩.常量元素SiO2、K2O、Na2O、Al2O3明显偏高,MgO较低,TiO2中等的特征.里特曼指数σ1.39~3.01,为钙碱性系列,在log t-logσ图解中全部落入造山带区.碧罗雪山一带流纹岩的大离子亲石元素含量变化比较小,微量元素表现为大离子亲石元素Rb、Sr、Ba明显富集,高场强元素Th也相对富集.Eu负异常明显,轻稀土元素富集,稀土分配模式为轻稀土富集右斜型.微量元素在Rb-(Yb+Nb)图解及Rb-(Yb+Ta)图解中大多数落入碰撞带花岗岩区和火山弧花岗岩区.综合岩石地球化学特征及构造环境判别图解指示,碧罗雪山地区忙怀组火山岩为澜沧江洋由西向东俯冲消减,由被动大陆边缘转化为活动大陆边缘过程中形成的岛弧火山岩.

Geochemical Characteristics and Tectonic Significance of Manghuai Formation Volcanic Rock in Biluo Snow Mountain Area in Western Yunnan

A bstract： The midd le Triassic Manghuai Formation volcanic rocks which belongs to typical island arc volcanic rocks of Lancangjiang island arc belt is located at the Biluo snow mountains,Western of Yunnan Province,SW China. The volcanic mainly erupted in manghuai formation,formed rhyolite. Major elements of SiO2,K2O,Na2O and Al2O3 were high,while low content of MgO and middle content of TiO2. The range of Rittmann index σ is 1.39~3.01,which belongs to calc-alkaline Series. In the log t-log σ figure,all samples are ranged into orogenic belt. Seen from the amounts of lithophile elements（Rb,Sr,Ba）,they have a narrow range in the area of Biluo snow mountains. High field strength element Th is obviously enriched. Eu negative anomaly is obvious. REE distribution patterns show oblique to the HREE side and enrichment in LREE. It is syn-collisional granite environment. Trace elements in Rb-（Yb＋Nb） diagram and Rb-（Yb＋Ta） diagram most fall into COLG and VAG. Synthesis of geochemical characteristics and tectonic setting discrimination diagrams indicate that Biluo snow mountains. Explain that Manghuai Formation volcanic rocks is subduction by Lancang Ocean basin from west to east. Biluo snow mountains region is the volcanic island arc system during the passive continental margin into an active continental margin.