云南保山卧牛寺组玄武岩成因:地幔柱活动的产物?

Petrogenesis of the basalts of Woniusi Formation at Baoshan area, Yunnan: Is it of mantle plume origin?

保山卧牛寺组玄武岩为低钛拉斑玄武岩 ,具有大陆板内玄武岩的特征 ,总体与峨眉山玄武岩中的低钛玄武岩相似。其分异程度较高 ,富集大离子亲石元素和高场强元素 ,有明显的Nb、Ta负异常 ,Zr和Hf正异常 ,Eu无异常或弱负异常。该玄武岩由 3个大喷发旋回形成 3个岩石单元 ,其中下部早期第 1单元由致密块状玄武岩、斜斑玄武岩、杏仁玄武岩和凝灰岩组成 ,第 2和第 3单元中见少量橄榄玄武岩和粒玄岩以及辉绿岩脉。有较高的86 Sr/ 87Sr值(0 .70 5 966～ 0 .70 6 657)、较低的1 43Nd/ 1 44 Nd值 (0 .51 2 2 1 2～ 0 .51 2 2 83) ,εNd(t)多为负值 ,源区为介于EMⅠ和EMⅡ端员之间的富集岩石圈地幔。上述地球化学和同位素特征均可与峨眉山玄武岩下部低钛拉斑玄武岩对比 ,表明该期岩浆作用可能同为地幔柱活动的产物 。

The basalts of Woniusi Formation at Baoshan area, located in western Yunnan Province, are composed of low_TiO 2 tholeiites generally similar to the low_Ti tholeiites from Emeishan large igneous province. They are highly_differentiated magmatites rich in large ion lithosphere elements and high field strength elements, have significant Zr and Hf positive anomalies as well as high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd ratios, and show characteristics similar to those of continental_rift basalts. Geochemically it consists of three eruption cycles which can be divided into three rock units. The early_stage unit 1 basalts are characterized by massive plagioclase_phenocryst basalts, amygdaloid basalts and basaltic tuff. The middle_stage unit 2 and the late_stage unit 3 consist of minor olivine basalts and trachybasalt as well as diabase dykes. From the similar geochemical and Sr_Nd isotopic features with Emeishan basalt, they are interpreted as the products that evolved from the same parent magmas and experienced crystal differentiation of plagioclase and clinopyoxene. Their parent magmas were generated from an enriched lithosphere mantle associated with mantle plume. This suggests that the Emeishan large igneous province may have remarkable westward extension.