滇东北码口地区峨眉山玄武岩岩石地球化学特征

Petrogeochemical characteristics of the Emeishan basalts in the Makou area, Northeast Yunnan

通过对滇东北码口地区峨眉山玄武岩的矿物学和岩石地球化学特征研究,对其成因以及母岩浆起源做出了合理的解释。该玄武岩SiO2的含量为48.88%~52.80%,为基性熔岩。在TAS图解中大部分样品点落入碱性玄武岩中,小部分为亚碱性玄武岩。其镁指数I(Mg~#)平均值为0.45%,比原始岩浆的0.67%~0.70%低,表明原始岩浆经历了一定分异;相对富集轻稀土而亏损重稀土(LREE/HREE=6.69~8.07),轻重稀土发生了轻微分异作用,可见轻微的负铕异常(δEu=0.72~0.93);在蛛网图中可见K、P和Sr等元素不同程度上的亏损,Ti/Y比值(Ti/Y含量为490.06~627.77)该玄武岩属于高钛玄武岩。研究表明,码口地区玄武岩为下地幔石榴石地幔橄榄岩部分熔融产物,形成于板块拉张环境,与地幔活动有关,在成岩过程中遭受地壳物质混染,并且发生了橄榄石、斜长石和单斜辉石的分离结晶作用。

This paper studied mineralogy and geochemical characteristics of Emeishan basalt in Makou area,northeastern Yunnan,this paper gives a reasonable explanation for petrogensis of these rocks and the origin of parent magma.The content of basalt SiO2 is 48.88%~52.80%,which is basic lava.In the TAS diagram,most of the samples fall into alkaline basalt series,and a small part is subalkaline basalt series.The content of Mg~#average value of 0.45%,which is lower than that of the original magma(Mg~#0.67%-0.70%).This indicates that the original magma undergoes some differentiation.The LREE/HREE ratio is 6.69-8.07,and the light and heavy rare earths are slightly differentiated,showing a slight negative Europium anomaly.(Delta Eu=0.72-0.93);K,P,Sr and other elements are depleted to varying degrees in the spidergrams map.The ratio of Ti/Y(Ti/Y content is 490.06-627.77)indicates that the basalt belongs to high-Ti basalt.The results show that the basalts in this area are partial melting products of lower mantle garnet mantle peridotite,formed in the continental intra-plate extensional tectonic setting and related to mantle activity.During diagenesis,they are contaminated by crustal materials,and olivine and plagioclase are separated and crystallized.