广西平南玄武岩的地球化学特征

GEOCHEMICAL CHARACTERISTICS OF ALKALINE BASALTS IN PINGNAN COUNTY, GUANGXI

平南碱性玄武岩产于 NE向的区域性断裂中 ,岩石类型有两类 ,一为碧玄岩 ,全岩 K- Ar年龄为 50 .8Ma,含各类包体 ,有以下特征 :1)含地幔岩包体 ,2 ) [Mg]=0 .78,3)在 Fe O/mol- Mgo/mol图解上液相线橄榄石的成分位于 Fo90～ Fo94之间 ,与橄榄岩包体中的橄榄石成分完全一致。4 ) Ni=0 .0 4 ( % ) <0 .0 5( % ) ,5)其熔浆密度为 2 .62 g/cm3,粘度为 2 .52 Pa· s,携带 4 cm直径包体上升的速度为 37.7cm/s,上升的速度较快 ,具原生碧玄岩浆的特点 ,岩浆液相线的温度为12 86℃ ,起源压力为 1.8GPa,玄武岩富集轻稀土可能与地幔性质有关。一为粗面玄武岩 ,不含包体 ,全岩 K- Ar年龄为 4 9.9Ma,岩浆液相线的温度为 1162℃ ,是进化岩浆。玄武岩的 Nd、Sr同位素显示为亏损地幔源区。它们都是大陆拉张环境下的产物 ,与中国东部新生代玄武岩的形成背景一致。

The Pingnan alkaline basalt occurred in a NE trending regional fault belt. Basalt has been divided into two types. One has all kinds of xenoliths including crustal xenolith and mantle derived xenolith,its whole rock K Ar age is 50.8 Ma. It has the following signals: a) with some lherzolite xenolith, b) [Mg]=0.78, c)with Fo90～Fo94, d) melting magma density=2.62 g/cm 3, viscosity =2.52 Pa·s,entraining xenolith with diameter 4 cm, v=37.7 cm/s. So this basalt is identified as primary magma. Its olivine phenocryst crystallization temperature is 1 286℃. It may be interpreted as temperature of magma liquids line. Its original pressure is 1.8 GPa. Another trachybasalt has no any xenolith. Its whole rock K Ar age is 49.9 Ma. Plagioclase crystallization temperature of trachybasalt is 1 286 ℃. It may be regarded as temperature of magma liquids line. Nd Sr isotope displays basalt derived from enriched mantle. Some discrimination figures from invariable primary elements and trace elements show that both the two basaltic volcanos erupted due to crust extension, which is the same as those of Eastern China in the Cenozoic.