燕山地区大红峪组火山岩古风化作用新认识

大红峪组火山岩 ,在成岩后受古风化作用影响引起化学成分的变化 ,特别是深风化带岩石后期K2 O沉积的影响 ,造成从未风化到深风化岩石在TAS图解、碱度系数、岩系指数及岩石类型等方面相应地发生变化。在TAS图解上 ,岩石向碱性增加、SiO2 减少方向变化。碱度系数变大 ,岩系指数的变化使火山岩反映为从钙碱性到超碱性。岩石类型也出现不确定性。深风化带岩石根据岩石化学成分计算标准矿物时 ,出现白榴石、霞石等碱性矿物 ,而实际矿物未见这一现象 ,也是后期K2 O沉积影响的结果。排除古风化的影响 ,作者认为大红峪组火山岩应为钾质的粗面岩和粗面玄武岩 ,属钙碱系列。粗面玄武岩为铝过饱和型 ,粗面岩为正常型。不存在明显的岩浆分离结晶作用 ,其原始岩浆应以基性为主。由于上述的原因 ,影响了对岩浆来源、构造环境的判断。推断其构造环境 ,是在总的海进条件下 ,出现多次的海退 ,伴有多次火山喷发活动 ,水动能比较大 ,气候干旱 ,封闭、半封闭的海盆中富含钾的环境。

The new discussion on the palaeoweathering of volcanic rocks of Dahongyu formation in Yanshan area

The chemical composition of Dahongyu volcanic rocks in Yanshan area changed from the fresh volcanic rock to the weathered rock. Also, they changed in TAS, in alkalinity index, in serial index, and in rock type. The alkali added and SiO 2 reduced in TAS. The alkalinity index added because K 2O had been sedimented from the basin. There are leucite and nepheline in normative mineral but they don't occur in real mineral that resulted from changed chemical composition. The serial index changed from Calc-alkalic series to ultra-alkali. We have drawn a wrong conclusion with regard to magmatic origin and tectonic setting. We regard Dahongyu volcanic rocks as trachybasalt and trachyte, Calc-alkalic series, potassic. Trachybasalt is peraluminous type, and trachyte normal type. The determination of magma origin and tectonic enriornment are influenced by palaeoweathering,which always occurs in the transgression and dry climate and closed or semi-closed basin rich in potassic enviornment with many times of regressive and volcanic activeties.