四川九寨沟隆康、漳扎火山岩研究

STUDY ON VOLCANIC ROCKS IN LONGKANG AND ZHANGZHA,JIUZHAIGOU,SICHUAN PROVINCE

论述了四川九寨沟隆康、漳扎火山岩的岩石学特征、时代及其构造学意义。火山岩呈层状、似层状 ,与海相碳酸盐岩、硅质岩互层 ,岩石细碧岩化 ,属古海相火山岩 ;以基性、中基性为主 ;既有碱性的 ,也有拉斑的 ;既有钾质的 ,也有钠质的。火山岩的稀土元素、微量元素分布型式介于大陆裂谷型与拉脊山寒武纪、陕甘川邻接区元古宙古洋盆类型之间 ,是小型陆间洋盆阶段的产物。火山岩的时代经详细调查 ,证实该地有两期火山岩。其中一期火山岩细碧岩化较强 ,在与其互层的碳酸盐岩中发现大量泥盆纪牙形石 ,时代属泥盆纪。火山岩的 K-Ar法同位素年龄值为 2 2 4～ 2 77Ma,代表古洋盆闭合造山和火山岩细碧岩化的年代。该期火山岩形成于南秦岭泥盆纪时的南秦岭洋早、中期阶段 ,在中三叠世印支运动主幕南秦岭主体造山阶段遭受绿片岩相变质而变成细碧角斑岩。另一期火山岩较新鲜 ,其 K-Ar法同位素年龄值为 2 2 6Ma,是晚三叠世早、中期南秦岭洋再度开裂的产物 ,晚三叠世末期印支运动末幕的南秦岭终结造山运动不强烈 ,因而火山岩的变质程度很低。

The paper presents the petrologic characteristics,ages and its tectonics significance of the volcanic rocks in Longkang and Zhangzha,Jiuzhaigou,Sichuan Province.The volcanic rocks appearing alternately with marine carbonate and siliceous rock present stratiform and bedded rocks under spilitization,and so they should belong to paleo marine volcanic rocks.Their acidity is mainly basic and intemediate basic.While their alkalinity series contain alkalic ones and tholeiitic ones.The R Na type has both potassic and sodic type.Their REE and trace element distribution patterns intervene between centinental rift pattern and vicinal region of Shanxi,Gansu and Sichuan Proterozoic Eon paleo ocean basin pattern of Laji Mountains Cambrian Period.The volcanic rocks are assumed to be the products of small sized intercontinental ocean basin period.The investigation verifies that volcanic rocks in this place are of two periods.Those of one period possess strong spilitization.A large quantity of conodonts of Devonian Period are found in the carbonate appearing alternately with them.Accordlingly,the actual period of the volcanic rocks belongs to Devonian Period. Their K Ar isotopic age is 224-277 Ma,and represents the age of paleo ocean basin closure orogenesis and the age of volcanic rocks spilitization.Volcanic rocks in this period are formed in the early and middle stage of Southern Qinling Mountains ocean of Southern Qinling Mountains Devonian Period,whereas the rocks received metamorphism of greenschist facies and turned into spilite Keratophyre in the orogenesis stage of Southern Qinling Mountains'main body,middle Triassic Period,Indosinian movement main episode.However volcanic rocks of another period are very fresh,with K Ar isotopic age of 226 Ma.They are believed to be the products of Southern Qinling Mountains ocean reopening in early and middle stage of late Triassic Period.As last episode of Indosinian movement in late Triassic Period was not severe in southern Qinling Mountains'final orogenesis movement,the metamorphic rank of volcanic rocks is very low.