准噶尔西北缘早侏罗世玄武岩的发现及地质意义

DISCOVERY OF LISA BASALT NORTHWESTERN EDGE OF JUNGGAR BASIN AND IT’S GEOLOGICAL SIGNIFICANCE

克拉玛依西蚊子沟早侏罗世玄武岩的发现为解决新疆北部中生代岩浆活动问题提供了依据.该岩体由灰绿色玄武岩和深灰色橄榄玄武岩组成,具有高TiO2、FeO、P2O5,富K2O、Na2O的特点,属典型的大陆玄武岩;εNd(t)值为4.4～4.9.87Sr/86Sr初始比值为0.70541～0.70543,钕、锶同位素组成表明属幔源火山岩;稀土元素La/Yb变化于1.21～6.65,平均值为3.04,;微量元素丰度值比一般比玄武岩高3～20倍,橄榄玄武岩与玄武岩样品的稀土和微量元素特征有着较大差异,表明原始玄武岩浆发生过分异过程.40Ar/39Ar法测定获(192.7±1.3)Ma(2σ)坪年龄,表明火山岩的喷发形成时代为早侏罗世.准噶尔盆地西北缘早侏罗世早期的基性岩浆喷溢是准噶尔盆地由挤压向伸展转化的标志性地质事件,表明从古生代末期开始至中生代初期形成的大规模的向盆地推覆运动已结束,准噶尔西北缘由前陆盆地向陆内坳陷盆地转化,这对准噶尔盆地油气形成、运移和聚集有重要影响.

The discovery of the Lias basalt at Wenzigou, Kelamay provided the evidence for answering the long pendent question about Mesozoic magmatic events of Northern part of Xinjiang. The Lias basalt consists of gray-green basalt and dark gray peridotite-basalt. According to petrochemistry of the basalt, TiO2, FeO and P2O5 are highly concentrated, as well as K2O and Na2O. it shows the characters of continental basalt; εNd（t） value is between 4.4 and 4.9 and ^87Sr/^86Sr value is between 0.70541 and 0.70543, it indicates that the lava came from mantle; lanthanide La/Yb varies between 1.21 and 6.65 and the average is 3.04,it shows the concentration of light lanthanide and depletion of heavy lanthanide; Generally, the abundance of trace elements are 20-30 times higher than they are in common basalts, As well as the abundance of Rb, Ba and Th. The abundance of Sr, Zr and Yb are low. By 40Ar/39Ar method, the age of （192.7＋1.3） Ma（2σ）of the basalt was obtained. That is Lias.The eruption of basic lava happened in early Lisa is the symbolic event which indicate Jungaer basin began to transform from extrusion setting to stretch setting. It also shows the large scale thrust activity which formed between late Paleozoic and early Mesozoic ended. Therefore, the northwestern edge of Junggar began to transform from foreland basin to continental sag basin. It shows great significance to the formation, travel and concentration of oil and gas in Junggar basin.