蒙西花岗斑岩成因与锆石U-Pb年代学研究

STUDY OF TECTONIC ENVIRONMENT AND U-Pb DATING OF ZIRCONS FROM THE MENGXI PLAGIOCLASE GRANITE PORPHYRY

蒙西斜长花岗斑岩体位于东准噶尔琼河坝多金属矿集区东部。斑岩体内可见铜矿化,侵入中-上奥陶统荒草坡群地层中。该岩体w(K2O)为1.59%～5.06%,属于钙碱性-高钾钙碱性系列,A/CNK平均值为1.22。ΣREE的含量为29.99×10-6～90.99×10-6,平均值62.00×10-6,稀土含量较低。Ba/Th比值为140.19～629.74,平均值为395.72。Ba/Th比值大于300,一般代表俯冲带流体对岩浆源区的贡献比较显著。(La/Yb)N比值为1.46～6.56,平均值为4.69,轻重稀土分馏不明显。δEu值为0.77～1.13,平均值为0.99,具有弱的Eu异常。研究认为花岗岩为地幔物质与上地壳沉积物通过俯冲的混合造成。锆石阴极发光图像显示,锆石为无色透明,无裂纹,岩浆震荡韵律环带发育和较清晰的核部,进行了LA-ICP-MS U-Pb同位素测年研究,认为斜长花岗斑岩形成于410±11Ma。锆石核部年龄为443.6±2.3Ma,为继承性锆石年龄。

Copper mineralization in Mengxi plagioclase granite porphyry, which intruded the Huangcaopo Group of Mid-late Ordovician,occurred in the east Qiongheba multi-metal deposits area of East Junggar. Analysis showed that the ratio of K2O ranges from 1.59% to 5.06%,be- longing to calc-alkaline to high-K calc-alkaline series. Also,the average value of A/CNK is 1.22, the ratio of ∑REE ranges from 29.99× 10-6 to 90.99 ×10-6 with an average value of 62.00 × 10-6 ,the ratio of Ba/Th ranges from 140. 19 to 629.74 with an average value of 395.72, sugges- ting that the fluids in subduction zones is significant to magma source. The ratio of （La/Yb）N ran- ges from 1.46 to 6.56 （average value 4.69） revealed that the fractionation of light and heavy rare earth was not obvious. The ratio of δEu ranges from 0.77 to 1.13 （average value 0.99） demon- strated that the abnormality of Eu is weak. The magma was the mixture of mantle material and upper crust sediment formed through subduction. Cathodoluminescence image showed that zircons were colorless, transparent, crack free, with well developed magma concussion rhythm annulus and core. Through LA-ICP-MS U-Pb isotopic dating, plagioelase granite porphyry was formed in 410 Ma± 11 Ma and the age of single grain zircon is 410 Ma±16 Ma,indicating that the Mengxi granite porphyry formed in late Silurian.