内蒙古前进场岩体岩石学与锆石U-Pb年代学研究

Petrology and LA-ICP-MS Zircon U-Pb Geochronology of the Qianjinchang Pluton,Southeastern Inner Mongolia

前进场黑云母花岗岩岩体位于内蒙古东南部,华北板块与西伯利亚板块碰撞的缝合部位。岩体北西边缘与林西组地层的接触带发育道伦达坝中型铜–钨–锡多金属岩浆热液矿床。岩石地球化学研究表明,黑云母花岗岩为高钾钙碱性系列的过铝质花岗岩,Na2O+K2O含量为6.12%~9.31%,Al2O3含量为14.42%~15.59%,A/CNK值为1.09~1.22,δ均值为2.41,在花岗岩ACF图解中落入S型花岗岩区。全岩总稀土含量为102.65~237.92μg/g,低Sr(89.91~192.80μg/g)高Yb(2.17~3.46μg/g),δEu值为0.46~0.81,亏损Ba,Sr,Eu,富集La,显示S型花岗岩特征,在Rb–(Y+Nb)图解中显示后碰撞岩浆地球化学性质。利用LA-ICP-MS锆石U-Pb年代学方法,确定岩体形成年代为(273±7.7)^(278±4.3)Ma,在早二叠世形成于华北板块和西伯利亚板块碰撞后拉张伸展的构造背景。

The Qianjinchang biotite granite from southeastern Inner Mongolia is located in the suture zone between the Siberian and North China Cratons. The Daolundaba Cu-W-Sn polymetallic deposit formed at the contact zone between the north-west margin of the pluton and the Lower Permian Linxi Formation. The biotite granite is high-K calc-alkaline and peraluminous, with Na2O＋K2O： 6.12% to 9.31%, Al2O3： 14.42% to 15.59%, A/CNK： 1.09 to 1.22, and a mean 5value of 2.41. The rock samples lie within the S-type granite region in the ACF plot. They are depleted in Sr （89.91 to 192.80 μg/g）, Ba, Eu （δEu： 0.46 to 0.81） and δREE （102.65 to 237.92 μg/g）, but slightly enriched in Yb （2.17 to 3.46 μg/g） and La, consistent with the characteristics of S-type granites. The rocks further fall in the region of post-collisional granite in the Rb vs. Y＋Nb diagram. LA-ICP-MS zircon U-Pb geochronology shows that the pluton emplaced at （273±7.7） to （278 ±4.3） Ma. These Permian granite rocks probably formed in an extensional setting that postdated the collision between the Siberian and North China Cratons.