内蒙古道郎呼都格地区A型花岗岩年代学、地球化学及地质意义

Geochronology and geochemistry of the Daolanghuduge A-type granite in Inner Mongolia,and its geological significance

道郎呼都格钾长花岗岩体位于华北克拉通北缘白乃庙构造带。SHRIMP锆石U-Pb定年获得139.6±1.7Ma岩体侵位年龄。岩体富硅(SiO2=75.79%～78.07%)、富碱(K2O+Na2O=7.39%～8.29%)、贫钙(CaO=0.22%～0.59%);稀土配分曲线呈现"海鸥式"分布特征,显示强烈的Eu负异常(δEu=0.03～0.12);微量元素特征显示具有较高Ga(21.2×10-6～26.6×10-6)、Zr(173×10-6～417×10-6)、Nb(32.3×10-6～42.4×10-6)和Y(24.6×10-6～53.9×10-6)含量,较低的Sr(14×10-6～44×10-6)、Ba(18×10-6～211×10-6)含量,在微量元素原始地幔标准化蛛网图上显示明显的Ba、Sr、P、Eu和Ti的负异常。以上特征表明道郎呼都格钾长花岗岩为A型花岗岩,为高温低压下长英质地壳的部分熔融及其后长石、榍石等的分离结晶作用的产物。结合区域构造演化,本文认为该区钾长花岗岩形成于板内伸展背景。在晚中生代期间,华北板块北缘的构造体制经历了重要的转变,由挤压体制转变为岩石圈减薄和地壳伸展,在伸展体制下,软流圈地幔上涌对上覆长英质地壳的直接加热作用促使其部分熔融形成该区A型花岗岩。

The K-feldspar granite intrusion from Daolanghuduge,Inner Mongolia,is located in the Bainaimiao arc of the northern margin of the North China Craton（NCC） .The SHRIMP analyses of zircons from the intrusion yield a weighted mean 206Pb/238U age of 139.6±1.7Ma.The intrusion is characterized by high SiO2 contents（75.79%～78.07%） ,high alkali contents（K2O＋Na2O=7.39%～8.29%） but low CaO contents（0.22%～0.59%） .The K-feldspar granite is marked by a ＂sea-gull＂ pattern of REE distribution with a δEu ranging from 0.03 to 0.12,showing a relatively significant negative Eu anomaly.These rocks are enriched in Ga（21.2×10-6～26.6×10-6） ,Zr（173×10-6～417 ×10-6） ,Nb（32.3×10-6～42.4×10-6） and Y（24.6×10-6～53.9×10-6） but depleted in Sr（14×10-6～44×10-6） and Ba contents（18×10-6～211×10-6） ,and exhibit remarkably negative Ba,Sr,P,Eu and Ti anomalies on the primitive mantle-normalized trace element diagram.All these characteristics resemble A-type granites which originated from the partial melting of felsic crust under high temperature and low pressure conditions and the subsequent fractional crystallization of feldspar,titanite,etc.Combined with regional tectonic evolution,we suggest that the K-feldspar granite formed in an intraplate extensional setting.The northern margin of the NCC went through a transitional tectonic regime from compression to extension during Late Mesozoic.Under this tectonic regime,the upwelling asthenosphere provided enhanced heat flux and triggered the partial melting of the overlying felsic crust and then produced the Daolanghuduge K-feldspar granite intrusion.