内蒙北山白云山北A型花岗岩年代学、地球化学及其地质意义

摘要

白云山北正长花岗岩位于红柳河-白云山-洗肠井蛇绿构造混杂岩带以北,长期以来对北山地区早古生代造山结束时间的限定一直存在争议.笔者选择白云山北正长花岗岩进行了LA-ICP-MS锆石U-Pb年代学和地球化学研究.结果表明,白云山北正长花岗岩具有高硅、高碱、低A12O、贫CaO和MgO的特征,显示出准铝质-过铝质、碱质花岗岩的特点;∑REE较高,LREE略富集,轻稀土元素分馏明显,重稀土分馏较明显,呈右倾“海鸥型”,Eu负异常明显;强烈富集Rb、Th、U、K等大离子亲石元素,强烈亏损Ba、Sr、P、Ti,并弱亏损Ta、Nb等元素.10 000 Ga/A1较高,为3.64～3.96,锆石饱和温度较高,为808～881℃,具A型花岗岩特征.LA-ICP-MS锆石U-Pb测年表明其形成年龄为(377.3±2.1)Ma,地质时代为晚泥盆世.对岩体特征、年代学、地球化学和地质背景综合分析结果表明,该岩体形成于后造山或造山作用演化晚期阶段,构造体制为挤压环境向伸展环境的转换.本次工作新厘定出的白云山北晚泥盆世A型花岗岩,对限定北山早古生代造山活动的时间具有重要意义.%Baiyunshan orthoclase granite is located in the north of the Hongliuhe-BaiyunshanXichangjing ophiolitic mé1ange zone,there has been long controversial about the ending time of the early Paleozoic orogenic in Beishan area.In this paper,LA-ICP-MS zircon U-Pb chronology and geochemistry were studied in Baiyunshan orthoclase granite.The results show that the Baiyunshan orthoclase granite has the characteristics of high silicon,high alkali,low Al2O3,poor CaO and MgO,showing the characteristics of metaluminous-over aluminum and alkaline granite;high ∑ REE,LREE slightly enriched,LREE fractionation significant HREE fractionation obviously,obvious negative anomalies of Eu,strongly enriched in Rb,Th,U,K and other large LILE,strongly depleted in Ba,Sr,P,Ti and weak losses of Ta,Nb.10 000Ga / A1 higher,3.64～3.96,zircon saturation temperature is higher,808～ 881℃,Exhibited the feature of Atype granites.By LA-ICP-MS zircon U-Pb dating,a weighted mean 206pb / 238U age of (377.3 ± 2.1) Ma (MSWD =0.034),is considered to be the formation age of the rock,that is to say Northern Baiyunshan granites were formed in late Devonian.Based on Characteristics of out crops,geochronology,geochemistry and geological setting,the authors hold that the Northern Baiyunshan granitic pluton might be a post-orogenic or late stage orogenic pluton,which formed in the tectonic setting of transformation from collision to extension in late Devonian.The newly developed Devonian A-type granite of the Baiyun Mountain in this work is of great significance to the time of the early Paleozoic orogenic activity.