内蒙古北山造山带中段石板井地区A型花岗岩锆石U-Pb年龄及对北山洋闭合时间的限定  

LA-ICP-MS zircon U-Pb age of A-type granite from the Shibanjing area of middle Beishan orogenic belt,Inner Mongolia,and its constraint on closure time of Beishan Ocean

作　　者：陈超 滕学建 潘志龙 修迪 何娇月 赵华平 张欢 张金龙 杨菊 CHEN Chao;TENG Xuejian;PAN Zhilong;XIU Di;HE Jiaoyue;ZHAO Huaping;ZHANG Huan;ZHANG Jinlong;YANG Ju(Institute of Regional Geological Survey of Hebei Province,Langfang 065000,Hebei,China;Tianjin Center,China Geological Survey,Tianjin 300170,China;North China Center for Geoscience Innovation,Tianjin 300170,China)

机构地区：[1]河北省区域地质调查院,河北廊坊065000 [2]中国地质调查局天津地质调查中心,天津300170 [3]华北地质科技创新中心,天津300170

出　　处：《地质通报》2020年第9期1448-1460,共13页Geological Bulletin of China

基　　金：中国地质调查局项目《阴山成矿带小狐狸山和雅布赖地区区域地质矿产调查》(编号:DD20160039);《内蒙古1∶5万西林陶勒、梧桐井、石桩子井、石板井幅区域地质矿产调查》(编号:DD20160039-01)。

摘　　要：石板井A型花岗岩体位于内蒙古北山造山带中段,岩性组成以正长花岗岩为主,二长花岗岩次之。LA-ICP-MS锆石U-Pb测年获得该岩体的侵位年龄为395.6±4.9 Ma(MSWD=3.6,n=15),时代为早泥盆世。岩石属偏铝-弱过铝质高钾钙碱性系列,具有高硅(SiO 2=72.71%~76.43%),富碱(K 2O+Na 2O=7.80%~9.23%),低铝(Al 2O 3=12.09%~13.73%),贫镁(MgO=0.06%~0.51%)和钙(CaO=0.44%~1.69%),K 2O>Na 2O的特点;稀土元素配分曲线呈轻稀土元素富集的右倾型,形态呈“海鸥式”分布,Eu强烈亏损(δEu=0.02~0.35,平均0.16);富集高场强元素Zr、Hf、U、Th和大离子亲石元素Rb、K等,而元素P、Ti、Ba、Sr明显亏损。上述地球化学特征指示该岩体属A型花岗岩,源于下地壳在高温条件下部分熔融及其后长石、磷灰石、榍石等的分离结晶。构造判别图解指示具有A 2型花岗岩的特征,形成于后碰撞伸展构造环境,指示牛圈子-洗肠井蛇绿岩带所代表的北山洋闭合时限在早泥盆世之前,早泥盆世该区的构造已由挤压体制转变为伸展体制。The Shibanjing A-type granite intrusion is located in the middle section of the Beishan orogenic belt of Inner Mongolia.Its lithologic composition is dominated by syenogranites,followed by monzogranite.The age of the granite obtained by LA-ICP-MS zircon U-Pb dating is 395.6±4.9 Ma(MSWD=3.6,n=15),suggesting Early Devonian.The rocks belong to the meta-aluminium-weak peraluminous high potassium calc-alkaline series and are characterized by high silicon(SiO 2=72.71%~76.43%),rich alkali(K 2 O+Na 2 O=7.80%~9.23%),low aluminum(Al 2 O 3=12.09%~13.73%),poor magnesium(MgO=0.06%~0.51%),calcium(CaO=0.44%~1.69%)and K 2 O>Na 2 O.The chondrite-normalized REE patterns of the granite belong to the“seagull”pattern of the right-type,with significant negative Eu anomalies(δEu=0.02~0.35,averaging 0.16).The granitic rocks are enriched in high field strength elements(e.g.,Zr,Hf,U and Th)and large ion lithophile elements(e.g.,K and Rb)but depleted in P,Ti,Ba,Sr.All these characteristics resemble features of A-type granites which originated from the partial melting of lower crust under high temperature conditions and the subsequent fractional crystallization of feldspar,apatite,titanite and some other rocks.The tectonic discriminant diagram indicates that it has the characteristics of A 2 granite,which was formed in the post-collision extension tectonic environment,indicating that the closure time of the Beishan Ocean represented by the Niuquanzi-Xichangjing ophiolite belt was prior to the Early Devonian,and that the structure of the area in the Early Devonian had changed from a compressional system to an extensional system.

关 键 词：北山造山带 A型花岗岩 锆石U-PB测年 后碰撞 北山洋 

分 类 号：P588.12[天文地球—岩石学]