冀西北宣化盆地侏罗纪九龙山组凝灰岩形成时代、构造环境及地质意义

Formation Age,Tectonic Setting and Geological Significance of The Jurassic Jiulongshan Formation Tuff in Xuanhua Basin,Northwest Hebei

冀西北宣化盆地九龙山组陆相沉积地层发育,但地层的时代归属缺少化石和同位素测年资料佐证。研究区杨家营−响水铺一带九龙山组中下部发育流纹质凝灰岩夹层,为准确确定宣化盆地九龙山组的时代和形成环境,笔者对流纹质凝灰岩采集了锆石U−Pb同位素测年样品和岩石地球化学分析样品,获得了(161.9±0.8)Ma的206Pb/238U加权平均年龄值(LA−ICP−MS法),为晚侏罗世早期。岩石化学特征表明,流纹质凝灰岩属高Si、高K钙碱性过铝质岩石;稀土元素配分模式曲线为右倾型,轻稀土强烈富集,负Eu异常不明显;富集大离子亲石元素(Rb、K、Th、U),亏损Sr和高场强元素(Nb、Ti)。岩石地球化学特征显示其形成于挤压构造背景。锆石Hf同位素εHf(t)值为−33.79~−2.71,二阶段亏损模式年龄(tDM2)为1179~3323 Ma,表明流纹质凝灰岩岩浆多为古元古代—中元古代加厚的下地壳熔融形成。综合分析认为,宣化盆地九龙山组形成于中侏罗世晚期—晚侏罗世早期的挤压构造背景,该研究成果为研究九龙山组的时代归属、形成环境提供了新资料。

The continental sedimentary strata of the Jiulongshan Formation in the Xuanhua basin in Northwest Hebei are well developed,but the age of the strata is lacking in evidence from fossils and isotope dating data.In the Yangjiaying−Xiangshuipu area of the study area,rhyolitic tuff interlayers develop in the middle and lower parts of the Jiulongshan Formation.In order to accurately determine the age and formation environment of the Jiulongshan Formation in the Xuanhua Basin,the author collected zircon U−Pb isotope dating samples from the rhyolitic tuff and rock geochemical analysis samples.The 206Pb/238U weighted average age(LA−ICP−MS method)of(161.9±0.8)Ma was obtained,which is the early Late Jurassic.The petrochemical characteristics show that the rhyolitic tuff is a high−silicon,high−potassium calc−alkaline peraluminous rock;the distribution pattern curve of rare earth elements is right−dipping,with strong enrichment of light rare earths and no obvious negative Eu anomaly;the enrichment of large ions is lithophilic elements(Rb,K,Th,U),depleted Sr and high field strength elements(Nb,Ti).The geochemical characteristics of the rocks show that it was formed in a compressional tectonic setting.The zircon Hf isotopeεHf(t)value is between−33.79~−2.71,and the second−stage depletion model age(tDM2)is between 1179~3323 Ma,indicating that the rhyolitic tuff magma is mostly Paleoproterozoic−Mesoproterozoic thickened lower crust Melted to form.According to comprehensive analysis,the Jiulongshan Formation in Xuanhua basin was formed in the late Middle Jurassic−early Late Jurassic compressional structural setting.The research results provide new data for the study of the age attribution and formation environment of the Jiulongshan Formation.