内蒙古赤峰地区泥盆纪晚期火山岩的发现及其地质意义

运用LA-ICP-MS锆石U-Pb测年获得华北地块北缘内蒙古赤峰莲花山地区朝吐沟组2件流纹质熔结凝灰岩的形成时代，分别为366Ma±2Ma和364Ma±2Ma。测年结果表明，莲花山朝吐沟组火山岩形成于泥盆纪晚期，而不是早石炭世。同时，这一结果也将赤峰地区朝吐沟组的时代限定为泥盆纪晚期，而非早石炭世。莲花山泥盆纪晚期火山岩总体上以富硅、碱、铝，贫铁、镁、钙、钛及过铝质为特征，与本区晚石炭世—二叠纪钙碱性火山岩（如青凤山组或大石寨组火山岩）的特征明显不同。华北地块北缘泥盆纪晚期火山岩的形成可能与早古生代末期—晚古生代初期白乃庙岛弧和华北克拉通北缘弧—陆碰撞后的伸展环境有关。这一构造环境与华北地块北缘泥盆纪碱性岩及基性—超基性岩侵位背景相似。

Recognition of the latest Devonian volcanic rocks in Chifeng area,northern North China block,and its geological implications

LA-ICP-MS zircon U-Pb dating of two rhyolitic tuff samples from the Chaotugou Formation in Lianhuashan area of Chifeng on the northern margin of the North China block yielded weighted mean 206Pb/238U ages of 366Ma±2Ma and 364Ma±2Ma, respectively. The new geochronological results indicate that the Lianhuashan volcanic rocks were erupted during the latest Devonian instead of the Early Carboniferous as previously considered. Because the volcanic rocks are located within the Chaotugou Formation which was previously considered to be Early Carboniferous in age, the age of the Chaotugou Formation should evidently be latest Devonian. The Lianhuashan latest Devonian volcanic rocks are characterized by high content of SiO2, Na2O+K2O, Al2O3, low content of T Fe2O3（0.36%~0.70%）, MgO, CaO, TiO2 and A/CNK>1.1, quite different from the geochemical characteristics of the Late Carboniferous-Permian calc-alkaline volcanic rocks on the northern margin of the North China block.