华北克拉通北缘与陆缘弧相关的早二叠世流纹岩——锆石U-Pb定年及地球化学证据

Early Permian rhyolite related to the continental marginal arc on the northern margin of the North China Craton——Evidence of zircon U-Pb dating and geochemical characteristics

华北克拉通北缘内蒙古赤峰北部朝阳沟村地区的原认为是志留纪的八当山流纹岩,现测得其中的锆石SHRIMP U-Pb年龄为277.3±3.3 Ma,应当属于早二叠世.岩石化学分析表明,流纹岩SiO2含量为75.24%~76.07%,K2 O和Na2 O含量分别为3.40%~5.31%和3.66%~4.88%,属于高钾钙碱性火山岩.岩石具高Al2 O3(12.92%~13.28%)、低CaO(0.13%~0.47%),铝饱和指数(A/CNK)为0.99~1.06,显示准铝质到弱过铝质特征.Hf同位素分析结果表明,锆石εHf(t)均为负值,为-10.7^-16.1,两阶段模式年龄(TDM2)为1736~2016 Ma,表明该火山岩起源于华北克拉通古老地壳物质的部分熔融.结合区域地质资料分析,朝阳沟村地区八当山火山岩主体岩性为中酸性火山岩,其形成于早二叠世古亚洲洋向南俯冲于华北克拉通北缘(包含华北克拉通北部和白乃庙岛弧带)之下的安第斯型活动大陆边缘构造环境.

Objectives:Predecessors have different conclusions on the age and tectonic setting of the Badangshan volcanics,which restricted the understanding the tectonic evolution of the middle segment of the northern margin of the North China Craton.The study of Badangshan rhyolite also contributes to the closure age of Paleo-Asian ocean.Methods:Based on the field work,through the microscope observation,the whole rock chemical analysis,the Hf isotopic analysis and SHRIMP U?Pb zircon ages of the original Silurian Badangshan rhyolite from Chaoyanggoucun area in northern Chifeng City of Inner Mongolia on the northern margin of the North China Craton.Results:The dating results reveals that the rhyolite were formed in Early Permian(277.3±3.3 Ma).The rhyolite belongs to high potassium calc-alkaline series characterized by high SiO2(75.24%?76.07%),variable K2O(1.68%~4.01%)and Na20(2.23%-5.02%).The rocks are characterized by high A12O3(12.92%?13.28%)and lower CaO(0.13%~0.47%)with the aluminum saturation index(A/CNK)being 0.99?1.06.Chemically,they belong metaluminous to weakly peraluminous volcanic rocks.Zircon eHf(t)values are negative(-10.7?-16.1)and two stage model age(TDM2)are from 1736 to 2016 Ma,indicating that the rhyolite originated from partial melting of the ancient materials of the North China Craton.Conclusions:A synthetic study of the geochemical characteristics of the Badangshan intermediate-acid volcanic rocks in Chaoyanggoucun area and the regional geological data reveals that the volcanic rocks formed in the setting of Andean marginal arc resulted from Paleo-Asian ocean subducting southward under the northern margin of the North China Craton which including the northern North China Craton and Bainaimiao arc belt.The rhyolite originated from partial melting of the ancient crustal materials of the North China Craton based on the geochemistry feature and zircon Lu—Hf isotopic results.