华北克拉通北缘赤峰地区显生宙麻粒岩的发现及其意义

The discovery of Phanerozoic granulite in Chifeng area of North Craton and its implication

华北克拉通构造体制从稳定向活动的转折是值得关注的问题。文中结合华北克拉通北缘赤峰和宁城晚古生代和早中生代两期麻粒岩捕虏体及其寄主闪长岩的研究,讨论其形成的不同构造背景,从而探讨体制转折的起始时期。赤峰柴胡栏子二叠纪镁铁质麻粒岩及其寄主闪长岩的锆石SHRIMP U-Pb年龄分别为(262.3±3.1)Ma和(256.6±6.5)Ma,宁城河南营子三叠纪镁铁质麻粒岩及其寄主闪长岩的锆石SHRIMPU-Pb年龄分别为(229.7±2.6)Ma和(221±3)Ma。在上述两个地区还发现晚古生代二叠纪的闪长岩脉等。通过对比研究认为:二叠纪岩浆活动具有大陆裂谷初始阶段的特点,高场强元素含量比较低;而三叠纪高场强元素的增高显示地幔部分熔融程度的提高。二叠纪岩浆活动与此时全球构造演化及邻兴蒙造山带出现的双峰式火山活动密切相关。而晚三叠世岩浆活动则与底侵背景下陆内伸展作用有关,是构造体制转折的标志。

It is found that the zircon SHRIMP U-Pb ages of the Permian mafic granulite and its host rock-diorite-in Chaihulanzi of Chifeng area are 262.3±3.1 Ma and 256.6±6.5 Ma respectively,and that the zircon SHRIMP U-Pb ages of the Triassic mafic granulite and its host rock-diorite-in Henanyingzi of Ningcheng area are 229.7±2.6 Ma and 221±3 Ma respectively.Other forms of magmation in this period were also discovered in these areas,including Permian accretion edges around the Archaeozoic zircon and Permian diorite-veins.Comparative study suggests that the Permian magmation possessed the characteristics of the initial stage of a continental rift with relatively low content of high-field-strength elements,and that the increased Triassic high-field-strength elements indicate a higher degree of mantle partial melting.Those granulites in combination recorded important information about the transformation of North China Craton from stable to active in its early stage.Isotope-geological study indicates that such transformation began with the crust-mantle interaction.The Permian magmation here was related to the global tectonic evolution and bimodal volcanic activity in Hinggan Mongolia Orogen at that period.The Triassic magmation here was related to intracontinental extension under the setting of underplating.