内蒙古金厂沟梁地区晚三叠世脉岩地球化学特征及成岩动力学背景

金厂沟梁位于张家口-赤峰-朝阳金矿集区东端, 区内发育大量规模不等的脉岩, 岩性以闪长岩、闪长玢岩为主.对其进行了锆石U-Pb年龄、主微量元素及Sr-Nd同位素分析.脉岩锆石LA-ICP-MS U-Pb年龄值可分为三组: 2 458~2 524 Ma、253±6 Ma(MSWD=3.0)及228±1 Ma(MSWD=0.26).2.5 Ga锆石反映成岩过程中有古老地壳物质参与, 253 Ma锆石可能与古亚洲洋闭合时的岩浆事件有关, 228 Ma则为脉岩的形成年龄.脉岩SiO₂(51.22%~68.48%)、MgO(1.35%~8.13%)含量变化较大, 且具有高Na₂O+K₂O、Al₂O₃及低的TiO₂、P₂O₅含量等特征.岩石LREE及LILE富集, HFSE亏损.脉岩(87Sr/86Sr)_i比值较为一致(0.704 95~0.705 92), 而ε_Nd(t)(-0.2~-9.5)及T_2DM(1.02~1.77 Ga)值变化范围较大.主微量元素及同位素研究结果表明: 部分熔融的岩石圈地幔熔体底侵到壳幔边界, 诱发古老地壳物质的部分熔融, 随后发生的幔源熔体与壳源熔体的混合是脉岩最可能的源区过程.主微量元素构造判别图解指示岩石形成于陆弧向板内演化的构造环境; 结合区域上同时代脉岩群、碱性岩带及A2型花岗岩的侵位, 认为晚三叠世华北板块北缘已完成与蒙古弧地体的碰撞并进入造山后伸展阶段. 

Petrogenesis and Geodynamic Setting of Late Triassic Dykes of Jinchanggouliang,Eastern Inner Mongolia

Jinchanggouliang region, which consists of Jinchanggouliang, Erdaogou and Xiaoerdaogou gold deposits, is located in the north of North China Craton (NCC) and south of Inner Mongolia-Daxinganling orogenic belt. Numerous early Mesozoic diorites and diorite porphyries occurred and yielded LA-ICP-MS zircon U-Pb age of 228±1 Ma (MSWD=0.26) in the region. Both diorites and diorite porphyries are dominated by shoshonitic rocks, which contain SiO₂ ranging from 51.22% to 68.48% and MgO from 1.35% to 8.13%, and are characterized by high contents of Na₂O+K₂O and Al₂O₃ but low concentrations of P₂O₅. These rocks are enriched in LREE, LILE but depleted in HFSE. All samples possess uniform (87Sr/86Sr)_i isotopic ratios (0.704 95-0.705 92) but relatively wider range of ε_Nd(t) (-0.2 to -9.5) and T_2DM ages (1.02-1.77 Ga). These results indicate that the magma source derived from mixing of lithospheric mantle and lower crust melt. Diagrams of major and trace elements reveal that dykes formed in transitional environment between land arc and intra-plate. Combined with some new discoveries in the neighborhood, such as dyke swarms, alkaline intrusive complex and A2-type granite, this study proposes that NCB had completed the collision with Siberia plate in Late Triassic and the north of NCC went into a post-orogenic extension stage.