大兴安岭北段下侏罗统柴河组的厘定及地质意义

Determination and geological significance of Chaihe Formation of the Lower Jurassic in north Daxing'anling Range

大兴安岭北段首次厘定出下侏罗统柴河组,为一套沉积岩及中性岩为主的火山岩岩石组合。在底部沉积岩层位中发现早侏罗世植物分子,为Podozamites sp.(苏铁衫属),Equisetites sp.(似木贼属)。火山岩-粗安岩锆石U-Pb同位素年龄为183.8±3.9Ma,时代为早侏罗世。锆石Hf同位素显示,初始^(176)Hf/^(177)Hf值分布于0.282935~0.282991之间,ε_(Hf)(t)值为+9.7^+11.4,Hf二阶段模式年龄T_(DM2)为495~612Ma,说明这些火山岩的原始岩浆来自新生中下地壳的部分熔融,表明额尔古纳地块的主体增生年代为新元古代。柴河期火山岩形成于活动陆缘弧环境,是早侏罗世蒙古-鄂霍茨克洋向南东方向俯冲的产物。下侏罗统柴河组的厘定,说明大兴安岭地区存在早侏罗世火山岩地层及相应的火山活动事件。该组的确立及引入,对大兴安岭中生代构造岩浆活动的研究及中生代火山岩地层划分对比、区域构造演化探讨具有重要的地质意义。

The Chaihe Formation of the Lower Jurassic is first defined in the northern Daxing＇ anling Range, which is a suite of sedimentary rocks and intermediate volcanic rocks. At the bottom of sedimentary rocks, we found plant molecular in the strata of Early Jurassic, namely Podozamites sp. and Equisetites sp. SHRIMIP zircon U-Pb age of the trachyandesites is 183.8±3.9Ma. The εHf（t） values of the zircons of trachyandesites range from ＋9.7~＋11.4. The TDM2 model ages yield 495~612Ma.This fact indicated that the rocks could have originated by partial melting of juvenile crust, and the accretion age of the Eergu＇Na block＇s main part was Neoproterozoic. The volcanic rocks of Chaihe Formation formed in the environment of active continental arc, relating to the subduction of Mongolia Okhotsk ocean to the Southeast direction in the Early Jurassic. The determination of the Chaihe Formation of the Lower Jurassic indicates the volcanic activity in the Lower Jurassic in the Daxing＇anling Range. The establishment and introduction of Chaihe Formation is of great geological significance for the study of tectonic-magmatic activities in the Mesozoic, the comparison of Mesozoic volcanic strata and regional tectonic evolution.