大兴安岭北段“佳疙瘩群”变质岩系成因研究

Located in the Erguna and Xing'an massifs in the northern part of Great Xing'an Range, the strongly metamorphic and deformed rocks have long been considered to be Neoproterozoic Jiageda Complex. The development of dating technology with higher precision allow us to find that "Jiageda Complex" is partly Precambrian, but mostly it is Paleozoic to Early Mesozoic in age. Due to the complexity of the forming and evolution of "Jiageda Complex" in the north part of Great Xing'an Range, our project is based on field geological survey and microscopic petrography research, using modern analytical techniques to study geochemistry, mineral chemistry and isotope chronology, understanding protolith formation, its forming age, its tectonic setting and the processes of the deformation, metamorphism, superimposition and transformation of each metamorphic type, establishing the initial state-metamorphic-deformation thermal event sequence of all the metamorphic types of "Jiageda Complex". Also we can examine, distinguish and elucidate genesis of Precambrian crystalline basement type, early Paleozoic para-metamorphic type and late Paleozoic-early Mesozoic ortho-metamorphic type of "Jiageda Complex" and reveal the genesis relationship with the tectonic attribute of Precambrian crystalline basement, its tectonic framework and the Paleozoic crustal structure in the northern part of Great Xing'an Range so as to provide significant geological basis for the analysis of regional metallogenic geological setting.

分布在大兴安岭北部额尔古纳-兴安地块中强烈变质变形的岩石长期被认为是新元古代形成的佳疙瘩群变质岩系。随着高精度测年技术的发展，发现"佳疙瘩群"变质岩系形成时代部分为前寒武纪，但大部分为古生代-早中生代。本项目依据大兴安岭北段"佳疙瘩群"变质岩系形成与演化的复杂特性，在野外宏观地质调研与室内显微岩相学研究基础上，利用现代分析测试技术，进行岩石地球化学、矿物化学以及同位素年代学等研究，厘清"佳疙瘩群"中各类变质岩系的原岩建造-形成时代-构造背景-变质变形叠加改造过程；建立各类变质岩系原岩起始状态-变质-变形热事件演化序列；甄别与解析前寒武纪结晶基底型、早古生代副变质岩型和晚古生代-早中生代正变质岩型等"佳疙瘩群"变质岩系的成因；揭示大兴安岭北部"佳疙瘩群"各类变质岩系的形成与前寒武纪结晶基底构造属性、区域构造格局、古生代期间地壳结构的耦合关系，为区域成矿地质背景分析提供重要的地质依据。

分布在大兴安岭北部额尔古纳-兴安地块中强烈变质变形的“佳疙瘩群”变质岩系，其原岩建造主体包含两部分：其一，出露于额尔古纳八大关牧场“佳疙瘩群”是一套经历了构造变质变形改造的正变质岩系，原岩为二长质-花岗闪长质-英云闪长质燕山期侵入体，形成于晚三叠世—晚侏罗世蒙古—鄂霍茨克造山带同碰撞-后碰撞构造演化过程中挤压—伸展构造转换机制下，在早白垩世德尔布干断裂带强烈活动使其遭受了低角闪岩相变质作用并发生了变质变形。其二，分布于全胜林场、莫尔道嘎、佳疙瘩镇一带的“佳疙瘩群”变质岩系，原岩建造为陆源碎屑岩夹少量中-基性火山岩沉积建造，初步认为形成于岛弧-活动大陆边缘环境，与早古生代中国东北地区所处的古亚洲洋多期俯冲消减作用造成的汇聚构造背景相一致。研究区大部分“佳疙瘩群”变质岩系并非传统意义上的前寒武纪结晶基底，可能多数为晚志留世-早泥盆世夹少量火山岩的沉积地层变质变形的产物。中国东北地区并不存在大规模的前寒武纪结晶基底，仅有少量分布于额尔古纳-兴安地块北缘、松嫩地块东缘和佳木斯地块，其形成时代均不早于新元古代。