CAJ | 학술논문

华北板块北缘与其北部地块之间古亚洲洋闭合的时限,涉及到大陆形成过程中的古洋陆格局及演化过程的重建。晚二叠世-早三叠世大兴安岭地区的构造演化过程是承上启下的转折时期,即是古亚洲洋向古太平洋构造体系转变的关键时期。本文通过对大兴安岭中段龙江地区上二叠统林西组和下三叠统老龙头组砂岩样品进行系统的碎屑锆石 U-Pb同位素年代学研究,证实林西组和老龙头组存在280 Ma、370 Ma和500 Ma的三组峰值年龄,分别代表物源区存在早二叠世大石寨组火山岩、晚泥盆世-早石炭世岩浆弧以及北侧地块的统一基底,林西组和老龙头组碎屑锆石最小年龄分别为254 Ma和247 Ma,结合野外实测剖面研究结果,认为上二叠统林西组和下三叠统老龙头组连续沉积,整合接触。1∶20万地质图显示内蒙古东部大兴安岭大部分地区缺失三叠系,笔者发现研究区晚侏罗世满克头鄂博组、白音高老组与下伏晚二叠世林西组呈角度不整合接触,结合大兴安岭中部零星出露的早三叠世地层、二连盆地参1井中也存在早三叠世地层,综合分析认为研究区应存在早三叠世地层,上二叠统与下三叠统应为连续沉积,现今多数地区早三叠世地层的缺失是由于后期构造抬升剥蚀造成的。根据上二叠统林西组和下三叠统老龙头组中碎屑锆石存在1800 Ma左右的典型华北板块基底年龄和两者的整合接触关系,结合内蒙古贺根山缝合带中花岗闪长岩244 Ma的年龄以及双井子岩体、哈拉图岩体、吉林中部大玉山岩体均为后碰撞的地球化学特征,认为华北板块北缘与其北侧地块群最终闭合时间应为P3-T1。
화북판괴북연여기북부지괴지간고아주양폐합적시한,섭급도대륙형성과정중적고양륙격국급연화과정적중건。만이첩세-조삼첩세대흥안령지구적구조연화과정시승상계하적전절시기,즉시고아주양향고태평양구조체계전변적관건시기。본문통과대대흥안령중단룡강지구상이첩통림서조화하삼첩통로룡두조사암양품진행계통적쇄설고석 U-Pb동위소년대학연구,증실림서조화로룡두조존재280 Ma、370 Ma화500 Ma적삼조봉치년령,분별대표물원구존재조이첩세대석채조화산암、만니분세-조석탄세암장호이급북측지괴적통일기저,림서조화로룡두조쇄설고석최소년령분별위254 Ma화247 Ma,결합야외실측부면연구결과,인위상이첩통림서조화하삼첩통로룡두조련속침적,정합접촉。1∶20만지질도현시내몽고동부대흥안령대부분지구결실삼첩계,필자발현연구구만주라세만극두악박조、백음고로조여하복만이첩세림서조정각도불정합접촉,결합대흥안령중부령성출로적조삼첩세지층、이련분지삼1정중야존재조삼첩세지층,종합분석인위연구구응존재조삼첩세지층,상이첩통여하삼첩통응위련속침적,현금다수지구조삼첩세지층적결실시유우후기구조태승박식조성적。근거상이첩통림서조화하삼첩통로룡두조중쇄설고석존재1800 Ma좌우적전형화북판괴기저년령화량자적정합접촉관계,결합내몽고하근산봉합대중화강섬장암244 Ma적년령이급쌍정자암체、합랍도암체、길림중부대옥산암체균위후팽당적지구화학특정,인위화북판괴북연여기북측지괴군최종폐합시간응위P3-T1。
Timing of the closure of the Paleo-Asian Ocean between the northern margin of the North China Plate and the blocks to the north is important for the reconstruction of the tectonic evolution of ocean and continent. The Late Permian and Early Triassic were the two important periods for the tectonic evolution of the Great Xing’an Ranges, during which the Paleo-Asian Ocean tectonic regime switched to the Paleo-Pacific Ocean regime. In this paper, the U-Pb dating of detrital zircons from the sandstones in the Upper Permian Linxi Formation and the Lower Triassic Laolongtou Formation in the Longjiang area of the central Great Xing’an Ranges was carried out. The dating results show three age populations of 280 Ma, 370 Ma, and 500 Ma. It is suggested that the provenances of the rocks should be the Early Carboniferous to Early Permian magmatic arcs, the volcanic rocks of the Late Devonian, and the basement of the uniform blocks in the study area, respectively. The youngest U-Pb zircon ages of zircons from the Linxi Formation and Laolongtou Formation are 254 Ma and 247 Ma, respectively. The 1∶200 000 geologic mapping of the Great Xing’an Ranges shows that the Triassic strata are absent in most of the region. Regionally, the Late Jurassic Manketouebo Formation and the Baiyingaolao Formation overlay unconformably the Late Permian Linxi Formation. Considering the scattered outcrops of the Early Triassic strata and the Early Triassic sequences in the bore-holes in the Erlian Basin, we propose that the Early Triassic strata occur in an area larger than was previously considered in NE China. The absence of the Early Triassic strata in most of the region might be attributed to the subsequent regional uplifting and erosion. According to the detrital zircon U-Pb age of 1800 Ma, which is typical for the basement in the North China Plate, from both the Linxi and Laolongtou Formations, along with the granitoids with 244 Ma from the Hegenshan suture zone, and the plutons with the post-orogenic geochemical features from the Shuangjingzi, Halatu, Dayushan areas as well, we conclude that the final collision between the northern margin of the North China Plate and the blocks to the north took place during the Late Permian to Early Triassic.