黑龙江龙江盆地中侏罗统万宝组时代确定新证据及其地质意义

摘要

Jurassic stratigraphy of the eastern Daxinganling region,which covers the western margin of Heilongjiang Province,China,has not been a high priority research.For many years,coal bearing strata determination of the Middle Jurassic relies on regional correlation with the typical Middle Jurassic coal-bearing Wanbao Formation of western Jilin Province.Now for the first time,we discovered a set of Middle Jurassic coal-bearing strata in a new round of geological survey in the region.In this paper,we determined the accurate forming age of 2 tuffs from the Wanbao Formation of the Longjiang Basin to be 165.2±1.7 Ma and 162.1± 1.6 Ma.In addition,abundant mega plant fossils were collected from these strata,which were found to represent the Neocalamites-Coniopteris-Raphaelia assemblage,indicating Early-Middle Jurassic age.Sporepollen study on the mega-fossils reached similar assessment.We conclude that the Wanbao Formation formed in the Middle Jurassic,most probably in the late Middle Jurassic.Geochemical analyses show that the 2 tuffs were made of peraluminous calc-alkaline volcanic rocks containing high Si,Al,low Ca,P,and enriched light rare earth elements with REE pattern gently tilting to the right and negative Eu anomaly.The enriched large ion lithophile elements Cs,Th,U,and depleted high field strength elements Nb,Ti,Zr,imply that the source rocks are crust genetic types.Volcanic activity of Wanbao period may be related to the dual role of the evolution of the ancient Pacific Plate and the evolution of the Mongolian-Okhotsk suture zone,and is more closely related to the evolution of the Mongolian-Okhotsk suture zone.Currently the Wanbao Formation of the Middle Jurassic represents an important newly discovered strata for the oil and gas exploration.Here,we provide reliable geochronological and biostratigraphic evidence not only for the classification and comparison of the Middle Jurassic coal-bearing strata in the eastern Daxinganling Region,but also for the formation and evolution history of the Longjiang Basin.%黑龙江西部大兴安岭地区侏罗纪地层研究程度不高,以往中侏罗世含煤地层的确定主要借助于与吉林西部万宝组的对比.近年来随着新一轮大兴安岭地区含煤及油气地层的区域地质调查工作的开展,笔者在大兴安岭地区中段的龙江盆地(黑龙江省龙江县以西)发现一套新的中侏罗世含煤地层,该地层由砂砾岩和火山碎屑岩夹煤层组成.文中对龙江盆地万宝组火山岩夹层内2件凝灰岩进行了LA-ICP-MS锆石U-Pb定年分析,获得凝灰岩形成时代,其年龄分别为(165.2±1.7) Ma和(162.1±1.6) Ma;笔者采得的植物大化石为Neocalamites-Coniopteris-Raphaelia组合,时代显示为早—中侏罗世;孢粉化石经初步鉴定,时代倾向于早—中侏罗世;综合同位素年龄及地层古生物研究,笔者认为龙江盆地万宝组的形成时代为中侏罗世晚期.地球化学分析显示:万宝组凝灰岩具有高Si、A1,低Ca、P过铝质钙碱性火山岩特点,富集轻稀土元素,配分曲线呈平缓右倾型,并具有明显负Eu异常;富集大离子亲石元素Cs、Th、U,亏损高场强元素Nb、Ti、Zr,反映火山岩为壳源成因类型.万宝期火山活动可能与古太平洋板块俯冲作用和蒙古鄂霍茨克缝合带演化双重作用有关,而与蒙古—鄂霍茨克缝合带演化联系更密切.目前中侏罗统万宝组是油气勘探的重要新层系,本研究不仅为大兴安岭东坡地区中侏罗世含煤地层的划分、对比提供同位素年代学和生物地层学依据,而且为龙江盆地形成演化历史及油气资源勘查提供了基础地质新资料.