摘要
神农架群的底界问题一直以来悬而未决。神农架群底界能否获取,已成为神农架群能否完美成为待建系候选层型的关键。距神农架东南几十公里的黄陵穹隆东北部地区自下而上发育中-新元古代西汊河组、吴家台组、浇园山组、南沱组和陡山沱组等地层,且胡正祥等(2012)认为该地吴家台组为神农架群中下部地层,那么该地是否存在神农架群的底界自然就引起大家的极大关注。本文基于该地浇园山剖面西汊河组石英片岩样品(0529-1),吴家台组底部砾岩(0228-4)和砂岩(0228-5)3件样品,应用LA-ICP-MS方法进行了碎屑锆石年龄的测试分析,同时结合地层单元间接触关系和岩石学与沉积学特征等标志,最终约束吴家台组形成时代。结果表明,崆岭北部樟村坪以北浇园山剖面西汊河组与上覆吴家台组呈不整合接触;西汊河组碎屑锆石年龄谱仅显示太古宙和2.0Ga两个明显的峰值;但吴家台组碎屑锆石年龄谱不仅包含了与西汉河组相同的太古宙和2.0Ga两个年龄峰值,而且还含有0.8Ga的弱峰值,由此断定西汊河组和吴家台组形成时限是完全不同的。前者应年轻于2.0Ga,但由于其为角闪岩相变质岩,又不同于扬子克拉通最终(1.8Ga)固结前的变质结晶基底岩石组合,因此推测西汊河组大致为中元古代,同理,吴家台组应形成于0.8Ga以后,结合吴家台组之上具有典型的南沱组冰碛杂砾岩,因此其时代应界于青白口纪晚期-南华纪早期之间,进一步结合各岩组砾岩中砾石组分的证据认为,吴家台组应相当于区域上莲沱组。同时研究表明,西汊河组和吴家台组物源主要来自黄陵穹隆核部中太古代TTG片麻岩和新太古代的花岗质片麻岩,古元古代碎屑锆石主要来源于崆岭杂岩中部的火山-岩浆岩以及黄陵穹隆南翼的新元古代岩浆岩。
Since long time,the definition of the base of Shennongjia Group is controversial.Solving this problem is the key point to decide whether Shennongjia group could be a potential stratotype section.The profile in the northeast part of Huangling Dome,tens of kilometers southeast to Shennongjia area,is composed of the Meso-Neoproterozoic deposits containing Xichahe,Wujiatai,Jiaoyuanshan,Nantuo,and Doushantuo Formations.According to Hu et al.(2012),Wujiatai Formation in this area belongs to the Middle-Lower part of Shennongjia Group.This infers that the base of the Shennongjia Group is probably located in Wujiatai Formation.Three detrital zircon samples were collected in the Jiaoyuanshan profile in this area,north to Zhangcunping located in the north of Kongling.They are from the quartz-schist(0529-1)of Xichahe Formation,the conglomerate(0228-4)and sandstone(0228-5)of the base of Wujiatai Formation,respectively.The age information provided by the LA-ICP-MS analysis of these zircons,combined with the contact relationship of adjacent strata,as well as the petrological and sedimentary features of the deposits,helped to confine the formation time of Wujiatai Formation.In the Jiaoyuanshan profile,Wujiatai Formation overlies Xichahe Formation unconformably.The U-Pb age spectrum of zircon from Xichahe Formation shows two obvious peaks corresponding to Archean Eon and 2.0 Ga respectively,while that of the zircon from Wujiatai Formation displays not only the two peaks mentioned above,but also an extra weak peak corresponding to 0.8 Ga.Therefore,Xichahe Formation is assumed to be younger than 2.0 Ga.As Xichahe Formation is dominated by amphibolite,which is lithologically different from the metamorphic crystal basement formed before the final consolidation of Yangtze craton(1.8 Ga),it is estimated to be formed during Meso-proterozoic.Wujiatai Formation should be younger than 0.8 Ga,and the moraine of Nantuo Formation overlying it confined its formation time between Late Qingbaikou and Early Nanhua Period.Furthermore,the composition of conglomerates of Wujiatai Formation infers that it should be equivalent to Liantuo Formation in age.The provenances of Xichahe and Wujiatai Formations are the Meso-Archean TTG gneiss and the Neo-Archean granitic gneiss from the core of Huangling Dome,the Paleo-Proterozoicvolcanic and magmatic rocks from the central of Kongling complex,and the NeoProterozoic magmatic rocks from the south flank of Huangling Dome.
作者
旷红伟
耿元生
柳永清
王玉冲
夏晓旭
彭楠
范正秀
陈骁帅
白华青
KUANG Hongwei;GENG Yuansheng;LIU Yongqing;WANG Yuchong;XIA Xiaoxu;PENG Nan;FAN Zhengxiu;CHEN Xiaoshuai;BAI Huaqing(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037;China University of Geosciences ,Beijing 100083)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2019年第1期72-93,共22页
Acta Geologica Sinica
基金
国家重点研发计划:深地资源勘查开采专项(2016YFC0601001)
国家自然科学基金(41472082)
中国地质调查局项目(1212011120142
DD20160120-01)
神农架世界地质公园项目共同资助
关键词
扬子北缘
崆岭地区
神农架群底界
吴家台组
碎屑锆石
North margin of Yangtze
Kongling area
basement of Shennongjia Group
Wujiatai Formation
Detrital zircon
