内蒙古阿拉善左旗乌兰尚德地区志留系圆包山组笔石化石的发现及其地质意义
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摘要
阿拉善盟阿拉善左旗杭乌拉山以北哈日奥布日格断凹陷盆地两侧,北东向出露770km2的哑地层,前人将其划为二叠系双堡塘组,但该套地层的岩性和岩石组合及层序特征,与内蒙古岩石地层所述的双堡塘组的现在定义和地质特征及区域变化完全相悖。采用现代沉积学理论和层序地层学方法,开展岩石组合、基本层序、沉积构造与沉积环境研究,划分出3个段级填图单元。根据发现的笔石化石和被晚泥盆世中粗粒黑云母花岗闪长岩侵入的关系,将地层时代确定为早志留世,改划为志留系圆包山组。泥盆纪区域变质变形作用使其发生浅变质和片理化变形,并形成轴向北东的复式向斜构造。后期叠加构造变形,形成紧闭倒转褶皱。志留系圆包山组的确定对本区地层划分和区域构造演化研究有重要意义。
A puzzling stratum is identified as Permian Shuangbaotang Formation,which is outcropped in NE direction for 770 km~2 on both sides of Hariaoribuge sag in northern Hangwula Mountain of Alxa Left Banner. However, the summarized lithology, rock association and sequence characteristics of strata is in contradiction with the definition, geological characteristics and regional change of Shuangbaotang Formation named by Inner Mongolian lithostratigraphic researchers. In this study, 1:50,000 regional mineral geological survey adopted modern sedimentology theory and sequence stratigraphy working method to study rock association,basic sequence, sedimentary structure and sedimentary environment. The authors divided the strata into three lithologic section-level geological units. According to the first discovery of graptolite and the invading contact of biotite granodiorites in the Late Devonian(U-Pb/379±2.9 Ma), the strata age should be Early Silurian and it was changed into Silurian Yuanbaoshan Formation.The Devonian regional deformation and metamorphic process produced low-grade metamorphism and schistositilization deformation and formed NE-trending compound syncline structure. Then it experienced superimposition ofstructural deformation and formed closed-reverse fold. The establishment of Silurian Yuanbaoshan Formation plays an important role in stratigraphic subdivision and regional tectonic evolution study.
A puzzling stratum is identified as Permian Shuangbaotang Formation,which is outcropped in NE direction for 770 km~2 on both sides of Hariaoribuge sag in northern Hangwula Mountain of Alxa Left Banner. However, the summarized lithology, rock association and sequence characteristics of strata is in contradiction with the definition, geological characteristics and regional change of Shuangbaotang Formation named by Inner Mongolian lithostratigraphic researchers. In this study, 1:50,000 regional mineral geological survey adopted modern sedimentology theory and sequence stratigraphy working method to study rock association,basic sequence, sedimentary structure and sedimentary environment. The authors divided the strata into three lithologic section-level geological units. According to the first discovery of graptolite and the invading contact of biotite granodiorites in the Late Devonian(U-Pb/379±2.9 Ma), the strata age should be Early Silurian and it was changed into Silurian Yuanbaoshan Formation.The Devonian regional deformation and metamorphic process produced low-grade metamorphism and schistositilization deformation and formed NE-trending compound syncline structure. Then it experienced superimposition ofstructural deformation and formed closed-reverse fold. The establishment of Silurian Yuanbaoshan Formation plays an important role in stratigraphic subdivision and regional tectonic evolution study.
引文
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(1)曹景轩,何显洲,王士政.中华人民共和国区域地质调查报告(1:20万哈日奥日布格幅).1982.
(2)曹景轩,何显洲,王士政.中华人民共和国区域地质调查报告(1:20万扎敏敖包幅).1982.
(3)董尚林,陈长虹,李新虎,等.内蒙古自治区阿拉善盟傲干奥日布格等三幅1:5万区域矿产调查报告.2008.
[2]周振寰.北山地区的早志留世笔石相地层[J].西北地质,1982,(3):3-11.
[3]潘桂棠,肖庆辉,陆松年,等.中国大地构造单元划分[J].中国地质,2009,36(1):1-28.
[4]潘桂棠,陆松年,肖庆辉,等.中国大地构造阶段划分和演化[J].地学前缘,2016,23(6):1-23.
[5]何世平,周会武,任秉琛,等.甘肃内蒙古北山地区古生代地壳演化[J].西北地质,2005,(3):6-15.
[6]许伟,魏建设,韩伟,等.银额盆地及周缘石炭系和二叠系沉积之后构造改造初探[J].地质通报,2018,37(1):132-143.
[7]梁国冰,丁仨平,李佐臣,等.甘肃北山小西弓南早泥盆世—晚志留世糜棱岩化石英闪长岩体年代学、地球化学特征及其地质意义[J].西北地质,2017,50(1):249-260.
[8]程海峰,辛后田,梁国庆,等.内蒙古北山地区黑红山一带斑状花岗闪长岩地球化学特征、LA-ICP-MS锆石U-Pb年龄及其地质意义[J].地质通报,2018,37(10):1895-1904.
[9]代文军,龚全胜,李海林,等.甘肃北山南带中段志留系的发现及地质意义[J].西北地质,2002,(3):42-47.
[10]张亚波,闫海卿,胡雷雷,等.内蒙古额济纳旗地区早石炭纪火山岩喷发旋回与岩石化学特征[J].西北地质,2018,51(3):1-15.
[11]闫海卿,陈元,范模春,等.内蒙古额济纳旗拐子湖地区花岗岩地球化学特征及其地质意义[J].中国工程科学,2015,17(2):97-105.
[12]牛亚卓,魏建设,史冀忠,等.甘肃北山地区北部上石炭统火山岩LA-ICP-MS锆石U-Pb年龄及其构造意义[J].地质通报, 2013,32(11):1720-1727.
[13]尹海权,周洪瑞,徐东卓,等.中亚造山带中段南缘圆包山组碎屑锆石U-Pb年龄、物源及其构造演化意义[J].地质学报,2017,91(10):2196-2211.
(1)曹景轩,何显洲,王士政.中华人民共和国区域地质调查报告(1:20万哈日奥日布格幅).1982.
(2)曹景轩,何显洲,王士政.中华人民共和国区域地质调查报告(1:20万扎敏敖包幅).1982.
(3)董尚林,陈长虹,李新虎,等.内蒙古自治区阿拉善盟傲干奥日布格等三幅1:5万区域矿产调查报告.2008.