Zircon and baddeleyite U-Pb geochronology of the Shennongjia Group in the Yangtze Craton and its tectonic significance
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摘要:
出露于扬子北缘神农架地区的神农架群是扬子地区保留比较完整的中元古代地层，其上部被青白口系马槽园群不整合覆盖。本文报导了神农架群砂屑白云岩、凝灰岩及侵入神农架群中的基性岩墙锆石及斜锆石U-Pb年龄。测年表明，神农架群下部大岩坪组碎屑锆石在1.4Ga、1.8Ga、2.0Ga、2.7Ga出现统计峰值；神农架群野马河组凝灰岩锆石U-Pb年龄为~1220Ma；侵入于石槽河组的基性岩墙斜锆石及锆石U-Pb年龄分别为1115Ma和1083Ma。根据新的测年结果，结合区域地质分析，我们得出以下几点主要结论：(1)可以将神农架群的沉积时代严格限定在1.4~1.1Ga之间，并推测神农架群碎屑物主体来自扬子克拉通古老基底，另有部分碎屑物质可能来自华夏地块或劳伦的前寒武纪基底；(2)神农架群和马槽园群之间的角度不整合面大致确定在1.1~1.0Ga之间，这一不整合面可能代表了扬子与华夏之间最早发生拼合的构造事件，是Rodinia超大陆汇聚事件的构造响应；(3)侵入于石槽河组的基性岩墙侵入时代为1115~1083Ma，这一期基性岩侵入事件在劳伦、非洲、澳大利亚以及南极洲都有记录。神农架地区的这一时期基性岩侵入事件是Rodinia超大陆汇聚过程中的产物还是和该时期全球性的超级地幔柱有关尚需要进一步研究；(4)神农架群沉积时代的确定，为建立我国1.4~1.1Ga期间的标准地层剖面提供了可能的候选剖面。(5)神农架群大岩坪组~1.45Ga碎屑锆石年龄峰为华夏地块在Columbia超大陆中位于劳伦和南极之间的观点提供了新依据。
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关键词:
- 扬子克拉通 /
- 神农架群 /
- 锆石和斜锆石U-Pb定年 /
- 罗迪尼亚超大陆 /
- 哥伦比亚超大陆
Abstract:The Shennongjia Group, spreading on the northern margin of the Yangtze Craton, is a well-preserved succession of the Mesoproterozoic strata in South China, unconformably overlain by the Macaoyuan Group. Zircon and baddeleyite U-Pb dating results of dolarenite, tuff of the Shennongjia Group and mafic dike intruding the Shennongjia Group are reported in this paper. The dating results demonstrate that there are four age peaks 1.4Ga, 1.8Ga, 2.0Ga and 2.7Ga in the detrital zircon age spectra of the Dayanping Formation of the Lower Shennongjia Group. The zircon U-Pb age of the tuff of the Yemahe Formation in the upper part of the Shennongjia Group is ca.1220Ma. The baddeleyite and zircon U-Pb ages of the mafic dike intruding the Shicaohe Formation are 1115Ma and 1083Ma respectively. Based on our new age data, combining with the regional geological setting analysis, we arrive at the following preliminary conclusions. (1) The age span of the Shennongjia Group can be precisely put in 1.4~1.1Ga; the sedimentary materials of the Shennongjia Group came mainly from the ancient basement of the Yangtze Craton with minor probably from the Cathaysian or Laurentian. (2) The unconformity between the Shennongjia Group and the overlying Macaoyuan Group was formed at about 1.1~1.0Ga probably in the process of the initial assembly between the Yangtze Craton and the Cathaysia Massif, which might be the tectonic response of the Rodinia supercontinental assembly event. (3) The emplacing time of the mafic dike intruding the Shicaohe Formation is 1115~1083Ma; coeval mafic intrusion events are widespread in Laurentia, Africa, Australia and Antarctica. It is still an unresolved problem whether this phase mafic intrusion event in the Shennongjia area resulted from the Rodinia supercontinental assembly or was genetically related with the coeval global-scale mantle plume. (4) The determination of the sedimentation age of the Shennongjia Group provides a potential candidate for the establishment of the standard section of the 1.4~1.1Ga strata of China. (5) The~1.45Ga detrital zircon U-Pb age peak of the Dayanping Formation provides new evidence to support the opinion that the Cathaysia was located in between Laurentia and Antartica in the Nuna (Columbia) supercontinent configuration.
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图 1
中国构造纲要图和研究区位置(a)和神农架地区前寒武纪地质略图(b)据(李铨和冷坚, 1987; Qiu et al., 2011修改)
Figure 1.
Tectonic sketch map of China, showing the study area (a) and simplified geological map of Precambrian basement of the Shennongjia area, northern Yangtze Block (b) (modified after Li and Leng, 1987; Qiu et al., 2011)
图 2
神农架地区前南华系地层柱状图
Figure 2.
Pre-Nanhua stratigraphic histogram of the Shennongjia area
图 3
测年样品野外露头和显微照片
Figure 3.
Outcrop pictures and photomicrographs of the dating samples
图 4
测年样品锆石和斜锆石CL图像和透射光显微照片
Figure 4.
Cathodoluminescence images and photomicrographs of the dating zircons and baddeleyites
图 5
野马河组凝灰岩样品11HB06-3锆石U-Pb年龄谐和图
Figure 5.
U-Pb concordias of zircons from tuff sample 11HB06-3 of the Yemahe Formation
图 6
侵入石槽河组辉长辉绿岩样品11HB24锆石U-Pb年龄谐和图
Figure 6.
U-Pb concordia of zircons from gabbro-diabase sample 11HB24 intruding the Shicaohe Formation
图 7
侵入石槽河组辉长辉绿岩样品11HB24斜锆石U-Pb年龄谐和图
Figure 7.
U-Pb concordia of baddeleyites from gabbro-diabase sample 11HB24 intruding the Shicaohe Formation
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