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扫描电子显微镜下的黄铁矿形态丰富多样,主要为正常(标准)草莓状黄铁矿(图 3a~c)、过大生长草莓状黄铁矿(图 3d)、充填草莓状黄铁矿(图 3e,f)、自形晶黄铁矿(图 3g~j)以及复合草莓状黄铁矿(图 3k,l)。完全充填草莓状黄铁矿(图 3f)、部分自形晶黄铁矿(图 3g)和它形晶黄铁矿(图 3h)内部存在残余孔洞。这些内部残余的小孔洞可能说明草莓状黄铁矿可以演化成为部分充填草莓状黄铁矿、完全充填草莓状黄铁矿、它形晶草莓状黄铁矿,甚至自形晶黄铁矿。大型草莓状黄铁矿(大于10 μm)与小型草莓状黄铁矿相伴而生。复合草莓状黄铁矿一般由多个正常草莓状黄铁矿聚合在一起而形成(图 3l)。一些复合草莓状黄铁矿内部还存在松散的黄铁矿微晶(图 3k)。样品中黄铁矿的形态主要以正常草莓状黄铁矿以及充填草莓状黄铁矿为主。杨林剖面草莓状黄铁矿粒径最小值分布范围是1~5 μm,平均为2 μm,样品间变化不大(表 1);粒径平均值分布范围是5~9 μm,平均为6.6 μm,样品间变化较大。草莓状黄铁矿粒径最大值分布范围是10~40 μm,平均为20 μm。在垂向上,吴家坪组中下部石灰岩的草莓状黄铁矿粒径比较大,而在吴家坪组上部和长兴组下部的薄层硅质岩和硅质石灰岩中草莓状黄铁矿粒径较小(图 4)。在长兴组中部的瘤状石灰岩基本上不含草莓状黄铁矿。
图 3 秭归杨林剖面吴家坪组和长兴组黄铁矿扫描电子显微相片
Figure 3. Pyrite SEM images (backscatter mode) in the Wujiaping and Changxing Formations at Yanglin section, South China
表 1 湖北秭归县杨林剖面吴家坪组—长兴组草莓状黄铁矿粒径统计表
Table 1. Framboid size data from the Wujiaping and Changxing Formations at Yanglin section, Hubei
地层组 样品 均值/μm 标准偏差/μm 偏度 最大值/μm 数量/个 解释 长兴组 Y54 6.3 2.0 1.1 12.5 76 贫氧—氧化 长兴组 Y52 6.4 2.3 1.0 14.7 129 贫氧—氧化 长兴组 Y51 5.4 2.2 1.5 14.0 116 硫化 长兴组 Y50 5.4 2.3 1.9 17.2 126 贫氧—氧化 长兴组 Y49 5.7 2.2 1.1 13.4 122 贫氧—氧化 长兴组 Y48 4.2 1.7 1.4 10.4 154 硫化 长兴组 Y47 5.0 2.6 1.6 16.1 135 贫氧—氧化 长兴组 Y46 6.5 2.5 1.3 16.7 130 贫氧—氧化 长兴组 Y45 5.0 1.9 1.1 12.8 122 硫化 长兴组 Y44 7.2 2.4 0.7 13.8 79 贫氧—氧化 长兴组 Y43 6.5 3.4 1.2 19.1 125 贫氧—氧化 长兴组 Y42 4.4 2.4 2.8 18.2 173 硫化 长兴组 Y41 5.5 3.2 2.0 19.8 151 贫氧—氧化 长兴组 Y40 5.4 2.5 1.5 17.2 118 贫氧—氧化 长兴组 Y39 5.2 2.2 2.0 17.9 143 硫化 吴家坪组 Y38 6.1 2.8 1.0 15.6 149 贫氧—氧化 吴家坪组 Y37 6.4 2.5 1.5 18.6 124 贫氧—氧化 吴家坪组 Y36 5.4 2.2 1.9 17.5 118 贫氧—氧化 吴家坪组 Y35 5.3 2.1 0.9 12.5 106 硫化 吴家坪组 Y34 4.4 2.4 2.2 15.2 149 硫化 吴家坪组 Y33 6.4 2.6 1.1 17.3 143 贫氧—氧化 吴家坪组 Y32 9.0 3.3 0.9 18.5 99 贫氧—氧化 吴家坪组 Y31 6.6 3.0 2.4 26.3 148 贫氧—氧化 吴家坪组 Y30 6.7 3.0 1.3 19.5 286 贫氧—氧化 吴家坪组 Y29 5.8 2.4 1.1 13.4 149 贫氧—氧化 吴家坪组 Y28 6.0 2.0 1.1 13.4 109 贫氧—氧化 吴家坪组 Y27 6.6 2.3 1.3 16.2 127 贫氧—氧化 吴家坪组 Y26 6.3 3.2 1.6 18.0 167 贫氧—氧化 吴家坪组 Y25 6.7 3.6 2.5 27.1 231 贫氧—氧化 吴家坪组 Y24 6.8 3.9 2.0 23.8 169 贫氧—氧化 吴家坪组 Y23 6.0 2.8 1.8 19.8 138 贫氧—氧化 吴家坪组 Y22 5.8 2.2 2.5 19.6 125 贫氧—氧化 吴家坪组 Y21 6.4 2.6 2.3 20.8 131 贫氧—氧化 吴家坪组 Y20 7.1 4.5 4.0 39.3 123 贫氧—氧化 吴家坪组 Y19 7.4 3.4 1.5 21.8 146 贫氧—氧化 吴家坪组 Y18 7.4 4.0 2.0 28.9 158 贫氧—氧化 吴家坪组 Y17 7.8 3.1 1.4 23.2 119 贫氧—氧化 吴家坪组 Y16 7.8 3.3 2.1 21.6 68 贫氧—氧化 吴家坪组 Y15 8.7 3.9 1.6 23.8 94 贫氧—氧化 吴家坪组 Y14 6.5 2.4 1.1 14.9 122 贫氧—氧化 吴家坪组 Y13 7.0 2.8 0.8 15.4 164 贫氧—氧化 吴家坪组 Y12 8.1 3.9 1.9 27.3 132 贫氧—氧化 吴家坪组 Y11 6.9 3.1 1.6 18.8 106 贫氧—氧化 吴家坪组 Y10 7.5 3.3 2.3 25.1 114 贫氧—氧化 吴家坪组 Y09 7.2 2.7 1.1 18.8 140 贫氧—氧化 吴家坪组 Y08 7.8 4.4 3.0 36.0 179 贫氧—氧化 吴家坪组 Y07 6.7 2.8 1.9 20.6 132 贫氧—氧化 吴家坪组 Y06 8.1 4.3 1.8 27.7 199 贫氧—氧化 吴家坪组 Y05 7.3 3.9 1.8 21.5 92 贫氧—氧化 吴家坪组 Y04 7.6 4.0 2.0 22.2 117 贫氧—氧化 吴家坪组 Y03 7.9 4.5 1.9 32.4 132 贫氧—氧化 吴家坪组 Y02 8.2 4.4 2.3 29.1 155 贫氧—氧化 吴家坪组 Y01 8.2 3.0 1.0 17.8 144 贫氧—氧化
Redox Conditions and their Implications at the Wujiaping-Changxing Formation Boundary in Zigui, Hubei Province
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摘要: 二叠纪发生了两次重大生物灭绝事件,一次是位于吴家坪初期即瓜德鲁普世末生物灭绝事件,另一次是位于长兴末期即晚二叠世末生物灭绝事件。在这两次生物灭绝事件之间的大约八百万年时间里,其环境变化研究相对较少,古海洋的氧化还原条件变化目前尚未清楚。通过分析湖北秭归县杨林剖面的黄铁矿形态来探讨吴家坪组-长兴组界线氧化还原条件及其意义。研究结果表明:吴家坪组与长兴组石灰岩沉积时期的古海水主要是贫氧至氧化环境。在吴家坪组上部至长兴组下部地层沉积时期古水体出现间歇性硫化现象。该硫化缺氧事件不仅出现在秭归地区,还广泛分布于鄂西盆地、扬子台地南缘南盘江盆地、阿拉伯地台以及东格陵兰盆地地区。这说明吴家坪-长兴组界线沉积时期的间歇性硫化缺氧事件有可能是一个全球性事件。该事件可能与二叠末事件有一定的关联。Abstract: Two mass extinctions occurred in the Permian, one at the beginning of the Wujiapingian, the extinction at the end of the Guadalupian, and the other at the end of the Changxingian, the end-Permian extinction. During the eight million years between the two extinctions, environmental changes have attracted few studies, and the paleoocean redox conditions are still unclear. This study analyzed pyrite morphology and framboid size distribution across the boundary of the Wujiaping and Changxing Formations at Yanglin in Zigui county, Hubei province, and discusses the redox conditions. The results show that the conditions of the water column during the deposition of carbonates in the Wujiaping and Changxing Formations were mainly dysoxic-oxic. Episodic euxinia occurred from the upper Wujiaping to the lower Changxing formations. The euxinic events at the Wujiapingian-Changxingian boundary occurred not only at Zigui, but also in other locations in the West Hubei Basin, the Nanpanjiang Basin in the southern Yangtze Platform, the Arabia margin and in East Greenland, implying a global event. This euxinic event may be linked to the Late Permian event.
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Key words:
- Wujiaping Formation /
- Changxing Formation /
- framboidal pyrite /
- redox conditons /
- Zigui
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图 1 研究剖面地质背景
(a)研究区古地理图及研究区位置(据文献[21]);(b)研究剖面野外露头;(c)研究区地层岩性特征。C.硅质岩;M.泥晶灰岩;W.粒泥灰岩
Figure 1. Geological setting of study section
(a) Late Permian paleogeography of South China and location of the studied section (after reference [21]); (b) outcrop section at Yanglin; (c) lithological log of the studied strata. C = chert; M = lime mudstone; W= wackestone
图 3 秭归杨林剖面吴家坪组和长兴组黄铁矿扫描电子显微相片
(a)正常草莓状黄铁矿,微晶之间孔隙明显;(b)沉积碎屑被草莓状黄铁矿包裹;(c)黄铁矿;(d)过大生长草莓状黄铁矿,在最外层的结构中显示为较大的微晶;(e)充填草莓状黄铁矿;(f)充填草莓状黄铁矿显示有残余孔隙,能反映草莓状黄铁矿的生长起源;(g)草莓状黄铁矿与自形晶黄铁矿;(h)它形晶草莓状黄铁矿与正常黄铁矿并存;(i)草莓状黄铁矿与自形晶黄铁矿;(j)草莓状黄铁矿与自行晶黄铁矿;(k)复合草莓状黄铁矿;(l)复合草莓状黄铁矿
Figure 3. Pyrite SEM images (backscatter mode) in the Wujiaping and Changxing Formations at Yanglin section, South China
图 4 杨林剖面吴家坪—长兴组草莓状黄铁矿粒径分布盒须图
盒须图中的盒子范围为25%到75%,两侧的须分别为最小值和最大值,氧化还原柱是根据图 5的结果划分
Figure 4. Pyrite framboid size distribution in the Wujiaping and the Changxing Formations at Yanglin section, South China
In the box-and-whisker plots, the boxes range from 25% to 75% and contain 50% of the data. The two ends of the horizontal lines represent minimum and maximum values, the redox column is according to Fig. 5
表 1 湖北秭归县杨林剖面吴家坪组—长兴组草莓状黄铁矿粒径统计表
Table 1. Framboid size data from the Wujiaping and Changxing Formations at Yanglin section, Hubei
地层组 样品 均值/μm 标准偏差/μm 偏度 最大值/μm 数量/个 解释 长兴组 Y54 6.3 2.0 1.1 12.5 76 贫氧—氧化 长兴组 Y52 6.4 2.3 1.0 14.7 129 贫氧—氧化 长兴组 Y51 5.4 2.2 1.5 14.0 116 硫化 长兴组 Y50 5.4 2.3 1.9 17.2 126 贫氧—氧化 长兴组 Y49 5.7 2.2 1.1 13.4 122 贫氧—氧化 长兴组 Y48 4.2 1.7 1.4 10.4 154 硫化 长兴组 Y47 5.0 2.6 1.6 16.1 135 贫氧—氧化 长兴组 Y46 6.5 2.5 1.3 16.7 130 贫氧—氧化 长兴组 Y45 5.0 1.9 1.1 12.8 122 硫化 长兴组 Y44 7.2 2.4 0.7 13.8 79 贫氧—氧化 长兴组 Y43 6.5 3.4 1.2 19.1 125 贫氧—氧化 长兴组 Y42 4.4 2.4 2.8 18.2 173 硫化 长兴组 Y41 5.5 3.2 2.0 19.8 151 贫氧—氧化 长兴组 Y40 5.4 2.5 1.5 17.2 118 贫氧—氧化 长兴组 Y39 5.2 2.2 2.0 17.9 143 硫化 吴家坪组 Y38 6.1 2.8 1.0 15.6 149 贫氧—氧化 吴家坪组 Y37 6.4 2.5 1.5 18.6 124 贫氧—氧化 吴家坪组 Y36 5.4 2.2 1.9 17.5 118 贫氧—氧化 吴家坪组 Y35 5.3 2.1 0.9 12.5 106 硫化 吴家坪组 Y34 4.4 2.4 2.2 15.2 149 硫化 吴家坪组 Y33 6.4 2.6 1.1 17.3 143 贫氧—氧化 吴家坪组 Y32 9.0 3.3 0.9 18.5 99 贫氧—氧化 吴家坪组 Y31 6.6 3.0 2.4 26.3 148 贫氧—氧化 吴家坪组 Y30 6.7 3.0 1.3 19.5 286 贫氧—氧化 吴家坪组 Y29 5.8 2.4 1.1 13.4 149 贫氧—氧化 吴家坪组 Y28 6.0 2.0 1.1 13.4 109 贫氧—氧化 吴家坪组 Y27 6.6 2.3 1.3 16.2 127 贫氧—氧化 吴家坪组 Y26 6.3 3.2 1.6 18.0 167 贫氧—氧化 吴家坪组 Y25 6.7 3.6 2.5 27.1 231 贫氧—氧化 吴家坪组 Y24 6.8 3.9 2.0 23.8 169 贫氧—氧化 吴家坪组 Y23 6.0 2.8 1.8 19.8 138 贫氧—氧化 吴家坪组 Y22 5.8 2.2 2.5 19.6 125 贫氧—氧化 吴家坪组 Y21 6.4 2.6 2.3 20.8 131 贫氧—氧化 吴家坪组 Y20 7.1 4.5 4.0 39.3 123 贫氧—氧化 吴家坪组 Y19 7.4 3.4 1.5 21.8 146 贫氧—氧化 吴家坪组 Y18 7.4 4.0 2.0 28.9 158 贫氧—氧化 吴家坪组 Y17 7.8 3.1 1.4 23.2 119 贫氧—氧化 吴家坪组 Y16 7.8 3.3 2.1 21.6 68 贫氧—氧化 吴家坪组 Y15 8.7 3.9 1.6 23.8 94 贫氧—氧化 吴家坪组 Y14 6.5 2.4 1.1 14.9 122 贫氧—氧化 吴家坪组 Y13 7.0 2.8 0.8 15.4 164 贫氧—氧化 吴家坪组 Y12 8.1 3.9 1.9 27.3 132 贫氧—氧化 吴家坪组 Y11 6.9 3.1 1.6 18.8 106 贫氧—氧化 吴家坪组 Y10 7.5 3.3 2.3 25.1 114 贫氧—氧化 吴家坪组 Y09 7.2 2.7 1.1 18.8 140 贫氧—氧化 吴家坪组 Y08 7.8 4.4 3.0 36.0 179 贫氧—氧化 吴家坪组 Y07 6.7 2.8 1.9 20.6 132 贫氧—氧化 吴家坪组 Y06 8.1 4.3 1.8 27.7 199 贫氧—氧化 吴家坪组 Y05 7.3 3.9 1.8 21.5 92 贫氧—氧化 吴家坪组 Y04 7.6 4.0 2.0 22.2 117 贫氧—氧化 吴家坪组 Y03 7.9 4.5 1.9 32.4 132 贫氧—氧化 吴家坪组 Y02 8.2 4.4 2.3 29.1 155 贫氧—氧化 吴家坪组 Y01 8.2 3.0 1.0 17.8 144 贫氧—氧化 -
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