Chamosite-Ooidal Limestones at the Bottom of Ordovician Pagoda Formation in the Southwestern Yangtze Platform: Genesis and Paleoenvironmental Implications
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摘要: 对四川省兴文县上奥陶统宝塔组底部含鲕绿泥石灰岩开展成因研究，有助于了解扬子台地西南部同时期的沉积环境演化过程.通过沉积学分析，并辅助以电子探针和扫描电镜等矿物学研究方法，发现富鲕绿泥石鲕粒及球粒为同生期沉积物，共生的生物碎屑组合指示海水为氧化性水体.丰富的微生物相关组构表明同期微生物席发育，且其代谢活动可致使水岩界面附近形成还原性水体，这是同生鲕绿泥石形成的必要条件，鲕绿泥石形成所需的Fe、Al元素来自早期风化壳在海侵阶段的大规模输入.宝塔组底部富鲕绿泥石灰岩是同期全球海平面演化在扬子台地的具体表现，佐证了早桑比期的海平面下降，是晚桑比期扬子台地快速海侵的重要标志.Abstract: A set of the chamosite-ooidal limestones was discovered at the bottom of the Pagoda Formation (Upper Ordovician) in Xingwen County, Sichuan Province, and its genesis can facilitate the understanding the evolution of the depositional environment in the southwestern Yangtze platform during the same period. Based on a detailed sedimentological analysis, this paper presents a study on the genesis of the chamosite-ooidal limestones, combined with mineralogical characterization via electron microprobe and scanning electron microscope studies. It is found that chamosite-rich ooids and peloids are syngenetic sediments, and symbiotic bioclastic assemblages indicate that the seawater was oxidizing. The abundant microbial-related fabric indicates that microbial mats develop during the same period, and their metabolic activities can lead to the formation of reducing water bodies near the water-sediment interface, which is a necessary condition for the formation of syngenetic chamosite. The Fe and Al elements needed for chamosite formation are derived from large-scale inputs of the early weathering crust during the transgression stage. The chamosite-ooidal limestones at the bottom of the Pagoda Formation are the concrete manifestation of global sea-level evolution in the Yangtze platform at same time. It is an important indicator of sea level fall in the Early Sandbian and rapid transgression in the Late Sandbian in the Yangtze platform.
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Key words:
- Yangtze platform /
- Pagoda Formation /
- chamosite ooids /
- microbial /
- sea-level change /
- sedimentology
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图 1 扬子台地中‒上奥陶统主要岩石地层单位对比
据汪啸风(2016)、苏文博等(1999)；R. 海平面上升；F. 海平面下降
Fig. 1. Correlation of litho-stratigraphic units in the Middle-Upper Ordovician of Yangtze platform
图 2 桑比期‒早凯迪期华南扬子地块古地理格架
据陈旭等(1990)、Chen et al.(2010)修改；图 2显示的晚奥陶纪铁质鲕粒分布的地理位置沈健伟(1994)
Fig. 2. Paleogeographic framework of the Yangtze block, South of China in the Sanbian-Early Katian Stage
图 4 宝塔组底部富鲕绿泥石鲕粒灰岩显微镜下特征(单偏光)
a.球度较好、包壳圈层发育、粒度较大的鲕粒，边缘被后期方解石脉(f)顺包壳圈层穿插；b. 包壳圈层较少、粒度较小的椭球形鲕粒；c. 未发育包壳、粒度较小的球粒；d. 海百合颗粒；e. 鲕粒边缘发生破损；f. 埋藏成岩阶段形成的方解石脉；g. 苔藓虫颗粒
Fig. 4. Photomicrograph of chamosite-ooidal limestone in the bottom part of Pagoda Formation (plane polarized light)
图 5 鲕粒圈层SEM背散射图像及能谱曲线
a. 为鲕粒圈层中片状鲕绿泥石与泥晶方解石的互层结构；b. 为少量方解石后期重结晶充填于鲕绿泥石圈层孔隙中；c. 为鲕粒圈层中的鲕绿泥石呈片状沿鲕粒圈层切线方向排列；d. 鲕粒圈层中的部分鲕绿泥石转化为赤铁矿；e.为蓝细菌钙化现象.①、②、③、④分别为图 5a和图 5d中对应测点的能谱曲线；①指示主要矿物为方解石；②指示主要矿物为赤铁矿，但其中的Al峰值表明原生鲕绿泥石转化为赤铁矿后残余的富铝矿物；③和④指示主要矿物为鲕绿泥石；“+”为测点位置
Fig. 5. BSE images and energy spectrum curves of cortical layers in ooid by SEM
图 6 富鲕绿泥石颗粒背散射图像及显微镜下特征
a. 鲕粒电子探针背散射图像及测点位置；b. 球粒电子探针背散射图像及测点位置；图中“○”为测点位置，1~16为测点编号；c、d分别为图a、b对应鲕粒和球粒的单偏光显微镜下特征；图d中球粒周围具显著的管状微生物组构特征；e. 鲕粒核部保留的微生物组构特征；f. 鲕粒周围基质中的自形黄铁矿，反射光；g为图f红色框内黄铁矿镜下特征，反射光
Fig. 6. Backscattering images of chamosite-rich grains and their characteristics under the microscope
图 8 基于Mg/Fe vs. Al/Si化学成分对比判别鲕绿泥石形成地质环境
据Damyanov and Vassileva(2001)修改
Fig. 8. Mg/Fe vs. Al/Si bivariate plots of chamosites from different geological settings
图 9 扬子台地西南缘桑比期沉积环境变化及鲕绿泥石鲕粒形成过程
a.桑比期早期低水位导致古陆暴露并形成富Fe、Al风化壳，为鲕绿泥石形成准备了重要物质基础；b.桑比期末期海侵导致水平面上升，古陆及风化壳被海水淹没并搬运Fe、Al质到浅海水域中；c.鲕绿泥石鲕粒形成过程模式图，在海水富Fe、Al背景下，微生物席发育为海水‒沉积物界面提供还原水体环境，从而导致鲕绿泥石结晶沉淀
Fig. 9. Evolution of sedimentary environment related to the formation of the chamosite ooidal limestones in Sandbian in southwest margin of Yangtze platform
表 1 铁质颗粒中各矿物成分电子探针分析结果（%）
Table 1. Chemical analytical results (%) of mineral components in iron particles
测点 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 SiO2 0.288 0.044 23.233 0.624 22.825 23.375 0.257 22.745 0.042 23.714 0.083 93.200 1.011 0.211 21.196 22.085 TiO2 bdl bdl 0.667 0.441 0.237 0.361 0.186 0.199 0.207 0.047 0.037 0.075 0.130 0.106 0.052 0.064 SrO 0.075 bdl 0.056 0.161 bdl 0.022 0.053 0.072 0.047 0.021 0.064 0.105 0.002 0.048 0.007 bdl Cr2O3 0.003 0.015 bdl 0.039 0.009 bdl 0.004 0.031 bdl 0.008 bdl bdl bdl bdl 0.022 0.022 MgO 0.933 0.885 6.289 0.356 5.850 6.221 0.663 6.151 0.492 6.617 0.751 0.012 0.854 0.936 6.135 6.390 P2O5 0.099 0.032 bdl 0.065 bdl bdl 0.017 0.057 0.035 0.009 0.090 bdl 0.011 0.035 bdl bdl MnO 0.017 0.030 0.070 0.144 bdl 0.021 0.094 0.019 0.133 bdl 0.096 bdl 0.131 0.155 0.005 0.002 Al2O3 0.006 bdl 17.795 0.409 17.314 17.412 0.105 16.471 0.054 19.366 0.010 0.006 0.711 0.181 15.916 17.799 SO3 0.018 0.049 bdl 0.004 0.006 bdl 0.063 bdl 0.065 0.015 0.029 bdl 0.099 0.125 0.027 0.030 FeO 0.249 0.317 35.913 7.187 36.250 36.337 1.633 31.268 0.749 34.037 0.371 0.083 2.958 3.030 32.416 34.515 K2O 0.053 0.149 0.079 0.009 0.032 0.046 0.002 0.090 bdl 0.038 0.001 bdl 0.018 0.014 0.093 0.090 CaO 52.055 53.414 0.291 45.987 0.119 0.190 55.078 4.789 56.866 0.229 52.930 0.223 49.928 51.611 7.733 2.699 BaO bdl 0.001 bdl bdl 0.037 0.006 0.017 bdl 0.015 0.017 0.063 bdl bdl bdl bdl bdl Na2O 0.038 0.025 0.033 0.026 0.054 0.046 0.016 0.043 0.019 0.072 0.012 0.059 bdl 0.065 0.018 0.049 Total 53.861 54.961 84.427 55.452 83.125 84.036 58.186 81.935 58.725 84.189 54.537 93.761 55.853 56.517 83.620 83.745 注：测点3、5、6、8、10、15、16为鲕绿泥石，1、2、4、7、9、11、13、14为方解石，12为石英；FeO代表全铁；bdl代表低于检测限. 
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表 2 鲕绿泥石原子比例计算结果
Table 2. Calculated atomic ratios of chamosites in this study
测点 3 5 6 8 10 15 16 Si 2.738 2.758 2.772 2.757 2.756 2.575 2.639 ⅣAl+ⅥAl 2.471 2.466 2.434 2.353 2.653 2.279 2.507 Fe 3.541 3.665 3.606 3.172 3.311 3.295 3.452 Mg 1.105 1.054 1.100 1.111 1.147 1.111 1.138 Mg/Fe 0.312 0.287 0.305 0.350 0.346 0.337 0.330 Al/Si 0.903 0.894 0.878 0.854 0.963 0.885 0.950 注：以28个氧原子为基础，计算鲕绿泥石的原子比例；Fe代表全铁.
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