Chronological constraints on multi-cellular organism fossil from Liangshan Formation of Paleoproterozoic Yimen Group in central Yunnan Province
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摘要:
滇中易门地区古元古界易门群亮山组中赋存有保存较好的多细胞生物化石，是大氧化事件，或广义的Lomagundi事件中生物与环境协同演化的产物。由于亮山组中普遍缺乏火山岩夹层及多期构造热事件的扰动，给精确测定这些多细胞生物的时代带来一定困难。以易门县铜厂地区罗洼垤组顶部灰白色凝灰岩1件、亮山组灰白色凝灰岩1件、粉砂质凝灰岩1件、灰黄色钙质粉砂质凝灰岩2件共5件样品260个锆石同位素测试结果为研究对象，结合区域构造热事件的分析，较准确地厘定了易门地区古元古界易门群亮山组中多细胞生物发育的时代为2.18~2.05Ga，与非洲加蓬Franceville地区发现的群居多细胞生物的时代一致，表明地球早期重大地质事件的同时性。
Abstract:Multi-cellular organism fossils from Liangshan Formation of Paleoproterozoic Yimen Group in central Yunnan Province are special products of the co-evolution between early life on the earth and the environment in Great Oxygenation Event (GEO), or generalized Lomagundi Event. However, it is difficult to directerly determine the geological time when the multi-cellular organism lived, because of thegeneral lack of volcanic interlayer in Liangshan Formation and the perturbation of multi-period tectonic thermal events. In this paper, the authors accurately revealed that the multi-cellular organism lived 2.18~2.05Ga years ago, as shown by the 260 zircon grains isotopic testing results, from two incanus tuff samples at the top of Luowadie Formation and in Liangshan Formation respectively, an incanus silty tuff sample in Liangshan formation, two sallow calcareous silty tuff samples in Liangshan Formation, in Tongchang Township, Yimen County, in central Yunnan Province, in combination with regional tectonic thermal events analysis. This epoch is equal to that of multi-cellular organism, found in Franceville area, Gabonese Republic, Africa, suggesting that early important events on the earth had geological synchroneity.
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致谢: 本文是《云南省区域地质志》（第二版，修编）、云南1:5万二街、易门县等4幅区调子项目的阶段性成果，工作过程中始终得到全国地质志项目办公室、区调子项目所属工程的支持，中国地质科学院李廷栋院士、丁孝忠研究员，天津地质矿产研究所陆松年研究员，中国地质调查局发展研究中心肖庆辉研究员，成都地质矿产研究所潘桂棠、孙志明研究员等亲自深入野外进行考查、指导工作；陆松年研究员向笔者介绍了目前国际上对新太古代顶、底界线划分方案的新趋势（2420Ma、2780Ma）；云南大学云南省古生物研究重点实验室刘煜博士提供了竹节状多细胞生物化石的CT扫描照片，在此一并致谢。
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图 1 罗洼垤组（Pt1l）与亮山组（Pt1ls）接触关系素描图及采样位置
Pt1ls—亮山组；Pt1l—罗洼垤组；1—第四系残坡积层；2—炭泥质板岩/硅泥质板岩；3—锰泥质板岩/粉砂质板岩；4—泥质板岩/凝灰质板岩；5—粉砂质凝灰岩/凝灰岩；6—断层及运动方向/化石产出层位及岩层产状；7—锆石同位素采样位置、编号及年龄值
Figure 1. Sketch of sampling location and stratigraphic contact relationship between Luowadie Formation and Liangshan Formation
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图 2 D2501地质点亮山组（Pt1ls）信手剖面及采样位置
Pt1ls—亮山组；1—第四系残坡积层；2—炭泥质板岩；3—粉砂质板岩；4—凝灰质板岩；5—钙质粉砂质凝灰岩；6—凝灰岩，7—锆石同位素采样位置、编号及年龄值；8—化石产出层位；9—岩层产状
Figure 2. Schematic map of sampling location and section at D2501 geological point
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图 3 样品PM022-26-2锆石207Pb/235U-206Pb/238U图解
Figure 3. Zircons 207Pb/235U-206Pb/238U diagrams from sample PM022-26-2
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图 4 样品PM022-28-1锆石207Pb/235U-206Pb/238U图解
Figure 4. Zircons 207Pb/235U-206Pb/238U diagrams from sample PM022-28-1
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图 5 样品D2501-1-3锆石207Pb/235U-206Pb/238U图解
Figure 5. Zircons 207Pb/235U-206Pb/238U diagrams from sample D2501-1-3
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图 6 样品D2501-1-2锆石207Pb/235U-206Pb/238U图解
Figure 6. Zircons 207Pb/235U-206Pb/238U diagrams from sample D2501-1-2
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图 7 样品D2501-1-1锆石207Pb/235U-206Pb/238U图解
Figure 7. Zircons 207Pb/235U-206Pb/238U diagrams from sample D2501-1-1
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图 8 B组锆石207Pb/235U-206Pb/238U图解
Figure 8. 207Pb/235U-206Pb/238U diagrams from B-group zircons
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图版I
a.古元古界易门群亮山组（Pt1ls）中的竹节状多细胞生物化石，外形与竹节石或现代红藻类似，易门县，亮山；b.古元古界易门群亮山组中的竹节状简单多细胞生物化石的CT扫描照片，易门县，亮山；c.PM022-26-2的A1组锆石的CL图像，多数颗粒具核-幔结构，幔部发育振荡环带，Th/U=0.47~0.72，易门县，亮山；d.PM022-28-1的B1组锆石的CL图像，具有核-幔-（边）结构，幔部发育振荡环带；Th/U=0.11~0.94，易门县，亮山；e.D2501-1-2的B2组锆石CL图像，多数颗粒具有核-幔-边结构，幔部发育振荡环带，Th/U=0.13~1.43，易门县，铜厂；f.D2501-1-3的C组锆石CL图像，C5组（下排）具有核-幔结构，而C6组（上排）不具核-幔结构，Th/U=0.41~1.2
图版I.
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