彭楠, 旷红伟, 柳永清, 耿元生, 夏晓旭, 王玉冲, 陈骁帅, 郑行海. (2018)华北克拉通南缘汝阳群大型具刺疑源类时代再厘定及早期真核生物群演化意义* [J]. 古地理学报, 20(4): 595-608
Peng Nan, Kuang Hong-Wei, Liu Yong-Qing, Geng Yuan-Sheng, Xia Xiao-Xu, Wang Yu-Chong, Chen Xiao-Shuai, Zheng Hang-Hai. (2018)Recognition of geological age for acanthomorphic acritarchs from the Ruyang Group,southern margin of North China Craton and its implication for evolution of early eukaryotes[J]. Journal Of Palaeogeography, 20(4): 595-608.   
Doi: 10.7605/gdlxb.2018.04.043
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华北克拉通南缘汝阳群大型具刺疑源类时代再厘定及早期真核生物群演化意义*
彭楠1, 旷红伟1, 柳永清1, 耿元生1, 夏晓旭1,2, 王玉冲1,2, 陈骁帅1, 郑行海2
1 中国地质科学院地质研究所,自然资源部地层与古生物重点实验室,北京 100037
2 中国地质大学(北京),北京 100083
通讯作者简介： 旷红伟,女,1969年生,教授,主要从事沉积地质学研究。E-mail: kuanghw@126.com。

第一作者简介： 彭楠,男,1983年生,助理研究员,主要从事沉积地质学与古地理学研究。E-mail: pengnan19830120@126.com。

收稿日期:2018-04-22
基金资助:*国家重点研发计划深地资源勘查开采重点专项(编号:2016YFC0601001)、国家自然科学基金(编号:41472082)、中国地质调查局项目(编号:1212011120142,DD20160120-01)和神农架世界地质公园项目共同资助
摘要

华北克拉通南缘汝阳群中以大型具刺疑源类为代表的真核生物群的时代归属问题一直存在争议。新近在豫西汝州阳坡村洛峪群洛峪口组中获得的凝灰岩锆石 SHRIMP U- Pb年龄为 1639± 13Ma,结合熊耳群和汝阳群已获得的其他年龄数据,将这类形态复杂的真核生物群出现的时间限定在 1.75— 1.64Ga,即中元古代的早期。这说明,过去一直被认为的代表进化程度较高但仅属于新元古代的一些真核生物群,其实在中元古代早期就已经出现了;由此可以进一步推测,在更古老的地层中应有更原始的微体化石出现,从而华北克拉通南缘中元古界(> 1.60Ga)将成为探寻最古老真核生物祖先的重要窗口。同时,将这些在中元古代早期就已经出现并在以后较长地史时期都存在的真核生物群用于标定地层形成时代或作为相关地层的对比标志,值得重新考虑和商榷。

关键词: 大型具刺疑源类; 早期真核生物; 中元古代; 汝阳群; 华北克拉通
中图分类号:P512 文献标志码:A 文章编号:1671-1505(2018)04-0595-14
Recognition of geological age for acanthomorphic acritarchs from the Ruyang Group,southern margin of North China Craton and its implication for evolution of early eukaryotes
Peng Nan1, Kuang Hong-Wei1, Liu Yong-Qing1, Geng Yuan-Sheng1, Xia Xiao-Xu1,2, Wang Yu-Chong1,2, Chen Xiao-Shuai1, Zheng Hang-Hai2
1 Key Laboratory of Stratigraphy and Paleontology,Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037
2 China University of Geosciences(Beijing),Beijing 100083
About the corresponding author： Kuang Hong-Wei,born in 1969,is a professor and doctoral supervisor in the Institute of Geology,CAGS. She is mainly engaged in sedimentology. E-mail: kuanghw@126.com.

About the first author： Peng Nan,born in 1983,is a research assistant in the Institute of Geology,CAGS. He is mainly engaged in sedimentary geology and palaeogeography. E-mail: pengnan19830120@126.com.

Fund:Co-funded by the National Key Research and Development Program of China(No.2016YFC0601001),the National Natural Science Foundation of China(No.41472082),China Geological Survey Projects(Nos.1212011120142,DD20160120-01) and the Project of Shennongjia Global Geopark
Abstract

The geological age of eukaryotes represented by large acanthomorphic acritarchs from the Ruyang Group,southern margin of North China Craton,has been controversial. New zircon SHRIMP U-Pb age of tuff(ca. 1639±13Ma)was obtained from the Luoyukou Formation of the Luoyu Group,Yangpo,Ruzhou,west Henan Province. This new age,combined with the ages from the Xiong'er and Ruyang Group,can well constraint the time of these eukaryotes with complex morphology from the Ruyang Group at 1.75-1.64Ga,which belongs to early Mesoproterozoic. Moreover,it shows that some eukaryotes with relatively higher degree of evolution,which were previously thought to be preserved in the Neoproterozoic,may have appeared in the Early Mesoproterozoic. We further suspect that there might be more primitive microfossils preserved in the older strata(>1.6Ga)in the southern margin of the North China Craton,which provides an important window to explore the ancestor of the eukaryote. Thus,caution should be taken when applying these Mesoproterozoic eukaryotes for stratigraphic correlation.

Key words: large acanthomorphic; early eukaryotes; Mesoproterozoic; Ruyang Group; North China Craton

20世纪70年代以来, 学者们相继在扬子和华北克拉通“ 中— 新元古代” 地层中发现了一系列重要的微体古生物化石, 这为探讨该阶段真核生物群的演化等问题提供了重要的证据(胡云绪和付嘉媛, 1982; 尹磊明, 1991; 阎玉忠和朱士兴, 1992; 尹崇玉和高林志, 1995, 1999, 2000; Xiao et al., 1997; Yin, 1997; Yin and Guan, 1999; 尹磊明和袁训来, 2003; 尹磊明等, 2003, 2004; Yin et al., 2005; Peng et al., 2007, 2009; 刘鹏举等, 2009; 罗翠和朱茂炎, 2010; 高维等, 2011; 李猛等, 2012; Liu et al., 2013; Pang et al., 2013, 2015; Agić et al., 2015, 2017; 刘欢等, 2015; Shi et al., 2017)。然而, 相比扬子地块埃迪卡拉纪微体古生物群相关研究取得的重要成果, 华北克拉通南缘中— 新元古代微体古生物群的相关研究却始终得不到统一认识, 其中一个重要原因, 就是相关地层的年代学及地层对比关系一直存在争议, 导致许多重要的地质发现和生物演化线索出现认识上的巨大差异。如豫— 陕— 晋交界地区相关地层中以具刺疑源类为代表的微体生物群的时代归属与对比就存在较大争议。随着华北克拉通南缘中— 新元古界中一批具刺疑源类化石(阎玉忠和朱士兴, 1992; 尹崇玉和高林志, 1995, 1999; Yin, 1997; 尹磊明和袁训来, 2003; 尹磊明等, 2004; 高维等, 2011; Pang et al., 2013, 2015)、宏观藻类化石(尹崇玉和高林志, 2000)及后生生物遗迹化石(胡建民等, 1991, 1996; 齐永安, 2005)的发现及研究, 该地区的中— 新元古界成为研究元古代早期生命演化的一个重要窗口。由于对生命演化认识的差异, 对于这些早期生命遗存化石的地层时代是新元古代还是中元古代一直存在争议(关保德等, 1988; 阎玉忠和朱士兴, 1992; 尹崇玉和高林志, 1995, 1999, 2000; Xiao et al., 1997; Yin, 1997; 尹磊明, 1999, 2006; Yin and Guan, 1999; 尹磊明等, 2003, 2004; 尹磊明和袁训来, 2003; Yin et al., 2005; 孟凡巍等, 2005; Yin and Yuan, 2007; 李猛等, 2012)。

尹崇玉和高林志(2000)认为汝阳群白草坪组中的大型具刺疑源类组合与西伯利亚地台乌楚— 马亚地区上里菲拉汉金群(950ma)比较接近。同时, 在苏格兰西北部新元古界Torridon群中报道的一个微体化石组合(Strother et al., 2011), 也与汝阳群微体化石组合十分相似(李猛等, 2012)。目前, 中元古界中已发现的具刺疑源类全部是一些个体小、刺不分叉的简单类型, 而大型、具复杂壳壁结构和刺饰的疑源类全部出现在新元古界(尹崇玉和高林志, 1999)。然而, 汝阳群中具刺疑源类个体大、很多具有双层壁结构和复杂的刺饰, 与新元古代南沱冰期后具刺疑源类辐射期出现的一些类型不仅形态特征相似, 而且可能存在演化联系, 因此, 汝阳群微体化石组合面貌更接近于新元古代早期的具刺疑源类特征(尹崇玉和高林志, 2000)。另外, 在豫西鲁山下汤洛峪口组下部地层中发现可靠的宏观藻类化石, 其形态特征与震旦纪陡山沱期出现的某些宏观绿藻化石相似, 也被作为洛峪口组归属于新元古界的证据(尹崇玉和高林志, 2000)。除此之外, 汝阳群中还发育有相当数量的其他大陆普遍发育的新元古代常见微古植物分子, 如Trachyhystrichosphaera stricta Hermann, cf. Cymatiosphaeroides pilatopium Zang and Walter, Simia nerjenica A. Weiss 及Tasmanites sp.等。根据以上古生物特征, 一些研究者认为豫西地区赋存具刺疑源类和宏观藻类化石的地层形成于新元古代(尹崇玉和高林志, 1999, 2000; 高维等, 2011)。

而另一部分学者更倾向于认为这套地层的时代为中元古代。依靠汝阳群中海绿石所测得的K-Ar年龄1100— 1200ma(马国干等, 1981; 关保德等, 1988), Yin(1997)最初认为汝阳群微体化石组合时代应介于中— 新元古代之间(1200ma左右)。随后一系列研究成果(尹磊明等, 2003, 2004; 尹磊明和袁训来, 2003; Yin et al., 2005; Yin and Yuan, 2007), 特别是山西永济地区水幽剖面中汝阳群化石组合, 与印度Bahraich群以及澳大利亚Roper群化石组合具有相同的具刺疑源类Tappania plana、Tappania tubata以及螺旋藻Spiromorpha segmentata(Yin, 1997; Prasad and Asher, 2001), 而Bahraich群的同位素推测年龄为1350— 1150ma(Prasad and Asher, 2001), Roper群的锆石U-Pb年龄为1492ma(Javaux et al., 2001), 这进一步将汝阳群微体化石群的时代限定在中元古代。同时, Xiao等(1997)也认为, 水幽剖面含疑源类化石地层之上的碳酸盐岩中碳稳定同位素, 与燕山地区中元古代高于庄组碳酸盐岩碳稳定同位素组成具有相似的特征, 所以汝阳群的地质时代应为中元古代。

由上述的争论和分歧可以看到, 获取相关地层的精确年代学数据, 来限定这套地层中出现的以疑源类为代表的真核生物群的时代, 显得尤为重要。近年来, 已有学者对华北克拉通南缘相关地层开展了一定的锆石高精度测年工作, 并获取了一批测年数据(苏文博等, 2012; 汪校锋, 2015; 李承东等, 2017)。文中通过豫西洛峪口组中新获取的凝灰岩样品, 开展锆石SHRIMP U-Pb定年及汝阳群— 洛峪群地层年代学研究, 进一步限定和证明了汝阳群— 洛峪群中以大型疑源类为代表的真核生物群的时代为中元古代早期, 为重新认识早期生命演化起源和演化谱系提供了精确的时间标尺。同时, 该认识也有助于华北中新元古代地层的划分和对比, 并对华北克拉通中新元古代构造背景演化及其与超大陆的关系研究也具有十分重要的意义。

1 区域地质背景及地层特征

华北克拉通位于中国的中北部, 构造演化复杂、岩石类型多样、后期改造强烈, 是中国最大、最古老的克拉通(Zhai and Santosh, 2011)。研究区位于华北克拉通南缘的熊耳裂谷带内, 属于渑池— 确山地层小区, 主要由太古代至古元古代变质岩构成华北克拉通南缘结晶基底(> 1.80Ga), 其上被中、新元古代沉积盖层角度不整合覆盖。沉积盖层由底部熊耳群火山— 沉积岩系(赵太平等, 2002; Zhao et al., 2002; He et al., 2009)和其上覆汝阳群、洛峪群以及平行不整合覆盖在洛峪群之上的震旦系黄连垛组、罗圈组或寒武系辛集组等地层组成, 登封一带为五佛山群, 在栾川一带发育高山河组、官道口群、栾川群(图 1-a)(河南省地质矿产厅, 1997)。此次的研究剖面位于河南汝州阳坡村附近, 虽有断层影响, 但本区的洛峪群与汝阳群之间、汝阳群与熊耳群之间、洛峪群与罗圈组等地层单元之间接触关系仍非常明确(图 1-b)。

图 1 豫西及研究区地质简图(据李承东等, 2017)及河南汝州阳坡村剖面位置Fig.1 Geological sketch map of western Henan Province(after Li et al., 2017) and location of section at Yangpo Village, Ruzhou City of Henan Province

熊耳群是华北克拉通1.8Ga左右固结后在南缘最早形成的三叉裂谷中发育的火山沉积岩系, 代表了华北克拉通固结后最早的拉张事件。熊耳群以火山熔岩占绝对优势, 沉积夹层及火山碎屑岩很少。熔岩以玄武安山质、安山质岩石为主, 次为英安— 流纹质岩石, 典型的玄武岩很少(河南省地质矿产局, ; 赵太平等, 2002)。

汝阳群角度不整合在熊耳群之上, 由下至上可进一步划分为小沟背组、云梦山组、白草坪组和北大尖组(关保德等, 1988)(图 2)。小沟背组主要为一套冲积扇和山间河流形成的砖红色砾岩、砂质砾岩, 以及少量含砾粗砂岩。云梦山组主要是由石英砂岩夹少量页岩及底部的砂砾岩和砾岩组成, 砂岩和砾岩均呈紫红色, 底部含有不稳定的铁矿层。白草坪组主要为紫红色泥岩、(泥)页岩, 汝阳北部多是紫红色泥岩(页岩)和紫红色石英砂岩的互层, 局部夹砾岩和白云岩, 其下部以紫红色、灰绿色页岩、粉砂质页岩大量出现为标志, 上部以紫红色、灰绿色页岩、粉砂质页岩结束为标志。在豫西— 晋南地区的白草坪组中不仅保存有具细网状纹饰的具刺疑源类化石Dictyosphaera delicata, Shuiyousphaeridium, Tappania以及少量的带藻化石Taeniatum sp.等(阎玉忠和朱士兴, 1992; Yin, 1997; 李猛等, 2012), 还出产有可靠的后生动物化石Ruyangia cylindrica Hu et Meng(胡建民等, 1991)。北大尖组主要以石英砂岩为主, 夹有灰白色长石石英砂岩和少量绿色页岩, 保存有具刺疑源类Archaeohystrichosphaeridium macroeticulatum, Shuiyousphaeridium sp., Tappania sp., 以及球形、舟形疑源类和多种带状、管状藻类化石等(阎玉忠和朱士兴, 1992; 尹磊明和袁训来, 2003; 尹磊明等, 2004; Yin et al., 2005)。另外, 在豫西地区的云梦山组和北大尖组中还发现了Ruyangichnus loyuensis Yang, Torrowangea rosei Webby, Changchengia ichnosp., Squamodictyon(?)ichnosp.遗迹化石(杨式溥和周洪瑞, 1995)。

图 2 豫西中元古界岩石地层序列简图
微体化石数据来源: 胡建民等, 1991; 阎玉忠和朱士兴, 1992; 杨式溥和周洪瑞, 1995; 尹磊明和袁训来, 2003; 尹崇玉和高林志, 2000; 李猛等, 2012; Yin, 1997; 尹磊明等, 2004; Yin et al., 2005。年龄数据来源: [1]苏文博等, 2012, 锆石U-Pb 年龄 LA-MC-ICPMS; [2-1]汪校锋, 2015, 锆石U-Pb 年龄 LA-MC-ICPMS; [2-2]汪校锋, 2015, 碎屑锆石最小U-Pb年龄 LA-MC-ICPMS; [3]李承东等, 2017, 锆石U-Pb 年龄 LA-MC-ICPMS; [4]Lan et al., 2014, 碎屑锆石中磷钇矿U-Pb 年龄 SIMS; [5]李猛等, 2013, 碎屑锆石最小U-Pb年龄 LA-MC-ICPMS; [6]Hu et al., 2014, 碎屑锆石最小U-Pb年龄 LA-MC-ICPMSFig.2 The Mesoproterozoic generalized stratigraphic column of western Henan Province

洛峪群与下伏汝阳群和上覆震旦系均呈平行不整合接触, 自下而上分为崔庄组、三教堂组和洛峪口组(图 2)。崔庄组岩性主要是紫红色、灰绿色页岩, 底部为含有海绿石的铁质石英砂岩, 其上为紫红色页岩, 在局部地区含有透镜状泥灰岩。三教堂组岩性主要为淡紫红色厚层条带状中粒石英砂岩, 夹铁质结核中粗粒石英砂岩, 顶部为海绿石石英砂岩, 其上、下均以石英砂岩为标志, 分别与下伏崔庄组和上覆洛峪口组整合接触。洛峪口组岩性主要是紫红色、灰绿色薄层状页岩、泥质白云岩和厚层状白云岩, 中上部白云岩中含有丰富的叠层石, 上部含多层薄层砖红色或灰绿色凝灰岩, 其上被黄连垛组或罗圈组等平行不整合覆盖。在豫西洛峪口组中发现了宏观藻类化石, 包括Chuaria, Shouhsienia, Tawuia, Ovidiscina等(尹崇玉和高林志, 2000)。

2 样品的岩性特征

在此次采集样品的剖面中, 洛峪口组保存较为完整, 可细分为4段(图 3): 一段为分布稳定的杂色泥页岩, 其底部整合于三教堂组含砾石英砂岩之上; 二段主要为富含叠层石的灰红色中厚层状白云岩, 夹有灰红色泥页岩薄层及“ 沉凝灰岩” ; 三段为富含叠层石的灰黄色厚层状白云岩; 四段为灰白色— 浅砖红色中薄层状白云岩— 白云质灰岩, 下部夹少量的灰色泥页岩, 近顶部薄层白云岩中可见厚度约10cm的绿色凝灰岩夹层。其上则被震旦系罗圈组杂色厚层冰碛砾岩平行不整合覆盖。样品 170325-3来自阳坡剖面洛峪口组四段近顶部的绿色凝灰岩夹层(N34° 03'13.7″; E112° 35'31.5″), 岩性为流纹质玻屑凝灰岩(图3-a), 具玻屑凝灰结构, 主要由火山玻屑、火山尘和少量的晶屑组成。玻屑呈鸡骨状、棒状、不规则状(图 3-b)。

图 3 河南汝州阳坡村洛峪口组实测剖面及采样位置(据左景勋等, 1997; 苏文博等, 2012; 略有修改)
a— 凝灰岩夹层(绿色); b— 凝灰岩显微结构特征(单偏光)Fig.3 Outcrop characteristics of the Luoyukou Formation and sample position at Yangpo Village, Ruzhou City of Henan Province(modified from Zuo et al., 1997; Su et al., 2012)

3 实验方法及结果
3.1 测试方法

首先样品采用常规的重选和磁选技术分选出锆石。之后将锆石样品颗粒和锆石标样粘贴在环氧树脂靶上, 进行打磨和抛光, 使锆石暴露出一半晶面。然后对锆石进行透射光、反射光以及阴极发光(CL)显微照相, 阴极发光和透、反射光照相均在北京离子探针中心扫描电镜实验室完成。最后通过图像对比, 分析锆石内部结构, 选出合适的测年位置点。

样靶在北京离子探针中心SHRIMP Ⅱ 仪器上测试。采用澳大利亚国家地质调查局标准锆石TEM做外标进行同位素分馏校正, 每分析5个样品点后对锆石标准TEM做分析。普通铅根据实测 204Pb 校正, 采用 207Pb/206Pb(> 1200ma)的年龄加权平均值, 其误差为2σ 。锆石样品U-Pb年龄谐和图的绘制以及年龄加权平均值计算采用Isoplot软件(Ludwing, 2003)完成。

3.2 测试结果

锆石CL图像显示锆石以不规则形态为主, 见少量的柱状, 长轴长度多在100~160μ m之间(图 4)。该样品共测试25颗锆石, Th、U含量分别为23~110 μ g/g和54~208 μ g/g, Th/U≥ 0.4(表 1)。从所测的25个数据看(表 1), 该样品的U-Pb同位素体系保持得也很好, 除4个测试点外(点1, 点4, 点18, 点25), 其余测点均落在谐和线上。21颗锆石 207Pb/206Pb 表面年龄加权平均值为1639± 13ma(图 5), 该年龄为岩石的形成年龄。

表 1 河南汝州阳坡村洛峪口组凝灰岩锆石SHRIMP U-Pb分析结果 Table 1 Results of zircon SHRIMP U-Pb analyses of tuff from the Luoyukou Formation at Yangpo Village, Ruzhou City of Henan Province

图 4 河南汝州阳坡村洛峪口组凝灰岩锆石CL图像及测点位置Fig.4 CL images and dating spots of zircons from tuff of the Luoyukou Formation at Yangpo Village, Ruzhou City of Henan Province

图 5 河南汝州阳坡村洛峪口组凝灰岩锆石U-Pb谐和曲线图Fig.5 U-Pb concordia diagram of zircons from the Luoyukou Formation at Yangpo Village, Ruzhou City of Henan Province

4 讨论
4.1 华北克拉通南缘汝阳群中具刺疑源类的时代归属

在华北克拉通南缘的汝阳群中保存有多样化的、发育有机质壁的微体化石群, 其中包括以具刺疑源类为代表的形态复杂的真核生物群(Xiao et al., 1997; Yin, 1997; Yin et al., 2005; Yin and Yuan, 2007; Schiffbauer and Xiao, 2009; 李猛等, 2012; Pang et al., 2013, 2015; Agić et al., 2015)。一直以来, 这些形态多样的真核生物被认为是新元古代的生物分子(尹崇玉和高林志, 1999, 2000), 但是随着研究的开展, 在中元古界中同样发现有特征类似或者相同的微体化石, 如在洛峪群中发现的宏观藻类化石(Ovidiscina, Chuaria, Tawnia, Shouhsienia)(尹崇玉和高林志, 2000), 在华北燕山地区中元古界高于庄组(1.56Ga)(梅冥相, 2008)和加拿大中元古界Hunting组(1.20Ga)(Butterfield, 2000)中也有出现, 汝阳群中保存的具刺疑源类Tasmanites和Trachyhystrichosphaera(尹崇玉和高林志, 1999), 在格林兰岛西北的中元古界Thule超群(Samuelsson et al., 1999)、澳大利亚西北部中元古界Bangemall群(Buick and Knoll, 1999)以及华北燕山地区中元古界串岭沟组(Peng et al., 2009)中都有出现。而以Dictyosphaera macroreticulata, Shuiyousphaeridium macroreticulatum和Tappania plana为代表的汝阳群早期真核生物群, 也广泛分布于其他地区的中元古代地层中(阎玉忠和朱士兴, 1992; Xiao et al., 1997; Yin, 1997; Javaux et al., 2001, 2017; Prasad and Asher, 2001; 孟凡巍等, 2005; Prasad et al., 2005; Yin et al., 2005; Nagovitsin, 2009; Nagovitsin et al., 2010; Pang et al., 2013; Adam, 2014; Sanchez et al., 2014; Agić et al., 2015, 2017; Butterfield, 2015; Adam and Butterfield, 2016; Adam et al., 2017)。因此, 这些早期的真核生物群在中元古界其他层位中已经出现。

前人曾采用全岩Rb-Sr法、海绿石K-Ar法以及碳酸盐岩的Pb-Pb法等对汝阳群和洛峪群的有关岩石进行过同位素年龄测定, 获得了一批从中元古代中晚期到新元古代的年龄数据(马国干等, 1980; 关保德等, 1988; 刘鸿允等, 1999)。由于这些方法中所测元素的易活动性, 因此测年的精确性、可靠性受到质疑。

21世纪以来, 测年技术迅猛发展, 同时在洛峪群上部层位中获得了可用于测年的凝灰岩夹层。近年来, 不同学者多次对洛峪群开展了精确地质年代学测量: 苏文博等(2012)在洛峪群中上部的凝灰岩中获得了1611± 8ma的锆石LA-ICP-MS的年龄值。Lan等(2014)对洛峪群三教堂组砂岩及其边部增生的磷钇矿进行了碎屑锆石年龄测定, 认为汝阳群和洛峪群形成于1750— 1400ma(但是1400ma可能是后期与热液活动有关的磷钇矿的形成年龄, 并不是成岩年龄)。汪校锋(2015)对洛峪群顶部洛峪口组上部和顶部的凝灰岩进行了锆石LA-ICP-MS年龄测定, 分别获得了1662± 20ma(MSWD=2.5)和1640± 16ma(MSWD=1.3)的年龄结果。李承东等(2017)对河南汝州洛峪口村和阳坡村洛峪口组近顶部的凝灰岩夹层进行了锆石LA-ICP-MS年龄测定, 分别获得了1638ma(MSWD=0.44)和1634ma(MSWD=1.3)的年龄结果。笔者新近测试表明, 河南汝州阳坡村洛峪口组近顶部凝灰岩夹层的锆石SHRIMP U-Pb年龄为1639± 13ma(MSWD=0.99)。综合上述数据, 以1640ma作为洛峪口组顶部凝灰岩的形成时代是可靠和准确的, 这也同时限定了洛峪群的顶界年龄, 也就是说, 洛峪群及下伏的汝阳群应形成于1640ma之前。

对于汝阳群, 虽然目前还没有汝阳群上、下界的可靠年龄, 但是值得注意的是, 汝阳群白草坪组石英砂岩4个样品碎屑锆石年龄数据中最小的年龄数值分别为1817± 22ma、1838± 23ma、1924± 17ma和1829± 28ma(李猛等, 2013); 而云梦山组碎屑锆石最小年龄为1711± 37ma(汪校锋, 2015)、1717± 53ma和1711± 37ma(Hu et al., 2014)。这些碎屑锆石尽管不能准确限定汝阳群形成的下限, 但是这些样品中都缺少中元古代的碎屑锆石, 因此可以推断汝阳群大致形成于1800— 1711ma之后。

结合不整合于汝阳群之下熊耳群的年龄1.80— 1.75Ga(任富根等, 2000; He et al., 2009; Wang et al., 2010; 柳晓艳等, 2011; Cui et al., 2011, 2013), 可认定汝阳群与洛峪群形成于1.75— 1.64Ga, 属于中元古代的早期(按GTS2012, Wagoner Kranendonk, 2012)。这说明, 华北克拉通南缘汝阳群中以具刺疑源类为代表的形态复杂的真核生物群出现的时限为1.75— 1.64Ga。

4.2 对早期真核生物群演化的再认识

新元古代被认为是前寒武纪生命演化最活跃的时期, 特别是真核生物得以空前繁盛(Porter et al., 2003; Butterfield, 2004, 2005a, 2005b; Knoll et al., 2006; Cohen and Knoll, 2012), 各类原生生物(Protistis)辐射发展, 同时出现越来越多的宏观多细胞藻类(后生植物)和早期后生动物的化石记录(Glaessner, 1984; Gehling, 1999; Grazdhankin, 2004; Narbonne, 2005; Fedonkin et al., 2007; Germs et al., 2009; Cortijo et al., 2010), 特别是大型具刺疑源类在埃迪卡拉纪早中期的快速辐射与多元化, 在华南、澳大利亚、东欧、印度等很多地区都有发现(Yuan and Hofmann, 1998; Yin, 1999; Xiao, 2004; Grey, 2005; Veis et al., 2006; Zhou et al., 2007; Willman and Moczydł owska, 2008; Vorob'eva et al., 2009; Sergeev et al., 2011; Yin et al., 2011; Moczydlowska and Nagovitsyn, 2012; Liu et al., 2013; Shukla and Tiwari, 2014; Xiao et al., 2014; Johsi and Tiwari, 2016)。也正是由于这一传统的认识和当时缺少精确的年代地层学数据, 导致在华北克拉通南缘汝阳群— 洛峪群中发现的一些真核生物群化石, 特别是一些代表进化程度较高的一类微体生物化石群, 如大型具刺疑源类化石(刺分叉有细网)、宏观藻类化石以及微古植物分子, 因为其可对比或相似于新元古代的一些微体古生物化石群, 一直被认为是属于新元古代(尹崇玉和高林志, 1999, 2000)。基于目前最新的地层年代学资料, 现在都应该归属于中元古代早期, 并且具刺疑源类在中元古界中也越来越多地被发现(阎玉忠和朱士兴, 1992; Xiao et al., 1997; Yin, 1997; Javaux et al., 2001; Prasad and Asher, 2001; 孟凡巍等, 2005; Prasad et al., 2005; Yin et al., 2005; Nagovitsin, 2009; Nagovitsin et al., 2010; Pang et al., 2013; Sanchez et al., 2014; Adam et al., 2014, 2017; Agić et al., 2015, 2017; Butterfield, 2015; Adam and Butterfield, 2016; Jaraux and Knoll, 2017), 特别是在全球中元古界中发现的以Tappania为代表的大型具刺疑源类, 被认为是“ 迄今所发现的最古老的、确定无疑应归属于真核生物的实体化石” (Butterfield, 2015)。同时一些最早期的真核生物的发现, 如Schizofusa, Valeria lophostriata, Grypania, Shuiyousphaeridium等(Du et al., 1995; Prasad et al., 2005; Yin et al., 2005; Bengtson et al., 2007; Peng et al., 2007, 2009; Nagovitsin, 2009; Adam, 2014; Agić et al., 2015; Vorob'eva et al., 2015; Zhu et al., 2016), 使得古元古代— 中元古代转换期成为早期真核生物演化阶段中最受关注的时期之一, 也进一步说明古元古代晚期— 中元古代时期也是真核生物起源和早期演化的重要时期。此外, 在澳大利亚Stirling Range组中裸眼可见的大型盘状化石(megascopic discoidal fossils)(Bengtson et al., 2007)、加拿大Hunting组中的红藻生物化石(1.20Ga)(Butterfield, 2000)、华北燕辽地区中元古界中的宏观藻类化石(阎玉忠, 1995; 牛邵武, 2002; 梅冥相, 2008), 以及在燕山地区的中元古界常州沟组(Lamb et al., 2009)、团山子组(1.63Ga)(Zhu and Chen, 1995; Qu et al., 2018)和高于庄组(1.56Ga)(Du et al., 1995; 孙淑芬等, 2006; Zhu et al., 2016)中发现的多细胞真核生物化石, 都表明大型多细胞真核生物的演化在中元古代就已经开始, 并且在酸性低氧条件下的古— 中元古代海洋环境中盛行(Qu et al., 2018)。

基于以上事实, 应对中— 新元古代真核生物群的演化重新认识: (1)古元古代末期— 中元古代是真核生物起源和早期演化的重要时期, 在中元古代早期真核生物的演化已经进入了一个新的水平(Shi et al., 2017); (2)在中元古代已经出现组织结构复杂、体型巨大的真核生物群, 说明过去认为这些代表进化程度较高的仅属于新元古代的真核生物群, 其实出现的时间可能要更早, 可能在中元古代早期就已经出现了; (3)可能会有更加原始的微体化石出现在更古老的地层中, 而华北克拉通南缘的相关地层(> 1.60Ga)将成为探寻最古老真核生物祖先的重要窗口; (4)将一些在中元古代就已经出现并在以后较长地史时期都存在的真核生物群, 用于准确标定地层的形成时代或相关地层的对比, 是需要重新认识和思考的(苏文博等, 2012; 李承东等, 2017)。

5 结论

1)最新获取的洛峪口组顶部凝灰岩锆石SHRIMP U-Pb年龄(1639± 13ma), 进一步限定华北克拉通南缘汝阳群中以大型具刺疑源类为代表的真核生物群出现的时限在1.75— 1.64Ga。

2)在中元古代地层中一些代表进化程度较高的真核生物群的发现, 表明部分真核生物群要比以往认识的出现时代早得多, 即应该会有更加原始的微体生物化石群出现在更古老的地层中。若将一些真核生物群用于准确标定地层的形成时代或相关地层的对比, 需要谨慎对待。

致谢 河南省地质调查院王世炎教授级高级工程师在野外过程中给予指导和帮助,匿名审稿人为本文提出了有益的修改意见,在此一并表示感谢!

作者声明没有竞争性利益冲突.

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