牟传龙, 葛祥英, 余谦, 门欣, 刘伟, 何江林, 梁薇. (2019)川西南地区五峰—龙马溪组黑色页岩古气候及物源特征: 来自新地2井地球化学记录* [J]. 古地理学报, 21(5): 835-854
Mou Chuan-Long, Ge Xiang-Ying, Yu Qian, Men Xin, Liu Wei, He Jiang-Lin, Liang Wei. (2019)Palaeoclimatology and provenance of black shales from Wufeng-Longmaxi Formations in southwestern Sichuan Province:From geochemical records of Well Xindi-2[J]. Journal Of Palaeogeography, 21(5): 835-854.   
Doi: 10.7605/gdlxb.2019.05.057
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川西南地区五峰—龙马溪组黑色页岩古气候及物源特征: 来自新地2井地球化学记录*
牟传龙1,3, 葛祥英1,2, 余谦1, 门欣1,3, 刘伟1, 何江林1, 梁薇1,3
1 中国地质调查局成都地质调查中心,四川成都 610081
2 中国地质大学(北京),北京10083
3 山东科技大学,山东青岛 266590

第一作者简介 牟传龙,男,1965年生,研究员,博士生导师,主要从事沉积地质与油气地质研究。E-mail: cdmchuanlong@163.com。

收稿日期:2019-03-18
基金资助:*中国地质调查局工程项目“南方页岩气基础地质调查工程”下设二级项目“四川盆地下古生界海相页岩气基础地质调查”(编号: DD20160176)资助
摘要

晚奥陶世—早志留世之交是地球历史时期中的关键时段,该时段伴有全球生物大灭绝、冈瓦纳冰川以及火山事件。晚奥陶世末期的冰川事件历时相对较短,在中国尚未找到冰川存在的直接证据。为了更好地研究奥陶—志留纪交替时期是否存在古气候的变化,选取了川西南地区新地 2井岩心中新鲜的上奥陶统五峰组—下志留统龙马溪组泥页岩样品,通过其主量、微量及稀土元素特征分析,采用多种化学风化指数判定源区风化作用强度及古气候条件。新地 2井五峰—龙马溪组所有岩石样品成分变异指数( ICV)均大于 1,表明源岩成分成熟度低,属构造活动时期的首次沉积。稀土元素表现为轻稀土富集、重稀土平坦并伴有明显的负 Eu 异常,配分模式与花岗岩相似,表明源岩以亲花岗岩、长英质为主; A-CN-K图解、主量元素 Al2O3/TiO2值、微量元素 Cr/Zr值和 Th/Sc值也体现长英质成分为主的物源特征。化学蚀变指数( CIA)、化学风化指数( CIW)和斜长石蚀变指数( PIA)等多种化学风化作用指标均指示,晚奥陶世—早志留世古风化作用强度经历中等—低等—中等、古气候温暖湿润—寒冷干燥—温暖湿润的变化过程。 CIA指数的低值指示的五峰组顶部(平均值 64.14)、观音桥组(平均值 61.7)和龙马溪组底部(平均值 64.61)即赫南特期存在短暂的寒冷气候,间接证实了冈瓦纳冰川作用在中国华南地区存在相应的地球化学记录。

关键词: 川西南地区; 新地 2井; 古气候; 物源特征; 地球化学
中图分类号:P512.2;P532 文献标志码:A 文章编号:1671-1505(2019)05-0835-20
Palaeoclimatology and provenance of black shales from Wufeng-Longmaxi Formations in southwestern Sichuan Province:From geochemical records of Well Xindi-2
Mou Chuan-Long1,3, Ge Xiang-Ying1,2, Yu Qian1, Men Xin1,3, Liu Wei1, He Jiang-Lin1, Liang Wei1,3
1 Chengdu Institute of Geology and Mineral Resources,Chengdu 610081,China
2 China University of Geosciences(Beijing),Beijing 100083,China
3 Shandong University of Science and Technology,Qingdao 266590,China

About the first author Mou Chuan-Long,male,born in 1965,professor,doctoral supervisor,is engaged in sedimentary geology and petroleum geology. E-mail: cdmchuanlong@163.com.

Fund:Financially supported by the second-level project“Fundamental Geology Survey of the Lower Paleozoic Marine Shale Gas in Sichuan Basin”(No. DD20160176),which is set under the China Geological Survey Project“The Fundamental Geology Survey Project of the Shale Gas of South China”
Abstract

The Late Ordovician-Early Silurian transition was a critical interval in Earth’s history,marked by mass extinction,the Gondwana Glaciation and volcanic events. The end of Ordovician glaciations had a short-lived duration and there is no direct evidence for the glaciation in China. In order to confirm whether the palaeoclimatic changes ever existed during the end of the Ordovician-Early Silurian, we carried out major and trace elemental analyses on the core mudstone samples from the Wufeng-Longmaxi Formations of Well Xindi-2 in southwestern Sichuan Province,and reconstructed palaeoweathering conditions and palaeoclimatology through various chemical weathering indices. The ICVs(index of chemical varition)of all samples are greater than 1,indicating that they are compositionally inmature and first cycle deposits in tectonically active areas. The REE patterns are similar to the granite’s,which are characterized by slight LREE enrichments and accompanied by flat HREE trends with weakly negative Eu anomalies,suggesting that the source rocks are mainly pro-granite and felsic. The A-CN-K triangular diagram,Al2O3/TiO2, Cr/Zr, and Th/Sc ratios indicate that the provenance of the rocks is also primarily felsic. Indicated by the CIA(Chemical Index of Alteration),CIW(Chemical Index of Weathering)and PIA(Plagioclase Index of Alteration),the sediments experienced from moderate to weak then to moderate chemical weathering in the source area,and palaeoclimate shifted from warm to cold and then to warm again during the Late Ordovician-Early Silurian. The low CIA values of the Upper Wufeng Formation(CIAaver=64.14),Guanyinqiao Formation(CIAaver=61.7)and the bottom of Longmaxi Formation(CIAaver=64.61)reflect that there was a short cold climate time in Hirnantian Age,and represent indirect geochemical records in the South China of Gondwana Glaciation.

Key words: southwestern Sichuan Province; Well Xindi-2; palaeoclimate; provenance; geochemistry

奥陶— 志留纪之交扬子地块及其周缘地区沉积了五峰— 龙马溪组黑色泥页岩, 该套黑色泥页岩厚度稳定、分布广泛, 是中国现阶段页岩气勘探开发的重点层系。前人对于该套黑色泥页岩的研究多集中在有机质、成熟度、矿物成分、孔隙度及古生产力等多个方面, 重点分析探讨五峰— 龙马溪组黑色页岩古氧化还原条件、沉积环境以及有机质富集机制(王清晨等, 2008; 严德天等, 2008, 2009; 张金川等, 2008; Yan et al., 2009a, 2009b, 2015; 陈代钊等, 2011; 刘树根等, 2011; 张春明等, 2012; 郭旭生等, 2014; Ran et al., 2015; Zhou et al., 2015; Chen et al., 2016; Luo et al., 2016; Wang et al., 2016a, 2016b, 2017, 2019; Liu et al., 2017; Yang et al., 2017; Zhao et al., 2017; Huang et al., 2018; Zhu et al., 2018; Wu et al., 2019; Zhao et al., 2019), 对于该时期五峰组— 龙马溪组古气候、古风化作用强度以及物源特征等的研究相对较少, Yan等(2010)和Zou等(2018)对华南东部地区(主要涉及川东、黔北及渝东南地区)五峰— 龙马溪组地表剖面样品进行过相应的地球化学分析, 其CIA指数显示观音桥组沉积时期(即赫南特期)存在短暂的寒冷干燥气候期。由于川西南地区构造地质条件复杂、地表断裂发育, 这增加了五峰— 龙马溪组连续剖面选择以及相应黑色页岩样品采集的难度, 因此本研究选择新地2井岩心中五峰组— 龙马溪组泥岩样品进行研究, 相较于地表样品, 岩心样品的地层序列连续且较少遭受到风化作用, 元素几乎未发生迁移, 通过对其地球化学数据及相应化学风化指数的分析可以更准确地恢复当时的古风化作用强度、古气候条件及源岩信息。

1 区域地质背景

早古生代华南地区分为扬子和华夏2陆块, 震旦— 中奥陶世华南地区处于拉张状态, 寒武纪扬子陆块在南华纪碎屑岩垫板之上发育以碳酸盐岩为主的缓坡沉积, 进而发展成为镶边型碳酸盐岩台地模式(许效松等, 2001; 陈洪德等, 2009; 牟传龙和许效松, 2010; 闫剑飞等, 2010; 牟传龙等, 2011; 黄福喜等, 2011; 严德天等, 2011; 余谦等, 2011), 晚奥陶世开始, 加里东构造运动的发生, 扬子陆块和华夏陆块由伸展变为挤压状态, 随着挤压碰撞作用的加剧, 川中、黔中和雪峰等边缘隆起面积不断扩大(图 1), 海平面相对上升, 扬子地区结束了被动大陆边缘镶边型碳酸盐岩台地沉积模式, 转为前陆盆地构造演化阶段, 扬子地区整体处于被各边缘隆起所围限的半局限盆地环境(刘伟等, 2010, 2012; 牟传龙和许效松, 2010; 牟传龙等, 2011; Wang et al., 2012), 沉积岩性也由碳酸盐岩变为陆源细碎屑岩(五峰— 龙马溪组黑色页岩沉积)。

图 1 川西南地区新地2井位置Fig.1 Location of Well Xindi-2 in southwestern Sichuan Province

四川盆地是在上扬子克拉通基础上发展起来的大型多旋回叠合盆地, 是上扬子克拉通内受北东向和北西向交叉的深大断裂控制形成的菱形构造盆地(汪泽成等, 2002; 何登发等, 2011; 隆轲等, 2011), 新地2井位于云南省大关县木杆镇漂坝村, 构造位置位于四川盆地边缘, 川西南地区构造地质条件非常复杂, 断裂发育, 地表上很难找到连续的五峰— 龙马溪组剖面, 新地2井以五峰组— 龙马溪组作为目的层、取心率达100%, 从岩心上观察, 黑色页岩垂向沉积序列连续, 可作为川西南地区五峰— 龙马溪组典型剖面予以研究。

2 地层特征

新地2井五峰— 龙马溪组为连续沉积, 自下而上沉积上奥陶统五峰组、观音桥组和下志留统(兰多维列统, Llandovery Series)龙马溪组。五峰组下伏地层为临湘组, 岩性以灰色— 深灰色瘤状泥灰岩为主, 腕足类、海百合等生物零星分布, 厚度约7.18 m。

五峰组深度范围为2069.96~2081.84 m, 岩性主要为灰黑色— 黑色泥页岩(图2-a)、含钙泥页岩, 层面上笔石生物发育(图 2-b), 黄铁矿颗粒呈星点状分布, 垂直层面上也见黄铁矿透镜体顺层发育, 黑色页岩层内发育有数层斑脱岩, 斑脱岩以灰色— 灰白色为主(图 2-a), 厚度1~2 cm。水平缝和垂直缝较发育, 缝宽2~3 mm, 缝间多充填方解石。

图 2 川西南地区新地2井五峰— 龙马溪组岩石宏观及微观特征
a— 五峰组内薄层斑脱岩, 2080.27~2081.06 m; b— 五峰组内笔石生物, 2075.30 m; c— 观音桥组赫南特贝腕足生物, 2069.85 m; d— 龙马溪组底部黑色页岩内发育的黄铁矿透镜体, 2065.43 m; e— 龙马溪组黑色泥岩内石英颗粒(扫描电镜), 样品编号B30, 2069.02 m; f— 观音桥组腕足生物碎片(显微镜), 样品编号B27, 2069.67 m; g— 五峰组草莓状黄铁矿(扫描电镜), 样品编号B8, 2079.67 m; h— 龙马溪组草莓状黄铁矿(扫描电镜), 样品编号B33, 2067.30 mFig.2 Macro- and micro-characteristics of rocks from the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province

观音桥组深度范围为2069.44~2069.96 m, 为深灰色— 灰黑色含钙泥岩, 富含赫南特贝腕足动物群(图 2-c), 底部见黄铁矿透镜体发育。

龙马溪组有利黑色页岩段深度范围为2050~2069.44 m, 岩性主要为灰黑色— 黑色含钙碳质泥页岩, 页理面上见大量黄铁矿晶粒呈星点状分布, 垂直层面多见黄铁矿透镜体(图 2-d), 局部也见有粉砂岩条带, 层间见3~5层斑脱岩。

3 样品采集与实验方法

对新地2井奥陶— 志留系界线附近的五峰组、观音桥组和龙马溪组进行了高密度采样, 样品间距一般在0.5~1 m之间, 紧邻界线处加密至0.1 m, 共采集54件样品。所有样品粗碎后用去离子水清洗, 干燥后研磨至200目以备进行相应的分析测试。

主量、微量和稀土元素分析在国家地质实验测试中心完成, 主量元素通过Phillips 4400 X荧光光谱仪分析测试, FeO采用HF、H2SO4对样品稀释后用重铬酸钾滴定法测定, 灼失量通过对样品加热至1000 ℃后1 h称量其前后重量的变化求得。微量和稀土元素分析采用电感耦合等离子体质谱仪PE300D(ICP-MS)来测定, 上述分析中主量元素的分析精度优于5%, 微量元素的分析精度可达10%。

将样品无污染粉碎至200目, 主量元素SiO2、Al2O3、TFe2O3、Na2O、K2O、CaO、MgO、TiO2、MnO和P2O5等采用X射线荧光法(XRF)在X荧光光谱仪(3080E)上测定, FeO采用滴定法分析, LOI为高温加热后重量和灼烧后重量之差。微量元素和稀土元素利用电感耦合等离子体质谱法(ICP-MS)在离子质谱仪(X-series)上测试完成。扫描电镜测试在核工业北京地质研究院采用Nova Nano SEM450超高分辨率扫描电子显微镜完成, 检测方法依据GB/T17361-2013和GB/T18295-2001。

X衍射分析在国土资源部重庆矿产资源监督检测中心完成, 测试使用ZJ207 Bruker D8 advance型X射线衍射仪, 仪器采用Ni滤波Cu靶辐射, 工作电压为40 kV, 工作电流为40 mA, 发射狭缝与散射狭缝均为1° , 接受狭缝0.3 mm, 扫描方式为步进扫描, 扫描速度采用3° (2θ )/min, 采样步宽为0.02° (2θ ), 测定标准遵循SY/T5163-2010, 数据分析采用软件High Score。

4 实验结果及分析
4.1 岩石学特征

薄片特征显示五峰组和龙马溪组黑色泥页岩单偏光下整体具泥质结构, 主要由粒径小于0.0039 mm的黏土矿物和碎屑颗粒(主要为石英和长石)等组成(图 2-e), 显微鳞片状的黏土矿物呈定向排列, 粉末状集合体的碳质有机质等黑色物质沿层面渲染呈黑色条带状, 胶结物以方解石和白云石为主, 白云石晶形明显且自形程度较好, 呈菱形晶状, 方解石多成微晶状存在于黏土矿物中。观音桥组灰黑色含钙泥岩显微镜下可见腕足类生物碎片(图 2-f)。

对五峰组和龙马溪组13个页岩样品进行X射线衍射分析(表 1), 结果表明五峰组和龙马溪组黑色页岩主要矿物含量为石英29.1%~71.3%, (平均44.3%)、钾长石0~3.4%(平均1.3%)、钠长石0.8%~6.0%(平均3.1%)、方解石4.0%~36.2%(平均17.6%)、白云石2.3%~34.3%(平均14.2%)、黄铁矿1.4%~5.9%(平均3.3%)、黏土矿物7%~33.5%(平均16.5%); 黏土矿物主要由伊利石和伊蒙混层组成, 伊利石相对含量22%~40%(平均33%), 伊蒙混层相对含量40%~65%(平均54.2%), 绿泥石及绿蒙混层含量次之, 绿泥石相对含量2%~10%(平均5.2%), 绿蒙混层相对含量为4%~20%(平均9.2%)。

表 1 川西南地区新地2井五峰— 龙马溪组样品X衍射分析矿物成分特征 Table 1 Mineral compositions by X-ray diffraction analysis of samples from the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province

扫描电镜照片显示黄铁矿晶体晶粒极细, 呈自形或半自形, 多以草莓状集合体形式存在(图 2-g, 2-h)。

4.2 地球化学特征

新地2井五峰— 龙马溪组54个岩石样品的主量、微量和稀土元素分析结果见表 2、表3和表4, 其中五峰和龙马溪组黑色页岩的SiO2含量为20.79%~81.98%, 平均含量为48.9%, Al2O3含量为1.83%~20.55%, 平均含量为7.93%。

表 2 川西南地区新地2井岩心内五峰— 龙马溪组岩石样品及参考样品主量元素特征 Table 2 Characteristics of major elements of samples from the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province and reference samples

黑色页岩内钙质含量相对较高, 其中CaO 含量介于1.85%~33.58%之间, 平均含量为14.40%; K2O普遍高于Na2O, 其中K2O含量介于0.54%~5.36%之间, 平均含量为2.27%; Na2O含量较低, 介于0.14%~0.73%之间, 平均含量为0.35%; Fe2O3含量变化也相对较大, 介于0.05%~8.05%之间, 平均含量为1.35%; FeO含量相对较稳定, 介于0.47%~2.34%之间, 平均含量为1.21%。MnO、TiO2、P2O5含量均较低, MgO 含量介于0.93%~6.20之间, 平均为3.55%; MnO不超过0.26%, 多集中在0.02%~0.06%之间, TiO2含量介于0.1%~0.73%之间; P2O5含量介于0.03%~0.65%之间。与PAAS相比, 五峰组和龙马溪组的黑色页岩具有低SiO2、Al2O3、Fe2O3、K2O、Na2O、MnO和TiO2, 高CaO等特征。其SiO2(48.9%)和Al2O3(7.93%)含量普遍低于PAAS(62.8%和18.9%); Fe2O3在五峰组和龙马溪组黑色页岩内平均含量仅为1.35%, 明显低于PAAS内Fe2O3的含量(7.2%); 黑色页岩的K2O和Na2O含量(平均分别为2.27%和0.35%)均小于PAAS相应K2O和Na2O含量(3.7%和1.2%); MnO含量最大0.26%, TiO2含量最大0.73%, 小于PAAS内MnO 和TiO2平均含量2.2%和1%; 但CaO含量(14.40%)却明显高于PAAS的CaO含量(1.4%)。

观音桥组5个含钙泥岩样品中SiO2含量为36.66%~58.11%, 平均含量为48.9%; Al2O3含量介于6.15%~9.36%之间, 平均含量为7.54%; CaO含量介于2.28%~18.64%之间, 平均含量为13.23%; K2O和MgO含量基本相等, 其中K2O含量介于1.81%~2.74%之间, 平均含量为2.19%, MgO含量介于0.91%~3.38%之间, 平均含量为2.18%; Fe2O3含量变化较五峰组和龙马溪组黑色页岩更大, 介于0.89%~12.96%之间, 平均含量为4.93%; FeO含量相对较稳定, 介于0.83%~1.26%之间, 平均含量为0.97%; Na2O含量较低, 介于0.46%~1.07%之间, 平均含量为0.72%; MnO、TiO2、P2O5含量均小于1%。

从表 3中可以看出五峰组、观音桥组和龙马溪组3个组泥质岩样品中微量元素总体特征表现富集大离子亲石元素Sr、Ba等元素, 高场强元素Nb、Zr、Hf和Th相对亏损。其中Rb、Cs的平均质量分数略低于PAAS; Rb的平均含量约97.44× 10-6, Cs的平均含量5.43× 10-6, 分别相当于PAAS平均含量的0.6和0.36倍; Sr的平均质量分数为270.21× 10-6, 约相当于PAAS内Sr含量的1.35倍, 而Ba的平均质量分数为1559.89× 10-6, 高达PAAS内Ba含量的2.86倍。高场强元素均低于PAAS的平均含量; Nb平均含量为8.62× 10-6(0.45× PAAS)、Zr平均含量为84.30× 10-6(0.4× PAAS)、Hf平均含量为2.66× 10-6(0.53× PAAS)、Th平均含量为10.22× 10-6(0.7× PAAS)。

表 3 川西南地区新地2井岩心内五峰— 龙马溪组岩石样品微量元素特征 Table 3 Characteristics of trace elements of samples from the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province

中所有岩石的稀土元素总量$\sum$REE 变化较大, 其范围介于(40.89~310.23)× 10-6之间, 平均含量为121.75× 10-6, 轻稀土相对于重稀土富集, 轻稀土含量为(31.27~266.07)× 1 0-6, 平均为107.84× 10-6, 重稀土为(7.77~44.16)× 10-6, 平均为13.91× 10-6。LREE/ HREE值为3.25~13.94, LaN/ YbN值为2.3~16.32, δ Eu值为0.47~0.93。在球粒陨石标准化的稀土配分图解(图 3)中, 从稀土元素的含量上看, 样品的轻、重稀土含量均略低于PAAS的轻、重稀土含量。所有样品表现出相对比较一致的展布特征, 呈明显的轻稀土富集、重稀土平坦的“ V” 型右倾特征, 并伴有δ Eu 轻度亏损。这种配分模式与花岗岩的稀土元素配分模式较为相似, 从稀土元素分布特征来看五峰组、观音桥组以及龙马溪组3个组的泥质岩类主要来源于长英质岩石。

表 4 川西南地区新地2井岩心内五峰— 龙马溪组岩石样品稀土元素特征 Table 4 Characteristics of rare earth elements of samples from the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province

图 3 新地2井五峰— 龙马溪组样品球粒陨石标准化稀土配分图解(标准化值来自Taylor and Mclenann, 1985)Fig.3 Chondrite-normalized REE distribution patterns of samples from the Wufeng-Longmaxi Formations in Well Xindi-2(normalization values after Taylor and McLenann, 1985), southwestern Sichuan Province

5 讨论
5.1 古风化作用强度及其对古气候的指示

化学蚀变指数(CIA)、斜长石蚀变指数(PIA)、风化指数(CIW)和成分变异指数(ICV)等都可以判断碎屑沉积物源区的物质组成以及化学风化作用强度。

ICV成分变异指数通常用于判定碎屑岩的源岩成分是首次沉积的沉积物还是再循环的沉积物, 根据定义, ICV=(Fe2O3T+K2O+Na2O+CaO* +MgO+MnO+TiO2)/Al2O3, 其中主成分为摩尔分数, CaO* 为硅酸岩中的CaO。当ICV> 1, 表明碎屑岩的成分成熟度低, 其内含有很少的黏土矿物, 反映沉积物是在构造活动时期的首次沉积; ICV< 1表明碎屑岩的成分成熟度高, 碎屑岩中含有大量黏土矿物, 沉积物质经历了再循环作用或者是遭受了强烈的化学风化作用条件下的首次沉积(Cullers and Podkovyrov, 2000, 2002), 受后生作用的影响较大。新地2井54个样品的ICV值均大于1, 平均值为3.72, 表明样品成分成熟度低, 源岩为首次沉积的沉积物, 并未受到再沉积作用的影响。

化学蚀变指数(chemical index of alteration, CIA)最初是由Nesbitt和Young(1982)提出用于判断物源区的化学风化作用强度, 其公式: CIA=Al2O3/[(Al2O3+CaO* +Na2O+K2O)]× 100, 式中主要成分均以摩尔分数表示, CaO* 代表硅酸盐岩中的CaO 含量(全岩中的CaO去掉化学沉积的CaO的摩尔分数), Mclennan等(1993)指出硅酸盐矿物中CaO和Na2O的平均组成比例, 依据沉积物样品中的CaO/Na2O的摩尔比值来进行校正, 因此, 在计算CaO* 时采用如下法则, 当CaO的摩尔分数大于Na2O的摩尔分数时, m(CaO* )=m(Na2O), 反之m(CaO* )=m(CaO)。随着研究的不断深入, CIA值对于古气候也有相应的指示(Nesbitt and Young, 1984; Young and Nesbitt, 1999)。CIA值介于50~65之间, 反映低等的化学风化作用强度和寒冷干燥的气候条件, CIA值介于65~85之间, 反映中等强度的化学风化和温暖湿润的气候条件, CIA值介于85~100之间, 反映强烈的化学风化强度和炎热潮湿的热带亚热带气候条件。

通常情况下, 物源区母岩物质是复杂的, 用CIA进行定量分析物源区古风化作用强度及古气候时, 还应考虑到沉积分异作用、再旋回作用、沉积区进一步风化作用以及成岩期钾质交代等作用的影响。上面提到新地2井五峰— 龙马溪组所有样品ICV值均大于1表明该区未受到再旋回作用的影响, 而成岩作用过程中钾交代作用会带入新的钾元素, 从而导致CIA值偏低, 因此需要对其进行校正。在A-CN-K三角图解中, Nesbitt 和Young(1984, 1989)通过长石分解的热力学和动力学过程和自然界风化剖面的地球化学特征得出风化剖面和与之相关的沉积物的预测风化趋势线, 图 4-1内近似平行于A-CN连线的实线a和b代表未发生钾交代作用的岩石风化趋势, 实线c代表发生钾交代作用的风化趋势, 其中实线3和实线4之间的虚线代表着发生了钾交代作用的泥质岩的CIA值范围, Fedo等(1995)指出通过K顶点与样品在A-CN-K三角图中投点连线的反向延伸线与未发生钾交代作用的风化趋势线的交点的纵坐标, 代表了发生钾交代作用之前的岩石的CIA值, 即CIAcorr(图4-1中实线2与实线1之间的线段代表了 CIAcorr的范围)。CIAcorr可通过公式进行计算(Panahi et al., 2000), 相应的计算公式为: CIAcorr=[Al2O3/(Al2O3+CaO* +Na2O+K2Ocorr)]× 100; K2Ocorr=[m* Al2O3+m(CaO* +Na2O)]/(1-m); m=K2O/(Al2O3+CaO* +Na2O+K2O); 其中相应的氧化物以摩尔分数为单位, K2Ocorr为未发生钾交代作用的泥质岩中K2O的含量, m 表示母岩中K2O的比例(徐小涛和邵龙义, 2018)。

图 4 川西南地区新地2井五峰— 龙马溪组岩石样品A-CN-K图解(据Nesbitt and Young, 1984; Fedo et al., 1995修改)
1— 遭受钾交代作用样品的示意性A-CN-K图解; 2— 新地2井A-CN-K图解Fig.4 A-CN-K ternary diagram for samples of the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province(modified from Nesbitt and Young, 1984; Fedo et al., 1995)

从图 4中新地2井五峰— 龙马溪组样品点的连线看其风化趋势线几乎平行于A-CN连线, 表明其CIA值基本未受到后期成岩作用过程钾质交代作用的影响, 通过公式计算得出的校正值CIAcorr与原CIA值基本一致(图 4-2; 表2)。

从图 5可以看出新地2井五峰— 龙马溪组CIA指数自下而上呈现一定的规律性, 五峰组下段样品B1— B20其CIA值介于67.48~73.37之间(平均值69.72), 向上至五峰组顶部样品B21— B23其CIA值介于61.51~68.74之间(平均值64.14), 观音桥组5个样品B24— B28其CIA值介于58.30~64.66之间(平均值61.70), 龙马溪组底部3个样品B29— B31的CIA值介于63.47~65.49之间(平均值64.61), 龙马溪组下段样品B32— B54其CIA值介于65.36~72.85之间(平均值68.18), 按照以上CIA值变化规律, 五峰组至龙马溪组下段古风化作用强度依次经历中等— 低等— 中等, 古气候温暖潮湿— 寒冷干燥— 温暖潮湿的变化过程。

图 5 川西南地区新地2井五峰— 龙马溪组岩石地层和风化指数CIA、PIA、CIW 变化(生物地层据陈旭等, 2017。同位素、海平面变化、环境变化等据Brenchley et al., 2006; Harper et al., 2014)Fig.5 Stratigraphic, lithological correlations and paleoweathering indices of CIA, PIA and CIW of the Wufeng-Longmaxi Formations in Well Xindi-2, southwestern Sichuan Province(chronostratigraphic subdivision after Chen et al., 2017. Isotope, sea level and environmental change after Brenchley et al., 2006; Harper et al., 2014)

化学风化指数(chemical index of weathering, CIW)是Harnois(1988)年提出的对化学风化作用强度的判别新指数, 其计算公式:CIW=100× [Al2O3/(Al2O3+CaO* +Na2O)], 式中的Al2O3、 Na2O和 CaO* 均采用摩尔分数, CaO* 代表硅酸盐岩中的CaO 含量。显生宙页岩的CIW值接近于85, CIW> 85, 显示强烈的化学风化作用强度。新地2井五峰组下段样品B1— B20的CIW值介于85.87~91.04之间(平均值89.13)(图 5), 向上至五峰组顶部样品B21— B23的CIW值介于76.18~87.41之间(平均值80.06), 观音桥组5个样品B24— B28的CIW值介于71.61~81.33(平均值76.87)之间, 龙马溪组底部3个样品B29— B31其CIW值介于80.47~82.24之间(平均值81.43), 龙马溪组下段样品B32— B54的CIW值介于81.71-91.77之间(平均值86.45), 按照CIW值的变化规律, 五峰组至龙马溪组下段古风化作用强度依次经历强烈— 低等— 强烈的变化过程。

斜长石蚀变指数(plagioclase index of alteration, PIA)单独指示斜长石的风化状况(Fedo et al., 1995)。其计算公式:PIA=100× (Al2O3-K2O)/(Al2O3+CaO* +Na2O-K2O), 式中主要成分均以摩尔分数表示, CaO* 代表硅酸盐岩中的CaO 含量。通常情况下新鲜岩石的PIA值为50, PAAS的PIA值为79, 黏土矿物如高岭石、伊利石及蒙脱石的PIA值则接近100。新地2井五峰组下段样品B1— B20的PIA值介于80.58~87.52之间(平均值84.93), 向上至五峰组顶部样品B21— B23的PIA值介于68.98~82.71之间(平均值73.65), 观音桥组5个样品B24— B28的PIA值介于63.21~74.84之间(平均值69.44), 龙马溪组底部3个样品B29— B31的PIA值介于73.33~76.13之间(平均值74.89), 龙马溪组下段样品B32— B54的PIA值介于75.61~88.88之间(平均值81.50), 按照PIA值的变化规律, 五峰组至龙马溪组下段古风化作用强度依次经历中等— 低等— 中等的变化过程(图 5)。

5.2 源岩成分

细碎屑沉积岩中包含了丰富的源区物质组成、构造环境以及相应构造演化的信息(Dickinson and Suczek, 1979; Cox et al., 1995; Hofmann, 2005; Sugitani et al., 2006), 其地球化学特征(主量、微量和稀土等元素)被广泛应用于判定源岩成分类型、恢复沉积盆地性质及构造背景(Bhatia, 1983, 1985; Bhatia and Crook, 1986; Wronkiewicz and Condie, 1987; Roser and Korsch, 1988; McLennan, 1989; McLennan and Taylor, 1991; McLennan et al., 1993; Cullers, 1995; Armstrong-Altrin et al., 2004, 2012; Purevjav and Roser, 2012; Zaid, 2012, 2015; Armstrong-Altrin, 2015)。Hayashi 等(1997)指出Al2O3/TiO2值在3~8之间物源可能是铁镁质岩石, 8~21之间是中性火成岩, 21~70是长英质岩石。新地2井五峰组样品Al2O3/TiO2值介于13.65~41.11之间(平均值22.81), 观音桥组Al2O3/TiO2值介于18.12~20.87之间(平均值19.32), 龙马溪组样品Al2O3/TiO2值介于20.38~60.44之间(平均值23.59), 3个组的Al2O3/TiO2值反映其物源可能是中性— 长英质岩石。A-CN-K三角图解对于源区成分的判断也是非常有用的(Nesbitt and Young, 1984, 1996; Fedo et al., 1995; Young and Nesbitt, 1999; 冯连君等, 2003; 黄云飞等, 2017), 其中玄武岩、安山岩、花岗闪长岩和花岗岩等重要的基岩根据化学成分投影在了A-CN-K图中相应位置。根据新地2井五峰组、观音桥组和龙马溪组3组样品点的分布(图 4), 得到了1条相应的风化趋势线, 该趋势线与Pl-Ksp连线的交点即为源岩斜长石和钾长石的比率, 从图中可以看出连线与花岗岩的成分相对比较接近, 从而判定新地2井五峰— 龙马溪组样品的沉积物源可能是花岗岩。

微量元素Cr 和Zr主要反映铬铁矿和锆石的含量, 其值可以反映镁铁质与长英质对沉积物的相对贡献(Wronkiewicz and Condie, 1989)。新地2井除五峰组B13样品的Cr/Zr 值为1.12外, 其余样品均小于1, 说明其物源主要来自于长英质岩石。Taylor 和McLennan(1985)以及Fedo等(1997)研究表明, Th/Sc 值是最适合于判别物源区性质的, 其中五峰— 龙马溪组3个组岩石的Th/Sc 值均大于1或在1附近, 指示物源以长英质组分为主。综上, 以主量、微量和稀土元素各特征元素比值及A-CN-K三角图解判定, 新地2井五峰组、观音桥组和龙马溪组岩石样品的源岩成分主要是长英质类岩石。

5.3 地质意义

晚奥陶世— 早志留世这一特殊时期全球范围内也发生了多个大事件, 晚奥陶世赫南特期发生了地球历史上第2次生物大灭绝事件, 同时期冈瓦纳大陆冰川事件波及全球, 赫南特期存在短暂时间的冰期, 但在中国范围内却始终找不到冰川存在过的直接证据。古生物方面, 戎嘉余(1984)指出晚奥陶世赫南特期观音桥组内发育的赫南特贝腕足动物是凉水型浅水生物群落, 间接证实中国华南地区在晚奥陶世— 早志留世初也曾经发生过短暂的冰川活动。岩石的CIA指数不仅仅能够反映岩石的古风化作用强度, 同样也可以指示当时的古气候变化。笔者通过对新地2井五峰组— 龙马溪组新鲜岩石样品进行主量、微量和稀土元素测试, 得出的CIA值自下而上具有一定的规律性, 五峰组下段(CIA平均值69.72)— 五峰组顶部(CIA平均值64.14)— 观音桥组(CIA平均值61.7)— 龙马溪组底部(CIA平均值64.61)— 龙马溪组下段(CIA平均值68.18), CIA值的变化反映了新地2井自五峰组底部到龙马溪组下段化学风化作用强度依次经历中等— 低等— 低等— 低等— 中等, 古气候温暖潮湿— 寒冷干燥— 寒冷干燥— 寒冷干燥— 温暖潮湿的变化过程。根据最新的古生物笔石带的划分原则, 赫南特阶包含了N. extraordinarius和N. persculptus这2个笔石生物带, 对应的岩石地层单位为五峰组的顶部、观音桥组和龙马溪组底部(Chen et al., 2006), Brenchley等(2006)和Harper等(2014)指出奥陶纪末期的最大冰盖期位于Hirnantian阶自N.extraordinarius底部到N. persculptus下段, 而此时恰好对应于新地2井相应的CIA、CIW和PIA最低值区。新地2井的CIA数据显示五峰组顶部— 观音桥组和龙马溪组底部有1次短暂的寒冷干燥气候的发生, 这也从地球化学角度间接证实了华南地区在赫南特期存在冈瓦纳冰川的响应。

6 结论

1)新地2井五峰组— 龙马溪组样品地球化学特征显示, 主量元素方面, 与PAAS相比, 泥质岩类具有低SiO2和Al2O3、高CaO等特征, 微量元素总体富集大离子亲石元素Sr、Ba等元素, 高场强元素Nb、Zr、Hf和Th相对亏损。稀土元素方面所有岩石表现出相对比较一致的展布特征, 呈明显的轻稀土富集、重稀土平坦的“ V” 型右倾特征, 并伴有δ Eu 轻度亏损。

2)新地2井五峰组— 龙马溪组下段所有岩石样品ICV值均大于1, 反映沉积物成分成熟度低, 是在构造活动时期的首次沉积, 化学指数CIA、CIW 和PIA自下而上均呈现相同的规律性, 反映晚奥陶世末— 早志留世初古风化作用强度依次经历中等— 低等— 中等、古气候条件经历温暖潮湿— 寒冷干燥— 温暖潮湿的变化过程。CIA指数的变化表明五峰组顶部— 观音桥组和龙马溪组底部有1次短暂的寒冷干燥气候的发生, 间接证实华南地区在赫南特期存在冈瓦纳冰川的地球化学响应。

3)新地2井五峰组— 龙马溪组泥页岩A-CN-K 图解、稀土元素特征以及主量元素Al2O3/TiO2值、微量元素Cr/Zr值和Th/Sc 值, 表明沉积物的物源区以长英质岩石为主。

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