南华盆地新元古代成冰纪成锰作用及其成矿背景

付勇，郭川

1)贵州大学资源与环境工程学院，贵阳，500025； 2)贵州大学喀斯特地质资源与环境教育部重点实验室，贵阳，500025

内容提要: 南华盆地成冰系大塘坡组锰矿是我国最重要的锰矿产出层位之一，它形成于成冰纪Sturtian冰川事件之后，其成矿背景及形成机理一直是研究的重点。在系统总结Sturtian冰川事件起始与结束时间、南华裂谷盆地结构演化及古气候演变等重大地质事件的最新研究进展的基础上，综合分析了南华盆地大型沉积型锰矿成矿作用过程与这些重大地质事件之间的联系。揭示了南华盆地Sturtian冰期的启动和结束与全球其他地区基本一致，分别发生在～717 Ma和～660 Ma之前。同时，对南华系大塘坡锰矿成矿时代进行了约束，大约形成于～660 Ma之前。在新元古代中期Rodinia超大陆裂解作用的影响下，南华裂谷盆地内部发育一系列由同沉积断层控制的地垒—地堑次级盆地。沿同沉积断层运移的热液流体为大塘坡锰矿的形成提供了大量的成矿物质，并控制着大塘坡锰矿的发育分布。化学蚀变指数(CIA)、锂同位素(δ7Li)及锇同位素组成[n(187Os)/n(188Os)]等风化指标显示，南华盆地Sturtian冰期晚期至间冰期大塘坡期早期的气候为寒冷干燥，随后转为温暖湿润并很快变为寒冷干燥。至大塘坡中晚期，气候逐渐由寒冷干燥恢复至温暖湿润，并一直保持至大塘坡晚期。整体来看，Sturtian冰期结束后，南华盆地表层海水逐渐氧化，深部沉积水体出现局部间歇式氧化环境，裂陷阶段热液和陆源输入的Mn2+被氧化为MnO2发生沉淀，并在底部伴随着有机质的埋藏及早期成岩作用而最终形成菱锰矿。

新元古代是地球演化过程中的一个重要时期，伴随着强烈和广泛的冰川作用(Hoffman et al., 1998)、真核生物扩张(Sahoo et al., 2016)及条带状铁建造(BIF)的再现(Cox et al., 2013)等。这些重大地质事件与当时的气候、海水化学性质及板块构造运动等密切相关，长期以来一直是地球科学研究的重点。其中成冰纪发生了两次全球范围的冰川事件(即Sturtian和Marinoan冰期)，分别对应着华南地区的江口冰期和南沱冰期(赵彦彦等，2011; 张启锐等，2016)，这两次冰期事件很大程度上改变了当时的大陆风化作用，并进一步造成了海洋的化学性质和氧化还原状态的显著变化(Hoffman et al., 2017)。华南地区的这两次冰期之间发生了大塘坡间冰期，对应着我国重要的成锰期，在此期间华南地区形成了丰富的沉积型锰矿床。多年勘探显示，华南地区南华系地层中发育多个大中型沉积型锰矿床(付勇等, 2014)，如贵州大塘坡(周琦等, 2013; Yu Wenchao et al., 2019; 张予杰等, 2020)、重庆秀山(凌云等, 2016; Ma Zhixin et al., 2019; 赵志强等, 2019)、湖南湘潭(史富强等, 2016)等，其中锰矿石主要赋存在大塘坡组第一段(或相当层位)的黑色岩系中，使得黔、湘、渝等地区成为我国最重要的锰矿产地之一。由于大塘坡组锰矿具有重要的经济价值和科学意义，使得南华纪时期与其形成密切相关的Sturtian冰川事件、盆地演化、古气候及海洋氧化还原状态等内容一直是研究的热点(Li Chao et al., 2012; 杜远生等, 2015; 齐靓等, 2015; 周琦等, 2016; Lan Zhongwu et al., 2020)。

1 南华盆地Sturtian冰期时限及南华 系锰矿成矿时代

目前Sturtian冰期时限及沉积矿床的形成年龄主要通过一系列的间接定年方法(如Rb-Sr、Sm-Nd、锆石U-Pb法等)，主要通过对冰期地层、含矿层系或相邻地层中的火山灰或成岩矿物来实现，但这些间接定年方法受其测试对象稀缺性和自身性质的制约，而使其应用存在很大的局限性(Rasmussen, 2005)。相比之下，Re-Os定年方法能直接限定富有机质沉积岩(如黑色页岩)的沉积年龄(Cohen et al., 1999; Kendall et al., 2004; 李超等, 2014; Fu Yong et al., 2016; 王富良等, 2016)。近期研究显示，在后期成岩作用(如热成熟、低变质作用等)过程中，黑色页岩中的Re-Os体系仍能保持封闭，即不会发生Re和Os元素的丢失或获取(Creaser et al., 2002; Rooney et al., 2010; 李超等, 2014)，这使得Re-Os定年方法被广泛用来限定前寒武系沉积地层的沉积年龄(Rooney et al., 2010; Zhu Bi et al., 2013; Tripathy and Singh, 2015)。

1.1 Sturtian冰期启动时间

Sturtian冰期地层在全球范围内广泛分布，但典型的冰期沉积主要发育在澳大利亚、纳米比亚、加拿大、中国南部、阿曼和蒙古等地区(Hoffman and Li, 2009; 赵彦彦和郑永飞, 2011; Hoffman et al., 2017)。这次冰期对应着华南地区的江口冰期，代表沉积为江口群(长安组+富禄组)或两界河组与铁丝坳组(Lan Zhongwu et al., 2014; 汪正江等, 2015)。近年来，针对成冰纪冰川事件的期次、起始与结束时间及全球对比，前人通过冰期沉积或相邻地层中的火山灰等开展了大量的研究工作，并取得了一系列的进展(图1和表1)。其中，Lan Zhongwu 等(2014)获得了桂北地区丹洲群顶部凝灰质粉砂岩和湖北宜昌地区莲沱组顶部凝灰岩的SIMS U-Pb年龄，分别为716.1 ± 3.4 Ma和715.9 ± 2.8 Ma；Lan Zhongwu 等(2015)获得的湖北宜昌地区莲沱组顶部凝灰岩SIMS U-Pb年龄为714 ± 8 Ma；覃永军等(2015)获得的黔东南地区下江群顶界U-Pb年龄为717 Ma；Jiang Zhuo-Fei等(2016)获得的川西地区开建桥组顶部凝灰岩夹层的SHRIMP和LA-ICP-MS U-Pb年龄分别为715.0 ± 9.8 Ma和718.8 ± 9.4 Ma；Song Gaoyuan 等(2017)获得的湘西地区板溪群顶部碎屑锆石的LA-ICP-MS U-Pb年龄为714.6 ± 5.2 Ma；蔡娟娟等(2018)获得的桂北地区长安组底部冰成杂砾岩碎屑锆石的LA-ICP-MS U-Pb最小加权平均年龄为719.6 ± 6.1 Ma；Lan Zhongwu 等(2020)获得的长安组之下拱洞组顶部的CA-ID-IRMS锆石U-Pb年龄(720.16 ± 1.42 Ma)，并通过Monte Carlo模拟将华南地区Sturtian冰期的发生时间限定在717.61 ± 1.65 Ma。因此，华南地区Sturtian冰期的发生时间应为～717 Ma。

表1 新元古代Sturtian冰期起始和结束年龄汇编Table 1 Compilation of chronometric dates for the onset and termination of the Neoproterozoic Sturtian glaciation

针对Sturtian冰期的起始时间，国外学者也开展了大量的研究。其中，Fanning 和Link(2004)获得的劳伦西亚Grand Canyon地区Pocatello组Scout Mountain段之上冰碛岩斑状流纹岩中岩浆锆石的SHRIMP U-Pb年龄为717 ± 6 Ma；Bowring等(2007)获得的阿曼Ghubrah冰碛岩中火山碎屑凝灰岩中的锆石TIMS U-Pb年龄为713.7 ± 0.5 Ma，这代表了最接近阿曼斯图特冰期起始时间的年龄数据，但由于取样点位于混积岩之中，因此阿曼斯图特冰期的起始时间应早于713 Ma。Macdonald等(2010，2018)获得了加拿大西北部地区Mount Harper群底部及其之下地层火山灰高分辨率锆石TIMS年龄，将加拿大地盾斯图特冰期的起始时间限定在717.4 ± 0.1 Ma和716.9 ± 0.4 Ma。因此，这些年龄数据显示，不同纬度不同大陆上的Sturtian冰期可能是一次同时启动的、快速的全球性事件，其发生的时间应为 ～ 717 Ma(图1；Macdonald et al., 2010; Lan Zhongwu et al., 2020)。

图1 新元古代Sturtian冰期起始和结束年龄分布图Fig. 1 The ages of the onset and termination of the Neoproterozoic Sturtian glaciation 数据来源：1—蔡娟娟等，2018；2—Lan Zhongwu et al., 2020；3, 17—Fanning and Link, 2004；4, 5—Macdonald et al., 2010；6, 7—Lan Zhongwu et al., 2014；8—Song Gaoyuan et al., 2017；9—Allen et al., 2002；10—Bowring et al., 2007；11—Lund et al., 2003；12—Ferri et al., 1999；13, 29—Fanning and Link, 2008；14, 15—Evans et al., 1997；16—尹崇玉等，2006；18—余文超等，2016b；19—Cox et al., 2018；20—Zhou Chuanming et al., 2004；21—Yu Wenchao et al., 2017；22—Rooney et al., 2014；23—高林志等，2013；24, 25 32, 37—Rooney et al., 2020；26—裴浩翔等，2017；27—Semikhatov, 1991；28—Zhou Chuanming et al., 2020；30—Wang Dan et al., 2019；31—Rooney et al., 2015；33—Zhou Chuanming et al., 2019；34—Fanning and Link, 2006；35—李明龙等，2021；36—Kendall et al., 2006；38—Zhang Sihong et al., 2008 Data are compiled from: 1—Cai Juanjuan et al., 2018&; 2—Lan Zhongwu et al., 2020; 3, 17—Fanning and Link, 2004; 4, 5—Macdonald et al., 2010; 6, 7—Lan Zhongwu et al., 2014; 8—Song Gaoyuan et al., 2017; 9—Allen et al., 2002; 10—Bowring et al., 2007; 11—Lund et al., 2003; 12—Ferri et al., 1999; 13, 29—Fanning and Link, 2008; 14, 15—Evans et al., 1997; 16—Yi Chongyu et al., 2006&；18—Yu Wenchao et al., 2016b&; 19—Cox et al., 2018; 20—Zhou Chuanming et al., 2004; 21—Yu Wenchao et al., 2017; 22—Rooney et al., 2014; 23—Gao Linzhi et al., 2013&; 24, 25 32, 37—Rooney et al., 2020; 26—Pei Haoxiang et al., 2017&; 27—Semikhatov, 1991; 28—Zhou Chuanming et al., 2020; 30—Wang Ping et al., 2019; 31—Rooney et al., 2015; 33—Zhou Chuanming et al., 2019; 34—Fanning and Link, 2006; 35—Li Minglong et al., 2021&; 36—Kendall et al., 2006; 38—Zhang Shihong et al., 2008

1.2 Sturtian冰期结束时间及南华盆地南华系 锰矿成矿时代

(1)Sturtian冰期结束时间。由于华南地区Sturtian冰期地层(如富禄组、铁丝坳组或古城组)中尚未发现用于高精度定年的同沉积火山岩，因此Sturtian冰期结束的时间通常由上覆间冰期地层大塘坡组底部的凝灰岩层限定。Zhou Chuanming 等(2004)获得的华南贵州东部松桃地区大塘坡组底部凝灰岩中锆石的ID-TIMS U-Pb年龄为662.9 ± 4.3 Ma，之后通过CA-ID-IRMS U-Pb法将之修正为659.96 ± 0.46 Ma(Zhou Chuanming et al., 2020)，但由于其采样部位处于大塘坡组底部位，说明Sturtian冰期结束时间应发生于之前。随后，尹崇玉等(2006)也获得了贵州东部松桃地区黑水溪剖面大塘坡组底部凝灰岩SHRIMP U-Pb年龄为667.3 ± 9.9 Ma。之后，不同学者进一步开展了一系列Sturtian冰期结束时间的定年工作。例如，Zhang Shihong等(2008)获得的华南湖北西部地区紧邻南沱组下部湘锰组(大塘坡组相当层位)凝灰岩层中锆石的SHRIMP U-Pb年龄为654.5 ± 3.8 Ma；余文超等(2016b)和Yu Wenchao等(2017)获得的华南贵州东部松桃地区将军山剖面和寨浪沟剖面大塘坡组底部含锰页岩层锆石的LA-ICP-MS U-Pb年龄分别为664.2 ± 2.4 Ma和662.7 ± 6.2 Ma；Zhou Chuanming 等(2019)获得的华南云南东部地区大塘坡组底部盖帽白云岩凝灰岩层锆石的CA-ID-TIMS U-Pb年龄为658.8 ± 0.5 Ma。最近，Rooney 等(2020)获得了贵州东南部地区和湖南西部地区大塘坡组底部含锰页岩层凝灰岩中锆石的CA-ID-TIMS U-Pb年龄，分别为660.98 ± 0.74 Ma、658.97 ± 0.76 Ma、657.17 ± 0.78 Ma，其中660.98 ± 0.74 Ma为最接近大塘坡组底界的年龄。同时，Rooney等(2020)还获得了贵州东北部距铁丝坳组顶部3 m的黑色页岩的Re-Os年龄为660.6 ± 3.9 Ma，也进一步证实了富有机质沉积岩Re-Os年龄方法的可靠性。在华南湖北恩施地区，李明龙等(2021)获得的大塘坡组底部凝灰岩锆石的LA-ICP-MS U-Pb年龄为658.1 ± 2.6 Ma。这些年龄数据在误差范围内完全一致，因此华南Sturtian冰期结束应发生在～660 Ma前。

世界其他地区也获得了与华南地区类似的年龄，也进一步限定了Sturtian冰期结束的时间。例如，劳伦大陆获得的Mackenzie Mountains地区Hay Creek群Twitya组底部黑色页岩的Re-Os年龄为662.4 ± 3.9 Ma(Rooney et al., 2014)；前苏联乌拉尔地区Laplandian冰碛岩之下火山灰层锆石的TIMS U-Pb年龄为660 ± 15 Ma(Semikhatov, 1991)；澳大利亚南部地区紧邻Appila (Sturt) 组冰碛岩之上Wilyerpa组中火山灰层锆石的CA-ID-TIMS U-Pb年龄和SIMS U-Pb年龄分别为663.03 ± 0.11 Ma(Cox et al., 2018)和659.7±5.3 Ma(Fanning and Link, 2008)；澳大利亚Adelaide Rift Complex地区Appila (Sdturt) 组冰碛岩之上的Tapley Hill组底部Tindelpina段黑色页岩的Re-Os年龄为657.2 ± 5.4 Ma(Kendall et al., 2006)；蒙古Tuva—MongoliaTaishir组底部黑色页岩的Re-Os年龄为659.0 ± 4.5 Ma(Rooney et al., 2015)。这些年龄与华南地区获得的年龄在误差范围内完全一致(图1)，因此，全球范围内Sturtian冰期的结束可能也是一个等时事件，其应发生在～660 Ma之前，其持续时间约为57 Ma(Rooney et al., 2014; Zhou Chuanming et al., 2019; Rooney et al., 2020)。

(2)南华盆地南华系锰矿成矿时代。由于Sturtian冰期结束的时限主要通过上覆大塘坡组底部凝灰岩和富有机质沉积岩(如黑色页岩)获得，这些年龄也对南华盆地南华系锰矿成矿时代进行了限定。近期，裴浩翔等(2017)获得的贵州东部道坨锰矿大塘坡组一段含锰黑色页岩的Re-Os同位素等时线年龄为660.6 ± 7.5 Ma；Wang Dan等(2019)获得的贵州东部松桃将军山剖面大塘坡组底部含锰页岩层凝灰岩中锆石的SIMS U-Pb年龄为659.3 ± 2.4 Ma。这些年龄数据在误差范围内是一致的，因此结合前人报道的年龄数据可将南华盆地南华系锰矿限定在～ 660 Ma，该年龄可对该时期南华盆地甚至全球范围内的成矿地质事件提供很好的年龄约束，同时能为全球对比研究体统很好的年龄框架。

2 南华纪南华裂谷盆地结构演化

已有研究显示，南华裂谷盆地的形成与演化与Rodinia超大陆的裂解密切相关(王剑等, 2001; 杜远生等, 2018)。其中，Rodinia超大陆的形成于中元古代末期(1300～900 Ma)的全球范围造山运动，这次构造运动几乎波及所有的大陆板块 (Hoffman, 1991; Li et al., 2008)。随后，新元古代时期(～750 Ma)发生了全球性的裂谷作用，导致Rodinia超大陆发生裂解，并最终在600 Ma完全解体(Li et al., 2008; Zhao Guochun et al., 2018; Wang Wei et al., 2020)。在Rodinia超大陆的形成—裂解过程中，扬子板块和华夏板块在830 Ma发生碰撞形成华南板块和江南造山带(王自强等, 2012; 孙海清等, 2013; 赵军红等, 2015; Li Qiwei and Zhao Junhong, 2020)，并自820 Ma开始发生多次幕式大陆裂解作用(Wang Jian and Li Zhengxiang, 2003)。在此背景下，扬子板块内形成了以南华裂谷盆地为代表的沿东南方向展布的裂谷系统(王孝磊等, 2004; 杜远生等, 2015; Zhao Guochun et al., 2018)，同时南华裂谷盆地内部也发育一系列由同沉积断层控制的地垒—地堑次级盆地(图2；周琦等, 2016)。

由于青白口纪(～800 Ma)第一次裂陷活动的作用，在湘西、黔东、桂北地区(江南构造带西段)分别沉积了一系列以深水沉积组合和火山岩及凝灰岩沉积为特征的板溪群、下江群、丹洲群(杜远生等, 2015)。随后，南华纪(～725 Ma)发生了第二次裂陷活动，在湘黔边界地区沉积了大塘坡组以黑色泥质岩系为特征的深水沉积(图3；Zhang Shihong et al., 2008)。而震旦纪(～635 Ma)发生了第三次裂陷活动，在江南构造带东侧形成了以硅质泥质岩系等深水沉积为特征的震旦系—奥陶系地层。整体来看，扬子地块东南缘裂谷盆地(I级)可划分为武陵次级裂谷盆地、天柱—怀化隆起及雪峰次级裂谷盆地3个II级结构单元，它们可进一步识别出间隔分布的III级地堑与地垒结构，如溪口—小茶园次级地堑盆地、松桃—石阡次级地堑盆地、万山—岑巩次级地堑盆地及黎平—从江次级地堑盆地(图2；周琦等, 2016)。在这些地堑盆地内，大塘坡组沉积厚度自盆地中心向盆地边缘逐渐降低(图3)。另一方面，由黔东地区向鄂西地区大塘坡组厚度逐渐降低，岩相组合在区域上也表现出明显的差异性，二分性逐渐消失(旷红伟等, 2019)。

图2 扬子地块东南缘裂谷盆地结构示意图及主要锰矿(剖面)分布图(修改自杜远生等, 2015)Fig. 2 The tectonic architecture of rift basin of southeastern Yangtze Block and distribution map of Mn deposits (modified from Du Yuansheng et al. 2015&) 1—重庆秀山小茶园锰矿；2—重庆秀山盐井沟剖面；3—重庆秀山笔架山锰矿；4—贵州松桃道坨锰矿；5—贵州松桃两界河锰矿；6—贵州松桃西溪堡锰矿；7—贵州江口桃映剖面；8—贵州铜仁万山石竹溪锰矿床；9—贵州新晃板桥剖面；10—贵州从江八当锰矿点；11—湖南花垣民乐锰矿；12—湖南湘潭锰矿床。 ① 溪口—小茶园次级地堑盆地；② 松桃—石阡次级地堑盆地；③ 万山—岑巩次级地堑盆地；④ 黎平—从江次级地堑盆地 1—Xiaochayuan Mn deposit, Xiushan, Chongqing; 2—Yanjinggou Section, Xiushan, Chongqing; 3—Bijiashan Mn deposit, Xiushan, Chongqing; 4—Daotuo Mn desposit, Daotuo, Guizhou; 5—Liangjiehe Mn Deposit, Songtao, Guizhou; 6—Xixibao Mn deposit, Songtao, Guizhou; 7—Taoying Section, Jiangkou, Guizhou; 8—Shizhuxi Mn deposit, Wanshan, Tongren, Guizhou; 9—Banqiao Section, Xinhuang, Guizhou; 10—Badang Mn deposit, Congjiang, Guizhou; 11—Minle Mn deposit, Huayuan, Hunan; 12—Xiangtan Mn deposit, Hunan. ① Xikou—Xiaochayuan sub-graben basin; ② Songtao—Shiqian sub-graben basin; ③ Wanshan—Cenggong sub-graben basin; ④ Liping—Congjiang sub-graben basin

图3 南华裂谷盆地南华纪两界河—大塘坡期盆地结构图(a，据周琦等, 2016修改)和大塘坡期早期盆地结构图 (b，据邹光均等, 2020修改)Fig. 3 The architecture of the Nanhua rift basin during the Lianghejie—Datangpo Age of the Nanhua Period (a, modified from Zhou Qi et al., 2016&) and during the early Datangpo Age (b, modified from Zou Guangjun et al., 2020&)

3 南华纪南华盆地风化作用记录

“雪球地球”假说认为，新元古代成冰纪冰川事件之后，地球迅速转为温室气候条件，并同时伴随着强烈的化学风化作用过程(Hoffman et al., 1998; Hoffman and Schrag, 2002)。前人研究显示，大陆风化作用对海洋营养物质的输入有着非常重要的影响，并进一步制约着海水的氧化还原状态，如高营养物质输入会导致海洋表层海水高的初始生产力和富有机质页岩的沉积(Yeasmin et al., 2017; Huang Taiyu et al., 2019; Li Chao et al., 2020)。已有研究显示，南华盆地大塘坡组底部(或一段)锰矿的形成受沉积水柱氧化还原状态的制约(何志威等, 2014; Wu Chengquan et al., 2016; 余文超等, 2020)。为了明确大塘坡组(或相当层位)下部含锰岩系及黑色页岩形成时期海洋南华盆地的氧化还原状态，前人采用一系列地球化学指标开展了大量的研究工作，如元素地球化学(朱祥坤等, 2013; 何志威等, 2014; Wu Chengquan et al., 2016; 赵志强等, 2019)、碳同位素(Chen Xi et al., 2008; 裴浩翔等, 2020)、硫同位素(Li Chao et al., 2012; 张飞飞等, 2013; Wang Ping et al., 2019)、铁组分(Li Chao et al., 2012; Ma Zhixin et al., 2019)、氮同位素(Wei Wei et al., 2016)、锶-钕同位素(Yu Wenchao et al., 2016; 余文超等, 2016a)、钼同位素(Cheng Meng et al., 2018; Ye Yuntao et al., 2018; Tan Zhaozhao et al., 2021)、锂同位素(Wei Guangyi et al., 2020)等。结果显示，南华盆地沉积水体的氧化还原状态大致经历了3个阶段：① 冰期阶段主要为缺氧环境；② 成锰阶段主要为表层海水氧化、深部水体缺氧的分层海洋结构；③ 含锰岩系上覆黑色页岩沉积时期主要为缺氧环境(Li Chao et al., 2012; Cheng Meng et al., 2018; Tan Zhaozhao et al., 2021)。近期研究表明，冰期向间冰期转换阶段，大陆风化作用强度并非是迅速升高的，而是存在着多次短期的波动(Huang Kangjun et al., 2016; Li Chao et al., 2020)。针对南华纪南华盆地的大陆风化强度变化，前人已开展了化学蚀变指数(CIA；齐靓等, 2015; 李明龙等, 2019; Wang Ping et al., 2020)、锂同位素(δ7Li)(Wei Guangyi et al., 2020)及锇同位素组成[n(187Os)/n(188Os)；裴浩翔等, 2017]等方面的研究，并取得一定的认识。

3.1 化学蚀变指数(CIA)和锂同位素(δ7Li)

大陆化学风化作用过程主要受控于湿度和温度(Nesbitt and Young, 1982; Sheldon and Tabor, 2009)。在热带—亚热带潮湿气候背景下，高沉淀、高气温及高产率的酸性表层水体有利于强烈风化作用(Schoenborn and Fedo, 2011)，可将砂质颗粒中的钾长石有效的转化为粘土矿物(Johnsson et al., 1991)。相比之下，在干旱和极地气候背景下，有限的沉淀作用及低温条件通常导致弱的化学风化作用，而且物理风化作用比化学风化作用更为有效，从而会形成化学上不成熟和相对欠风化的沉积物(Nesbitt and Young, 1989)。因此，Nesbitt和Young(1982)提出的化学蚀变指数(Chemical index of alteration；CIA)可作为评估土壤和沉积物化学风化强度的指标，并具有古气候意义(Sheldon and Tabor, 2009; Zhai Lina et al., 2018; Wang Ping et al., 2020)。在不同气候条件下，风化搬运之后形成的沉积物具有不同的CIA值(Nesbitt and Young, 1989；冯连君等，2006)。例如，炎热湿润气候条件下形成的沉积物的CIA值通常为80～100；温暖潮湿气候条件下一般为70～80；寒冷干燥气候条件下形成的沉积物的CIA值为55～70(冯连君等，2006)。但需要注意的是，沉积物的CIA值可能会受到物源组成、搬运过程中的水动力颠选作用及成岩期钾交代作用等因素的影响(McLennan, 1993; Bahlburg and Dobrzinski, 2011; 李明龙等, 2021)。相比之下，锂(Li)元素在自然界中主要以Li+离子形式存在，而且无化合价变化(苟龙飞等, 2017)。已有研究显示，大陆风化作用可造成Li同位素的最大分馏，但生物作用几乎不会造成Li同位素的分馏(苟龙飞等, 2017；Wei Guangyi et al., 2020)。在风化搬运过程中，轻的Li同位素(6Li)会在次生粘土矿物中优先富集，而重的Li同位素(7Li)则会被搬运至海洋中，从而导致海相自生粘土矿物通常具有比陆源风化产物更高的δ7Li值(苟龙飞等, 2017；Wei Guangyi et al., 2020)。因此，海相细粒碎屑沉积岩中的Li同位素组成主要受控于其中陆源碎屑物质及海相自生粘土矿物的比例及其Li同位素组成。

李明龙等(2021)开展了鄂西走马地区钻孔ZK701南华系大塘坡组及相邻层段(下覆古城组和上覆南沱组)的CIA分析(图4)。古城组顶部较低的CIA值(平均值为57.6；n=2)指示南华盆地Sturtian冰期晚期的气候条件仍为寒冷干燥，并一直持续至间冰期大塘坡期早期(CIA平均值=59.1；n=2)。随后，气候发生短暂的波动变化，先由寒冷干燥转为温暖湿润并很快变为寒冷干燥。至大塘坡中晚期，气候逐渐由寒冷干燥恢复至温暖湿润，并一直保持至大塘坡晚期。之后，可能受Marinoan冰期的影响，气候缓慢向寒冷干燥转变。这种变化趋势与鄂西走马地区钻孔ZK701(李明龙等, 2019)及黔东地区钻孔ZK4207与ZK1408 (齐靓等, 2015)南华系的CIA值反映的气候演化基本一致(图4)。由于大塘坡组底部锰矿层矿物相主要为碳酸锰，可能会对CIA计算产生较强的影响。为了消除这种影响，Wang Ping 等(2020)对黔东地区钻孔ZK2115锰矿层做了去碳酸盐组分处理。尽管处理前后的CIA值存在差异(处理过的样品值偏高)，但在大塘坡组底部均存在一个低值区域，达到一个高值之后，再转为振荡的低值，并逐渐升高，指示气候变化与ZK701类似的趋势(图4)。这种趋势在黔东地区道坨剖面大塘坡组的δ7Li也有很好的响应(Wei Guangyi et al., 2020)。整体来看，δ7Li值自大塘坡组底部表现出升高→降低→再升高的趋势，指示在大塘坡期早期寒冷干燥气候背景下大陆风化作用逐渐减弱，然后在气候转为温暖潮湿气候背景下大陆风化作用增强，并在气候再次转为寒冷干燥状态下大陆风化作用随之减弱(图4)。随后，大塘坡期中晚期在温暖潮湿背景下大陆风化作用振荡变化，但维持一个较高的强度。

3.2 锇同位素组成[n(187Os)/n(188Os)]

在氧化性海水中，Re和Os通常以ReO-4和HOsO-5等高价态的稳定形式存在，在还原性海水中则会被还原成较难迁移的低价离子(Peucker-Ehrenbrink and Ravizza, 2000; Yamashita et al., 2007)。由于Re、Os的亲有机性，富有机质沉积岩在沉积过程中会吸附一定数量的低价Re、Os离子。另一方面，富有机质沉积岩(如黑色页岩)中Os元素主要为水成成因，从而导致富有机质沉积岩具有与同时期海水相同的Os同位素比值[即初始n(187Os)/n(188Os)值；李超等, 2014]。因此，可通过Re-Os等时线年龄法获得富有机质沉积岩(如黑色页岩)的初始n(187Os)/n(188Os)值，以此来约束同时期海水的Os同位素比值(Cohen et al., 1999; Peucker-Ehrenbrink and Ravizza, 2000; Cohen, 2004; Fu Yong et al., 2016; Wei Shuaichao et al., 2017; Rotich et al., 2020)。与Sr同位素一样，地质历史时期海水中的Os同位素组成并非保持不变，其变化非常剧烈(Cohen, 2004)。海水中的Os同位素组成主要有3个物源：①河流输入的高放射性的古老地壳风化Os[现今河流的n(187Os)/n(188Os)：1.4～1.6；Peucker-Ehrenbrink and Ravizza, 2000]；②洋中脊热液蚀变输入的非放射性Os[n(187Os)/n(188Os)约为0.127；Esser and Turekian, 1993; Cohen, 2004]; ③宇宙尘埃带来的非放射性Os[n(187Os)/n(188Os)约为0.127；Shirey and Walker, 1998; Cohen, 2004]。整体来看，海水中的Os同位素主要是这3项来源的综合结果，地质历史时期突发性地质事件通常会改变不同端元的供给通量，从而可能造成海水Os同位素的相应波动(Ravizza and Turekian, 1989; Finlay et al., 2010)。现今正常海水的n(187Os)/n(188Os)为1.05～1.06，反映氧化风化状态下海水中的Os同位素组成主要来源于放射性陆壳的风化作用(Levasseur et al., 1998; Peucker-Ehrenbrink and Ravizza, 2000)。而太古宙海水的n(187Os)/n(188Os)仅为0.1～0.15(Anbar et al., 2007; Yang et al., 2009)，则指示缺氧背景下海水中的Os同位素组成主要来源于洋中脊的热液蚀变作用(Hannah et al., 2004; Kendall et al., 2009)。因此，海水中Os同位素特征的变化可很好的用来示踪地质历史时期海洋和气候环境的演化特征(van Acken et al., 2013; Gibson et al., 2019; Rotich et al., 2020)。

尽管前寒武纪海水的Os同位素组成数据较少，其n(187Os)/n(188Os)值整体表现出阶梯式增大的趋势(图5)，可能反映了大气氧气含量的幕式增加过程(Li Chao et al., 2012; Pufahl et al., 2014; Fan Haifeng et al., 2018; Cole et al., 2020; Uahengo et al., 2020; Wei Guangyi et al., 2021)。裴浩翔等(2017)获得的贵州道坨锰矿大塘坡组底部黑色页岩的初始n(187Os)/n(188Os)值为0.781，该值明显低于现今正常海水的n(187Os)/n(188Os)值(图5)，指示大塘坡组底部沉积时期海水中Os同位素组成来源以海底洋中脊热液蚀变为主，这也与当时南华盆地广泛发育的裂谷活动相一致(杜远生等, 2015; 周琦等, 2016)。另一方面，较低的初始n(187Os)/n(188Os)值也揭示了当时相对寒冷干燥气候背景下相对较弱的风化强度。Rooney 等(2020)总结了华南、蒙古及加拿大Sturtian冰期地层的Os同位素化学地层(图6)，可见n(187Os)/n(188Os)值从Sturtian冰碛岩顶部的～1.4逐渐降至距冰碛岩～2.5 m的～0.4，随后逐渐升高。n(187Os)/n(188Os)值的变化特征揭示了大陆风化作用强度的变化趋势，即Sturtian冰期结束阶段大陆风化作用相对较强，进入间冰期之后，大陆风化作用可能明显减弱，随后再次增强(Li Chao et al., 2012)。Sturtian冰期至间冰期大陆风化作用强度的变化特征与Marinoan冰期非常类似(Huang Kangjun et al., 2016)。

图5 前寒武纪富有机质沉积岩和准同生—早期成岩黄铁矿的初始n(187Os)/n(188Os)值Fig. 5 The initial n(187Os)/n(188Os) of the Precambrian organic-rich sedimentary rocks and syndepositional—early diagenetic pyrite 引用数据来源于Kendall et al., 2009、Rooney et al., 2010, 2011、裴浩翔等, 2017 Data are compiled from Kendall et al., 2009, Rooney et al., 2010, 2011 and Pei Haoxiang et al., 2017&

图6 Sturtian冰期结束后海水的Os同位素化学地层(修改自Rooney et al., 2020)Fig. 6 The composite Os isotope chemostratigraphy of the post-Sturtian successions (modified from Rooney et al., 2020) 引用数据来源于/Data from: Peucker-Ehrenbrink and Ravizza, 2000、Meisel et al., 2001、Rooney et al., 2014, 2015

4 锰矿床成矿模式及南华盆地南华系 锰矿成矿机理

4.1 锰矿床成矿模式

根据赋矿岩石类型的不同，我国锰矿床大致可分为有海相沉积型、火山—沉积型、碳酸盐岩中热水沉积型(或“层控”型)、与岩浆作用有关的热液型、受变质型及表生型6种类型(付勇等, 2014)。其中前3种类型主要为沉积型，中国与全球锰矿的分布和储量数据显示，这类锰矿床的工业价值最大，具有巨大的工业储量(付勇等, 2014；Maynard, 2010, 2014)，其成矿过程与大气氧气含量、海洋氧化还原状态及盆地类型等密切相关(Roy, 2006；付勇等, 2014)。针对沉积型锰矿床，目前国内外学者基于大量的实例研究提出了两种较为流行的锰矿成因模式：① 富锰缺氧海水中直接沉淀成矿(图7a)；② 成岩孔隙水中转化成矿(图7b, c, d；董志国等, 2020)。

图7 沉积型锰矿床主要成矿模式图Fig. 7 The main metallogenic models of sedimentary manganese deposits 修改自/modified from: Force and Cannon, 1988; Huckriede and Meischner, 1996; Roy, 2006; Maynard, 2014

(1)富锰缺氧海水中直接沉淀成矿。锰是一种对氧化还原敏感的变价元素，其存在的形式主要受体系的Eh—pH条件控制(Roy, 2006)。在氧化水体中，锰主要以氧化物(Mn2O3)沉淀的形式存在；在还原性水体中，则主要以溶解态Mn2+离子形式存在(Krauskopf, 1957)。在现代海洋缺氧水体(如开放海洋的最小含氧带或局限海洋分层水体的深部缺氧带)中Mn2+相对集中，但大量实例研究和实验模拟显示，现代海洋缺氧水体的Mn2+浓度并不足以沉淀出碳酸锰(MnCO3)矿物(Mucci, 2004; 王霄等，2018)。另一方面，虽然铁与锰的化学性质比较类似，由于Mn2+/Mn(OH)3的还原电位比Fe2+/Fe(OH)3高(Lu Zunli et al., 2010)，在适当的还原条件下铁和锰可发生分离(Krauskopf, 1957)。但大量的研究实例显示，沉积型锰矿床中并不存在明显的铁锰分带现象(Maynard, 2010)。因此，富锰缺氧海水中直接沉淀成矿模式还有待进一步的论证。

(2)成岩孔隙水中转化成矿。这种成矿模式主要包括3个阶段：① Mn2+离子在还原性缺氧水体中的富集；② 在氧化还原界面之上，运移过来的Mn2+离子被氧化形成锰氧化物(或氢氧化物)发生沉淀；③随着上覆沉积物的堆积，沉积物—水界面之下逐渐演变为一个还原的微环境，早期沉积的锰氧化物(或氢氧化物)被微生物还原成Mn2+离子，其与孔隙水中的碳酸根(CO32-)离子结合形成碳酸锰(即菱锰矿)发生沉淀(Roy, 2006; Maynard, 2014)。大量的研究实例显示，这种成矿模式主要发生在具氧化还原分层结构的盆地中(Roy, 2006; Maynard, 2014; 董志国等, 2020; 余文超等, 2020)。其中海洋的化学性质分层结构可形成于多种环境，如与极端地质事件(如冰川事件等；图7b)有关的水体循环受限、最小氧化带的扩张(图7c)及季节性富氧底流的输入(图7d)等(Force and Cannon, 1988; Li Chao et al., 2012; Maynard, 2014)。近年来，越来越多的学者认为，在沉积型锰矿床形成过程中微生物可能也起着非常重要的作用(Fan Delian et al., 1999; Yu Wenchao et al., 2019)。

4.2 南华盆地南华系锰矿成矿机理

在元古宙时期，地球大气圈逐渐发生氧化，并对地球表层环境、海洋化学条件和元素循环过程等产生了深远的影响(Kump, 2008; Lyons et al., 2014; Cole et al., 2020)。大量的研究显示，大气圈氧气含量的增加并不是一个渐进的过程，而是在两次快速的增氧之后才达到现今的水平(Canfield, 2005; Lyons et al., 2014)。它们分别为2.1～2.4 Ga左右的大氧化事件(Great Oxidation Event；GOE)和0.75～0.58 Ga左右的新元古代氧化事件(Neoproterozoic Oxygenation Event；NOE)(Kump, 2008; Lyons et al., 2014)。在大气圈增氧过程中，新元古代成冰纪地球上发生了几次全球性的冰川事件，冰雪甚至覆盖率中低纬度和磁道地区(即“雪球地球”；Hoffman et al., 1998)。其中可进行全球对比的两次冰期为Sturtian冰期和Marinoan冰期，分别对应华南地区的江口冰期和南沱冰期(赵彦彦和郑永飞, 2011; 张启锐和兰中伍, 2016; 旷红伟等, 2019)。在两次冰期之间的间冰期期间，华南地区南华盆地沉积了深水相大塘坡组(或相当层位)，主要分布在黔—湘—渝等地区。

5 结论

(1)南华盆地Sturtian冰期的启动和结束与全球其他地区基本一致，分别发生在～717 Ma和～660 Ma之前，持续时间约为57 Ma；且南华盆地南华系大塘坡锰矿大约形成于～660 Ma之前。

(2)在Rodinia超大陆裂解作用的影响下，南华裂谷盆地内部发育一系列由同沉积断层控制的地垒—地堑次级盆地，为锰质的沉淀提供了可容空间；同时，深部热液为大塘坡锰矿的形成提供了大量的成矿物质，并对大塘坡锰矿的发育具有明显的控制作用。

(3)南华盆地Sturtian冰期晚期至间冰期大塘坡期早期的气候主要为寒冷干燥，随后转为温暖湿润并很快变为寒冷干燥。至大塘坡期中晚期，气候逐渐由寒冷干燥恢复至温暖湿润，并一直保持至大塘坡期晚期。

(4)Sturtian冰期结束后，南华盆地表层海水逐渐氧化，深部沉积水体出现局部间歇式氧化环境，热液和陆源输入的Mn2+被氧化为MnO2发生沉淀，并在早期成岩阶段在微生物的作用下最终形成菱锰矿。

致谢：感谢评审专家和责任编辑对本文修改提出的宝贵建议。

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