蔡娟娟, 崔晓庄, 兰中伍, 王剑, 江卓斐, 邓奇, 卓皆文, 陈风霖, 江新胜. (2018)华南扬子陆块成冰纪冰川作用的启动时限及其全球对比* [J]. 古地理学报, 20(1): 65-86
Cai Juan-Juan, Cui Xiao-Zhuang, Lan Zhong-Wu, Wang Jian, Jiang Zhuo-Fei, Deng Qi, Zhuo Jie-Wen, Chen Feng-Lin, Jiang Xin-Sheng. (2018)Onset time and global correlation of the Cryogenian glaciations in Yangtze Block, South China[J]. Journal Of Palaeogeography, 20(1): 65-86.   
Doi: 10.7605/gdlxb.2018.01.005
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华南扬子陆块成冰纪冰川作用的启动时限及其全球对比*
蔡娟娟1, 崔晓庄2,3, 兰中伍4, 王剑2, 江卓斐2, 邓奇2, 卓皆文2, 陈风霖3, 江新胜2
1 中国地质科学院,北京 100037
2 中国地质调查局成都地质调查中心,四川成都 610081
3 成都理工大学沉积地质研究院,四川成都 610059
4 岩石圈演化国家重点实验室,中国科学院地质与地球物理研究所,北京 100029
通讯作者简介 崔晓庄,男,1984年生,成都地质调查中心助理研究员,主要从事沉积学与前寒武纪地质研究。E-mail: cgscuixz@126.com。

第一作者简介 蔡娟娟,女,1990年生,中国地质科学院硕士研究生,古生物学与地层学专业。E-mail: dzcaijj@126.com。

收稿日期:2017-07-28
基金资助:*国家自然科学基金项目(编号: 41502114,41772115,41672112,41673016)与中国地质调查局项目(编号: DD20160017)共同资助
摘要

江口冰期和南沱冰期是华南地区引人注目的 2次成冰纪冰川事件,但其确切启动时间及其全球对比关系仍未有定论。为此,对桂北地区成冰系(南华系)长安组底部和南沱组底部冰成杂砾岩开展了碎屑锆石 U-Pb年代学研究。长安组碎屑锆石 U-Pb年龄集中分布于 958— 717 Ma,显著峰值为 720、 753、 805及 848 Ma,最年轻一组 206Pb/238U 年龄的加权平均值为 719.6±6.1 Ma,可解释为长安组最大沉积年龄;南沱组碎屑锆石 U-Pb 年龄集中分布于 987— 649 Ma,显著峰值为 650、 720、 753、 779、 803、 823及 848 Ma,最年轻一组 206Pb/238U 年龄的加权平均值为 649.3±6.2 Ma,可解释为南沱组最大沉积年龄。结合已发表的相关年龄数据可知,江口冰期很可能启动于 ca.715 Ma,与塔里木、阿拉伯—努比亚、劳伦等陆块的 Sturtian冰川作用高度同步;南沱冰期的启动应晚于 650 Ma,与西伯利亚、澳大利亚、劳伦等陆块的 Marinoan冰川作用基本同步。另外,碎屑锆石 U-Pb年龄谱与 CL图像显示,长安组和南沱组的物质来源主要为下伏新元古界岩浆—沉积记录,揭示出冰川对下伏地层的强烈刨蚀作用和华南新元古代幕式构造岩浆热事件。扬子陆块成冰纪冰川刨蚀作用可能与 Rodinia 超大陆“裂离”有关的强烈伸展活动存在联系,并可能持续至 Marinoan 冰期结束。

关键词: 成冰纪; 冰成杂砾岩; 碎屑锆石; 江口冰期; 南沱冰期; 扬子陆块
中图分类号:P512.4;P534.3 文献标志码:A 文章编号:1671-1505(2018)01-0065-22
Onset time and global correlation of the Cryogenian glaciations in Yangtze Block, South China
Cai Juan-Juan1, Cui Xiao-Zhuang2,3, Lan Zhong-Wu4, Wang Jian2, Jiang Zhuo-Fei2, Deng Qi2, Zhuo Jie-Wen2, Chen Feng-Lin3, Jiang Xin-Sheng2
1 Chinese Academy of Geological Science,Beijing 100037
2 Chengdu Center, China Geological Survey,Chengdu 610081,Sichuan
3 Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,Sichuan
4 State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029
About the corresponding author Cui Xiao-Zhuang,born in 1984,is a research assistant of Chengdu Center, China Geological Survey. Now he is mainly engaged in researches on the sedimentology and Precambrian geology. E-mail: cgscuixz@126.com.

About the first author Cai Juan-Juan,born in 1990,is a master candidate of paleontology and stratigraphy at Chinese Academy of Geological Science. E-mail: dzcaijj@126.com.

Fund:Co-funded by the National Natural Science Foundation of China(Nos. 41502114,41772115, 41672112, 41673016)and the China Geological Survey Project(No. DD20160017)
Abstract

It's well known that the Jiangkou and Nantuo glaciations are two remarkable Cryogenian glaciations in South China. However,their precise onset time and global correlations remain undecided. In this study,detrital zircon LA￣ICP￣MS U-Pb dating was performed on the glacial diamictites from the basal part of the Chang'an and Nantuo Formations in the southeastern Yangtze Block. The new results show that detrital zircon grains from the Chang'an Formation give a concentrated206Pb/238U age range of 958 to 717 Ma with distinct peaks at 720,753,805 and 848 Ma. The youngest age population yields a weighted mean of 719.6±6.1 Ma,representing the maximum depositional age of the Chang'an Formation. Detrital zircon grains from the Nantuo Formation has a main206Pb/238U age range between 987 and 649 Ma,with distinct peaks at 650,720,753,803,823 and 848 Ma. The youngest age population gives a weighted mean of 649.3±6.2 Ma,representing the maximum depositional age of the Nantuo Formation. Taking into account the available age data,it is suggested that the Jiangkou glaciation was most likely initiated at ca.715 Ma,which is synchronous with the Sturtian glaciation in other blocks such as Tarim,Arabia-Nubia and Laurentia;the Nantuo glaciation was probably initiated later than 650 Ma,synchronous with the Marinoan glaciation in other blocks such as Siberia, Australia and Laurentia. Furthermore,detrital zircon U-Pb age spectrums and CL images indicate that sediments of the Chang'an and Nantuo Formations in the Yangtze Block were mainly derived from the underlying Neoproterozoic magmatic and sedimentary rocks,indicative of the intensive Neoproterozoic severe exaration and episodic tectonic-thermal events. The Cryogenian exaration in the Yangtze Block should be the response to the breakup of the Rodinia supercontinent prior to the termination of the global Marinoan glaciation.

Key words: Cryogenian; glacial diamictites; detrital zircons; Jiangkou glaciation; Nantuo glaciation; Yangtze Block
1 概述

成冰纪发生了多期冰川事件, 对地球早期的岩石圈、水圈、生物圈和大气圈产生了深刻的影响(Canfield and Teske, 1996; Hoffman and Schrag, 2002; Canfield et al., 2007)。其中, Sturtian与Marinoan冰川作用是最具代表性的2次冰川事件, 其沉积记录几乎遍及全球各陆块, 在华南、阿拉伯— 努比亚、澳大利亚、劳伦和塔里木陆块上的记录尤为完整(Hoffmann et al., 2004; Kendall et al., 2004, 2006; Zhang et al., 2008a; Xu et al., 2009; Lan et al., 2014)。然而, 其精确启动时间、期次、时限及分布仍存在争议, 是当前国际前寒武纪地质研究的热点(Lan et al., 2014, 2015a, 2015b; Rooney et al., 2015及该文文献)。

华南地区新元古代中期沉积— 火山记录分布广泛(图 1)。其中, 成冰纪(南华纪)地层自下而上依次为江口群(长安组、富禄组)、大塘坡组和南沱组, 分别对应江口冰期、大塘坡间冰期和南沱冰期沉积(刘鸿允等, 1980; 张启锐和储雪蕾, 2006; Zhang et al., 2008a; Lan et al., 2014, 2015a)。近年来, 围绕成冰纪冰川事件期次划分、启动时间及其全球对比开展了大量研究工作, 取得了系列进展(图 1; 表 1)。其中, 桂北丹洲群顶部年龄715.9± 2.8, Ma(Lan et al., 2014)、鄂北莲沱组顶部年龄714± 8, Ma(Lan et al., 2015b)、湘西板溪群五强溪组顶部年龄714.6± 5.2, Ma(Song et al., 2017)、开建桥组顶部年龄715.0± 9.8, Ma(Jiang et al., 2016), 暗示华南江口冰期的启动年龄为715, Ma左右; 而湖北长阳大塘坡组中部年龄654.2± 2.7, Ma(Liu et al., 2015)和湖南吉首湘锰组(大塘坡组)顶部年龄654.5± 3.8, Ma(Zhang et al., 2008b), 则将南沱组最大沉积年龄限定为654, Ma左右。尽管如此, 关于华南成冰纪2次冰川作用的时限、期次及全球对比仍存极大争议, 如有学者认为长安组的底界年龄在ca.780— 746, Ma之间(高林志等, 2013; 孙海清等, 2014; 刘建清等, 2015; 韩坤英等, 2016)。

图 1 华南前寒武纪地层及新元古代裂谷盆地分布简图(修改自Du et al., 2013; 崔晓庄等, 2016)
1— 澄江组顶部凝灰岩年龄725± 11, Ma(崔晓庄等, 2013); 2— 板溪群牛牯坪组上部凝灰岩年龄725± 10, Ma(Zhang et al., 2008a); 3— 莲沱组顶部层凝灰岩年龄724± 12, Ma(高维和张传恒, 2009); 4— 板溪群岩门寨组凝灰质粉砂岩年龄719± 10, Ma(Wang et al., 2012b); 5— 板溪群大江边组含粉砂铁质泥岩年龄719± 10, Ma(伍皓等, 2013); 6— 板溪群岩门寨组顶部凝灰岩年龄717.2± 8.9, Ma(柏道远等, 2015); 7— 丹洲群拱洞组顶部凝灰岩年龄716.8± 6.8, Ma(Wang et al., 2016); 8— 丹洲群拱洞组顶部浅变质凝灰质粉砂板岩年龄716.1± 3.4, Ma(Lan et al., 2014); 9— 丹洲群拱洞组顶部浅变质凝灰质粉砂板岩年龄715.9± 2.8, Ma(Lan et al., 2014); 10— 开建桥组顶部含岩屑凝灰岩年龄715.0± 9.8, Ma(Jiang et al., 2016); 11— 板溪群五强溪组顶部细砂岩年龄714.6± 5.2, Ma(Song et al., 2017); 12— 莲沱组上部晶屑凝灰岩年龄714± 8, Ma(Lan et al., 2015b); 13— 大塘坡组底部凝灰岩年龄667.3± 9.9, Ma(尹崇玉等, 2006); 14— 大塘坡组底部凝灰岩年龄664.2± 2.4, Ma(余文超等, 2016); 15— 大塘坡组底部凝灰岩夹层年龄662.9± 4.3, Ma(Zhou et al., 2004); 16— 大塘坡组顶部凝灰岩夹层年 龄654.5± 3.8, Ma(Zhang et al., 2008b); 17— 大塘坡组中部凝灰岩夹层年龄654.2± 2.7, Ma(Liu et al., 2015)。年龄信息详见表 1Fig.1 Distribution of the Precambrian strata and Neoproterozoic rift basins in South China (modified from Du et al., 2013; Cui et al., 2016)

表 1 Sturtian冰期和Marinoan冰期启动年龄约束 Table1 Age constraints on the initiation of Sturtian and Marinoan glaciations

碎屑锆石U-Pb年龄研究不仅可用于示踪沉积物源、恢复古地理格局以及反演构造过程(Bruguier et al., 1997; Ireland et al., 1998; 余文超等, 2014; 李忠和高剑, 2016), 而且还可为沉积地层提供时代约束(Dickinson and Gehrels, 2009), 尤其对前寒武纪地层的时代约束效果良好(Nelson, 2001)。文中对桂北地区成冰系长安组和南沱组底部的冰成杂砾岩进行了碎屑锆石U-Pb年代学研究, 以期准确限定2套冰成杂砾岩的最大沉积时间, 厘定扬子陆块江口冰期和南沱冰期的确切启动时限及其全球对比, 并探讨其碎屑来源及区域构造岩浆热事件。

2 区域地质背景与样品测试
2.1 区域地质背景

一般认为, 华南是Rodinia超大陆的重要组成部分, 中元古代晚期— 新元古代初由扬子陆块和华夏陆块沿江南造山带聚合而成(Li et al., 2008b)。随后, Rodinia超大陆的裂解引发的全球性裂谷事件(Li et al., 1999; Li et al., 2008a), 在华南形成了扬子北缘碧口— 汉南裂谷盆地、扬子西缘康滇裂谷盆地以及扬子东南缘南华裂谷盆地(图 1)。其中, 南华裂谷盆地的基底由四堡群及其相当地层(如梵净山群、冷家溪群、双桥山群、双溪坞群等)组成, 裂谷盆地充填包括冰期前丹洲群浅变质岩及其相当地层(如板溪群、下江群、澄江组、莲沱组等)和成冰系江口群— 南沱组及其相当地层(Wang and Li, 2003; 崔晓庄等, 2016)。

研究区位于南华盆地南段广西北部罗城县和三江县一带(图 2-a), 区内成冰纪地层层序完整(图 2-b), 自下而上划分为江口群、大塘坡组和南沱组。其中, 江口群由下部长安组和上部富禄组组成(张启锐和储雪蕾, 2006; Zhang et al., 2008a; Lan et al., 2014, 2015a)。研究剖面上, 长安组为一套厚达近千米的灰绿色含砾泥质砂岩(冰成杂砾岩), 成层性差, 坠石构造发育, 为冰海冰筏沉积(图 3-b), 其下伏拱洞组绢云母粉砂质板岩、变质细砂岩等发育良好的水平层理、斜层理等沉积构造, 与上覆地层对比存在明显的跳相现象, 两者呈平行不整合接触(图 3-a)。同时, 长安组与上覆富禄组底部铁质岩呈整合接触。南沱组整合于间冰期地层大塘坡组与冰后期地层陡山沱组之间, 为一套厚1000~2000, m的灰绿色、深灰色厚层块状含砾砂质泥岩或泥质砂岩, 为海相冰成杂砾岩(图 3-c, 3-d)。

图 2 桂北地区地质简图和采样位置(a)(改自《中国地质图集》编委会, 2002)及成冰纪地层柱状图(b)Fig.2 A simplified geological map and sampling sites in north Guangxi(a)(modified from China's Geological Atlas Editorial Board, 2002) and its stratigraphic column of the Cryogenian(b)

图 3 桂北地区长安组和南沱组的典型露头照片
a— 长安组含砾泥质砂岩与下伏丹洲群拱洞组顶部绢云母粉砂质板岩、变质细砂岩等呈平行不整合接触, 界线上下岩性突变, 发生明显跳相; b— 长安组灰绿色块状含砾泥质砂岩中的滴石构造, 坠石为板岩砾石, 上下层理绕砾石发生弯曲变形; c— 南沱组灰绿色厚层块状含砾砂 质泥岩, 杂基支撑, 砾石含量相对较少; d— 南沱组灰绿色厚层块状含砾泥质砂岩, 杂基支撑, 砾石含量较照片c中明显增多Fig.3 Typical outcrop photos of the Chang'an Formation and Nantuo Formation in north Guangxi

2.2 样品采集与测试

长安组样品HJC-N1采自罗城县黄金镇垌坪村长安组剖面底部(地理坐标: 24° 59'24″, 108° 53'38″), 岩性为含砾泥质砂岩; 南沱组样品FL-N1采自三江县富禄乡至三江县城的321国道旁(地理坐标: 25° 44'26″, 109° 10'35″), 为南沱组剖面底部含砾泥质砂岩。具体采样位置及层位见图 2。锆石挑选前, 已尽量将其中砾石剔除。

样品处理过程如下: 将样品粉碎至60目以下, 先用人工重砂方法富集锆石, 然后在双目镜下精选锆石。将挑好的锆石颗粒粘在双面胶上, 固定在透明的环氧树脂中, 和树脂一起打磨抛光, 直至露出锆石内部, 以适合LA-ICP-MS分析。对抛光后的锆石进行反射光、透射光显微照相和阴极发光(CL)图像分析, 选择代表性锆石颗粒和区域进行U-Th-Pb同位素分析。

锆石原位微区U-Th-Pb分析在中国地质大学(武汉)地质过程与矿产资源国家重点实验室进行, 分析方法为激光剥蚀— 电感耦合等离子体质谱(LA-ICP-MS)分析法。测年采用的ICP-MS为美国Agilent公司生产的Agilent7500a。激光剥蚀系统为德国MicroLas公司生产的GeoLas2005。实验中采用氦作为剥蚀物质的载气, 激光频率为6, HZ, 激光强度为50, mJ, 激光斑束直径为32, μ m, 分析流程详见Yuan等(2004)。同位素组成用标准锆石91500作为外标进行校正, 元素含量测定采用NIST SRM610作外标、29Si作内标进行校正。锆石测年中Pb同位素比值、U-Pb年龄和微量元素含量运用ICPMS-DataCal(ver8.3)(Liu et al., 2010)进行处理。年龄计算采用标准程序Isoplot(ver3.23)(Ludwig, 2003), 测试数据、年龄加权平均值误差均为1σ 。对年龄大于1000, Ma的锆石采用 207Pb/206Pb 年龄, 而对年龄小于1000, Ma的锆石则采用 206Pb/238U年龄(Black et al., 2003)。

3 分析结果
3.1 长安组

样品HJC-N1的锆石无色透明, 晶型较为完整, 粒径介于70~140, μ m之间, 长宽比为 1︰1~3︰1, 阴极发光图像(CL)显示典型的岩浆型震荡环带结构(图 4)。共选择80颗锆石进行了U-Th-Pb同位素分析, U含量介于41~763 μ g/g之间、Th 含量介于35~643 μ g/g之间(表 2)。谐和度大于90%的数据点共77个(表 2), 均分布在谐和线上及其附近, 年龄分布于2518— 717, Ma 之间(图 5-a; 图 6-a)。除HJC-N1-04和HJC-N1-24外, 其余75颗锆石的Th/U值均介于0.41~1.74之间(表 2)。小于1000, Ma的年龄集中于958— 717, Ma(图 5-b; 图 6-b), 在720、753、805及848, Ma处存在明显峰值, 并在920、953, Ma处存在微弱峰值(图 6-b)。大于1000, Ma的锆石共有6颗, 年龄分别为2518、2480、2461、2439、1998及1035, Ma。9颗最年轻锆石的 206Pb/238U年龄分布介于724— 717, Ma之间。这些锆石呈自形棱角状, 边界平直, 未见明显磨损, 内部显示清晰的震荡环带(图 4), 且Th/U值为0.65~1.60, 表明为未经过或仅经过轻微磨蚀的岩浆锆石。将其中谐和度最好的7颗锆石的 206Pb/238U年龄进行加权平均, 计算结果为719.6± 6.1, Ma(MSWD=0.07)(图 5-b)。

图 4 桂北地区长安组(上)和南沱组(下)样品典型锆石的CL图像Fig.4 Typical CL images of zircons of samples from the Chang'an Formation(above)and Nantuo Formation(below) in north Guangxi

图 5 桂北地区长安组和南沱组样品碎屑锆石U-Pb年龄谐和图
a— 长安组锆石U-Pb年龄总谐和图; b— 长安组小于1000, Ma 的锆石U-Pb年龄谐和图; c— 南沱组锆石U-Pb年龄总谐和图; d— 南沱组小于1000, Ma的锆石 U-Pb年龄谐和图Fig.5 Concordia diagrams for detrital zircons of samples from the Chang'an Formation and Nantuo Formation in north Guangxi

图 6 桂北地区长安组和南沱组样品碎屑锆石U-Pb年龄分布直方图
a— 长安组锆石U-Pb年龄分布直方图; b— 长安组小于1000, Ma 锆石U-Pb年龄分布直方图; c— 南沱组锆石U-Pb年龄分布直方图; d— 南沱组小于1000, Ma锆石U-Pb年龄分布直方图Fig.6 Age histograms for detrital zircons of samples from the Chang'an Formation and Nantuo Formation in north Guangxi

表 2 桂北地区长安组与南沱组样品碎屑锆石LA-ICP-MS U-Pb定年数据 Table2 Detrital zircon LA-ICP-MS U-Pb data of samples from the Chang'an Formation and Nantuo Formation in north Guangxi
3.2 南沱组

样品FL-N1的锆石晶形较为完整, 无色透明, 多呈短柱状晶体, 粒径介于80~150, μ m之间, 长宽比介于 1︰1~3︰1之间, CL图像显示典型的岩浆型震荡环带结构(图 4)。共选取70颗代表性锆石进行U-Th-Pb同位素分析(表 2), 其U、Th含量范围分别为23~1292 μ g/g、21~1624 μ g/g。共获取66个谐和度大于90%的年龄(表 2), 均落于谐和线上及其附近, 年龄分布于1622— 649, Ma之间(图 5-c; 图 6-c), 其所对应的Th/U值范围为0.37~2.23。小于1000, Ma的锆石共有63颗, 年龄集中于987— 649, Ma(图 5-d; 图 6-d), 在650、720、753、779、803、823及848, Ma处存在明显峰值, 同时, 在905、925及985, Ma处存在微弱峰值(图 6-d)。大于1000, Ma的锆石共有3颗, 年龄分别为1031、1439及1622, Ma。最年轻的4颗锆石的 206Pb/238U年龄集中分布于652— 649, Ma, 锆石略有磨损, 但边界平直, 内部震荡环带清晰可见(图 4), Th/U 值范围为0.67~2.23, 为经历了有限搬运的岩浆锆石。将其中谐和度最好的3颗锆石的 206Pb/238U年龄进行加权平均, 计算结果为649.3± 6.2, Ma(MSWD=0.01)(图 5-d)。

4 讨论
4.1 江口群冰成杂砾岩最大沉积年龄

多数学者认为华南江口群包括了江口冰期2幕冰川作用的沉积, 即长安组和富禄组上部古城段及其相当层位(铁丝坳组), 分别对应早期的长安冰期和晚期的古城冰期(张启锐和储雪蕾, 2006; Zhang et al., 2008a; Lan et al., 2014, 2015a)。其中, 长安组是华南最早的新元古代冰川沉积, 其冰成杂砾岩的最大沉积年龄不仅涉及中国南华系底界年龄的确定, 而且关系到南华系与国际成冰系底界年龄的对比。然而, 由于采样层位和测试手段的不同, 以往的长安组冰成杂砾岩最大沉积年龄数据极为分散, 甚至相互矛盾(表 1)。例如, 根据桂北罗城丹洲群拱洞组和上覆长安组的凝灰岩锆石SHRIMP U-Pb年龄将长安组最大沉积年龄约束于ca.780, Ma(高林志等, 2013), 桂北三江地区长安组沉积年龄被认为介于746— 780, Ma之间(韩坤英等, 2014), 湖南新化江口组底部火山角砾岩锆石SHRIMP U-Pb年龄为785± 11, Ma(刘建清等, 2015), 均支持将南华系底界标定为780, Ma的观点。孙海清等(2014)则认为将长安组底界年龄置于760, Ma更为合适。

然而, 许多高精度定年结果却显示, 很多冰期前地层的顶部年龄明显小于上述南华系底界年龄。例如, 湖南芷江牛牯坪组上部凝灰质粉砂岩和云南玉溪澄江组顶部凝灰岩的年龄均为725, Ma左右(Zhang et al., 2008a; 崔晓庄等, 2013), 表明早冰期长安组的沉积应晚于725, Ma。一系列最新研究结果也支持这一认识, 例如, 覃永军等(2015)认为黔东南下江群顶界年龄为717, Ma, Lan等(2014)对桂北丹洲群顶部凝灰质粉砂岩的定年结果为715.8± 2.5, Ma, 湖北宜昌莲沱组顶部凝灰岩年龄为714± 8, Ma(Lan et al., 2015b), 川西开建桥组顶部凝灰岩年龄为715.0± 9.8, Ma(Jiang et al., 2016)以及湘西板溪群顶部碎屑锆石年龄为714.6± 5.2, Ma(Song et al., 2017)。文中, 桂北长安组底部冰成杂砾岩碎屑锆石年龄谱的最小峰值年龄为720, Ma(图 6-b), 且南沱组年龄谱中亦存在720, Ma的峰值年龄(图 6-d)。选取长安组碎屑锆石最小年龄组中谐和度最好的7颗锆石计算其加权平均值为719.6± 6.1, Ma(图 5-b), 对应锆石呈自形棱角状, 其内部震荡环带显著, Th/U值均大于0.4, 应为未经磨损或仅经轻微磨损的岩浆锆石。因此, 江口群冰成杂砾岩的最大沉积年龄应为ca.720, Ma。

4.2 南沱组冰成杂砾岩最大沉积年龄

南沱组的顶界年龄已被公认为635, Ma(Condon et al., 2005; Zhang et al., 2005), 但其底界年龄一直未有定论。长期以来, 南沱组中难以找到可用于精确定年的同沉积火山岩, 故而其底部冰成杂砾岩最大沉积年龄的限定有赖于下伏地层大塘坡组的定年工作。在贵州松桃寨郎沟剖面上, 南沱组底界年龄被限定为662.9± 4.3, Ma(Zhou et al., 2004), 但其采样层位处于大塘坡组底部, 紧靠铁丝坳组(与古城段相当)之上的盖帽碳酸盐岩, 表明其作为江口群顶界年龄或大塘坡组底界年龄的约束更为合适。随后, 同地区黑水溪剖面和将军山剖面大塘坡组底界年龄分别为667.3± 9.9, Ma(尹崇玉等, 2006)和664.2± 2.4, Ma(余文超等, 2016)的报道与寨郎沟剖面一致, 进一步说明南沱组冰成杂砾岩的最大沉积年龄应远小于663, Ma。研究表明, 华南南沱组之下存在沉积缺失或冰川剥蚀等情况(Zhang et al., 2008b; Liu et al., 2015), 湖北长阳大塘坡组中部年龄654.2± 2.7, Ma(Liu et al., 2015)与湖南吉首大塘坡组顶部凝灰岩年龄654.5± 3.8, Ma(Zhang et al., 2008b)高度一致, 证明南沱组的最大沉积时间应晚于654, Ma。文中南沱组底部冰成杂砾岩的碎屑锆石年龄数据显示, 其最小年龄组非常集中地分布于652— 649, Ma之间(n=4), 其中谐和度最好的3颗锆石加权平均年龄为649.3± 6.2, Ma(图 5-d)。所选锆石略有磨损但边界平直, Th/U> 0.4, 应为经历了有限搬运的岩浆锆石。因此, 南沱组冰成杂砾岩最大沉积年龄应小于650, Ma。误差范围内, 该年龄值与大塘坡组顶部年龄(Liu et al., 2015)基本吻合。

4.3 江口冰期启动时间及其全球对比

江口冰期的启动标志着华南新元古代冰川事件的开始。因此, 其启动时间的精确性直接影响其与其他陆块新元古代冰川事件的对比。近年来, 一系列高精度年龄数据揭示江口群底界的大致年龄范围为725— 714, Ma(表 1)。文中长安组碎屑锆石U-Pb年代学结果证实, 华南江口冰期启动不早于720, Ma。结合相关可靠数据, 可以确认华南江口冰期很可能于715, Ma左右开始启动。值得注意的是, 其他陆块也广泛存在底界年龄介于725— 714, Ma的新元古代冰川作用记录(表 1; 图 7)。例如, 新疆库鲁克塔格地区阿拉通沟组冰成杂砾岩的底界年龄应不大于725, Ma(Xu et al., 2009); 加拿大西北部Mackenzie山脉地区Sturtian冰期记录的下限年龄处于723— 716, Ma之间(Heaman et al., 1992; Macdonald et al., 2010); 同样, 位于劳伦西部的美国Porteuf Narrow地区也发现了年龄相近的记录, 如Pocatello组Scout Mountain段冰碛岩(Sturtian)中斑状流纹岩的年龄为717± 4, Ma(Fanning and Link, 2004); 而在阿拉伯— 努比亚陆块上, 阿曼北部Jebel地区Huqf超群Ghubrah组冰碛岩(Sturtian)的年龄为713.7± 0.5, Ma(Bowring et al., 2007), 但考虑到其采样层位处于冰成杂砾岩底部之上, Lan等(2014)认为Sturtian冰期的真实启动年龄应在713.7± 0.5, Ma之前。误差范围内, 华南江口冰川作用的启动与塔里木、劳伦、阿拉伯— 努比亚等陆块上Sturtian冰川作用的启动高度同步, 证实江口冰期与Sturtian冰期的对应关系, 从而支持Sturtian冰川作用为一次快速、同步的全球性事件, 并可能启动于715, Ma左右。

图 7 新元古代Sturtian和Marinoan冰川作用启动时间的全球对比
由表 1中小于725, Ma的精确年龄数据绘制, 矩形方块代表冰期启动时间, 相应的竖线长度代表数据误差范围, 深灰色细条带则代表冰期启动年龄的最佳估计值。数据来源见表1Fig.7 Global correlation for onset time of the Neoproterozoic Sturtian and Marinoan glaciations

4.4 南沱冰期启动时间及其全球对比

最新研究成果揭示, 南沱冰期启动时间应小于654, Ma(Zhang et al., 2008b; Liu et al., 2015)。文中南沱组底部冰成杂砾岩碎屑锆石U-Pb年代学结果表明, 南沱冰期启动应晚于650, Ma。除华南以外, 其他陆块也发现有与南沱冰期启动年龄相近的新元古代冰川作用记录(表 1; 图 7)。例如, 美国Scout Mountain段冰碛岩(Sturtian)之上盖帽碳酸盐岩与似盖帽碳酸岩之间的沉凝灰岩年龄为667± 5, Ma(Fanning and Link, 2004), 表明此区内Marinoan冰期的启动应远在667, Ma之后; 而乌拉尔地区Laplandian(Marinoan)组冰成杂砾岩中花岗质砾石的年龄为660± 15, Ma(Semikhatov, 1991), 指示Marinoan冰期的起始时间应远小于660, Ma; 另外, 澳大利亚西部Umberatan群Enorama组黑色页岩Marino Arkose段中的碎屑锆石年龄为657± 17, Ma(Ireland et al., 1998), 并且此年龄值与来自澳大利亚南部和中部的多组数据在误差范围内相当(Fanning and Link, 2006; Kendall et al.; 2006; Fanning and Link, 2008), 表明Marinoan冰期在澳大利板块的启动应不早于657± 17, Ma。需要指出的是, 澳大利亚南部Marinoan冰期前沉积Tapley Hill组底部Tindelpina页岩段的年龄为643± 2.4, Ma(Re-Os, MSWD=1.1, Kendall et al., 2006), 是迄今为止鲜有报道的年龄明显小于其他大洲的新元古代间冰期沉积记录。考虑到Tapley Hill组总厚度大于1600, m, 若此Re-Os年龄的准确性可得到充分验证, 那么Marinoan冰期在澳大利亚南部的启动年龄可能远小于643, Ma, 可能暗示Marinoan冰川作用具有多期性或者穿时性的特征。综合分析可知, 华南南沱冰川作用的启动时间应与西伯利亚、澳大利亚、劳伦等陆块上Marinoan冰川作用的启动时间基本吻合, 进一步证实南沱冰期可与Marinoan冰期直接对比的观点。

4.5 碎屑锆石来源及其古地理意义

锆石年龄直方分布图(图 6; 图 8)显示, 长安组(HJC-N1)碎屑锆石年龄呈现明显的多峰式分布特征, 在720、753、805及848, Ma处峰值明显, 并在920和953, Ma处峰值微弱。南沱组(FL-N1)碎屑锆石在650、720、753、779、803、823及848, Ma处峰值明显, 同时, 在903、925及985, Ma处峰值微弱。可见, 2组样品碎屑锆石年龄分布模式基本一致, 说明两者可能具有相似的物质来源。

图 8 扬子陆块新元古代沉积序列的碎屑锆石U-Pb年龄分布对比
扬子北缘莲沱组及南沱组数据来自Liu等(2008); 扬子西缘澄江组数据来自于Wang等(2012a), 南沱组数据来自江卓斐(2016); 扬子东南缘丹州群拱洞组数据来自Wang等(2012b)Fig.8 Comparison of detrital zircon U-Pb age distribution from the Neoproterozoic sedimentary sequence in Yangtze Block

将文中碎屑锆石U-Pb年龄谱与扬子北缘、西缘及东南缘相关地层进行对比分析发现, 其新元古代年龄分布可进行完全对比: 均集中分布于750— 850, Ma之间, 且在ca.800, Ma处具有共同的最大峰值, 另外在ca.850、820及750, Ma处普遍存在峰值。研究表明, 新元古代幕式岩浆活动在扬子陆块周缘普遍发育, 且可能与Rodinia超大陆的裂解密切相关(Li et al., 2003a; Li et al., 2008b)。Li等(2008b)认为, 华南板块在中元古代晚期到新元古代期间处于Rodinia超大陆的中心位置, Rodinia聚合和裂解的产物为南华系提供了物源。例如, 东南缘ca.820, Ma的本洞、三防、元宝山花岗岩系列(Li, 1999)以及ca.800, Ma的刀岭山岩体(Li et al., 2008b), 侵入三门街组的ca.765, Ma流纹质英安岩(Zhou et al., 2007)等; 西缘地区ca.850, Ma的关刀山花岗岩(Li et al., 2003b)、ca.820, Ma的峨山花岗岩(Li et al., 2003a)、ca.800, Ma的苏雄组流纹岩(Li et al., 2002)以及ca.750, Ma的康定花岗质岩石(Li et al., 2003a)等; 北缘ca.820, Ma的铁船山流纹岩(Ling et al., 2003)、黄陵庙奥长花岗岩— 花岗闪长岩套(马国干等, 1989), ca. 780、 750, Ma的庐镇关花岗岩(Wu et al., 2007)等。文中样品的碎屑锆石多呈自形棱角状, 显示应为近源岩浆岩的剥蚀沉积产物。因此, 长安组与南沱组的碎屑锆石较好地记录了华南新元古代幕式构造岩浆热事件。

年龄谱对比分析还显示, 扬子陆块新元古代沉积序列中大于1000, Ma的碎屑锆石年龄分布存在明显差异。首先, 扬子北缘新元古代沉积地层中碎屑锆石的太古代年龄信号十分显著(图 8-a, 8-b), 显然宜昌地区太古代崆岭地体为其做出了显著贡献; 其次, 扬子西缘地层中碎屑锆石的古元古代年龄信号明显加强(图 8-c, 8-d), 这与扬子西缘可能存在2500— 2300、1900— 1700及1650— 1500, Ma的岩浆事件有关(Sun et al., 2009); 然而, 扬子东南缘新元古代地层碎屑锆石的前新元古代年龄信号极其微弱(图 8-e, 8-f, 8-g)。研究表明, 扬子陆块新元古代裂谷盆地系统盆地基底组成存在明显差异, 如南华裂谷盆地的基底由四堡群、冷家溪群、双溪坞群及其相当地层组成, 而康滇裂谷盆地则由昆阳群、会理群、东川群及其相当地层组成, 这应是大于1000, Ma碎屑锆石年龄分布不同的主要原因。

此外, 丹洲群与冰期沉积的碎屑锆石年龄谱分布基本吻合(图 8-e, 8-f, 8-g), 表明其下伏地层也应是冰期沉积的物质来源之一。在野外露头上, 南沱组与长安组均为结构成熟度较低的杂基支撑、基底式胶结的冰成杂砾岩, 同时表明了其物源较近。研究表明, 冰川对下伏地层会产生一定的刨蚀作用(Ingó lfsson and Lokrantz, 2003), 从而会造成界面上下地层的不整合接触。例如, 桂北罗城地区长安组冰成杂砾岩与下伏拱洞组即为平行不整合接触(汪正江等, 2013; 本文)。张世红等(2008)认为冰川发生的同时, 存在裂谷伸展断层作用, 形成的犁式断层的下盘由于卸载作用会发生旋转, 而位于断块旋转相对上升的部位将遭受冰川的大规模剥蚀, 使得盆地边缘某些部位的地层剥蚀厚度远大于冰期海平面下降所造成的蚀源区与沉积区的高度差。此外, 汪正江等(2013, 2015)认为板溪期沉积与长安组之间的区域性地层缺失或平行不整合与Rodinia超大陆由 “ 裂解” 向“ 裂离” 的构造转换有关, 是此伸展性构造转换的沉积响应。文中研究结果表明, 桂北地区长安组与南沱组具有极其相似的物质来源, 暗示南沱冰期时下伏新元古代岩浆岩和沉积岩可能仍在遭受剥蚀, 这些物质在冰川前缘堆积形成厚层状杂砾岩。同时, 南华裂谷盆地与Rodinia超大陆“ 裂离” 有关的伸展作用可能持续至Marinoan冰期结束。

5 结论

对桂北长安组底部和南沱组底部冰成杂砾岩碎屑锆石U-Pb年代学的研究为华南成冰纪冰川作用提供了新的信息, 主要结论如下:

1)桂北长安组底部冰成杂砾岩的最大沉积年龄为719.6± 6.1, Ma, 表明江口冰川作用启动不早于720, Ma, 进一步综合现有研究结果可知, 江口冰期很可能于715, Ma左右开始启动, 在误差范围内, 与塔里木、劳伦、阿拉伯— 努比亚等陆块上Sturtian冰川作用的启动具有高度同步性, 验证了江口冰期与Sturtian冰期的对应关系, 进一步证实Sturtian冰川作用为一次快速、同步的全球性事件。

2)桂北南沱组底部冰成杂砾岩的最大沉积年龄为649.3± 6.2, Ma, 指示华南南沱冰期启动应晚于650, Ma, 与现有研究结果取得一致, 并在误差范围内, 与西伯利亚、澳大利亚、劳伦等陆块上Marinoan冰川作用的启动基本同步, 验证了南沱冰期与Marinoan冰期的对比关系。

3)桂北成冰系长安组和南沱组的碎屑锆石U-Pb年龄谱体现了华南新元古代的构造热历史。年龄谱对比和CL图像分析表明, 研究区长安组和南沱组碎屑物质主要来源于冰川对区内下伏新元古代岩浆岩和沉积岩的刨蚀。这种强烈的刨蚀作用可能发生在与Rodinia超大陆“ 裂离” 有关的强烈伸展背景下, 并且扬子东南缘与Rodinia超大陆“ 裂离” 有关的伸展活动可能持续至Marinoan冰期结束。

致谢 LA-ICP-MS锆石U-Pb定年工作得到了中国地质大学(武汉)地质过程与矿产资源国家重点实验室胡兆初教授的帮助,谨致谢忱。

作者声明没有竞争性利益冲突.

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