岩石学报  2020, Vol. 36 Issue (4): 1059-1075, doi: 10.18654/1000-0569/2020.04.06   PDF    
胶北地体荆山群大理岩碳氧同位素地球化学特征及其对Lomagundi-Jatuli事件的指示
李凯月1, 汤好书2, 陈衍景1, 薛莅治1, 王玭3, 孙之夫4     
1. 北京大学造山带与地壳演化教育部重点实验室, 北京 100871;
2. 中国科学院地球化学研究所, 矿床地球化学国家重点实验室, 贵阳 550008;
3. 中国科学院南海海洋研究所边缘海与大洋地质重点实验室, 广州 510301;
4. 山东黄金集团有限公司, 莱州 261442
摘要: 古元古代大气圈快速大量充氧,地球表生系统发生一系列重要变化,其中之一是2.3~2.06Ga碳酸盐岩碳同位素正异常现象,称Lomagundi-Jatuli事件,颇受瞩目,成为当前国际研究前沿。胶北地体荆山群普遍经历高角闪岩相-麻粒岩相变质作用,其下部禄格庄组碳酸盐地层发育。本文研究了山后地区荆山群禄格庄组26件大理岩和7件白云质石英岩样品,获得δ13Ccarb值在-0.8~+3.4‰(V-PDB),平均+1.4‰,其中大理岩δ13C平均值为+1.6‰,白云质石英岩δ13C平均值为+0.9‰。氧同位素值变化于9.3‰~20.2‰(V-SMOW),平均14.0‰。样品碳-氧同位素值之间具有很好的正相关性,且有13件样品Mn/Sr比值大于6,说明高角闪岩相的变质作用可能使δ13Ccarb下降了3‰~5‰,原始沉积碳酸盐的δ13Ccarb值高达+8.4‰,明显正异常,是全球性Lomagundi-Jatuli事件的响应。
关键词: Lomagundi-Jatuli事件    大氧化事件    碳同位素异常    荆山群禄格庄组    胶北地体    
Carbon and oxygen isotope geochemistry of marbles in the Jingshan Group, Jiaobei Terrane and its indication to the Lomagundi-Jatuli Event
LI KaiYue1, TANG HaoShu2, CHEN YanJing1, XUE LiZhi1, WANG Pin3, SUN ZhiFu4     
1. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing 100871, China;
2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Science, Guiyang 550008, China;
3. Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;
4. Shandong Gold Group Co., Ltd, Laizhou 261442, China
Abstract: The rapid rise of atmospheric oxygen content in Paleoproterozoic (i.e., Great Oxidation Event) resulted in many dramatic changes in earth's surficial system, including the 2.3~2.06Ma positive δ13Ccarb excursion, termed Lomagundi-Jatuli Event, which has been a focus of international geological and geochemical studies. The Lugezhuang Formation, lower portion of the Jingshan Group, is a high amphibolite to granulite facies metamorphic carbonate association. Herewith we report the results obtained from isotope and element geochemical study of 26 marble and 7 dolomitic quartzite samples from the Lugzhuang Formation at Shanhou village. The δ13Ccarb values of the samples range from -0.8‰ to +3.4‰ (V-PDB), with an average of +1.4‰. The δ13Ccarb averages of marbles and dolomitic quartzites are +1.6‰ and +0.9‰, respectively. The δ18Ocarb values range from 9.3‰ to 20.2‰ (V-SMOW), with an average of 14.0‰. The δ18Ocarb and δ13Ccarb values positively correlate, and 13 samples show Mn/Sr ratios of >6. This indicates that the δ13Ccarb values of carbonates have been intensely modified by metamorphism, possibly causing a decrease of δ13Ccarb values by 3‰ to 5‰. Hence, the δ13Ccarb values originally sedimentary carbonates are possibly up to +8.4‰, showing positive δ13Ccarb excursion characterizing the global Lomagundi-Jatuli Event.
Key words: Lomagundi-Jatuli Event    Great Oxidation Event    δ13Ccarb anomaly    Lugezhuang Formation of Jingshan Group    Jiaobei Terrain    

科学家共识地球形成初期大气圈是缺氧还原性的,现今大气圈富氧,氧气含量达21%(体积)。富氧大气圈是人类生存的必需条件,其形成机制、过程和时间自然备受关注。1980年之前,科学家普遍认为地球大气圈和水圈的氧化过程是缓慢的、渐变的,至少始于3.8Ga,主要发生在2.6~1.9Ga期间(Cloud, 1968; Schidlowski et al, 1975; Frakes, 1979);1980年之后,受白垩纪末期恐龙灭绝事件研究的影响,陈衍景及合作者认识到古元古代水-气系统充氧事件及相关事件的突然性、短时性和系统性,2.3Ga前后突然大量出现红层、冰碛岩、含叠层石的厚层碳酸盐、菱镁矿、磷块岩、蒸发岩(石膏、硼酸盐等)、苏必利尔湖型条带状铁建造、石墨矿床以及沉积物负铕异常,提出了2.3Ga时地球表生环境由还原性突变为氧化性的观点(Chen, 1988Chen et al, 1988; 陈衍景, 1987, 1990; 季海章和陈衍景,1990陈衍景等, 1991, 1996)。

国际地层委员会前寒武纪分会高度重视2.3Ga环境突变事件及其分期意义,提出了增设2.3Ga为成铁纪(Siderian)与层侵纪(Rhyacian)界线的前寒武纪分期方案,获得了国际地球科学联合会的赞同(Cowie et al, 1989陈衍景等,1994)。新界线增设引发了2.3Ga地质事件性质研究的热潮,学者们成功尝试了多种元素和C、S、N、Mo等同位素方法(Schidlowski, 1988; Baker and Fallick, 1989a, bKarhu, 1993; Karhu and Holland, 1996Bekker et al, 2004Anbar et al, 2007; Guo et al, 2009),更广泛、深刻、精确地揭示或证实了2.3Ga表生环境突变事件(Chen and Zhao, 1997; Melezhik et al, 1999Pavlov and Kasting, 2002Huston and Logan, 2004唐国军等,2004Tang et al, 2011, 2013a, b, 2016; Martin et al, 2013Tang and Chen, 2013Young, 2013, 2019Chen and Tang, 2016; Condie, 2016; 陈衍景和汤好书,2018汤好书和陈衍景, 2018a, b)。据Karhu and Holland (1996)估算,在2.4~2.2Ga期间(即2.3Ga前后),大气圈自由氧含量从 < 10-13PAL增至15%PAL (PAL=Present Atmosphere Level),足见充氧量之大、速度之快。因此,Holland (2002)引入大氧化事件(Great Oxidation Event: GOE)的概念,专指2.3Ga左右大气圈成分由缺氧到富氧的变化(The change in the composition of the atmosphere from anoxygenic to oxygenated ca. 2.3Ga)。

大氧化事件概念的提出进一步激发了学者们对古元古代表生环境演化研究的兴趣,并提出了多种观点或模式。例如,Kump et al (2011)提出了脉动式氧化的观点,即:(1)2500Ma左右大气氧含量达到10-8~10-5PAL,能使黄铁矿氧化;(2)~2400Ma时或2320Ma前,非质量硫同位素分馏现象突然消失,表明大气圈氧含量急剧升高(Bekker et al, 2004Kump, 2008Guo et al, 2009);(3)~2000Ma时大气氧含量达~1%PAL。再如,Murakami et al (2011)基于古土壤研究提出了古元古代大气氧含量逐渐上升的观点,认为2100~2000Ma期间快速上升至~1%PAL。最近,我国学者提出了大氧化事件的两阶段氧化模式,即2.5~2.3Ga水圈氧化和2.3~2.05Ga大气圈氧化(Tang and Chen, 2013; 陈威宇和陈衍景, 2018; 陈威宇等, 2018; 汤好书和陈衍景, 2018a, bChen et al, 2019),得到了多数学者的赞同(Young, 2013, 2019Condie, 2016; 翟明国等, 2018)。

在揭示2.3Ga环境突变或大氧化事件的研究中,碳酸盐碳同位素(δ13Ccarb)发挥了关键作用。Schidlowski et al(1975, 1976)最早发现俄罗斯Karelia的Jatulian碳酸盐岩(2.65~1.95Ga)和津巴布韦Lomagundi省白云岩(~2.07Ga)的δ13Ccarb分别高达8.8‰和13.6‰,提出大量藻类发育和堆埋,导致CO2富集13C,沉积碳酸盐碳同位素正异常。Schidlowski et al(1975, 1976)的开创性研究直到1989年才引起重视,各大陆相继发现了古元古代δ13Ccarb正异常现象(陈威宇和陈衍景,2018及其引文),显示了事件的全球性(图 1)。由于该δ13Ccarb正异常事件最初发现于Lomagundi群和Jatuli群,故被称为Lomagundi或Jatuli事件,或者称为Lomagundi-Jatuli事件(简称LJE)。LJE发生时间的厘定是个难题(Melezhik et al, 2013),因为记录δ13Ccarb正异常岩石都是沉积岩,缺乏合适的定年矿物,往往只能通过其它地质体定年结果限定沉积作用的最大和最小年龄,得到的年龄范围通常跨度较大。Fennoscandian地盾记录LJE事件的碳酸盐地层沉积于2.2~2.06Ga之间(Karhu, 1993; Karhu and Holland, 1996);南美Marquette Range超群(Bekker et al, 2006Vallini et al, 2006)、南非Transvaal超群(Hannah et al, 2004; Frauenstein et al, 2009)、北美Huronian超群和芬兰Kuusamo带Sericite Schist组(Melezhik et al, 2013)的年龄数据显示LJE始于2.31~2.22Ga(Melezhik et al, 2013);Fennoscandian地盾Kuusamo带的结束时间是~2080Ma,Pechenga带和Imandra-Varzuga带则为~2058Ma(Melezhik et al, 2007)。Martin et al (2013)统计分析了最近20年获得的古元古代地层U-Pb和Re-Os年龄, 确认LJE发生在2306±9Ma至2057±1Ma之间,即2.3~2.06Ga。

图 1 Lomagundi-Jatuli事件记录的碳酸盐岩剖面分布示意图(据Tang et al, 2011) Fig. 1 Distribution of the carbonates recording Lomagundi-Jatuli Event in the world (after Tang et al, 2011)

华北克拉通是全球最古老陆块之一,广泛发育了古元古代变质沉积建造,理应经历并记录了GOE、LJE等事件,但长期缺乏这些事件记录的报道,甚至缺乏专门研究。长期缺乏研究的主要原因是我国2.06Ga之前的地层变质较深,只有辽河群、滹沱群等少数地层单元为绿片岩相,其余为角闪岩相或麻粒岩相。在如此强烈变质改造的情况下,GOE、LJE等事件的标志性地质地球化学记录还能否被保存?即使保存下来,又能否被揭示出来?即使被揭示出来,又能否被学者们接受或认可?困难重重,使不少学者望而却步。一些关注GOE、LJE等事件的我国学者,也往往绕开华北克拉通而开展非洲、澳州或美洲大陆低变质或未变质古元古代沉积地层的研究,获得了容易被认可的GOE、LJE等的证据(Guo et al, 2009; Luo et al, 2016)。但是,华北克拉通GOE、LJE研究仍然被搁置,也仍然无法回答华北克拉通是否存在GOE、LJE记录等问题。显然,直接开展华北克拉通中高级变质地层的地质地球化学研究或探索,即使获得的结果或认识不如未变质区,也是回答上述问题,提升华北克拉通研究程度的必由之路。

根据华北克拉通石墨矿床的碳同位素研究资料,陈衍景等(2000)首先讨论了华北克拉通记录LJE的可能性,特别指出了胶东荆山群存在碳酸盐碳同位素正异常的信息,并推测荆山群原始沉积碳酸盐的δ13Ccarb可能高于4.2‰。为检验这一推断,并探讨荆山群对古元古代大氧化事件及成矿大爆发事件的记录,我们对荆山群开展了专门研究,初步认为荆山群下部禄格庄组和野头组发育大量厚层大理岩,上部陡崖组大量发育石墨矿床,它们可能对应着记录了GOE的LJE和Shunga事件。本文旨在报道山后地区荆山群禄格庄组大理岩碳氧同位素及元素地球化学研究结果,探讨其对LJE的记录。

1 区域地质概况

胶北地体位于华北克拉通东部,北临渤海,西侧以郯庐断裂为界与鲁西地块相邻,东南端以烟台-五莲断裂为界与苏鲁超高压变质带相邻(杨进辉,2000唐俊等,2004刘平华等, 2011, 2013谢士稳等,2014)。胶北地体主要出露新太古代胶东群、古元古代荆山群和粉子山群(孔庆友,2006),以及大量中生代花岗岩(图 2)。

图 2 胶北地体区域地质图(据谢士稳等,2014修改) Fig. 2 Geological map of Jiaobei Terrain (modified after Xie et al, 2014)

荆山群是胶东半岛石墨矿、大理岩矿的主要赋矿层位(陈衍景等, 2000),主要分布在胶北地体的平度仙山、明村,莱西南墅,莱阳荆山、旌旗山,牟平光山、祥山,海阳晶山、乳山午极等地。主体为一套碎屑岩夹碳酸盐岩的岩石组合,原岩恢复为一套多旋回的富铝泥沙质碎屑岩-钙镁质碳酸盐建造,间夹基性火山岩,是陆源浅海沉积产物,经历了高角闪岩相-麻粒岩相变质(季海章和陈衍景,1990季海章等,1990; 卢良兆等,1996孔庆友,2006),自下而上可以分为禄格庄组、野头组和陡崖组(图 3)。禄格庄组分为下部的安吉村段和上部的光山段,安吉村段以(石墨、石榴、夕线)黑云片麻岩为主,间夹(石墨、黑云、石榴、透辉)变粒岩;光山段则以(蛇纹石、白云石)大理岩为主,夹有硅质大理岩、蛇纹石金云橄榄大理岩、角闪变粒岩和透辉变粒岩,偶夹有斜长角闪岩。野头组包括下部祥山段,以灰绿色透辉岩、透辉变粒岩为主,夹有斜长角闪岩、黑云斜长片麻岩以及长石石英岩;上部定国寺段,主要以蛇纹石大理岩、透辉石大理岩、方柱透闪大理岩等各类大理岩为主。陡崖组下部徐村段是石墨矿的主要含矿层位,主要岩性为石墨黑云变粒岩、石墨透辉变粒岩、透辉岩、黑云变粒岩、(石墨)黑云斜长片麻岩、石榴(夕线)二云(黑云)片岩,夹透闪石英岩、二云石英片岩等;上部水桃林段以石榴二云片岩、石榴黑云片岩为主,夹白云石英片岩等(张增奇和刘明渭,1996)。

图 3 荆山群地层柱(据李洪奎等,2013) Fig. 3 Stratigraphic units of the Jingshan Group (modified after Li et al, 2013)

前人针对荆山群地层做了许多年代学测试工作。Wan et al (2006)用SHRIMP锆石U-Pb年龄测试方法研究了烟台南部荆山群二云母夕线石榴片麻岩,获得最年轻的碎屑锆石207Pb/206Pb年龄为2175±16Ma。刘平华等(2011)获得莱西北墅南部夕线石榴斜长片麻岩的最年轻碎屑锆石207Pb/206Pb年龄为2100±10Ma。Tam et al (2011)对莱西南墅地区的麻粒岩和片麻岩碎屑锆石进行SHRIMP U-Pb年龄测试,获得最年轻的碎屑锆石年龄为2320±16Ma。Wan et al (2006)得到的荆山群二云母夕线石榴片麻岩的8颗变质锆石年龄在1859±11Ma~1902±8Ma之间,加权平均年龄为1882±12Ma。刘平华等(2011)获得栖霞小庄铺荆山群孔兹岩26颗变质锆石年龄为1847±8Ma~1879±5Ma,加权平均年龄为1868±3Ma。北墅夕线石榴斜长片麻岩的变质锆石年龄分为1853±8Ma~1922±12Ma和1833±9Ma~1844±11Ma两组,而由于最大年龄数据1922±12Ma的协和度小于90%,故将第二大年龄1901±20Ma作为这一组变质锆石的最大年龄。Tam et al (2011)测得栖霞太平庄麻粒岩21颗变质锆石年龄为1815±14Ma~1875±19Ma,加权平均年龄为1837±8Ma;莱西南墅陡崖组含石墨石榴石夕线石片麻岩的20颗变质锆石年龄为1790±19Ma~1853±27Ma,加权平均年龄1821±18Ma;莱西常家屯黑云片麻岩22颗变质锆石年龄1801±19Ma~1874±20Ma,上交点年龄为1836±68Ma;莱西南墅基性麻粒岩的4颗变质锆石年龄为1855±90Ma~2012±53Ma,加权平均年龄为1956±41Ma,但4颗锆石中的3颗年龄数据误差大于50Ma,可信度较低;平度双庙麻粒岩(08JB01-1-6)19颗变质锆石年龄为1633±150Ma~2125±130Ma,加权平均年龄为1884±24Ma。然而,双庙麻粒岩锆石年龄误差较大,最大可达230Ma,仅有7颗锆石年龄误差小于50Ma,且无协和度报道,本文仅做参考。

董春艳等(2010)对莱阳吕格庄地区侵入荆山群的变质闪长岩进行SHRIMP U-Pb年代学测试,获得14颗岩浆锆石年龄在1829±14Ma~1895±21Ma之间,加权平均年龄为1852±9Ma;莱西南墅变质辉长岩岩浆锆石年龄在1821±41Ma~1926±23Ma之间,11颗锆石的加权平均年龄为1865±11Ma。

荆山群碎屑锆石年龄分布较广,最大碎屑锆石年龄为2936±17Ma (Wan et al, 2006),最小碎屑锆石年龄是2100±10Ma (刘平华等,2011),说明物质来源复杂。变质锆石年龄范围相对集中,主要为1775~1875Ma,可能是较强变质作用时间,较可信的最大变质锆石年龄为1902±8Ma (Wan et al, 2006)。侵入荆山群的岩浆岩侵位年龄为~1.86Ga(董春艳等,2010)。本文将最年轻碎屑锆石年龄作为地层的最大沉积年龄,最大变质锆石年龄作为地层的最小沉积年龄,得到荆山群沉积年龄为2.10~1.90Ga。

2 样品和分析方法 2.1 样品及采样位置

本文研究的33件大理岩和白云质石英岩样品采自胶北地体北部的古元古代荆山群,位于平度市东北的南墅镇山后村附近(图 2图 4)。山后村与南墅镇之间分布多个石墨矿床,以南墅石墨矿床最为知名,它们赋存于荆山群陡崖组地层。本文样品则采自位于荆山群下部的禄格庄组光山段,所有样品采自深度为725.2m的钻孔(ZK 1604)。

图 4 山后大理岩矿矿区地质图(山东正元地质资源勘查有限责任公司,2011) Fig. 4 Simplified geological map of the Shanhou marble deposit

① 山东正元地质资源勘查有限责任公司.2011.山东省平度市南墅镇山后大理岩矿床普查报告

33件样品分为大理岩和白云质石英岩两类。其中,26件为纯度较高的白云质大理岩,手标本呈乳白色(图 5a, c, d),镜下白云石自形-半自形(图 5f-h),含有少量石英颗粒(图 5g, i-l);7件白云质石英岩(SiO2>30%)样品呈灰白色(图 5b, c, e),镜下石英颗粒呈不等粒状,碳酸盐矿物主要为白云石,次为方解石,方解石多呈网脉充填在石英颗粒间隙(图 5m-o)。详细描述见表 1

图 5 荆山群禄格庄组样品地质特征 (a)大理岩, 样品SH07;(b)含钙石英岩, 样品SH26,靠近含碳角砾岩;(c)变粒岩和含钙石英岩, 样品SH30;(d)白色块状大理岩, 样品SH07;(e)石英岩, SH30;(f)粗粒白云石大理岩,发育石英网脉,SH09;(g)白云石大理岩,SH21;(h)大理岩遭受方解石化,SH03; (i)白云石和石英颗粒,SH01;(j)早期细粒白云石(DoI)大理岩,可见晚期重结晶粗粒白云石(DoII)和石英,SH05;(k、l)硅化蚀变, SH25, SH19; (m)石英岩,石英颗粒被方解石胶结,样品SH40;(n)角砾状构造石英岩, SH26; (o)硅化蚀变,白云母蚀变, SH26. Do-白云石; Qtz-石英; Cc-方解石; Ms-白云母 Fig. 5 Geological characteristics of samples from the Lugezhuang Formation, Jingshan Group

表 1 荆山群禄格庄组样品地质特征 Table 1 Geological characteristics of samples from the Lugezhuang Formation, Jingshan Group
2.2 分析方法

碳-氧同位素分析在中国科学院地球化学研究所环境地球化学国家重点实验室完成。粉末样品首先通过德国Thermo Fisher Scientific的GasbenchⅡ在氦气流中冲洗8min,然后在真空系统中使用100%的磷酸与粉末样品在72℃恒温水浴中恒温反应4h以上,收集碳酸盐矿物释放出的CO2用于质谱分析。使用Gasbench Ⅱ连续流进样,经Thermo Fisher Scientific MAT 253型气体质谱分析仪分析得出样品的δ13C值和δ18O值,碳同位素结果均采用V-PDB标准,氧同位素采用V-SMOW标准,测试精度为0.2%。

全岩主量元素测试在澳实分析检测(广州)有限公司完成。首先把包含硝酸锂在内的助溶剂加入待测样品中,待充分混合后高温熔融,把熔融物倒入铂金模子形成扁平玻璃片后,采用PANalytical Axios X射线荧光光谱仪进行分析。

全岩微量元素测试在中科院地球化学研究所矿床地球化学国家重点实验室完成。采用的仪器是四级杆型电感耦合等离子质谱仪,仪器型号为ELAN DRC-e ICP-MS,样品分析数据的相对精度优于10%,详细测试方法见Qi et al (2000)

3 测试结果

荆山群禄格庄组33件样品碳-氧同位素分析结果,主量及Sr元素地球化学测试结果均列于表 2。大理岩的碳-氧同位素值在地层上的分布见图 6,频率分直方图见图 7

表 2 荆山群禄格庄组样品主要氧化物(wt%)、Sr(×10-6)及δ13CV-PDB (‰)和δ18OSMOW (‰)组成 Table 2 Composition of main oxides (wt%), Sr (×10-6), δ13CV-PDB (‰) and δ18OSMOW (‰) of samples from the Lugezhuang Formation, Jiangshan Group

图 6 禄格庄组地层柱状图及δ13CV-PDB (‰)和δ18OSMOW (‰)组成 Fig. 6 Stratigraphic column, δ13C (‰) and δ18O (‰) values of samples from the Lugezhuang Formation

图 7 禄格庄组碳酸盐样品δ13C (a)和δ18O (b)分布直方图 Fig. 7 Histograms for δ13C (a) and δ18O (b) values of samples from the Lugezhuang Formation

26件白云石大理岩样品具有较稳定的CaO和MgO含量,分别为21.06%~31.72%和12.87%~19.77%,平均分别为27.25±2.09%和17.07±1.59%。烧失量(LOI)和MgO/CaO(mol)较稳定,分别为32.85%~45.62%和0.78~0.94,平均分别为41.47±3.23%和0.88±0.04。SiO2含量在1.82%~28.63%之间变化,平均10.81±6.38%。相较而言,7件含钙石英岩CaO、MgO含量较低,分别为7.93%~13.35%和4.17%~7.34%,平均分别为9.99±1.34%和5.22±0.80%。相应烧失量较低,为11.04%~24.80%,平均为17.54±3.19%。SiO2含量在33.14%~73.95%之间,变化范围较大,平均51.20±12.71%。随着SiO2含量升高,MgO/CaO值逐渐下降,为0.65~0.80,平均0.73±0.06(表 2)。

样品Al2O3含量多 < 1%,26件大理岩样品的Al2O3含量在0.06%~2.68%之间,平均0.52±0.40%。而7件含钙石英岩样品Al2O3含量较高,在0.32%~15.28%之间,平均8.48±4.62%。Fe2O3T含量变化范围在0.60%~7.53%之间,大理岩样品平均为1.73±0.85%,含钙石英岩样品平均为4.22±1.70%,含量有所增加。所有样品的Na2O含量 < 0.5%,K2O的含量范围变化稍大,在0.01%~3.75%之间。MnO含量范围在0.02%~0.17%(154×10-6~1725×10-6),大理岩样品和石英岩样品MnO的平均值均为0.08%。此外,样品中还含有少量的P2O5(0.01%~0.13%)和TiO2(0.01%~1.07%)。微量元素实验数据显示,样品的Sr含量在55.67×10-6~471.7×10-6,大理岩样品平均值为175.0±87.53×10-6,含钙石英岩样品平均值为233.0±103.8×10-6。大理岩Mn/Sr范围为1.72~23.52,平均7.52±4.98;含钙石英岩Mn/Sr在2.22~12.00之间,平均值为4.91±2.52。

大理岩样品的δ13C值在-0.78‰~3.42‰之间,平均1.57±0.80‰(V-PDB);δ18O值相对较高、变化范围大,为9.76‰~20.23‰,平均14.84±2.77‰(V-SMOW)。含钙石英岩样品δ13C值为0.02‰~2.50‰之间,平均0.88±0.90‰(V-PDB);δ18O值较低且变化范围小,为9.28‰~12.86‰,平均10.82±0.70‰(V-SMOW)(表 2)。

4 讨论 4.1 碳酸盐岩碳氧同位素地球化学行为

在各种碳储库中,碳酸盐岩碳同位素组成最高,δ13C值平均为0.5‰ (表 3)。同时,碳酸盐岩的氧同位素组成也较高,δ18Ocarb值通常大于20‰。据此可以推断,碳酸盐岩与其它碳储库进行碳同位素交换时,容易释放13C,汲取12C,其δ13C降低,同时,与之平衡反应的其它碳储库δ13C升高。事实上,学者们已经共识碳酸盐岩沉积之后的成岩、变质和流体作用均导致δ13Ccarbδ18Ocarb降低,且δ13Ccarbδ18Ocarb值随变质程度升高而降低(Veizer, 1983; Banner and Hanson, 1990; Jacobsen and Kaufman, 1999; Veizer et al, 1999; 陈衍景等, 2000; Melezhik et al, 2001a, b, 2005汤好书等, 2008, 2009)。

表 3 地球主要碳储库的δ13CV-PDB (‰) Table 3 The δ13C values (PDB‰) of major carbon reservoirs

碳酸盐岩沉积之后的地质作用导致碳氧同位素降低的根本原因是:碳酸盐矿物的化学成分可表示为Mg、Ca、Fe、Mn的金属氧化物(MO)与CO2的结合物,即MO(CO2。根据软硬酸碱理论(参见戴安邦,1987Chen and Zhao, 1997),与O配合或结合时,C4+是比Mg2+、Ca2+、Fe2+、Mn2+更硬的酸,而18O是比16O更硬的碱,因此,在MO(CO2中,18O倾向于分配到CO2中。在碳酸盐岩经历成岩、变质作用而脱CO2时,由于18O随CO2逸失而导致剩余MO·CO2的相对富集16O,即δ18Ocarb降低。同样道理,13C比12C硬度大,更倾向于与18O结合,形成13C18O2;因此,碳酸盐岩以任何形式脱CO2时,都会造成残余碳酸盐岩δ13Ccarb降低。然而,目前对δ13Ccarbδ18Ocarb在后期地质作用中降低的幅度,尚缺乏定量估算公式或办法,只有一些统计性研究。

Schidlowski et al (1975)统计认为碳酸盐岩δ18Ocarb值在重结晶或变质过程中降低约2‰~3‰,Veizer et al (1999)则认为碳酸盐岩δ18Ocarb在成岩作用中降低~2‰。Bottinga (1969), Wada and Suzuki (1983), Schidlowski (1988)等研究表明,温度高于650℃的变质作用会使δ13Ccarb降低幅度>3‰。就Fennoscandian地盾区而言,几乎没有经历蚀变的碳酸盐(白云岩和石灰岩)δ13Ccarb为+5‰~+9.6‰(平均7.4±0.7‰),经历了绿帘-角闪岩相变质的碳酸盐δ13Ccarb值为+1‰到+5‰(Melezhik et al, 2013)。如果角闪岩相碳酸盐尚可保留初始沉积时13C富集特征的话,麻粒岩相碳酸盐则几乎不能保留13C富集信息,因其δ13Ccarb值比角闪岩相降低5‰以上,δ18Ocarb值降低幅度可达10‰以上,可从~25‰降低到~10‰,致使麻粒岩相碳酸盐平均δ18Ocarb值一般不超过18‰(Baker and Fallick, 1989a, b; Melezhik et al, 2001a, b)。由上可见,变质等级越高,δ13Ccarbδ18Ocarb值越低。

流体-岩石相互作用也会导致碳酸盐矿物δ13C和δ18O值下降(Melezhik et al, 1999; Kump et al, 2011)。例如,密西西比Burlington-Keokuk组白云岩较好记录了流体-岩石相互作用的历史(Banner and Hanson, 1990),重结晶白云岩δ18Ocarb值变化较大,δ13Ccarb值变化较小,说明δ18Ocarb相对于δ13Ccarb更易发生变化。因此,Hudson (1977)建议δ18Ocarb可作为岩石遭受流体作用的指示剂,冯伟民等(2003)Aharon (2005)主张以δ18Ocarb=18‰作为判别碳酸盐岩发生热液蚀变作用的阀值,而Melezhik et al (2005)则主张δ18Ocarb阀值为20‰。

LJE期间的碳酸盐碳、氧同位素值偏高。Schidlowski et al (1976)研究的LJE期间的δ18Ocarb值平均为22.2±1.6‰,最大值达26.9‰。Veizer et al(1992a, b)认为早前寒武纪白云岩δ18Ocarb较高,平均为26±2‰。Melezhik et al (1999)认为早前寒武纪白云岩δ18Ocarb平均值在28‰左右,并将25‰~26‰作为岩石遭受蚀变或变质与否的阈值。

一些学者(Veizer, 1983; Valley, 1986; Banner, 1995Jacobsen and Kaufman, 1999; Veizer et al, 1999; Ray, 2009)专门研究了碳酸盐岩δ18O较δ13C更易变化的原因。Banner and Hanson (1990)认为,在开放体系下,即使流体/岩石比值很低(F/R < 10),碳酸盐矿物也能与富水流体之间达到氧同位素分馏平衡,但碳同位素分馏平衡则要在流体/岩石比值高达103时才能实现,其原因是碳酸盐岩碳含量高,可以缓冲流体作用引起的δ13Ccarb变化。

在流体作用下,碳酸盐岩δ13Ccarbδ18Ocarb值降低方式可分为三类(汤好书等, 2008Tang et al, 2011, 2013a):(1) δ13Ccarbδ18Ocarb同步降低(Veizer and Hoefs, 1976; Guerrera et al, 1997; Jacobsen and Kaufman, 1999),发生在F/R比值较高的条件下;(2)近乎平行于δ18Ocarb轴的极端变化趋势,发生在F/R比值较低的条件下(Banner and Hanson, 1990),且流体富水贫碳质(如CO2);(3)近乎平行于δ13Ccarb轴的极端变化趋势,是碳质流体作用的特定现象。

4.2 荆山群碳酸盐碳氧同位素组成

造山纪发生了全球性陆块汇聚、碰撞、造山作用,最终形成哥伦比亚超大陆。荆山群在造山纪哥伦比亚超大陆会聚过程中发生了强烈的变形变质,周喜文等(2001)用石榴子石-黑云母矿物对计算出南墅地区荆山群孔兹岩系的峰期变质温度在700~750℃之间,李凯月等(2018)通过石墨拉曼光谱研究估算张舍石墨矿所经历的最高变质温度在730~790℃之间。可见,胶北地体含石墨矿的孔兹岩系所经历的最高变质温度可能在700~800℃之间,变质等级达到高角闪岩相-麻粒岩相。那么,在如此强烈、复杂地质改造作用中,荆山群碳酸盐岩地球化学特征和碳氧同位素组成发生哪些改变呢?

4.2.1 禄格庄组碳酸盐岩流体作用及碳同位素变化

蚀变作用常导致碳酸盐岩Sr含量降低,Mn、Fe、Rb含量增加(Brand and Veizer, 1980; Derry et al, 1992; Melezhik et al, 2001a, b, 2008; Bekker and Kaufman, 2007),Mn和Sr含量可以指示成岩作用的级别。Mn/Sr比值对碳酸盐岩的后期流体作用较为敏感(Kaufman et al, 1993; Bekker et al, 2008)。一般认为,Mn/Sr>6时,碳酸盐岩受后期流体作用影响较大(Kaufman and Knoll, 1995; Melezhik et al, 1999)。

在荆山群禄格庄组样品中,13件样品Mn/Sr比值超过6,但δ13C-Fe2O3Tδ13C-Mn/Sr、δ18O-Mn/Sr之间的相关性不明显,R值分别为0.14、0.34、0.25(图 8a-c),没有显示成岩作用对禄格庄组碳酸盐岩碳氧同位素的强烈影响。

图 8 禄格庄组样品元素协变图 Fig. 8 Correlations of elements in samples of the Lugezhuang Formation

早前寒武纪碳酸盐地层主要为白云岩,后期流体交代作用趋向于生成方解石(Melezhik et al, 2001a, b; 图 5j-l)。在图 8中,禄格庄组样品主要落入白云岩区域,Mg/Ca-Mn/Sr的相关系数R值为0.15,没有明显的相关性。δ13C-Mg/Ca、δ18O-Mg/Ca之间的相关系数R值分别为0.32和0.54(图 8e, f),显示中等程度的正相关,表明白云岩的方解石化越强烈,δ13C和δ18O值越低。

Mg/Ca与SiO2之间的相关系数为-0.71(图 8g),显著负相关,说明硅化导致MgCO3减少或者CaCO3的相对增多(方解石化)。在图 8h中,SiO2与LOI之间明显负相关,R=-0.98,原因之一是硅化导致岩石中CO2等挥发分含量降低,原因之二是记录了原始沉积SiO2与碳酸盐矿物之间的消长关系。从岩石薄片观测结果来看,石英交代、充填碳酸盐矿物的现象普遍、清楚,硅化证据充分。虽然无法排除SiO2等组分是原始沉积形成,但排他性的岩相学依据不足够充分。9件样品SiO2>25%,其δ18O值稳定在9.28‰~11.21‰,说明碳酸盐矿物与含SiO2流体之间基本达到了氧同位素平衡(图 8i);同时,这9件样品的δ13C值介于-0.06‰~2.50‰之间,明显低于其它研究样品。以上说明,流体作用使δ13C、δ18O值降低。

4.2.2 禄格庄组碳酸盐岩变质作用及氧同位素变化

禄格庄组33件样品δ18O值在9.28‰~20.23‰之间,即9.3‰~20.2‰之间,平均14.0‰。33件样品中,只有1件样品的δ18O值大于20‰,5件样品δ18O值大于18‰,表明沉积后经历了强烈的地质作用改造。如前所述,SiO2与Mg/Ca、LOI呈负相关关系,指示样品发生了方解石化、硅化或钙硅酸盐化。图 8i中,SiO2 < 30%时,即白云质大理岩,样品δ18Ocarb值随着SiO2含量增高而降低,直至δ18Ocarb降至11‰左右;SiO2>30%时,即白云质石英岩,δ18Ocarb值稳定在11‰附近,不再随SiO2含量变化而改变。这一现象表明,原始沉积白云岩可能经历了两次改造作用:(1)成岩或变质作用及其伴随的脱挥发份作用,造成δ18Ocarb值降低和SiO2含量相对增高;(2)以硅化为代表的交代作用,表现为SiO2含量不断增高,而碳酸盐δ18Ocarb值稳定在11‰附近,说明残余碳酸盐与流体之间的氧同位素分馏达到了平衡。

前述现象和认识与显微镜观察结果一致,显示样品后期硅化等热液作用较明显,主要依据是:(1)大理岩样品多由中-粗粒白云石组成,发育石英网脉(图 5f);(2)虽然少部分石英呈现重结晶特点(图 5i),但绝大部分为硅化形成(图 5j-l);(3)强硅化样品,如白云质石英岩,多具有角砾状构造或碎裂结构,发育在角砾岩带附近(图 5b, n, o图 6);(4)山后大理岩矿区及附近广泛发育多种碎裂岩、糜棱岩(图 4),并发育NNE或EW向含金石英脉。

4.3 荆山群碳同位素正异常对LJE的指示

在全球范围内,不同时代海相碳酸盐岩的δ13C均值为0.5±2.5‰。通常认为,δ13Ccarb值明显高于或低于平均值的碳酸盐岩形成于特殊沉积环境(Melezhik et al, 1999, 2013; Sanyal et al, 2009);当δ13Ccarb值大于3‰时,即可被视为大于普通碳酸盐岩的异常沉积(Karhu, 1993)。在LJE期间,碳酸盐岩δ13Ccarb值普遍大于4‰ (Melezhik et al, 2013),集中在5‰~18‰之间,甚至出现δ13Ccarb值达到28‰极端案例(Bekker et al, 2003)。Melezhik et al (1999)认为LJE由3或4个δ13Ccarb值正漂移旋回组成,每个旋回的δ13Ccarb值都正漂移至5‰以上,之后再回到0‰;在多旋回的循环往复中,δ13Ccarb值始终大于0‰,充分证明当时海洋碳储库富集13C。

近年,一些学者探索性地开展了华北克拉通古元古代变质地层碳氧同位素研究,陆续发现多处古元古代碳酸盐岩碳同位素正向漂移现象。例如,辽北辽河群关门山组(2.3~1.9Ga)42件白云岩δ13Ccarb值为4.6‰~5.9‰,平均为5.3‰(汤好书等,2008)。辽东辽河群大石桥菱镁矿区(2.3~1.85Ga)大石桥组白云岩δ13Ccarb值为0.6‰~1.4‰,平均为1.2‰,大石桥组原始沉积白云岩的δ13Ccarb值应在4.2‰左右(汤好书等,2009)。河南登封嵩山群(2.51~1.95Ga)碳酸盐岩δ13Ccarb值为-1.0‰~4.2‰,平均2.5‰,具有碳同位素正异常现象(Lai et al, 2012)。山西五台地区滹沱群大石岭组175件δ13Ccarb数据普遍为正值,变化于0.6‰~3.5‰,具有δ13Ccarb正漂移特征(陈威宇和陈衍景, 2018)。考虑到高角闪岩相-麻粒岩相变质作用可使δ13Ccarb值降低3‰以上(Wada and Suzuki, 1983),陈衍景等(2000)提出胶东南墅石墨矿床院后矿区荆山群大理岩的原岩碳酸盐δ13Ccarb值应大于4.5‰,因为兰心俨(1981)报道的2件大理岩δ13Ccarb值分别为0.8‰和1.5‰,已经显示了δ13Ccarb正异常。

本研究获得荆山群禄格庄组33件大理岩和石英岩之碳酸盐胶结物样品的δ13Ccarb值,变化于-0.8‰~3.4‰之间,平均为1.4±0.9‰。在33件样品δ13Ccarb值中,只有2件为负值,分别为-0.78‰和-0.06‰,其余为正值,最大为3.4‰,显示了正异常特征。图 9中,禄格庄组碳酸盐碳、氧同位素之间的相关系数(R)为0.55,其δ18O变化范围(9.3‰~20.2%)大于δ13C变化范围(-0.8‰~3.4‰),δ18O-δ13C散点图略显平行于δ18O轴的趋势。禄格庄组部分样品与世界LJE期间未变质或浅变质海相碳酸盐相分布范围一致,多数样品类似于角闪岩相-麻粒岩相变质的~2.15Ga的Lofoten-Vesteralen大理岩特征(Baker and Fallick, 1989a, b; Martin et al, 2013)。也就是说,虽然总体属于麻粒岩相的荆山群变质程度高于同时期的Lofoten-Vesteralen大理岩,但其碳氧同位素特征仍像后者一样保留了LJE事件特征,甚至个别样品的碳氧同位素特征竟然像未变质的LJE时期碳酸盐岩,δ13Ccarb正异常清楚。事实上,高级变质过程的脱水、脱挥发份、重结晶作用及流体作用导致禄格庄组碳酸盐碳氧同位素大幅度降低,部分样品达到了石英、碳酸盐、流体之间的氧同位素分馏平衡,δ18Ocarb稳定在11‰附近。考虑到δ18Ocarb降低如此强烈,可以推断禄格庄组大理岩δ13Ccarb值至少低于其原始沉积碳酸盐3‰~5‰。现今禄格庄大理岩δ13Ccarb值平均为1.4‰,初始沉积碳酸盐δ13Ccarb平均值应不低于4.4‰,可能为6.4‰,最大值甚至达8.4‰以上,较好地记录了LJE事件。

图 9 荆山群禄格庄组碳酸盐δ13C-δ18O同位素与全球Lomagundi海相碳酸盐对比(底图据Tang et al, 2013a) Fig. 9 δ13C vs. δ18O of the Lugezhuang Formation, Jingshan Group compared with those of the 2.31~2.06Ga Lomagundi marine carbonates (base map and data source see Tang et al, 2013a and references therein for details)
5 结论

(1) 胶北地体荆山群禄格庄组发育大量碳酸盐地层,其33件样品δ13Ccarb值为-0.8‰~3.4‰之间。其中,大理岩样品δ13Ccarb值平均为1.6±0.8‰,白云质石英岩δ13Ccarb值平均为0.9±0.9‰。碳氧同位素值之间具有很好的正相关性,部分样品Mn/Sr比值大于6。禄格庄组碳酸盐经历了重结晶-变质作用以及硅化作用,造成样品去碳酸盐化强烈。

(2) 高角闪岩相-麻粒岩相变质作用和流体交代作用是影响碳氧同位素值的主要因素,导致使禄格庄组大理岩碳同位素值至少下降了3‰~5‰,原始沉积碳酸盐δ13C平均值应该不低于4.4‰,正异常明显,记录了全球性的LJE事件。

致谢      样品采集过程中得到山东黄金集团有限公司史先洪工程师的热情帮助;实验测试工作得到了中国科学院地球化学研究所矿床地球化学国家重点实验室的大力支持,微量元素由该实验室胡静、黄静高级工程师指导完成,碳氧同位素测试由该实验室谷静高级工程师帮助完成;三位审稿人提出了宝贵的修改意见;在此一并致以诚挚感谢!

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胶北地体荆山群大理岩碳氧同位素地球化学特征及其对Lomagundi-Jatuli事件的指示
李凯月, 汤好书, 陈衍景, 薛莅治, 王玭, 孙之夫