【作者】 周琦；

【导师】 杜远生；

【作者基本信息】 中国地质大学 ， 古生物与地层学， 2008， 博士

【摘要】 黔东地区位于扬子板块东南边缘与江南造山带的结合部位。新元古代全球性的Rodinia超大陆裂解作用,在华南形成了新元古代被动陆缘裂谷盆地,断裂、拉张作用形成多个拉张盆地,控制了该地区南华纪早期冷泉碳酸盐岩（菱锰矿+白云岩）的形成和分布。该地区南华纪早期的两界河组和大塘坡组中均有冷泉碳酸盐岩发育和分布。其中两界河组的冷泉碳酸盐岩是发育在相当于Sturtian冰期的含砾碎屑岩沉积中,以白云岩透镜体或白云岩丘的形式产出,大小不等,内部含砾,之上为铁丝坳组冰碛含砾砂岩;大塘坡组为介于Sturtian冰期和Marinoan两个冰期之间的间冰期沉积,岩性主要为粉砂质页岩,底部为一套黑色炭质页岩沉积。大塘坡组的冷泉碳酸盐岩位于这套黑色岩系之中,由于该岩系中的冷泉碳酸盐岩是以菱锰矿体为主,另有少量白云岩透镜体,因此,这套黑色岩系又称黑色“含锰岩系”。两界河期冷泉碳酸盐岩的形态为透镜状或丘状,其顶面多上拱,两端微微上翘,与围岩突然接触,零星分布在含砾碎屑岩中:大塘坡期的冷泉碳酸盐岩主要有透镜体状及似层状等形态。平面上冷泉碳酸盐岩的岩性组合具有同心环状的展布规律:中心为菱锰矿—炭质页岩组合,锰含量最高,古天然气渗漏沉积构造十分发育;往外出现白云岩透镜体,即菱锰矿—白云岩—炭质页岩组合,菱锰矿以条带状构造为主,块状构造次之;再向外菱锰矿尖灭,为白云岩—炭质页岩组合;最外为炭质页岩—黑色页岩组合,有机质明显减少,黑色岩系厚度明显减薄。南华纪早期的大塘坡期“含锰岩系”中冷泉碳酸盐岩透镜体（菱锰矿、白云岩）与两界河期的冷泉碳酸盐岩（白云岩透镜体或白云岩丘）具有成生联系,互为标志,不单独出现。某一区域在Sturtian冰期含砾碎屑岩沉积中如出现冷泉碳酸盐岩沉积,则之上的大塘坡组底部的黑色“含锰岩系”中就一定会出现冷泉碳酸盐岩——菱锰矿沉积,反之亦然。甚至在贵州大塘坡和湖南民乐两大锰矿区,Sturtian冰期的含砾碎屑岩沉积中出现白云岩透镜体的层数都是相等的。充分说明了两期冷泉碳酸盐岩是在同一个古天然气渗漏系统中不同时期的冷泉碳酸盐岩沉积,是一个完整的冷泉碳酸盐岩形成体系。菱锰矿的出现,它是南华纪早期黔东乃至更大区域的多个天然气渗漏系统中时间较晚的一次含锰的冷泉碳酸盐岩事件沉积产物。因此,Sturtian冰期的含砾碎屑岩沉积中冷泉碳酸盐岩透镜体的有无,可作为预测大塘坡组有无菱锰矿等冷泉碳酸盐岩沉积的关键标志,意义重大,并在实践中取得了成功。与现代冷泉碳酸盐岩十分相似,南华纪早期的冷泉碳酸盐岩具有较为典型的渗漏沉积构造,且分布普遍。特别是在大塘坡地区菱锰矿矿体中普遍发育的被沥青充填的气泡状构造,孔洞构造、底辟构造、渗漏管构造、软沉积变形纹理（不协调状、放射状、平卧褶曲状、穿层状等）、泥火山等构造。在菱锰矿的气泡状构造中,气泡壁均为玉髓,且构成气泡壁的玉髓均呈从中心向外有规律的放射状结构。由于后期成岩压实作用的影响,气泡发生压扁变形,甚至压裂,变形的长轴方向与层理平行;两界河组中的白云岩丘也普遍见帐篷构造、孔洞构造、软沉积变形纹理等典型渗漏沉积构造。矿物组分较为简单,主要由菱锰矿、钙菱锰矿、镁钙菱锰矿、硫锰矿及少量锰白云石、锰方解石等组成,含少量粘土矿物、有机炭及草莓状黄铁矿、石英、磷灰石、重晶石、绿泥石等自生矿物;两界河组白云岩主要是白云石,具体也可分为含镁相对较高和含钙相对较高的两期白云石。由于正处于Sturtian冰期,白云岩透镜体中含有较多的陆源碎屑矿物。与现代冷泉碳酸盐岩同位素特征一样,南华纪早期冷泉碳酸盐岩的碳同位素具有明显负偏的特征:菱锰矿的δ¹³C在-8.14‰～-10.38‰之间,其中,气泡状菱锰矿的碳同位素负偏移程度最大,δ¹³C低于-10‰,其次为块状菱锰矿石,而条带状菱锰矿石碳同位素负偏移程度略低:大塘坡早期白云岩透镜体的δ¹³C的负偏移可达-8.19‰～-12.98‰。进一步说明Nh₁d¹中的菱锰矿透镜体与白云岩透镜体成因的相似性;两界河期的白云岩的C同位素值同样存在明显负偏现象,但δ¹³C的负偏移相对较小,δ¹³C一般为-2.19～-2.86‰,变幅较小,平均值为-2.65‰;充填在菱锰矿气泡中沥青的有机碳同位素δ¹³C的负偏值更达-30.98‰。菱锰矿石与白云岩的氧同位素均为负值,δ（¹8）0‰一般为-4.72～-15.32‰,总体上看,菱锰矿的δ¹⁸0‰较白云岩偏负,多数小于-8‰。冷泉碳酸盐岩（主要是菱锰矿）及围岩（炭质页岩）中黄铁矿分布普遍,硫同位素具有异常高的正值为特征。δ³⁴S值一般大于+40‰,甚至大于+60‰,也许是元古宙时期所记录到的全球最正的数值。围岩黑色炭质页岩和下伏的铁丝坳组含砾炭质砂岩中的黄铁矿,δ³⁴S同样具有异常高的正值,尽管比菱锰矿硫同位素值略低,均远高于同时期海洋硫酸盐δ³⁴S的平均值,应是在非正常的海洋高度封闭的环境中形成的。从菱锰矿体的底板炭质页岩到菱锰矿,δ³⁴S值为从+49.30‰升到+52.90‰～+49.5‰,而菱锰矿体之上的炭质页岩的δ³⁴S值迅速则降低至+39.20‰。即δ³⁴S值具有从相对较低到高,菱锰矿体沉积后又迅速降低的规律,松桃大塘坡和大屋锰矿区也有类似规律。冷泉碳酸盐岩的硫同位素具有异常高正值的特征,说明了华南新元古裂谷盆地的构造背景下,古天然气渗漏的特殊环境下的产物。黔东地区各锰矿区中,菱锰矿体的围岩—南华纪大塘坡早期黑色炭质页岩的微量元素Mn与Cr的比值（Mn/Cr）约为40,几乎为一常数,可作为预测或判断南华纪早期黑色岩系中是否存在冷泉碳酸盐岩——菱锰矿体的又一关键标志。同时,菱锰矿中的有机质中出现了明显以甲烷为生的生物标识—古菌类异戊二烯烷和藿烷,古菌类异戊二烯烷和藿烷被认为是以甲烷为生的典型微生物。高含量的有机碳从另一个侧面佐证了该地区在南华纪早期存在甲烷渗漏,引起微生物的大量繁殖,从而导致δ¹³C值明显负偏。研究区南华纪早期至少有两期火山喷发,而火山喷发可能是诱发天然气规模释放的原因,两个因素叠加在一起,可能正是结束南华纪大塘坡早期的封闭缺氧沉积环境,产生环境突变,导致全球变化,转入正常的间冰期沉积的主要因素。岩浆活动提供了锰质来源。在冷泉口周围,古天然气渗漏导致微生物大量繁殖,由于甲烷的缺氧氧化形成菱锰矿沉积。在无氧条件下,由于硫酸盐还原细菌作用甲烷发生缺氧氧化,形成菱锰矿或白云岩和单硫化物(CH₄+SO^2-4→HCO₃^-+HS^-+H₂O)。由甲烷产生的生物碳酸盐局部超饱和能增强碳酸盐沉淀(2HCO₃^-+MN²⁺→MnCO₃+CO₂+H₂O;2HCO₃^-+Ca²⁺→CaCO₃+CO₂+H₂O),形成菱锰矿或白云岩等。过饱和的HS^-也会增强黄铁矿沉淀,并常以草莓状形式出现。这一理论不但解决了过去成矿理论中菱锰矿的沉积环境与生物生存环境的矛盾,还诞生了一门新的边缘学科——天然气渗漏与金属成矿学,以全新的观点和理论基础,去研究天然气渗漏所产生的成岩成矿作用,具有较强的科学探索性和创新性。不仅扩大了天然气及水合物研究领域的时空范围,而且对其所导致的成岩成矿、环境评价和生物演化事件的分析研究提供了新思路,具有重要的理论意义和应用价值。更多还原

【Abstract】 The eastern Guizhou lies in the area between the southeastern Yangtze block and Jiangnan orogenic belt.As the breakup of Rodinia Supercontinent,several rift basins formed in the Neoproterozoic passive continental margin through resultant fracture and extension,which controlled the formation and distribution of cold seep carbonates（rhodochrosite +dolostone）in Lower Nanhua System in the northeastern Guizhou.Cold seep carbonates are present in both of the Liangjiehe and Datangpo formations of the Early Nanhua Period in the eastern Guizhou.The cold seep carbonates of the Liangjiehe Formation occur in the gravel-bearing clastic rocks correlating to the Sturtian glacial period. They forms laminated gravel-bearing dolomite lens or dolomite domes of variable size.Above them are moraines of the Tiesiao Formation.The Datangpo Formation,correlating to period between the Sturtian and Marinoan glaciations,is composed of mainly silt shales with a series of black carbonaceous shales at its bottom.The cold seep carbonates of the Datangpo Formation lies in the black shales.So,the block shales are also called manganese-bearing rock series because the most parts of cold seep carbonates are mainly rhodochrosites,with a few dolostone lens.The shapes of the Liangjiehe cold seep carbonates are lenticular or domed with convex upper boundaries and slightly tip-tilted lateral boundaries.They occur sparsely in gravel-bearing clastics with sharp contacts.The Datangpo cold seep carbonates are lenticular or layer-like.They exhibit concentric distribution in lithological association in the region.In the center are rhodochrosites - carbonaceous shales with the highest manganese content,and well developed sedimentary structures relating to ancient gas seep;Outwards lie dolostone lens,consisting of rhodochrosite- dolostone- carbonaceous shale association.The rhodochrosites are mainly banded with some massive ones.Then rhodochrosites decrease outwards,replacing by dolostonecarbonaceous shales.The peripheral belt is carbonaceous shales - black shales association,with lower organic matter content and thin thickness of the whole black rock series.The Datangpo cold seep carbonate lens（rhodochrosite - dolostone）of manganese-bearing rock series and the Liangjiehe cold seep carbonates（dolostone lens or dolostone domes）in Lower Nanhua system appear genetically associated.That is,if Sturtian cold seep carbonates occur in gravel-bearing clastic rocks,their overlap’s black manganese-bearing rock series of the basal Datangpo Formation would contain cold seep carbonates- rhodochrosite deposition;It is also true reversely.Even number of layers of dolostone lens in Sturtian gravel-bearing clastics from the Datangpo of Guizhou,and from the Mingle manganese ore of Hunan are equal.It demonstrates that the two described periods of cold seep carbonates resulted from the same ancient gas seep system,and the system is a complete cold seep carbonate producing system.The occurrence of rhodochrosite in the early Nanhua period thus was an event deposition of magnesium - bearing seep carbonates from one of gas seep systems in eastern Guizhou or even in broad area.And the lenticular seep carbonates appear in Sturtian gravel-bearing clastics may serve as an indicator of rhodochrosite or cold seep carbonates in the Datangpo Formation.Such a conjecture would be significant in scientific research,and has been proved in recent ore exploration in the study area.Similar to the modern cold seep carbonate,typical seep features are common in the cold seep carbonates of early Nanhua age in the study area.They include bubble structures filled by bituminous materials from rhodochrosite ore of the Datangpo area,void structure,diaper structure,seepage tube structure,soft-sediment deformation lamination（inharmonic,radial and recumbent fold,as well as pierced structure）,mud volcano structures and so on.Bubble walls of the bubble structure in rhodochrosite are radial arranged chalcedony.As the results of diagenetic compaction,the bubbles were flatten,or even fractured,with long axis parallel to bedding. Typical seep structures are also common in the dolostone domes in the Liangjiehe Formation, including tepee structures,void structures and soft-sediment deformation structures. Mineralogical association of rhodoehrosite deposition is relatively simple,consisting of rhodochrosite,greinerite,greinerite with magnesium alabandite,and minor mangandolomite, manganocaleite,as well as minor clay minerals,organic carbon,pyrite,quartz,apatite,chlorite authigenic mineral;The Liangjiehe dolostone consists of mainly dolomite,which may be grouped into magnesium- and calcium - rich dolomites respectively.Because of tha advance of the Sturtian glaciation,terrigenous clastic content in the dolostone lens is relatively high.Also similar to modern cold seep carbonates,the carbon isotope of the early Narthua cold seep carbonates described here shows distinct negative excursion.Theδ¹³C of rhodochrosite ranges from -8.14‰to - 10.38‰,with the lightest in the bubble rhodochrosite,commonly less than -10‰;Lighter values are from the massive rhodochrosite,and then banded rhodochrosite.Theδ¹³C of early Datangpo dolostone lens is -8.19‰～-12.98‰.It further demonstrates the similar origin of the rhodochrosite lens with that of the dolostone from the Datangpo Formation.The negative features ofδ¹³C are also present in the Liangjiehe dolostone.They are -2.19‰～-2.86‰, averaging at -2.65‰,slightly heavier than those in the Datangpo Formation.Theδ¹³C value of organic carbon from the asphalt filled in rhodochrosite bubble is as low as -30.98‰.Theδ¹⁸O‰of rhodochrosite and dolostone are both negative,ranging from -4.72 to -15.32‰.Theδ¹⁸O‰of rhodochrosite is generally lighter than those of dolostone,mostly less than -8‰.Pyrite,common in cold seep carbonates（rhodochrosite）and their host rocks（carbonaceous shale）,is highly positive in itsδ³⁴S value.Theδ³⁴S value of the pyrite is generally larger than +4‰,and may up to +60‰,perhaps the heaviest value recorded in Proterozoic.Pyrite from hosting black carbonaceous shale and from underlying gravel-bearing carbonaceous sandstone of the Tiesiao Formation is also highly positive inδ³⁴S value.Although itsδ³⁴S value is lower than that of rhodochrosites,it is still higher than the average value of the coeval marine sulphate. Based on the features of these determinedδ³⁴S value,it is reasonable to infer a non-normal,and highly isolated sedimentary condition.Upsections from carbonaceous shale at the bottom of rhodochrosite ore body to rhodochrosite,and finally to carbonaceous shale of rhodochrosite roof,δ³⁴S varies from +49.30‰,to +52.90‰～+49.5‰in the rhodochrosite ore body,and drop to +39.20‰in the roof rocks.Similar trend were obtained from Datangpo manganese ore district of Songtao and Dawu manganese ore deposits district.The characteristic highly positive value of sulphur isotope of the cold seep carbonates indicates that the cold seep carbonates formed in a kind of gas seep conditions in Neoproterozoic rift basins of the South China.In the eastern Guizhou region,the ratio of Mn to Cr is nearly a constant,around 40,either from the host rocks of the rhodochrosite ore body or from the black carbonaceous shale of early Datangpo stage in Nanhua period.This ratio seems to be another key indicator to tell if there is cold seep carbonates—rhodochrosite in the black series in the horizon.In addition,biomarks from rhodochrosite demonstrated the existence of methane,because such biomarks are believed from typical microorganism living mainly on methane.In another hand,high organic carbon content evinced the seepage of methane,which furnished the flourish of microorganism and in turn led to a negative excursion of ambientδ¹³C.There were at least two phases of volcanic explosion in the study area during the early Nanhua, which is also a possible factor leading to explosive escape of gas.Coincidence of tectonic and volcanic factors might have been the main cause,terminating the anoxic depositional condition of the early Datangpo age,and resultant rapid global variation of environments,and gradually evolved into normal interglacial condition.The activity of magma provided the source of manganese.Around the mouth of cold seep,ancient gas seep leads to the flourish of microorganism,rhodochrosite deposites were formed through anaerobic oxidation of methane.In anaerobic condition,methane was oxidized through sulphate reduction bacteria,resulting in accumulation of rhodochrosite or dolomite and monosulfide(CH₄+SO^2-→HCO₃^-+H₂O). Locally supersaturated condition caused by methanogenetic biomineralisation,resulted in the formation of rhodochrosite or dolomites(2HCO₃^-+Mn²⁺→MnCO₃+CO₂+H₂O;2HCO₃^-+Ca²⁺→CaCO₃+CO₂+H₂O).The oversaturated HS^- may also facilitates the precipitation of pyrite,in the form of strawberry.The theory not only solves the conflicts between inferences from deposit of rhodochrosite and existence of microorganisms,but also born a new frontier science—gas seep and metallogenesis.This theory,focusing on diagenesis and ore-forming process associated with gas seepage,is of creative significance.It not only broadens our viewpoint for gas hydrate as well as its special and temporal distribution,but also sheds new light on metallic mineralization, biological evolution and environmental assessment relating to gas seep.Thus it is of significance in theoretic research and routine practice.更多还原