【作者】 杨捷；

【导师】 曾佐勋； Timothy W.Lyons；

【作者基本信息】 中国地质大学 ， 地质学， 2019， 博士

【摘要】 新元古代是地球演化历史的关键转折时期,Rodinia超大陆的形成与裂解、冈瓦那超大陆的形成都在新元古代时期完成。伴随着超大陆的形成和裂解,新元古代时期地球表层系统发生了显著的改变,例如气候的剧烈变化、海洋和大气中氧气含量的快速上升、多细胞生物开始出现等。这其中,新元古代的“雪球事件”、碳同位素负漂移、以及“盖帽碳酸盐岩”等冰期相关的问题使得新元古代冰川成为近年来国际地质学界研究的热点。在我国这一时期的沉积记录主要表现为冰碛岩及冰碛岩之上的碳酸盐岩沉积,其在塔里木、华北和华南三大板块均见报道。目前的研究多集中在华南三峡地区和塔里木板块库鲁克塔格地区,华北板块分布相当局限,仅在中朝大陆南部边缘形成一套山岳冰川-冰海沉积。宁夏贺兰山地区广泛发育一套类冰碛岩沉积,砾石成分复杂,粒径大小悬殊,杂乱排列,产出层位十分稳定。为了更好的了解新元古代末次冰期以来华北古环境和古气候情况,本文在贺兰山地区详细测量了五条新元古代地层剖面,由北向南大致为由内陆盆地向陆架边缘过渡的古地理环境。正目观组冰碛岩的沉积特征表明它很可能是受到一定程度的构造活动改造的冰川沉积产物,冰碛岩中基质的化学蚀变指数（CIA）表明正目观组冰碛岩是在寒冷、干燥的古气候环境中沉积形成的。另外,在华北西缘贺兰山中段南部地区发现了白云岩层直接覆盖在正目观组冰碛岩之上的新沉积类型。这类白云岩在贺兰山地区分布不连续,在南部地区分布较广,而在中部和北部地区则分布较为局限。本文根据在这类白云岩中发现的与全球新元古代盖帽白云岩相似的特征,包括:（1）白云岩与下部冰碛岩岩性明显变化;（2）硅质碎屑物质含量降低,未见明显的角砾;（3）类似的“平底晶洞构造”;（4）白云岩镜下可观察到内部充填薄层状的白云质微晶;（5）δ¹³C呈负漂移,由下往上逐渐降低;将正目观组冰碛岩之上的白云岩层命名为正目观组盖帽白云岩。正目观组盖帽白云岩厚约1.6 m,主要由下层纹层状白云岩和上层厚层状白云岩组成。通过野外路线和剖面测量、室内岩石薄片鉴定以及主-微量元素含量、无机-有机碳同位素(δ¹³C_carb-δ¹³C_org)、总有机碳含量（TOC）等高精度地球化学分析手段,本文对盖帽白云岩的沉积特征、沉积环境以及形成原因进行了探讨。根据兔儿坑组内发现的埃迪卡拉纪晚期古生物化石“Helanoichnus”和“Shaanxilithes”,可以推断正目观组和华南的灯影组以及西北的皱节山组可能属于同期的地层。正目观组下层盖帽白云岩镜下薄片观察可见有陆源碎屑矿物,同时该层样品中地球化学数据显示:（1）Zr和REE呈现适当的正相关变化特征;（2）陆源物质相关元素（Al、Ti、Zr等）含量上升;（3）氧化还原敏感元素（U、V、Cr等）、REE以及Zr之间呈现正相关变化特征;（4）δ¹³C_org大幅度波动,TOC含量较低;（5）REE+Y配分模式图与河/湖水相似。这些特征表明正目观组下层盖帽白云岩受到了陆源碎屑物质加入的影响,同时间接反映了冰期末期华北克拉通风化作用逐渐加强。另一方面,上层盖帽白云岩主要由超过95%的微晶白云岩组成,上层白云岩样品中地球化学数据显示:（1）Fe和Mn富集;（2）P,Cu,Pb,Zn,Ni,V富集;（3）δ¹³C_carb沿剖面向上逐渐降低而TOC逐渐增加;（4）Eu正异常;（5）Y/Ho比值增大;（6）REE+Y配分模式图与深部海水相似。这些特征表明正目观组上层盖帽白云岩受到了海底热液混染的深部海水上涌的影响。综上所述,贺兰山地区正目观组盖帽白云岩的地球化学数据表明埃迪卡拉纪中-晚期冰期以来华北克拉通的西缘经历了古环境和古气候的剧变,至于这一变化是否影响到整个华北克拉通以及是否具有全球的普遍性还需要进一步研究。类似于贺兰山正目观组这样未有精确年龄制约的新元古代地层在全球其他地区还有很多,对这类地层进行深入研究并通过古生物化石、碳同位素等手段将其与全球已定年的标准地层剖面进行对比,对于认识全球新元古代古环境和古气候演化具有重要意义。更多还原

【Abstract】 Neoproterozoic era was a tumultuous period of Earth history,which witnessed the formation and breakup of the supercontinent Rodinia and the subsequent amalgamation of Gondwana.Accompanied by the supercontinent dispersal and amalgamation,many aspects of Earth system were changed,such as the severe climate change,the rapid increase of oxygen,and the emergence of complex life.Recently,researches on the Neoproterozoic “snowball earth”,Rodinia supercontinent and cap carbonates have made the Neoproterozoic glacier event a new focus around the international geology realm.In China,the Neoproterozoic successions are dominated by carbonates and mostly distribute at the Tarim,North China and South China blocks.Previous works mainly concentrated in the Quruqtagh area of the Tarim Block and the Three Gorges area of the South China Block.Because of the limited outcropping of the Neoproterozoic strata in the North China Block(where only a suit of mountain glaciers and ocean glacier deposits on the southern margin of Sino-Korean Block),researches on the Neoproterozoic glacier event on the North China Block are limited.Although Neoproterozoic glacial deposits were previously described in the Helan Mountains,the overlying cap carbonate in this region remained poorly known.To better understand the Neoproterozoic glacial events in North China,we investigated five sections along a north-south transect across the Helan Mountains capturing a transition from shelf margin to intrashelf basin.This effort revealed an excellent exposure of Neoproterozoic diamictite and associated carbonate succession(Zhengmuguan Formation)on the western shelf margin of North China.We propose that the Zhengmuguan Formation along the western margin of the NCC,as well as the likely contemporaneous Luoquan Formation along the southern margin of the NCC,are glacial sequences partially influenced by reworking into sediment gravity flows.The comparatively low CIA values of the Zhengmuguan diamictite,as for other Neoproterozoic examples,may reflect a cold and arid depositional environment.In this paper,a ~1.6-m-thick carbonate succession is distinguished from the diamictite sequence on the basis of lithologic characteristics,including the carbonate’s very low siliciclastic/conglomerate content and the bedding properties.The carbonate succession is patchy in its distribution and limited to the southern areas of the central Helan Mountains.Attributes of our carbonate succession that are found widely in Neoproterozoic cap carbonates around the world include(1)dramatically contrasting lithologic changes,such as that from the underlying Zhengmuguan diamictite to the carbonate sequence;(2)minor siliciclastic contents,including the lack of conglomerate;(3)the stromatactis-like cavities;(4)thinly laminated carbonate with rhythmic laminae;and(5)the comparable negative δ13C values.The presence of the late Ediacaran fossils Helanoichnus and Shaanxilithes suggest that the Tuerkeng Formation is equivalent to the Dengying Formation of South China and the Zhoujieshan Formation in Northwest China.If correct,the depositional age of the underlying Zhengmuguan Formation can be constrained to the mid-late Ediacaran based mostly on the fossils and may be correlative to the Gaskiers event.Additional work is needed to confirm this age relationship.The cap carbonate consists of a lower thinly laminated dolomite and an upper thickbedded dolomite.Based on our petrographic and geochemical measurements,the lower carbonate contains detrital minerals and exhibits(1)moderately positive correlation between Zr and REE;(2)elevated concentrations of continental inputs(as recorded in Al,Ti,and Zr);(3)sympathetic variations between redox sensitive elements(U,V,and Cr),REE,and Zr;(4)relatively large variations in δ13Corg for low levels of TOC,which could reflect detrital controls;and(5)riverine-or lacustrine-style REE+Y patterns(Fig.14A).These observations collectively suggest that deposition of the lower cap carbonate was strongly influenced by terrestrial inputs derived from enhanced continental weathering of the NCC with only limited mixing with seawater.In contrast,the upper cap carbonate consists of over 95% microcrystalline dolomite and displays(1)paired enrichments in Fe and Mn;(2)enrichments in P,Cu,Pb,Zn,Ni,and V;(3)a systematic decrease in negative δ13Ccarb and increase in TOC;(4)positive Eu anomalies;(5)positive shifts in Y/Ho ratios;and(6)deep marine REE+Y patterns(Fig.14B).Viewed together,these data suggest that the upper cap carbonate was deposited in waters dominated by upwelling of deep seawater strongly influenced by hydrothermal fluids.Our petrographic and geochemical results suggest the Zhengmuguan cap carbonate was initially deposited via glacial meltwater with significant inputs from continental weathering subsequently overprinted by upwelling anoxic deep seawaters along a continental margin.In summary,our data from a poorly known mid-late Ediacaran cap carbonate capture a high-resolution record of shifting climatic and oceanographic conditions of at least regional importance.The uncertainties of these interpretations are high,but the potential importance of these findings justifies the need to publish the data and our conclusions—along with the unavoidable caveats that come with any poorly dated frontier section.The Zhengmuguan Formation in North China may provide evidence for a mid-late Ediacaran glaciation of unconstrained but potentially appreciable magnitude.更多还原