Geochemistry of Late Permian to Early Triassic pelagic cherts from southwest Japan: implications for an oceanic redox change

Major, trace, and rare earth element abundances were determined for the southwestern Japanese pelagic chert sequence from the early Late Permian to early Early Triassic to investigate a redox change in deep-sea pelagic environments before and at the Permo-Triassic boundary (PTB) (251 Ma). The sequence was primarily deposited in the deep-sea of the superocean Panthalassa, and then was accreted to Japan in the Middle Jurassic. A remarkable lithostratigraphic change from red chert to siliceous∼carbonaceous claystone through gray chert is observed in this sequence. Constituent elements for these sedimentary rocks are essentially derived from two sources: (1) an ancient seawater via biogenic (mainly radiolarian), hydrothermal, and authigenic materials and (2) an average shale-like terrigenous material. The present measurement demonstrates significant stratigraphic changes of the Ce/Ce_non-ter* value (estimated Ce anomaly value of non-terrigenous component, recalculated by subtraction of terrigenous REEs from bulk REEs) and the (Mn/TE)_sample/(Mn/TE)_PAAS value (excess Mn component other than terrigenous one; TE=terrigenous elements including Ti, Al, Nb, Hf, Th; PAAS=Post-Archean Average Australian Shale) in the Permian chert. The Ce/Ce_non-ter* values increase from <0.2 to 1 and the (Mn/TE)_sample/(Mn/TE)_PAAS values decrease up-section, suggesting that the redox condition of deep-sea open-ocean changed from oxic to suboxic in an interval of approximately 10 Myr. The (∑Fe/TE)_sample/(∑Fe/TE)_PAAS and (Mn/TE)_sample/(Mn/TE)_PAAS values of carbonaceous claystone near or at the PTB are less than unity, suggesting that reductive dissolution of iron and manganese occurred under an anoxic condition. This supports the idea of the PTB oceanic anoxia in the superocean Panthalassa. The present data suggest that the anoxic condition prevailed in the deep-sea pelagic regions for an extremely long period, much more than 10 Myr, from the middle Late Permian to early Early Triassic. This long-term development of widespread oceanic anoxia may have been linked to the greatest mass extinction of the Phanerozoic.     

Rare Earth, Major and Trace Elements in the Kunimiyama Ferromanganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

Abstract. Rare earth, major and trace element geochemistry is reported for the Kunimiyama stratiform ferromanganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The deposit immediately overlies greenstones of mid-ocean ridge basalt (MORB) origin and underlies red chert. The ferromanganese ores exhibit remarkable enrichments in Fe, Mn, P, V, Co, Ni, Zn, Y and rare earth elements (excepting Ce) relative to continental crustal abundance. These enriched elements/ Fe ratios and Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are generally analogous to those of modern hydrothermal ferromanganese plume fall-out precipitates deposited on MOR flanks. However in more detail, Mn and Ti enrichments in the ferromanganese ores are more striking than the modern counterpart, suggesting a significant contribution of hydrogenetic component in the Kunimiyama ores. Our results are consistent with the interpretation that the Kunimiyama ores were umber deposits that primarily formed by hydrothermal plume fall-out precipitation in the Panthalassa Ocean during the Early Permian and then accreted onto the proto-Japanese island arc during the Middle Jurassic. The presence of strong negative Ce anomaly in the Kunimiyama ores may indicate that the Early Permian Panthalassa seawater had a more striking negative Ce anomaly due to a more oxidizing oceanic condition than today.     

塔西北地区早寒武世玉尔吐斯组热液作用及沉积模式

埃迪卡拉纪-寒武纪之交全球沉积了一套黑色泥页岩和硅质岩组合,塔里木盆地早寒武世玉尔吐斯组即包含这样一套组合,并被认为是塔里木古生界主力烃源岩之一,然而目前对这套岩石组合的成因及沉积过程的研究还相对薄弱。在野外研究的基础上,将玉尔吐斯组分为5段,分为两个海进-海退旋回,认为其总体沉积于缓坡背景之上。对第一段硅质岩进行主量元素及稀土元素分析,结果显示其含有较低的Al/(Al+Fe+Mn)和较高的Fe/Ti值(均值分别为0.29和108.24)。在Al-Fe-Mn和Fe/Ti-Al(Al+Fe+Mn)图解上,多数样品落入热液作用范围,表明硅质岩主要受热液作用控制。硅质岩REE配分模式显示其不具有Eu的正异常和LREE富集的特征,这显示了海水对热液的稀释作用。靠近热液盆内口的库勒和什艾日克剖面发育较厚的硅质岩和一系列滑移变形构造。早寒武世时期,南天山洋已经打开,硅质热液广泛发育于整个塔北的深水和浅水区,这些迹象表明该区早寒武世仍然有很强的伸展构造活动。     

广西晚古生代硅岩的地球化学及其形成的大地构造环境

广西晚古生代泥盆、石炭纪和二叠纪地层中广泛发育放射虫硅岩，在桂西百色、那坡一带这些地层中的硅岩与玄武岩伴生。而钦州、柳州及河池一带未见到玄武岩。对这些地区硅岩的主元素、微量元素及稀土元素的研究表明：不同地区的硅岩有不同的成因，百色、那坡地区的硅岩形成在深水远洋环境并受到海底火山及热液作用的影响；钦州板城地区的硅岩以生物成因为主；而柳州、河池的硅岩属于离火山、热液活动稍远的生物成因。玄武岩的发现及硅岩成因的判别对探讨广西晚古生代大地构造环境及演化具有重要意义。     

安徽省巢湖地区二叠纪栖霞组、孤峰组硅质岩地球化学特征对比及大地构造背景分析

为探讨下扬子地区二叠系硅质岩的硅质来源和形成背景,以巢湖地区二叠纪栖霞组和孤峰组硅质岩为研究对象,收集了前人大量有关栖霞组和孤峰组的研究成果和数据,并对采自栖霞组和孤峰组硅质岩样品进行了主量元素、微量元素、稀土元素测试。栖霞组、孤峰组硅质岩样品测试结果显示,硅质岩的稀土元素经北美页岩标准化后的配分曲线近似平坦型向左倾斜,有弱Ce负异常,且HREE弱富集,反映了热水沉积硅质岩的特征。研究区硅质岩样品的Al Fe Mn三角图中,样品投点分布较广,即Al比值变化范围较大,认为是岩浆在喷发过程中受硅-铝质陆壳混染作用的结果。结合沉积环境判别指标分析认为：栖霞组硅质岩沉积时期其沉积环境主要为大陆边缘环境;孤峰组硅质岩沉积时期其沉积环境为大陆边缘-海盆过渡环境。从栖霞组到孤峰组沉积时期,海侵规模不断变大,水体变深,海盆面积扩大,研究区在不同时期沉积环境存在差异。研究区栖霞组、孤峰组硅质岩的沉积与大陆边缘裂陷活动紧密相关。     

湖北竹山杨家堡组硅质岩成因及沉积环境分析

竹山地区位于扬子陆块北缘,寒武纪早期发育一套深水硅质岩系。该岩系由纯硅质岩夹少量白云岩、页岩所组成。硅质岩中微晶石英含量均超过85%,还含有少量碳质、燧石及微量绢云母。地球化学分析表明,该硅质岩的SiO₂含量较高,平均为93.4%;Al₂O₃和TiO₂含量较低,分别为0.19%~1.45%和0.01%~0.09%,TFe₂O₃为0.053%~0.342%,Al/(Al+Fe+Mn)比值为0.58~0.83,平均为0.71,Eu/Eu*为0.74~3.61,平均为1.65,表明其硅质主要来源于生物本身或其生命活动产物,海底热液活动亦提供了一部硅质物质;同时,硅质岩具有较低的TFe₂O₃/TiO₂比值(2.16~12.56,平均值为5.86)和Al₂O₃/(Al₂O₃+Fe₂O₃)比值(0.65~0.87,平均值为0.78),Y的正异常(Y/Ho值为34.81~60.19,平均为47.0)及Ce的负异常(Ce/Ce*值为0.28~0.92),表明它们形成于远离陆源物质干扰的边缘海盆环境,且Ce负异常从底部到顶部有逐渐减小的趋势,记录了寒武纪时期扬子北缘被动陆缘裂谷盆地不断扩大,水体逐渐变深的过程。     

塔里木下寒武统富有机质沉积层段地球化学特征及意义

塔里木盆地寒武系下统玉尔吐斯组底部富有机质沉积层段分布广泛,层位稳定,其中的硅质岩发育,并伴有磷矿产出.硅质岩的Al(Al Fe Mn)和Si/(Si Al Fe)比值分别在0.0023～0.0046和0.965～0.98之间,表明其形成于海底热水沉积环境,远离陆源区.富有机质沉积层段明显富集As、Hg、Pb、Zn、Cu、Co、P、V、Ba等微量元素,富集系数F(N)远大于1.Ba/Sr比值远远大于1,与现代海底热水沉积物中的Ba/Sr比值相似,具有明显的海底热水沉积特征;Th/U和V/Sc比值显示其形成与海洋缺氧事件有关,而导致缺氧事件发生的主要原因则是海底火山作用及其与之相伴的海底热水流体活动.     

Geochemical Indication of Sinian Bedded Siliceous Rocks in the Hunan-Guizhou-Guangxi Area and Their Environmental Significance

In the Hunan-Guizhou-Guangxi area there have developed very thick bedded siliceous rocks of the late Sinian. The rocks have a fairly pure composition, with an average content of siliceous minerals exceeding 95%. They are relatively rich in Fe and Mn, and poor in Al, Ti and Mg. The Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th ratios and the Al-Fe-Mn and Fe-Mn-(Ni+Co+Cu)×10 triangle diagrams all show that they are hydrothermal sedimentary siliceous rocks. In the rocks the total amount of REEs is low, the δCe shows an obvious negative anomaly and the 8Eu a weak anomaly, and LREE>HREE, all indicating that they are products of hydrothermal processes. The δ30Si and δ18O values, as well as the formation temperature of the rocks all clearly show that the silica forming the rocks comes from hot water. Besides, analyses of the depositional environment of the rocks using the MnO/TiO2 ratio and the δCe and δ30Si values yield the same conclusion that they are formed in environments from continental marginal slope     

Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai–Khentei belt of Central Mongolia: Evidence for Middle Paleozoic subduction–accretion activity in the Central Asian Orogenic Belt

Recent mapping projects undertaken in Central Mongolia have revealed the widespread occurrence of radiolarian chert within a Paleozoic accretionary complex. We present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the Gorkhi Formation in the Khangai–Khentei belt of the Central Asian Orogenic Belt.The Gorkhi Formation consists of sandstone shale, alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale, basalt, limestone, and clast-bearing mudstone. Radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt (sedimentary contact) and with conformably overlying siliceous shale and turbidite deposits. The tectonic stacking of basalt–chert and chert–turbidite successions is the most remarkable structural feature of the formation.The recovery of moderately well-preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest Silurian (Pridolian) to the Late Devonian (Frasnian). No age control exists for the siliceous shale, shale, and sandstone, although they are considered to be latest Devonian or slightly younger on the basis of stratigraphic relationships with underlying chert.The Gorkhi Formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed Archean–Neoproterozoic basement; however, the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate. From the lowermost to uppermost units, the stratigraphy comprises ocean floor basalt, pelagic deep-water radiolarian chert, hemipelagic siliceous shale, and terrigenous turbidite deposits. The biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep-water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years. These data, together with structural data characterized by tectonic repetition of the stratigraphy, indicate that these rocks formed as an accretionary wedge along an active continental margin, possibly that of the Angara Craton. The mid-oceanic chert was probably deposited in the Northern Hemisphere portion of the Paleo–Pacific Ocean that faced the Angara Craton and the North China–Tarim blocks. Thus, we propose that subduction–accretion processes along the Paleo–Pacific rim played an important role in the accretionary growth of the active continental margin of the Angara Craton, directly influencing the evolution of the Central Asian Orogenic Belt.     

鄂西地区上二叠乐平统大隆组硅质岩成因及有机质富集机理

晚二叠世是古生代环境、生物演化的关键时期,发育广泛的富有机质沉积,并成为南方重要的烃源岩层位。为了解该时期的富有机质堆积机理,我们选择了鄂西台内盆地晚二叠世大隆组富有机质硅质岩(TOC平均值为5.8%,峰值为18%)作为研究对象。硅质岩中常量元素Al/Fe均值为0.79,Mn/Ti均值为0.21,显示为大陆边缘环境下的生物成因,即非热水成因。∑REE值与Al2O3含量有着良好的正相关关系,表明当时陆源物质的输入对硅质岩的形成有很大影响。微量元素U/Th、V/(V+Ni)均说明硅质岩沉积环境为缺氧水体环境。通过对草莓状黄铁矿粒径的统计表明,大多数(64%～89%)的草莓状黄铁矿粒径小于5μm,反映缺氧甚至硫化的环境。而且,U、V、Mo的富集以及Ni、Cu元素的亏损说明水体的缺氧主要是由于海水分层,水体循环受阻造成的。TOC与氧化还原参数的变化步调基本一致,两者有着较好的正相关关系,而与生产力指标Ba/Al相关性很差,说明有机质富集主要是由水体的缺氧而不是初级生产力控制的。     

泰国北部难河构造带二叠纪放射虫、硅质岩和玄武岩

在泰国北部难河构造带Pha Som变质杂岩中发现保存很好的放射虫硅质岩、玄武岩地层层序.层状硅质岩含放射虫化石Follicucullus porrectus, 地质时代为中二叠世晚期至晚二叠世早期.其硅质岩SiO₂含量均在92.5%以上, Al/ (Al+Fe+Mn) 平均比值为0.51, Ce/Ce*比值为1.14, 为大陆边缘型硅质岩.玄武岩具有富集大离子亲石元素与高场强元素以及轻稀土富集等洋岛玄武岩的特点.说明难河构造带中-晚二叠世之交存在洋岛型火山岩和靠近大陆边缘的深海盆地硅质岩, 代表了小洋盆的沉积组合.该构造带闭合时间应在晚二叠世与晚三叠世之间.      

鄂西南利川二叠纪吴家坪组硅质岩成因及沉积环境

鄂西南利川吴家坪组硅质岩与海相石煤及多层火山粘土岩共生,产海绵骨针及放射虫化石,放射虫组合以泡沫虫类为主,内射球虫类次之,隐管虫类很少,代表外陆棚沉积环境。硅质岩的陆源元素Al、Ti含量较高,热水来源元素Fe、Mn含量较低,Al/(Al+Fe+Mn)值在0.64~0.83之间,平均值0.73。在Al-Fe-Mn三角图中,样品位于或紧邻非热水沉积区,在 SiO₂- (K₂O+Na₂O)、SiO₂-Al₂O₃, (K₂O+Na₂O)-Al₂O₃ 图解中大多数样品落在火山成因区,少数位于火山成因与生物成因区之间。相对于沉积岩的微量元素丰度,生命元素Mo、V富集,指示热水沉积的特征元素Ba、As、Sc亏损。∑REE较低,∑LREE/∑HREE平均值为3.02,Ce、Eu均轻度负异常（δCe:0.75~0.83,平均值0.79;δEu:0.61~0.95, 平均值0.84）,经北美页岩标准化后稀土元素配分曲线近水平。以上地球化学特征表明研究区硅质岩属火山成因,并伴有生物作用。氧化还原判别指标U/Th、V/(V+Ni)及Ce_anom,沉积环境判别指标MnO/TiO₂,La_N/Yb_N 与判别图解100×TFe₂O₃/SiO₂-100 Al₂O₃/SiO₂、TFe₂O₃/(100-SiO₂)-Al₂O₃/(100-SiO₂)、TFe₂O₃/TiO₂-Al₂O₃/(Al₂O₃+TFe₂O₃)、La_N/Ce_N-Al₂O₃/(Al₂O₃+TFe₂O₃)与∑LREE/∑HREE-100×Eu/∑REE一致表明硅质岩形成于大陆边缘缺氧环境。     

甘肃北山地区红山铁矿硅质岩的地球化学特征及沉积环境

通过野外地质调查和系统的岩石主量元素、微量元素、稀土元素等的研究,探讨红山铁矿床硅质岩的地球化学特征及其沉积环境。红山铁矿床中广泛发育一套与铁矿体呈共生关系的硅质岩。研究表明,与铁矿密切共生的呈层状和透镜状产出的灰色、紫红色硅质岩总体属于低硅类型的硅质岩。在SiO2-Al2O3判别图解中,SiO2与Al2O3负相关,样品数据点均落入热水沉积区域;稀土元素配分模式经北美页岩标准化后呈平缓型,与东太平洋热泉水、红海热卤水具有相似的正Eu异常和配分模式,δEu值均大于1,显示热水沉积的特征;硅质岩中的V、Ni、Cu、Ba等微量元素的含量具有陆缘海盆热水沉积的特征。     

甘肃北山地区红山铁矿硅质岩的地球化学特征及沉积环境

通过野外地质调查和系统的岩石主量元素、微量元素、稀土元素等的研究,探讨红山铁矿床硅质岩的地球化学特征及其沉积环境。红山铁矿床中广泛发育一套与铁矿体呈共生关系的硅质岩。研究表明,与铁矿密切共生的呈层状和透镜状产出的灰色、紫红色硅质岩总体属于低硅类型的硅质岩。在SiO2-Al2O3判别图解中,SiO2与Al2O3负相关,样品数据点均落入热水沉积区域;稀土元素配分模式经北美页岩标准化后呈平缓型,与东太平洋热泉水、红海热卤水具有相似的正Eu异常和配分模式,δEu值均大于1,显示热水沉积的特征;硅质岩中的V、Ni、Cu、Ba等微量元素的含量具有陆缘海盆热水沉积的特征。     

桂西南柳桥地区上二叠统大隆组层状硅质岩成因和沉积环境

对桂西南上二叠统大隆组层状硅质岩地球化学的研究发现:陆源主量元素Al、Ti等含量较高,并呈很好的相关性（R>0.90）;热液来源的主量元素Mn和Fe等含量偏低,并具有较小的负相关关系（R=－0.30）;陆源元素(Al、Ti、Hf、Zr、Th等)与总稀土元素含量具有较高正相关性（R为0.70～0.83）;Al Fe Mn三角图解指示研究区的硅质岩为非热液成因。这些说明陆源物质是硅质岩形成的重要物源,结合硅质岩中含有大量的硅质生物（放射虫和海绵骨针等）的事实,我们认为研究区硅质岩是在生物作用为主,并有大量物源物质和少量热液物质（可能与大断裂导致的玄武岩喷发有关）和火山物质混入的条件下形成的。Ce/Ce*、(La/Yb)Shale、(La/Ce)Shale和∑REE与细粒沉积物沉积环境的关系以及(La/Ce)Shale—Al2O3/(Al2O3 +Fe2O3)图解等说明研究区硅质岩沉积环境为大陆边缘的中下部。Th/U和Ceanom指示了硅质岩形成于氧化环境。      

Geological and Geochemical Investigation of Three Ophiolite‐Hosted Manganese Prospects,Southeast of Birjand,South Khorasan,East of Iran

The studied ophiolite‐hosted manganese prospects are located in southeast of Birjand, South Khorasan, in the east of Iran. The manganese ores within the ophiolitic sequence in this region occur as small discrete patches, associated with radiolarian chert and shale. Manganese ores in the host rocks are recognizable as three distinct syngenetic, diagenetic, and epigenetic features. The syngenetic manganese ores occurred as bands associated with light‐red radiolarian chert. The diagenetic Mn ores occurred as lenses accompanied by dark‐red to brown radiolarian chert. The epigenetic Mn ores occurred as veins/veinlets within the green radiolarian shale. The major manganese ore minerals are pyrolusite, braunite, bixbyite, ramsdellite, and romanechite showing replacement, colloidal, and brecciated textures. The high mean values of Mn/Fe (15.32) and Si/Al (15.65), and the low mean concentration values of trace elements, such as Cu (85.9 ppm), Ni (249.9 ppm), and Zn (149 ppm), as well as the high concentration values of Si, Fe, Mn, Ba, Zn, Sr, and As in the studied manganese ores furnished sufficient evidence to postulate that the sea‐floor Mn‐rich hydrothermal exhalatives were chiefly responsible for the ore formation, and the hydrogenous processes had negligible role in generation of the ores. The further geological and geochemical evidence also revealed that the ores deposited on the upper parts of the ophiolitic sequence by submarine exhalatives. The intense hydrothermal activities caused leaching of elements such as Mn, Fe, Si, Ba, As and Sr from the basaltic lavas (spilites). After debouching of the sea‐floor exhalatives, these elements entered the sedimentary basin. The redox conditions were responsible for separation of Fe from Mn.     

内蒙古阿拉善北部杭乌拉地区早古生代硅质岩地球化学特征及其构造意义

内蒙古阿拉善北部杭乌拉地区位于中亚造山带的中段南缘,处于西伯利亚板块、塔里木板块和华北板块之间,是研究中亚造山带和古亚洲洋演化的重要区域。文中以杭乌拉地区硅质岩地球化学特征为主要研究内容,结合前人研究结果,对阿拉善北部地区早古生代构造背景进行分析。杭乌拉地区下古生界西双鹰山组和班定陶勒盖组硅质岩镜下可见黏土、陆源粉砂和放射虫等,表现为明显的沉积成因特征。硅质岩地球化学特征一致性较差,其中Fe、Mn、Al等主量元素、Sc/Th值、(La/Ce)_n值和δCe值反映出硅质岩为大陆边缘沉积,(La/Yb)_n值反映硅质岩接近于远洋沉积背景,δEu 值表现为无热液作用影响,V、V/Y、Ti/V值多接近于洋中脊和大洋盆地硅质岩特征,U/Th、Ba/Sr值也属于热水成因硅质岩特征。硅质岩稀土元素配分曲线呈平坦状,无明显的右倾(大陆边缘轻稀土元素富集配分模式)和左倾(开放洋盆重稀土元素富集配分模式)特征。研究区早古生代地质记录相对较少,但是鉴于该地区晚古生代复杂的构造演化特征及其对早古生代古地理格局的继承,并结合上述硅质岩地球化学特征,认为研究区在早古生代为一个多岛洋环境。     

广西南宁地区泥盆系硅质岩地球化学特征及沉积环境

泥盆纪时,广西南宁附近发育了一套从早泥盆世埃姆斯期到晚泥盆世弗拉斯期末的硅质岩建造,硅质岩多为薄层,灰黑到黑色,至弗拉斯阶上部变为灰黄色,水平纹理发育,常夹火山凝灰岩或与之互层。这套硅质岩SiO2含量高（一般>90％）,其他主量化学成分均很低（一般<5%）。TFe2O3和含量相对比较富集（平均含量3.32％）, Al2O3、TiO2、MgO和MnO相对贫乏（平均含量分别为0.71％、0.04％、0.10％和0.02％）。常量元素Fe/Ti、（Fe+Mn）/Ti和Al/（Al+Fe+Mn）比值的平均值分别为91.65,92.12和0.25,微量元素Ti/V和U/Th比值的平均值分别为4.57和4.09,其中U/Th比值在弗拉斯期早期达到最大。这些数据说明,硅质岩在形成时受到了明显的海底热液活动影响,而这种热液活动在弗拉斯期早期达到最强,后逐渐减弱,而热液活动的强度可能主要受盆地裂解（或张裂作用）的影响;因此,盆地拉张作用在弗拉斯期早期达到最强,这与硅质岩的空间分布是吻合的。     

浙黔桂地区寒武纪硅质岩的地球化学特征及其形成背景

本文采用常量和稀土元素地球化学方法对浙黔桂地区寒武纪硅质岩的形成背景进行了研究。研究表明,硅质岩的A12O3/（A12O3＋Fe2O3）为0.56~0.87;Ce/Ce＊为0.74±0.19;Lan/Cen为0.90~1.58;Lan/Ybn平均为6.38,轻稀土明显富集,这些特征表明研究区硅质岩形成于大陆边缘环境或大陆边缘与深海过渡带。形成于裂陷盆地初期的硅质岩,比古华南残留洋盆和扬子陆块边缘的硅质岩轻稀土元素更加富集。构造拉张期形成的硅质岩为热液成因硅质岩,其稀土配分模式显示出轻稀土相对亏损、Eu正异常特征;构造稳定期形成的硅质岩为沉积成因硅质岩,稀土配分模式显示出轻稀土相对富集、Eu弱负异常特征。由硅质岩稀土配分模式的差异性反映出寒武纪时期的拉张为幕式拉张。位于扬子地台边缘的硅质岩所具有的Ce负异常,暗示着江南造山带寒武纪后曾发生过由东向西的大规模逆冲推覆。     

滇东北会泽灯影组硅质岩成因及沉积环境——来自岩石学和地球化学证据

通过1：50 000区域地质调查,发现云南省会泽县一带灯影组中普遍发育硅质岩。本文通过硅质岩岩石学特征和地球化学特征分析,重点探讨了灯影组各段硅质岩的成因机制和沉积环境。结果表明：震旦系-寒武系灯影组下段硅质岩中相对富集Al和Ti,总稀土（ΣREE）相对较高,Ce呈负异常,Eu呈正异常;中段硅质岩中相对富集Ca和Mg,总稀土较低,LREE>HREE,Ce呈负异常,Eu呈正异常;上段硅质岩总稀土较低,LREE>HREE,Ce大部分呈负异常,Eu呈正负异常。综合硅质岩岩石学、地球化学特征,并结合震旦纪区域地质构造演化,认为研究区内下段硅质岩表现为正常海水沉积、中段硅质岩表现为典型热水沉积、上段表现出正常海水沉积受热水沉积影响的特征。硅质岩形成于复杂大陆边缘的浅海陆棚环境。