Radiolarian Age of Chert-hosted Bedded Manganese Deposits from the Gen-otani Mine in the Tamba District, Northern Kyoto City, Japan

Abstract. Numerous bedded manganese deposits sporadically distributed throughout the Tamba district, southwestern Japan are intercalated within chert sequence. It is well known that radiolarian remains are commonly included in both bedded manganese deposits and host cherts. The Gen‐otani mine, one of these deposits, is located at Otani, Keihoku‐Shimonaka, northern Kyoto City. Chemical composition and age of the chert sequence at the mine were examined. Mainly according to SiO₂ and MnO contents together with lithology, the chert sequence is divided into three sections; lower massive chert, middle bedded manganese deposit and upper bedded chert sections. Radiolarian faunas consisting of middle Jurassic species such as Eucyrtidiellum unumaense, Dictyomitrella(?) kamoensis, Parvicingula dhimenaensis, Sethocapsa aitai, Sethocapsa kodrai, Transhsuum brevicostatum, Tricolocapsa plicarum, Unuma echinatus and others were extracted from both the middle manganese section and overlying bedded chert of the upper section. This examination reveals that the bedded manganese deposit at the Gen‐otani mine formed until Bajocian to early Bathonian (middle Middle Jurassic) in age.     

On the Age of the Ophiolitic Complexes in Northeastern Jiangxi:A Micropaleontological Analysis

The age assignment of cherts from ophiolitic/metamorphic complexes in northeastern Jiangxi is widely regarded as one of the crucial issues in the tectonic interpretation of South China. The ophiolitic and metamorphic complexes in northeastern Jiangxi have been traditionally regarded as part of the Proterozoic “Banxi (=Penhsi) Group” of the Jiangnan Uplift. However, recent reports of Late Paleozoic radiolarians from the rock complexes have caused some researchers to question the traditional interpretation of the tectonic and paleogeographic framework in the region; but other workers are suspicious about these findings. In order to test the validity of the putative radiolarians, we, an interdisciplinary study group including micropaleontologists, tectonic specialists and regional geologists, conducted field investigations and multiple re-samplings of the localities where the reported fossils were collected. Our comprehensive study shows that the chert of the ophiolitic complex and the associated low-grade metamorphic slate yield Mesoproterozoic and Neoproterozoic acritarchs, confirming the traditional view of their age assignment (Proterozoic); on the other hand, no recognizable radiolarian fossils have been discovered therein, thus raising questions about the recently published result of Paleozoic radiolarian findings. Probable causes for the putative radiolarian findings are thus discussed herein. Geochemical characteristics indicate that the chert sedimentation most probably occurred under a continental margin setting.     

Correlation between the bedded chert sequence of southwest Japan and δ13C excursion of carbonate sequence, and its significance to the Permian-Triassic mass extinction

Abstract Stratigraphic productivity variations of radiolarians below the Permian-Triassic boundary are examined with Ishiga Diagrams in bedded chert sequences of southwest Japan. The diagrams of two different outcrops, drawn from the thickness variation of chert beds, show common stratigraphic variation, which indicates the diagram is a useful tool for correlation of bedded chert sequence. The common stratigraphic productivity variation is also well correlated to a compiled δ¹³C excursion of shallow carbonate sequences. Bedded chert records a dramatic extinction event in a shallow surface zone of oceans below the Permian-Triassic boundary. The Permian-Triassic mass extinction is divided into three intervals based on the Ishiga Diagrams, the stratigraphic lithological variation of bedded chert sequences, and the δ¹³C curve. The preceding extinction interval in the late Djulfian stage was not as serious an event and the biosphere soon recovered. The event of the main extinction interval commenced in the Dorashamian stage and caused a serious destruction of the biosphere. An event of the aftermath interval during the Early Triassic caused a delay in the recovery from the main extinction interval.     

Late Paleozoic radiolarians from the Bentong-Raub suture zone, Peninsular Malaysia

Abstract Radiolarians extracted from marine siliceous sediments from the Bentong-Raub suture zone, Peninsular Malaysia have indicated a range of ages for olistostromal blocks of bedded chert, siliceous argillite and tuffaceous argillite, and chert clasts and lenses within the mélange from the suture zone. Late Devonian (Faniennian), Early Carboniferous (Tournaisian and Viséan) and Early Permian (Wolfcampian and Leonardian) ages are represented by seven radiolarian zones from ten localities along the suture zone. In stratigraphic order these include Holoeciscus 2–3 Assemblage Zones, Albaillella paradoxa Zone, Albaillella dejendrei Zone, Albaillella cartalla Zone, Pseudoalbaillella lomentaria Zone, Albaillella sinuata Zone and Pseudoalbaillella longtanensis Zone. Fifteen genera are represented by 35 species. The range of ages from Late Devonian to Early Permian suggests that an ocean existed between the Sibumasu and East Malaya terranes from at least Late Devonian to late Early Permian time and that closure of the ocean between the two terranes could not have occurred until after late Early Permian time. The range of ages and rock types from different depositional environments, indicate that the Bentong-Raub suture zone includes a disrupted accretionary complex.     

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.     

一种特殊类型硅质岩——浮石状燧石

一种较为少见的特殊硅质岩产于南京及巢湖地区的下二叠统孤峰组地层中。它的显著特征是岩石中存在大量菱形和不规则状孔隙,孔隙周边和孔隙间主要由微晶石英组成。通过对该岩石的成分和结构等特征的研究,认为它是一种交代碳酸盐岩成因的硅质岩,并定名为浮石状燧石。     

黔北瓜德鲁普世—乐平世转换时期大规模成硅事件硅质来源厘定

联合古陆(Pangea)大陆边缘及泛大洋和古特提斯洋的广大地区在二叠纪时期出现大规模硅质富集事件, 被称之为“全球二叠纪硅质沉积事件”。黔北遵义地区二叠纪瓜德鲁普世—乐平世转换时期成硅事件为该事件重要组成部分, 呈现出“白泥塘层”层状炭硅质灰岩、层状硅质岩和硅质条带或燧石结核三种不同类型硅质富集岩石。遵义地区硅质来源的研究能够深入认识全球性“二叠纪硅质沉积事件”, 同时为研究该时期遵义盆地的结构和演化提供了重要窗口, 对遵义锰矿成矿作用也有重要指示意义。文中采用岩石学和地球化学方法并通过硅同位素的协同指示意义对硅质来源进行厘定, 并对硅质岩成因与沉积环境进行归纳总结, 在研究区建立系统完整的硅质富集沉积体系。实验测试结果显示: “白泥塘层”炭硅质灰岩中δ30Si值介于–0.09 ~ –0.67之间, 层状硅质岩中δ30Si值介于0.03~1.47之间。结合主、微量元素特征和岩石学特征表明: “白泥塘层”炭硅质灰岩形成于深水盆地缺氧还原环境, 硅质来源于海底热液, 沉积中心位于热液喷口的附近, 沉积过程中受陆源硅质输入影响显著。层状硅质岩形成于浅海台地正常沉积氧化环境, 硅质来源于海底热液, 沉积远离热液喷口, 热液沿构造裂隙通道经过了一定的迁移距离, 与茅口组浅海台地相正常沉积的块状-厚层状灰岩发生交代作用, 从而形成硅质岩; 硅质条带或结核形成于浅海台地正常沉积氧化环境, 硅质主要来源于海底热液, 沉积过程中有陆源硅质的输入, 由长时间多期次的海底热液喷流作用带来的硅质迁移至盆地外围浅海台地形成。     

西准噶尔构造带包古图地层小区上泥盆统红山梁组的建立及地质意义

西准噶尔构造带包古图地层小区白碱滩北红山梁剖面上新发现一套晚泥盆世红色“硅质岩、粉砂质硅质岩、凝灰质硅质岩”海相沉积组合,与上覆包古图组深灰色—灰黑色含碳粉砂岩呈整合接触。经区域对比确认,该套地层显著有别于研究区乃至整个西准噶尔构造带地层分区已发现的全部上泥盆统各组岩石地层,符合“野外可识别、界面可区别、区域可对比、图面可表达”的原则,据此新建红山梁组。本次于红山梁组枕状玄武岩、玄武岩、安山岩中分别获得365. 9±5. 0 Ma、370. 6±7. 4 Ma和374. 6±3. 6 Ma的LA- ICP- MS锆石U- Pb年龄,佐证其时代为晚泥盆世。红山梁组与包古图组连续沉积之确认,消除了包古图组老于希贝库拉斯组这一存疑,建立了完整的下石统地层序列,新增了包古图地层小区泥盆纪的沉积记录,为区域沉积与构造演化研究提供了新资料。     

东秦岭二郎坪群硅质岩热水沉积地球化学特征及其地质意义

二郎坪群硅质岩成因研究对二郎坪群的构造背景和铜多金属矿床成因的确定具有重要意义。通过对二郎坪群中三种硅质岩的地质特征和岩石地球化学分析，认为二郎坪群硅质岩是典型的热水沉积硅质岩。常量元素地球化学特征值（N(Al)/N（Al+Fe+Mn））指示该硅质岩的沉积环境存在东西差异，南阳盆地以东弧后盆地的规模较大（N(Al)/N（Al+Fe+Mn）=0.30~0.45），沉积环境类似远洋盆地，硅质岩的热液成分比例大，受陆缘物质影响小；而南阳盆地以西弧后盆地的规模较小（N(Al)/N（Al+Fe＋Mn）=0.59），沉积环境为近大陆的边缘海，硅质岩Al含量相对较高，受到陆缘物质影响相对大。地质特征和稀土元素特征（负Eu异常、弱负Ce异常）揭示了二郎坪群硅质岩是弧后盆地型低温热液流体和海水混合形成，这为二郎坪群形成于弧后盆地构造环境的认识提供了新的重要证据。热水沉积硅质岩与铜多金属矿床的共生关系证明研究区铜金属矿床的成因是海底热液喷流沉积作用。     

西藏南部硅质岩的特征及相关问题探讨

西藏南部广泛发育中生代古海洋沉积硅质岩,在空间上集中分布于雅鲁藏布江缝合带和喜马拉雅沉积带,呈东西向条带状分布,断断续续延伸达数千公里。可大致厘定出3条硅质岩带:缝合带蛇绿岩北侧早白垩世硅质岩带、缝合带蛇绿岩南侧三叠纪至白垩纪硅质岩带和喜马拉雅沉积带白垩纪硅质岩带,产出块状、层状、角砾状和结核状4种类型的硅质岩。在沉积建造特征上,缝合带蛇绿岩北侧硅质岩带以蛇绿岩-硅质岩建造为主;缝合带蛇绿岩南侧硅质岩带主要以火山岩-硅质岩建造和改造型硅质岩建造为主;喜马拉雅沉积带硅质岩则以浊积岩-硅质岩建造为特征。藏南硅质岩具有重要的研究意义,其薄弱环节和科学问题主要包括:硅质岩的成因问题、硅质岩的微组构问题、硅质岩中的古海洋沉积事件信息、硅质岩对沉积环境的指相意义等。