Off-scraped Permian-Jurassic bedded chert thrust on Jurassic-early Cretaceous accretionary prism: Radiolarian evidence from le Island, central Ryukyu Island Arc

Abstract The pre-Neogene basement of the central Ryukyu Island Arc shows zonal structures analogous to those of the outer belt of southwest Japan. The innermost terrane (Iheya Zone) consists of isoclinally folded beds dipping northwestward; the anticlinal cores are composed mainly of Permian chert, whereas the synclinal parts are represented by Jurassic to Cretaceous sandstone-rich alternating siliceous shale and chert, bearing appropriate radiolarian fossils. At the east-central area of Ie Island, the basement rocks are exposed as a 172 m high peak, Tattyu. The flank area of Tattyu is composed of latest Jurassic to Berriasian siliceous shale and chert as part of an accretionary prism, while most of Tattyu is composed of a continuous and very compact sequence of Norian through Kimmeridgian (?) bedded chert which is rather gently inclined. Beyond an unexposed part below the Norian chert, Guadalupian chert is recognized. It is inferred that this pelagic chert (Tattyu sequence) was off-scraped and thrust on to the accretionary prism which developed on its flank area in an accretion process after the Early Cretaceous.     

A new exhalite type—siderite-sericite chert formed in fault-controlled lacustrine basin

On the basis of geological, fabric, lithographical and mineralogical study, a new exhalite type, called ’siderite-sericite chert’ according to its mineral assemblage, was reported for the first time. It was formed in a fault-controlled lacustrine basin and closely associated with sulfide ore characteristic of complex metal assemblage of Sn-Ag-Cu-Pb-Zn. Its geological setting, mineral assemblage, as well as associated ores are quite unique, and therefore worthy of detailed research. In the Dajing deposit, Linxi County, Inner Mongolia, it is this very exhalite that is regarded as a kind of acidic subvolcanic dike till now, and because of this, a genetic model of hydrothermal epigenetic filling by subvolcanic fluid prevails over the Dajing deposit. It is demonstrated again that any geochemical study might be senseless without support of detailed geological and lithographi-cal investigations. This is of particular importance at the present time when one is readily confused by various modern techniques.     

皖南浙西晚震旦世至早寒武世硅岩物源与环境类型初探

通过野外与室内的研究工作 ,认为皖南浙西地区晚震旦纪至早寒武纪的硅岩是复合成因的产物 ,其物源类型比较复杂 ,包括热水来源、生物来源及陆源输入 ,但以热水来源的SiO2 为主。产出的环境属于近洋脊与大陆边缘环境之间的过渡带。岩石学及地球化学的研究都证明了上述观点。     

滇西南耿马地区弄巴剖面早石炭世硅质岩的地球化学特征及古地理意义

滇西南耿马地区弄巴剖面由不同时代、不同岩性的地层断片组成，其中早石炭世2个断片由玄武岩和灰白色、紫红色放射虫硅质岩、硅质泥岩、凝灰岩组成。对硅质岩进行地球化学研究后发现，该地区硅质岩SiO2含量均在80％以上，MnO／TiO2的平均值为0．58，Al/(Al Fe Mn)的平均值约为0．48，Ce／Ce*的平均值为0．91，与已知大地构造背景的硅质岩地球化学特征对比，表明其为近大陆边缘的洋盆边缘型硅质岩。     

吉林省中部红帘石硅质岩的特征及意义

吉林省中部磐石县境内烟筒山地区的红帘石硅质岩存在于古生代地层之中。地球化学研究发现，该硅质岩具有较低的SiO2(80%左右)和较高的Al2O3(10％左右)。微量元素中V的含量也较低，稀土元素中不出现明显的Ce负异常，表明其形成于与陆源物质关系密切的大陆边缘环境。岩石的强烈变形和无层序特点暗示该区的古生代地层可能大多为构造混杂岩，不具备传统的地层划分和对比意义。矿物温压计算结果显示，该硅质岩经历了约430℃、大于6．OGPa的中压相系变质作用。不存在其他高压变质作用矿物，可能反映了本区为兴蒙造山带中古亚洲洋最后闭合的地点所在，并推测其最后闭合的时间应在古生代末一中生代初期。     

Reexamination of Radiolarian Biostratigraphy in Permian in Pelagic Chert Sequences at Dachongling Section, South China

Permian radiolarian biostratigraphy was reexamined in the bedded chert section in Qinzhou area of southeast Guangxi Zhuang Autonomous Region, South China. On the basis of the analyses of characteristic radiolarian composition, six radiolarian zones are recognized, namely, Pseudoalbaillella longtanensis Zone, Pseudoalbaillella globosa Zone, Follicucullus monacanthus Zone, Follicucullus scholasticus Zone, Follicucullus charveti Zone and Neoalbaillella ornithoformis Zone, in ascending order. Correlation of these radiolarian zones with those in Japan is presented. The Neoalbaillella ornithoformis Zone was firstly discovered in Dachongling Section, which may indicate that pelagic chert of the late Permian existed there, according to the co-occurring conodonts.     

陕西渭北奥陶系放射虫硅质岩与火山凝灰岩的成因环境

通过对层状放射虫硅质岩的岩石学和火山凝灰岩岩石类型的特征研究，认为渭北奥陶系的放射虫硅质岩是由远源的火山喷发物提供了ＳｉＯ２的来源，造成有利于硅质生物大量繁殖的条件，死后堆积海底，最终转变为层状放射虫硅质岩。从而进一步推断，本区中、上奥陶统的沉积形成于海平面大规模上升的远洋或半远洋的深水环境。火山凝灰岩则是岛弧火山喷发的产物，反映本区曾一度是华北板块活动大陆边缘弧后盆地北部边缘部分。     

西秦岭太阳顶群硅质岩的岩石学及地球化学特征

本文详细论述西秦岭太阳顶群(寒武—奥陶系)硅质岩的岩石学、矿物学和化学成分特征,重点讨论球粒硅质岩的成因。通过微量元素、稀土元素及硅同位素分析,查明硅质具有深部来源的特征,并指出喷溢到浅水盆地底部的海底热水的硅化作用是其主要的形成机制。     

Depositional Environments of Bedded Chens in Western Yunnan Segment of Paleo－Tethys， China： a Geochemical Approach

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Petrologic and Geochemical Characteristics and Origin of Gusui Cherts,Guangdong Province,China

The characteristic structures of the Precambrian cherts from the Gusui section, Guangdong ,Chi-na, include bedded structure ,laminated structure ,massive structure and pseudobrecciated structure.The chert is characterized by consistently low abundance of TiO2,Al2O3 and most trace elements.Howevver ,it is enriched in Ba,As,Sb,Hg and Se.In Al-Fe-Mn ternary diagrams,it falls into the “hydrothermal field“ .Correspondence analysis and factor analysis show that many elements show up in the factor that represents the leaching of country rocks by hydrothermal solutions,and are the very characteristic element association fo the geochemically anomalous South China basement.Petrologic and geochemical evidence suggests a hydrothermal origin for the chert.The chert may have been formed in a Precambrian fift or an extension zone developed within the Yunkai marginal geosyncline, with a fault system linking it to an unknown heat source at depth.