Displaced dolomites in radiolarian cherts of the Chichibu Belt on Shikoku Island,Southwest Japan

The northern tract of the Chichibu Belt on Shikoku Island, Southwest Japan, in places contains dolomites of Late Carboniferous age displaced into radiolarian cherts. The sections here examined are along the Niyodo gorge, central Shikoku. The sequence begins with thinly interbedded dolomitized radiolarian cherts and dolomites with a small amount of dolomitized radiolarian claystone and calcisiltite beds. These rocks, 5–10 m thick, are succeeded by a thick section of bedded and massive dolomites, commonly 40–50 m thick, which have thin intercalations of radiolarian claystone in the upper part. This dolomite sequence is depositionally overlain by a sequence, up to 50 m thick, comprising thinly interbedded radiolarian cherts and claystones, which, in turn, contain lenticular bodies of dolomite.Thin-section examination reveals that most of dolomites of the area have an arenitic or lithic texture, and should be termed doloarenite and dololithite. This means that dolomites are detrital. All lines of evidence suggest that the dolomites were not formed in the same depositional site in which the radiolarian cherts were being accumulated, but formed instead as allochthonous bodies which were displaced into a deep oceanic basin of chert deposition.The following sequence of events is postulated: (1) deposition of shallow-water calcareous sediments in a subtidal area; (2) dolomitization in a very shallow-water to supratidal environment; (3) displacement of dolomitized sediments, possibly mainly as debris flows into a deep-water, truely pelagic realm, in which siliceous radiolarian sediments were accumulating; (4) continued accumulation of siliceous sediments after the major influx of dolomitized sediments; and (5) minor influxes of dolomitized sediments during the continuous accumulation of siliceous sediments.     

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。     

赣东北蛇绿混杂岩带和变质岩系中"放射虫硅质岩"的再研究

赣东北蛇绿混杂岩和变质岩中的硅质岩时代问题已成为研究华南区域构造和古地理的焦点之一，长期以来被地质学界认为属于江南古陆元古代“板溪群”的范畴。近年来，关于该构造岩系中存在晚古生代放射虫动物群的报道，导致一些学者怀疑该地区传统的构造古地理格局需要重新解释。对此，许多地学工作者提出异议。为了验证上述放射虫动物群的报道的准确性，从古生物学、构造地质学和区域地质调查等多领域进行了野外调查和样品的多次重复系统采集；同时，对已发表的资料进行了再分析。研究显示，赣东北蛇绿混杂岩和变质岩中的硅质岩和扳岩的岩石薄片和微体古生物分析样品中未产出放射虫化石，但是在部分硅质岩和板岩样品中却发现了中-新元古代的疑源类化石。有关硅质岩的主量元素、稀上元素中Ce异常值、（La/Ce）x比值和微量元素等地球化学特征显示其沉积作用与陆源物质有关，沉积环境接近大陆边缘、远离深海远洋环境。     

OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS

OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS     

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro- and cryptocrystalline quartz, with local chalcedonic quartz (both length-fast and length-slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro- and cryptocrystalline quartz, length-slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg-rich environment. Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica-rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.     

新疆东昆仑木孜塔格蛇绿混杂岩发现早石炭世放射虫

在新疆木孜塔格蛇绿混杂岩的燧石中发现了早石炭世杜内晚期的放射虫化石,首次为这套蛇绿混杂岩提供了时代依据.结合在邻区的祁漫塔格和可可西里发现的放射虫,认为青藏高原北缘在早石炭世期间曾沿多处裂解.它们之间可能有狭窄的通道相连,是否构成统一的大洋尚待进一步研究.     

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.     

Silica diagenesis of Neogene diatomaceous and volcaniclastic sediments in northern Japan*

Marine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal-CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal-CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K. In subsurface sections, values of d(101) spacing of opal-CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal-CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal-CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal-CT; one is progressively ordered opal-CT and the other is additionally cemented opal-CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.     

西藏南部白垩纪深海沉积地层——冲堆组及其地质意义

藏南日喀则地区,有一套深海沉积地层断续出露于吉定—白朗一带。在西段,它基本上位于蛇绿岩北侧。在东段则由于构造原因而夹在蛇绿岩中间(图1)。     

Tectonic,diapiric and sedimentary chaotic rocks of the Rakhine coast,western Myanmar

The western margin of Myanmar is the northern extension the active Sunda (India-Eurasia) subduction zone. Coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave-cut terraces that allow characteristics of tectonic, sedimentary and diapiric mélanges to be recognized. Tectonic shear zones (tectonic mélanges) contain fragments of Cretaceous ophiolites (chrome-spinel-bearing peridotites and radiolarian cherts) that are in contact with thrust packets of Eocene turbidite units (broken formations). The turbidites contain shale-rich beds that have been sheared during soft-sediment deformation (sedimentary broken formations) and are sandwiched between undeformed thick sandy beds. These are mass transport deposits (MTDs) that most likely formed during deposition of the initial detritus of the Himalayan orogenic zone, probably trench slope basins on the accretionary prism. The ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface, forming diapiric mélanges. These diapiric mélange bodies contain only small fragments (<50 cm) that are randomly oriented and do not exhibit shear fabrics. Because the region lacks superimposed deformation characteristic of most orogenic belts, the origins of all three rock bodies can easily be distinguished.     

An attempt to quantitatively reconstruct the paleo-primary productivity by counting the radiolarian fossils in cherts from the latest Permian Dalong Formation in southwestern China

A quantitative reconstruction method of paleo-primary productivity is present on the basis of the investigation on the faunal record in cherts. By obtaining the accumulation rate of radiolarian shells in the cherts and considering all aspects of silica loss before burial, such as dissolution in the eutrophic zone or within the sediments, it is possible to reconstruct the production rate of radiolarian in the surface water, which would be used to estimate the paleo-primary productivity. This method is applied to the end Permian chert sequence from Dalong Formation in Dongpan area, southwestern China. The resulting paleo-primary productivity of the ocean in end Permian south China is about 477.9 g C/m²/a during the sedimentation of cherts, which, to some extent, agrees with the primary productivity reported for some modern oceans.     

The origin and diagenesis of cherts from Cyprus

The Troodos Massif of Cyprus is overlain by a variety of cherts in pelagic chalks, volcanogenic sediments, radiolarites and radiolarian mudstones, all of Campanian to Upper Eocene age. There are two chert types, granular chert and vitreous chert. X-ray diffraction (XRD) reveals the silica polymorphs, disordered cristobalite and quartz. Silicification of the chalks varies from incipient, to bedded, granular cherts, all with disordered cristobalite as the main silica phase. Quartzitic cherts are restricted to the base of Upper Palaeocene and Lower Eocene calciturbidite beds. Disordered cristobalite predominates in the radiolarian mudstones at the foot of the sequence. The form of disordered cristobalite in cavities ranges from microspherules of radiating bladed crystals, the ‘lepispheres’ of the Deep Sea Drilling Project (DSDP) to bladed overgrowths, and fibrous silica. In contrast, within the fine grained matrix, the disordered cristobalite takes the form of partly coalescent crude microgranules and microspherules. Most of the chalcedonic quartz in Cyprus is derived by recrystallization of previously inorganically precipitated disordered cristobalite rather than by direct precipitation. According to the concept of impurity-controlled maturation the composition of host sediment controls the incorporation of exchangeable cations and other impurities into inorganically precipitated disordered cristobalite. With time (up to 100 million years) internal solid state reorganization of the disordered cristobalite is accompanied by gradual expulsion of impurities, until the cristobalite dissolves followed by quartz precipitation. Complete conversion to quartz takes place first in porous calcareous sediments free of impurities, as in the Cyprus calciturbidites; in fine grained clay-rich sediments, like Cyprus radiolarian mudstones, disordered cristobalite persists much longer. Impurity-controlled maturation also helps explain the diagenesis of Cyprus chert nodules.     

Facies distinction and interpretation of primary cherts in a mesozoic continental margin succession,Othris, Greece

A Mesozoic continental margin sequence in the Othris Mountains, eastern Greece, contains a variety of both primary and replacement cherts occurring in several stratigraphic positions. Primary cherts, defined as these derived from siliceous sediment, show differences in bedform, sedimentary structures, mineralogy and chemistry. Examples described illustrate deposition by turbidity currents, traction currents and an in-situ pelagic “rain”. Chemical analyses show that major elements can be used to discriminate between different chert formations. Iron and manganese enrichment is a feature of cherts which have been deposited under steady sedimentation conditions.Stratigraphic relationships prove the older, current-deposited, Othris cherts to have been deposites on a volcanic ocean ridge where they probably accumulated in topographic depressions, and also between the ridge and the lower continental margin where they were deposited mainly by turbidites derived from an oceanic source. A younger suite which covers calcareous shelf, marginal and pelagic sediments is an in-situ deposit. These inferences illustrate how primary cherts can add to interpretations of ancient oceanic terrains by providing information on sea-floor topography and, potentially, circulation patterns. Some cherts can be used as facing indicators in sequences where no other wayup evidence is available.     

INVERSION OF STRIKE-SLIP MOVEMENTS ALONG THE YARLUNG ZANGBO SUTURE ZONE, TIBET

INVERSION OF STRIKE-SLIP MOVEMENTS ALONG THE YARLUNG ZANGBO SUTURE ZONE, TIBET     

渭北奥陶系的放射虫燧石岩

渭北奥陶系的放射虫燧石岩是在稳定的华北地台上形成的远洋沉积.它们产在赵老峪组深水碳酸盐地层的下部,并以层位稳定、纹层发育、富含放射虫骨骼、成岩交代组构清楚,以及在剖面上与薄层的硅质页岩呈韵律互层等为等征,表明是深水的放射虫软泥在成岩作用早期由生物蛋白石经溶解-再沉淀反应快速转变而成.这种产在地台上的远洋沉积虽不多见,却是海平面大规模上升及其伴生的重大地质事件的反映.渭北放射虫燧石岩的时代相当于中奥陶世卡拉道克期.当时正值全球性海平面上升,秦岭古海盆也在发生强烈扩张与俯冲,因此,华北地台南缘下沉,使渭北地区变成了弧后深水盆地的北部边缘.当时研究区的古地理位置也恰好处于低纬度的赤道附近.这些都为放射虫燧石岩的堆积提供了有利的条件.由于当时钙质浮游生物尚未大量兴起,所以其沉积作用不受CCD的控制,水深较之现代的类似物可能要浅得多.     

青海可可西里地区蛇绿岩的时代及形成环境

青海可可西里地区发现两条蛇绿混杂岩带,它们沿逆冲带分布。古生物、地层和同位素定年资料表明其时代为早石炭世-早二叠世。其岩石组合代表大洋岩石圈残迹。镁铁质岩主要形成于洋岛环境。硅质岩形成于深海、半深海环境,部分生成于洋岛或洋脊附近热水活动区。这是一套弧前蛇绿混杂岩,是古特提斯洋壳俯冲过程中形成的增生楔。这些资料表明,可可西里地区至少在早石炭世就已存在古特提斯洋,洋盆中有一些洋岛,洋盆在早二叠世末期基本闭合。     

The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima,southern Ryukyu Arc,Japan

The island of Ishigaki Jima, located in the western part of the southern Ryukyu Arc, Japan, is underlain by a basement comprising the Tumuru and Fu-saki formations. The former is a pelitic glaucophane schist with a metamorphic age of 220–190 Ma, and the latter is a weakly metamorphosed accretionary complex, composed mainly of chert, mudstone and sandstone with minor amounts of limestone and mafic rocks. The Fu-saki Formation was weakly metamorphosed at ∼140 Ma. Latest Carboniferous–Early Jurassic microfossils have been obtained from the limestones, cherts and siliceous mudstones of this formation, but no fossils have been collected from the phyllitic mudstones. The radiolarian fauna of the phyllitic mudstones described herein indicates a late Pliensbachian–early Toarcian (Early Jurassic) age. This result, when combined with existing data, enables the reconstruction of an oceanic plate stratigraphy, showing a succession of (in ascending order) Upper Carboniferous–Triassic cherts, Sinemurian–lower Pliensbachian siliceous mudstones and upper Pliensbachian–lower Toarcian phyllitic mudstones and sandstones. The radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the Fu-saki Formation to the base of the Tumuru Formation.     

对赣东北晚古生代放射虫的初步认识

基于以往大量的研究成果和基本地质事实,人们已认同江南造山带是一个新元古代的碰撞造山带,最近,一些研究者根据在赣东北新元古代蛇绿混杂岩带中发现的晚古生代含放射虫硅质岩,认为该蛇绿岩带中的硅质岩至少是晚古生代的产物,进而提出一个解体江南古陆、江南地区普存在晚古生代一中生代板溪洋的命题根据野外调查、室内化学分析等综合研究,本文认为赣东北含放射虫硅质岩是大陆边缘浅水相沉积产物,稀土元素特征不支持大洋地壳上的深水环境,强烈的印支期构造事件有可能使晚古生代含放射虫硅质岩发生构造运移,并与新元古代蛇绿岩彼此混杂。     

Radiolarites et sédiments hypersiliceux océaniques: une comparaison

The mineralogical and chemical composition of Jurassic radiolarian cherts has been studied in Morocco (Rif), Italy (Lombardy basin and Apennines), Greece (Pindus zone and Vourinos Massif), some in close association with ophiolites. We have compared these samples with Cretaceous cherts from the NW Pacific (Leg 32) and with Cenozoic diatomaceous oozes from the Sea of Japan (Leg 31). The silica in the radiolarian cherts is quartz or chalcedony. Most of these rocks also contain feldspars and hematite while the clay fraction is composed of illite and/or chlorite generally associated with swelling clays and, locally, with kaolinite. In oceanic sediments all mineralogical species of silica have been detected (from opal to quartz), the clays generally being the same as those of the radiolarian cherts, the feldspars also being present. Based on the chemical composition of the radiolarian cherts, three facies can be distinguished: massive cherts, pelitic radiolarites and ferruginous radiolarites, the latter occurring only near the contact with volcanic basement. The chemical composition of cherts and diatom oozes from the Pacific is very close to the composition of radiolarian cherts. Although the mineralogy of radiolarian cherts can be related to several models (detrital, diagenesis of pelagic clays etc.) the detrital origin of part of the clay fraction seems certain. The origin of silica and its relation to the palaeolatitudes and the relatively confined nature of the Tethys oceans as well as the influence of volcanic inputs are evaluated, Chemical and mineralogical composition of radiolarian cherts shows that the diagenesis of the clay fraction is not a significant source of silica. Accumulation of diatom oozes in the Sea of Japan and in other areas, shows that the distance from continents and very deep seas are not essential to the development of siliceous sedimentation.     

Ophiolitic melange and flyschoidal sediments of the Tithonian–Lower Cretaceous in Albania

Ophiolitic melange and flyschoidal sediments of the Tithonian-Lower Cretaceous age are widespread in the Eastern Albanides. They lie transgressively or normally on top of the ophiolitic sequence through radiolarian cherts of the Kimmeridgian-Tithonian age, or on top of the carbonate sequence of the periphery of the ophiolites through Middle Liassic-Dogger-Malm pelagic limestones with manganese nodules and radiolarian cherts. They are overlain by conglomerates or neritic limestones of the Lower Cretaceous age. Ophiolitic melange consists of ophiolitic conglobreccias, often of homogenous composition: serpentinite melange with a ‘sedimentary’serpentinite matrix, or basaltic ‘sedimentary’tuffagglomerate. Less commonly they have a heterogeneous composition with small amounts of fine-grained matrix and marls with calpionellids. In other examples, the ophiolitic melange contains heterogeneous exotic blocks including all the members of the ophiolite suite: serpentinite, ophicalcite, gabbro, plagiogranite, diabase, basalts, dacites, amphibolite, sulphide and chromite ores as well as blocks of radiolarites, limestones etc. in the argillic matrix. They are overlain by conglomerate-sandstone-marly flyschoidal deposits with abundant ophiolitic detritus and calpionellids. These deposits are linked with Tithonian-Lower Cretaceous tectogenesis, which led to the fragmentary uplift of ophiolites and partly of their sedimentary periphery, and to the formation of the faulted topography. The presence of the ophiolitic melange and flyschoidal sediments both over the ophiolites and the associated sedimentary rocks of their periphery indicate that the latter were not the basement of an ophiolite nappe during the Late Jurassic time.