Geochemistry of the Samarka terrane cherts (Sikhote-Alin) and the size of the accreted paleo-oceanic plate

We have studied the geochemistry of Late Triassic cherts from tectonic sedimentary complexes of different structural levels of the Samarka terrane. The results show that the contents and distribution patterns of major rock-forming oxides, trace elements, and REE in the cherts of the upper and lower structural levels are somewhat different, which is due to different facies environments of chert accumulation. In particular, the contents of Al₂O₃, TiO₂, and K₂O gradually decrease and the contents of Fe₂O₃ and MnO increase downsection. The contents of Zr, Rb, Hf, Th, and Cr, which were supplied into the bottom sediments with a terrigenous suspension of heavy-mineral fragments, are twice to ten-fold lower than their average contents in the upper crust. In contrast, the contents of Pb, Cu, and Ni, which got into the sediments mostly with metalliferous hydrothermal solutions, are rather high and even exceed their average contents in the middle and upper crust. That is, the contents of the first-group trace elements gradually decrease and the contents of the second-group trace elements increase from upper to lower structural level. The calculated negative Ce anomaly gradually decreases from lower (0.75) to upper (0.88) structural level. The geochemical parameters altogether indicate that the cherts accumulated in the same pelagic depositional environment but in its different parts. The Katen complex composing the lower structural level is the most remote from the continental margin, and the Amba-Matay complex forming the upper structural level is the most proximal. Based on the geochemical and biostratigraphic data and on the time of accretion of paleo-oceanic fragments, we have established the location of each complex within the not yet subducted oceanic plate and estimated the extensionof this plate. Throughout the Jurassic, about 6000 km of the oceanic lithosphere was subducted beneath the eastern margin of the Paleo-Asian continent and partly accreted to it.     

Comparative geochemical study of the cherty rocks of the Taukha terrane (Sikhote-Alin) and its paleogeodynamic significance

Acta Geochimica - The article presents the results of a comparative geochemical study of cherts and siliceous–clayey rocks composing the tectono-sedimentary complexes of various structural...     

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。     

The petrogenetic association of carbonatite and alkaline magmatism: constraints from the Masfut-Rawda Ridge, Northern Oman Mountains

Summary ?A new occurrence of carbonatites associated with intrusive ijolite and syenite has been discovered within the Hawasina Complex underlying the Semail Ophiolite Complex at the southern part of the Rawda-Masfut ridge, Northern Oman Mountains. The carbonatites occur as dikes and sills with lengths of several hundreds of meters and range in composition from calciocarbonatites to ferruginous calciocarbonatites. The carbonatites intruded the ijolite and the associated radiolarian cherts of the Early Cretaceous Sid’r Formation. The close spatial association of carbonatite, ijolite, syenite and radiolarian cherts along with geological, petrographical and geochemical data indicates that these rocks are of intra-oceanic origin. Petrological and field relationships between the carbonatite and associated alkaline silicate intrusives from the Masfut area are consistent with the carbonatites being generated as derivative magmas through liquid immiscibility. They appear to represent magmas related to the volcanism associated with regional crustal extension that preceded the genesis of the Semail Ophiolite. Received April 19, 2001; revised version accepted February 18, 2002     

Radiolarian biostratigraphic data from the Casiguran Ophiolite,Northern Sierra Madre,Luzon, Philippines: Stratigraphic and tectonic implications

Results from the first detailed radiolarian biostratigraphic study conducted in Luzon are reported. The data were obtained from cherts associated with the Casiguran Ophiolite, a dismembered ophiolite mass consisting of serpentinized peridotites, gabbros, dolerite dikes and pillow basalts exposed along the eastern coast of the Northern Sierra Madre, Luzon, Philippines. Cherts and limestone interbeds conformably overlie the ophiolite. The radiolarian assemblages from the cherts constrain the stratigraphic range of the cherts to the Lower Cretaceous (upper Barremian–lower Aptian to Albian). This new biostratigraphic result is in contrast with the Upper Cretaceous stratigraphic range previously reported in the region.Radiolarian biostratigraphic results from the Casiguran Ophiolite provide additional evidence for the existence of Mesozoic oceanic substratum upon which Luzon and neighboring regions within the Philippine archipelago were likely built. Interestingly, the result closely resembles those reported for the ophiolite in southeastern Luzon as well as the oceanic crust of the Huatung Basin situated east of Taiwan and the ophiolites in eastern Indonesia. In light of this, along with previously gathered geochemical data from the ophiolites, a common provenance is being looked into for these crust–upper mantle sequences in the western Pacific region.     

桂西地区上二叠统硅质岩地球化学特征及对本区烃源岩发育潜力的指示意义

通过对桂西地区南丹龙王坡11件上二叠统硅质岩样品和平果四塘6件上二叠统硅质岩样品的主量和微量元素分析研究,并结合已发表的崇左东攀部面大隆组的硅质岩地球化学数据,认为桂西地区上二叠统硅质岩为生物成因,形成于受陆源碎屑影响的大陆边缘沉积环境,其沉积水体为氧化条件.研究区内硅质岩的TOC为0.10%～0.19%,平均为0.1...     

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.     

云南墨江金矿床硅质岩的地质地球化学特征及其意义

墨江金矿床厂组中下段硅质岩具有典型的沉积构造，岩石富集∑Fe和As 、Sb、Bi、Ga等微量元素（相对于地壳克拉克值），而相对贫AI2O3；稀土元素以总量（∑REE）低，负铈异常为特征，与热水沉积的硅质岩特征相似。在判别硅质岩形成与作用的一系列主量和微量元素图解上，本区的硅质岩位于热水沉积作用范围，但四十八两山段硅质岩有趋近于正常化学沉积作用。利用硅质岩的氧同位素计算出它的形成温度为128－146℃。地质、地球化学特征表明本区硅质岩的形成与热水沉积作用有关，但四十八两山段硅质岩受到正常化学沉积作用的影响。、     

Characteristics of Rare Earth and Trace Element Patterns in Bedded Cherts from the Bottom of the Lower Cambrian in the Northern Tarim Basin, Northwest China and Their Genetic Significance

The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo,As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, T1, Bi and Th are highly depleted in the cherts.These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondritenormalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu*-10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. EREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu)N ratio decreases from 2.72 at the bottom to 0.67 at the top. NASC-normalized (La/Ce)N ratio increases from 1.36 at the bottom to 3.13 at the top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean-basin floor in the Franciscan Complex exposed at Marin Headlands, California (F-MH chert) (Murray et al., 1991). These geochemical signatures are inconsistent with our previous sedimentological data, which suggests a continental shelf setting. Based on multiple lines of evidence including high TOC content in the concomitant black shales, phosphorite at the bottom of black rock series, regional rise of sea level,and beginning of the southern Tianshan Ocean geotectonic cycle, the authors infer that the hydrothermal fluid was carried to the continental shelf by upwelling from a divergent pelagic ocean floor setting.     

西准噶尔玛依勒蛇绿混杂岩中硅质岩的岩石化学特征及其成因

新疆玛依勒早古生代硅质岩一般产于蛇绿混杂岩带中,为探讨其成因类型和沉积环境,对其进行了岩相学和地球化学研究。硅质岩主要为紫红色,主要由微晶自生石英和隐晶质组成。地球化学分析结果显示样品均受到了酸性火山凝灰质的影响,一般具有高SiO2、Al2O3,低Fe2O3,中等高TiO2等特征,可能是火山和生物化学共同作用的产物。综合其地质产状、岩相学特征以及上述地球化学特征,认为该区硅质岩产于俯冲带之上的构造环境而非大洋环境。     

香山群狼嘴子组硅质岩地球化学特征及形成环境

香山群自建群以来,时代归属一直存在争议,通过区域地质调查,把香山群地质时代限定为中晚寒武世。为深入探讨鄂尔多斯西南边缘香山群形成的沉积环境和构造背景,对香山群狼嘴子组硅质岩岩石学、地球化学特征进行了综合分析。分析结果表明硅质岩SiO2含量为82.44%～98.13%,Al/(Fe Mn Al)比值为0.46～0.69,Al2O3/(Al2O3 Fe2O3)比值为0.53～0.77;LREE/HREE为3.57～8.45,轻重稀土分异作用强,LaN/CeN值0.66～1.15,Ce/Ce*值0.66～0.93,为大陆边缘型硅质岩,成岩过程包含热水沉积和正常沉积作用。同时,硅质岩建造中碎屑岩和产出的辉绿岩床的地球化学特征表明:碎屑岩沉积背景也属被动大陆边缘区,辉绿岩属于伸展作用构造背景下的大洋拉斑玄武岩系列。综合地质资料认为,在中晚寒武世,阿拉善地块南缘与鄂尔多斯地块西南缘具有离散型大陆边缘性质。     

Geochemistry of basal Cambrian black shales and cherts from the Northern Tarim Basin,Northwest China: Implications for depositional setting and tectonic history

Black shales and thin-bedded cherts in the basal Cambrian are widespread worldwide and they carry important information on the formation of sedimentary basins and on the tectonic history. We studied the geochemical signatures of the early Cambrian black shales and bedded cherts from the Northern Tarim Basin, China, with the objectives of understanding the depositional setting of these rocks and inferring the tectonic history in the region. Twenty two black shales, ten cherts, and two nodular phosphorites were collected from two outcrops at Xiaoerbulake and Sugaitebulake in the Northern Tarim Basin, spanning vertical sections of 8.8 and 7.5 m, respectively. A suite of techniques, including field investigations, X-ray diffraction, electron microscopy, trace element, rare earth element (REE), and isotope geochemistry, were employed to characterize the geochemical signatures of these rocks. Field evidence indicates that the black shales and bedded cherts are over- and underlain by dolomites, suggesting a shallow marine depositional environment. Mineralogical and trace element data suggest that the Tarim black shales and cherts were deposited in a suboxic continental shelf environment, and hydrothermal activity may have extracted certain trace elements from mafic continental crust and concentrated them in the sedimentary basin. REE characteristics for the cherts are very similar to those that are known to be deposited in pelagic ocean floor settings, suggesting that the hydrothermal fluids may be derived from the infant southern Tianshan Ocean in the north of the Tarim Basin. Os isotope signatures at the time of deposition (¹⁸⁷Os/¹⁸⁸Os_i = 1.1–2.7) are typical of crustal signatures, and the radiogenic Os isotope signatures rule out the mantle as a possible source of Os and other metals. A positive correlation between ¹⁸⁷Os/¹⁸⁸Os and ε_Nd is consistent with upper crust-derived basin sediments that contain a variable contribution of hydrothermal fluids possibly derived from ancient mafic continental crust. These trace element, REE, and isotope systematics collectively suggest that incorporation of hydrothermal fluids derived from ancient, mafic continental crust combined with deposition in relatively reducing conditions may have controlled the chemical and isotopic compositions of these rocks. We infer that the hydrothermal fluid was carried to the continental shelf by upwelling during the initial stages of formation of the southern Tianshan Ocean, where the fluid interacted with thinned, mafic crustal basement lithologies and was subsequently incorporated into the black shales and bedded cherts in the Northern Tarim Basin. This study provides important geochemical evidence for the creation of the Tianshan Ocean, which is a result of break-up of the Rodinia Supercontinent during the early Cambrian.     

First finds of Late Tithonian and middle-late Albian radiolarian assemblages in volcanogenic-siliceous rocks of the Amur River right lower reaches and their tectonic significance

The lithological-stratigraphic study of volcanogenic-siliceous rocks developed on the left side of the Machtovaya River, a right tributary of the Amur River, yielded the first radiolarian assemblages of the late Late Tithonian, the late Late Tithonian-early Valanginian, and the middle-late Albian age. It is established that the stratigraphic succession of volcanogenic-siliceous rocks in this area is composed of upper Tithonian-Valanginian dark red to red-brown cherts with basalts in the lower part of the section and Albian dark gray clayey cherts, olive-gray siliceous-tuffaceous argillites, and tufaceous siltstones in its upper part. The replacement of cherts by their clayey varieties likely occurred in the Aptian. The composition, structure, and age of these strata and the rocks constituting the Kiselevka-Manoma accretionary complex are different, which indicates their different tectonic origin.     

The Nikolaevsky base-metal skarn deposit, Primorye, Russia

The study of the mineralogical and geochemical features of ores and their textural and structural relationships at the Nikolaevsky base-metal skarn deposit allowed us to establish the succession of mineral assemblages and to ascertain the complex distribution of elements in minerals, ores, and orebodies resulting from nonuniform development of metasomatic zones in the single ore-metasomatic process. The vertical mineralogical and geochemical zoning of the deposit is expressed in the replacement of economic Pb-Zn mineralization at the lower levels of the skarn orebodies with Ag-Pb-Zn mineralization at the upper levels of the major Vostok-1 orebody located at the contact of limestone and felsic volcanics and the block orebodies at the lower level of felsic volcanic rocks. Au-bearing vein Pb-Ag-Sb mineralization occurs in the near-surface zone of the deposit. Three mineralogical types of ores have been identified: sulfide-hedenbergite, quartz-carbonate-sulfide, and sulfide. The zonal distribution of these types within orebodies is combined with variations in mineralogy and in the distribution of major and minor elements. The statistical processing (the Geokhimiya-1 program) of 96 chemical analyses of monomineralic galena and sphalerite samples taken from skarn showed that an intimate Pb ? Bi assemblage is characteristic of the lower levels, whereas a Ag Bi assemblage is typical of the upper levels. The elemental assemblages correspond to definite mineral assemblages, determining the mineralogical and geochemical zoning of the deposit.     

东秦岭二郎坪群硅质岩地球化学特征及其沉积环境意义

东秦岭二郎坪群硅质岩成因分析对二郎坪群形成环境和构造背景研究具有重要意义。通过对二郎坪群中3类硅质岩地质特征、岩石地球化学特征（w(SiO₂)=61.35%~96.01%、N(Si)/N(Al)=2.77~51.53）研究,认为二郎坪群硅质岩为非纯硅质岩;稀土元素的配分模式和特征值（∑REE、∑LREE、∑HREE、δCe、δEu)、常量元素含量（w(Al₂O₃)、w(TiO₂)）和微量元素含量（w(Nb)、w(Rb)、w(Th)）的分析表明研究区硅质岩是热水成因型,但受陆源物质成分影响,陆缘物源对南阳盆地西硅质岩影响较大。地球化学综合分析揭示二郎坪群硅质岩产出于不同海相沉积环境,南阳盆地西硅质岩形成于大陆边缘海环境,而盆地以东硅质岩形成于远洋盆地环境,证实了二郎坪群形成于弧后盆地构造环境。     

湘桂地区泥盆系硅质岩稀土元素地球化学及沉积环境

对湘桂地区泥盆系钦防海槽和南丹－河池台盆及灵山－衡阳台盆中发育的硅质岩的稀土元素地球化学分析,给出这些硅质岩为深水盆地沉积环境产物的解释,其成因类型既包括了正常沉积硅质岩,也有部分为原生热水沉积硅质岩,为证实丹治台盆及灵衡垮台 盆形成为线性走滑作用所致提供了新的证据。     

Geochemical Features and Formation of the Auriferous Cherts in the Mojiang Gold Deposit,Yunnan

Auriferous cherts in the Middle Carboniferous Jinchang Formation are the dominant host rocks of auriferous quartz veins and mixed orebodies comprised of gold-bearing quartz veins and cherts in the Mojiang gold deposit.The rocks exhibit sedimentary texture and structure and are composed of hot-water deposited minerals.The FeO,Fe2O3,Au and Ag contents of the auriferous cherts are high;the Cr,Ni and Co contents are also high but significantly variable;MnO/TiO2 and TFe/TiO2 ratios are relatively higy.As viewed from a few diagrams that distinguish different chert formations,the auriferous cherts are in or near the range of hot-water deposited cherts.Because the correlation coefficients between Au contents and those of Cr, Ni of the rocks are negative,a great Au amount in the cherts might not be brought about by later hydrothermal alterations.The rare-earth elements,O and Si isotopic compositions of the auriferous cherts demonstrate that the cherts belong to hot-water deposited rocks.The later hydrothermal alterations made the petrochemical compositions of the cherts deviate from the characteristics of hot-water deposition.In general,the geological and geochemical features of the auriferous cherts demonstrate that the rocks were formed by hot water deposition.     

云南墨江金矿床含金硅质岩的地球化学特征和成因

中石炭统金厂组（Ｃ２ｊ）下部含金硅质岩是云南墨江金矿庆的主要围岩之一,具有沉积结构构造,含热水沉积矿物。岩石的ＦｅＯ、Ｆｅ２Ｏ３、Ａｕ和Ａｇ含量高;Ｃｒ、Ｎｉ和Ｃｏ含量高、变化大;ＭｎＯ／ＴｉＯ２和ＴＦｅ／ＴｉＯ２比值较大;Ａｕ含量与ＮｉＣｒ含量相关性低,Ａｕ可能不是后期热液作用带入的。在判别硅质岩形成作用的一系列元素和微量元素关系图上,含金硅质岩位于热水沉积作用的范围内或接近于热水沉积作用。岩石     

Geological and Geochemical Characteristics of the Cherts in the Mojiang Gold Deposit and Their Implications

Geological and geochemical characteristics of cherts of the low-middle parts of the Jinchang Formation indicate that cherts are associated with hydrothermal sedimentation,but the middle-upper parts of the Jinchang Formaiton are mixed with normal sediments.The cherts are characterized by high Fe,As,Sb,Si and Ga,low Al and totoal REE,negative Ce anomaly and HREE enrichment.Their δ^18O values show that the forming temperatures of the cherts range from 128℃ tp 146℃.     

华南热水沉积硅质岩建造及其成矿效应

华南地区热水沉积建造发育。文中介绍该地区热水沉积建造 ,特别是震旦系顶部、泥盆系榴江组和二叠系当冲组 3个重要层位的硅质岩建造 ,分析它们的地质地球化学特征。研究表明 ,华南三层位沉积硅质岩的共同特征是TiO2 、Al2 O3 和K2 O含量一致偏低 ,大部分微量元素含量偏低 (与地壳克拉克值相比 ) ,但Ba、As、Sb富集 ,具有较为典型的热水沉积成因特点。多元统计分析显示 ,大部分微量元素在第一个主因子上均有显著因子载荷 ,与它们在基底的富集或亏损无关 ,代表了古地热系热水循环中的淋滤因子。华南三层位热水成因硅质岩具有相似的REE地球化学特征。REE总量低 ,稀土配分模式落在典型热水沉积物的上、下限之间 ,多数样品呈现δCe和δEu负异常。正常沉积的混入使部分硅质岩的REE配分模式复杂化。最后 ,讨论了与热水沉积建造相关的成矿效应 ,为金属矿床成因和评价提供约束条件