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1.
在信江盆地中存在数层和石炭纪海相火山岩及其海底块状硫化物矿层相伴生,与石炭纪地层整合产出的层状硅质岩。由对硅质岩常量元素、微量元素、稀土元素、硅和氧同位素等地球化学特征研究表明,本区硅质岩具有一定的热水沉积硅质岩地球化学特征。在Al-Fe-Mn和Fe-Mn-(Ni+Co+Cu)三角图上,本区硅质岩属热水沉积硅质岩。由硅质岩MnO/TiO2比值、δCe值和δ30Si值分析表明,信江盆地石炭纪硅质岩的沉积环境主要为浅海。 相似文献
2.
The Upper Sinian bedded cherts are widely distributed on the southeast continental margin of the Yangtze plate, with a total thickness ranging from 20 to 150 m. The cherts are very simple in chemical composition, with the contents of siliceous minerals exceeding 90% and those of other mineral species being very low. The total content of trace elements is considerably variable. The contents of most trace elements are low as compared with their crust clarke values. But these cherts are very rich in the trace elements Ba, As, Ab, Ag and U, coupled with relatively high contents of Fe, Mn and low contents of Al, Ti, and Mg. It can be identified as the cherts of hydrothermal origin in terms of the values of Fe/Ti, (Fe + Mn)/Ti, Al/ (Al + Fe + Mn) and U/Th. In the Al-Fe-Mn, and Fe-Mn-(Cu + Ni + Co) × 10 diagrams and lg [U]-lg[Th], Cr-Zr diagrams, all the samples from the area studied fall within the hydrothermal sediment field. All these characteristics show that the cherts are mainly the product of hydrothermal processes. The fact that the δ30Si values of the chert range from 0.0‰ to 0.7‰ with §18O from 20.l‰ to 23.6‰ clearly manifests that the submarine hydrothermal water is the main source of silica. 相似文献
3.
西藏札达县夏浦沟的放射虫硅质岩和岛弧火山岩: 新特提斯洋内俯冲体系的记录? 总被引：1，自引：0，他引：1
在西藏阿里地区夏浦沟野外调查发现放射虫硅质岩、熔岩组合.放射虫硅质岩SiO2含量在89.47%~92.94%之间, Si/Al在43~67之间, Al/ (Al+Fe+Mn) 比值在0.68~0.74之间, MnO/TiO2平均比值为0.60, Ce/Ce*平均值为0.89, LaN/CeN平均值1.09, 指示它位于洋盆和大陆边缘过渡的环境, 总体更接近大陆边缘环境.伴生的火山熔岩以安山质为主, 具低TiO2 (0.75%~0.98%)、轻稀土弱富集和源自板片流体的易溶元素如Ba、U、Pb相对轻稀土富集, 亏损高场强元素(HFSE) Nb、Ta、Ti等, 指示形成于岛弧环境, 很可能是中生代新特提斯洋洋内俯冲系统的组分.放射虫动物群主要包括Alievium cf.regulare、Alievium cf.fatuum、Archaeospongoprunum cf.patricki、Archaeodictyimitra mitra Dumitrica等早白垩世分子, 其提供了岛弧活动时间上限的约束. 相似文献
4.
MARK J. F. LAWRENCE 《Sedimentology》1994,41(3):479-498
The Late Cretaceous to Early Eocene, dominantly micritic, Amuri Limestone Group (ALG) was deposited in an approximately NW trending trough, in eastern Marlborough, New Zealand. The ALG comprises: the Mead Hill Formation; the Teredo, Lower and Middle Limestone formations; and the Upper and Lower Marl formations. Chert and dolomite are concentrated in the Mead Hill Formation, which contains five of six recognized diagenetic zones: Zone I at the base of the ALG consists almost entirely of chert; Zone II consists solely of chert and dolomite; Zone III comprises chert and limestone; Zone IV is composed of chert plus dolomite; Zone V is a chertified mudstone; and the minor amounts of chert found in the Middle Limestone Formation comprise Zone VI. With the exception of Zones IV and V, chert decreases stratigraphically upwards and away from the basin centre. All the dolomites are composed of <1 mm diameter rhombohedra in discontinuous beds and lenses. Generally Ca-rich, and non- to slightly ferroan, the dolomite contains approximately 500–900 ppm Mn and 200–400 ppm Sr. δ13C values average 1–2%PDB with δ18O ratios of about -4%PDB. Mass balance calculations indicate that the Mg2+ for dolomitization was derived from sea water. Sr, Fe and Mn concentrations are interpreted as indicating dolomite formation in the marine environment, with no influence from meteoric waters. The intimate association with pyrite implies dolomite formation in association with sulphate reduction, in the upper sediment column. δ18O data show that the bulk of the dolomite formed at temperatures below 50°C. All chert samples contain in excess of 90 wt% SiO2, about 1 wt% Al2O3 and 1 wt% from losses on ignition. Generally all other major elements total less than 2 wt% oxide. δ18O values range from 26·8 to 29·0%SMOW. Chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate-bound components from the site of chertification, and the effective dilution by SiO2 of non-carbonate-bound insoluble residues. δ18O data indicate that chert formed in fluids of similar composition and temperature as the dolomite. The abundance of disseminated pyrite in cherts implies an association with sulphate reduction. Silica for chertification is thought to have initially come from dissolution of siliceous organisms. However, there is insufficient biogenic silica available to form the volumes of chert observed. It is suggested that the bulk of the silica came from SiO2-rich pore waters generated by clay mineral reactions in the thick underlying mudstones. The ALG compacted down through these pore waters. Chert and dolomite nucleation are considered to have been penecontemporaneous. Dolomitization was initially probably the faster process, continuing as long as sulphate reduction prevailed and there was an adequate supply of Mg2+. The nucleation of chert, although initially slower (probably due to a relatively lower initial SiO2 supply), continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix. The amount of chertification decreased progressively as SiO2 supplies diminished, both stratigraphically upwards and away from the basin centre. 相似文献
5.
The >3·0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although the cherts have undergone postdepositional silica enrichment, such as cementation and metasomatic silicification, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other primary to early diagenetic components were carbonates, sulphates (gypsum and anhydrite) and organic matter. Although these mineral associations, on the whole, correspond to those of modern marine evaporites, they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). This feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated; compared with modern ocean waters, the concentrations of Fe and Si were significantly higher and the pH value might have been lower. These conditions could be obtained by contributions of Fe- and Si-enriched hydrothermal solutions and continental run-off to the site of deposition. Grey cherts contain detrital quartz and altered Fe–Ti oxides and were formed in a period of input of terrigenous detrital materials. They are characterized by higher concentrations of TiO2, Al2O3, Cr, Ni, Zn, Rb and Zr compared with the other types of chert and by very low (< 4) Al2O3/TiO2 values. These features are attributed to the supply of terrigenous detrital materials that contain abundant Fe–Ti oxides (ilmenite and titanomagnetite) and fine TiO2 particles. Such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of Ti relative to Al. 相似文献
6.
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. 相似文献
7.
湘中南地区奥陶系由"细碎屑岩-硅质岩系-粗碎屑岩"构成,三者厚度变化具有明显的规律性:厚度等值线的展布逐渐趋于北东方向,厚度最大区域向南东方向迁移。区内岭口剖面烟溪组硅质岩SiO_2含量(89.08%~94.32%)和Al/(Al+Fe+Mn)值(0.52~0.79)较高,具有轻稀土略富集、无明显铈异常和铕异常的特点;大桥剖面烟溪组硅质岩SiO_2含量高(91.74%~95.14%),Al/(Al+Fe+Mn)值为0.34~0.56,具有轻稀土富集、无明显铕异常和间歇性铈负异常、Y/Ho比值低(20.65±1.63)的特点。硅质岩地球化学特征及图解说明其主要为正常海相生物成因,形成于开阔的大陆边缘背景。对比邻近地区相应层位数据发现,湘中南及其邻区中—晚奥陶世硅质岩成因与沉积背景相似,指示其形成于统一盆地中,结合地层等厚度图分析认为,盆地经历了被动大陆边缘—前陆盆地的转换,硅质岩系可能是前陆盆地初始阶段的产物,在其展布范围内无明显热液影响,暗示造成华夏地块抬升的地球动力学来源可能还在该套硅质岩系展布范围的更南部或东南部。 相似文献
8.
鄂西地区的上奥陶统五峰组、下志留统龙马溪组下部、下二叠统孤峰组和上二叠统大隆组中发育有薄层硅质岩。二叠系栖霞组、茅口组和吴家坪组灰岩中发育有结核状或似层状燧石。Fe Al Mn三角图 ,Cu、Ni、Co、Cr、Zr的关系 ,U Th判别图 ,Ba、As、Sb元素和REE配分模式等地球化学指标表明 :五峰组和龙马溪组层状硅质岩属于正常海水中生物化学和化学沉积 ;孤峰组和大隆组层状硅质岩、茅口组灰岩中燧石结核不属于典型的热水沉积相 ,但受热水作用的影响 ,而孤峰组硅质岩受热水作用明显。鄂西五峰组和龙马溪组硅质岩的沉积环境是四周被古陆或台地环绕的半封闭深水相滞流盆地。鄂西孤峰组和大隆组硅质岩的沉积环境是由裂陷作用形成的碳酸盐台地内部的盆地(或台沟 )滞流还原环境。CaO/(Fe +CaO)值和 (MgO/Al2 O3 )× 10 0值表明 :五峰组、龙马溪组和大隆组硅质岩沉积环境海水略有淡化。栖霞组、茅口组和吴家坪组含结核状或似层状燧石的生物灰岩 ,形成于陆棚上快速海侵和上升洋流形成的还原环境。 相似文献
9.
JIN Xiaochi WANG Yizhao XIE GuanglianInstitute of Geology Chinese Academy of Geological Sciences Baiwanzhuang Ro Beijing Geological Survey of Yunnan Yujiang Ro Yuxi Yunnan 《《地质学报》英文版》2003,77(4):440-456
Phanerozoic strata are distributed in several north-south trending zones in the central part of the Changning-Menglian Belt. Four types of Devonian to Triassic stratigraphic successions can be identified: (1) elastics with limestone lenses in the mid-section, changing up-section into alternations of fine elastics and cherts; (2) elastics with chert intercalations and limestone lenses, and topped by Permian basic volcanics; (3) elastics-basic volcanics-carbonates-clastics; and (4) limestones, dolomitic limestones-dark gray thin-bedded limestones, argillaceous limestones, mudstones and siliceous mudstones. Devonian to Triassic cherts occur in different horizons and different zones from east to west. These cherts are usually transitional to their neighboring elastics. There is no continuous Devonian to Middle Triassic chert sequence in the central zone of the Changning-Menglian Belt as Liu et al. (1991,1993) reported. Volcanics and the overlying carbonates described by some workers as "seamount" sequences 相似文献
10.
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. 相似文献
11.
滇东南麻栗坡位于右江盆地西南缘,该处蛇绿岩被认为是古特提斯洋壳的遗迹,对蛇绿岩所代表八布洋的研究是全面了解右江盆地与华南西南缘古特提斯构造演化的重要环节。研究区广泛发育一套夹有硅质岩的深水相沉积,对麻栗坡八布和西牛堡中—下二叠统他披组硅质岩的地球化学特征研究表明:硅质岩Al/(Al+Fe+Mn)值为0.43~0.70,Al2O3/TiO2值为25.5~44.8,Eu/Eu*值为0.81~1.02,为非热液成因硅质岩。硅质岩Ce/Ce*值分别为0.67~1.29和0.77~1.18,Y/Ho值分别为28.84~31.23和26.41~28.91,形成于大陆边缘环境。结合其他地区硅质岩地球化学特征,认为早—中二叠世八布洋盆的西南缘可能为活动大陆边缘,其洋壳的俯冲方向为南西向。 相似文献
12.
Radiolarian-rich sediments dominated pelagic deposition over large portions of the Tethys Ocean during middle to late Jurassic time as shown by extensive bedded chert sequences found in both continental margin and ophiolite units of the Mediterranean region. Which paleoceanographic mechanisms and paleotectonic setting favored radiolarian deposition during the Jurassic, and the nature of a Tethys-wide change from biosiliceous to biocalcareous (mainly nannofossil) deposition at the beginning of Cretaceous time, have remained open questions. Previous paleomagnetic analyses of Jurassic red radiolarian cherts in the Italian Apennines indicate that radiolarian deposition occurred at low peri-equatorial latitudes, similar to modern day deposition of radiolarian-rich sediments within equatorial zones of high biologic productivity. To test this result for other sectors of the Mediterranean region, we undertook paleomagnetic study of Mesozoic (mostly middle to upper Jurassic) red radiolarian cherts within the Aegean region on the Peloponnesus and in continental Greece. Sampled units are from the Sub-Pelagonian Zone on the Argolis Peninsula, the Pindos–Olonos Zone on the Koroni Peninsula, near Karpenissi in central Greece, and the Ionian Zone in the Varathi area of northwestern Greece. Thermal demagnetization of samples from all sections removed low-temperature viscous and moderate-temperature overprint magnetizations that fail the available fold tests. At Argolis and Koroni, however, the cherts carry a third high-temperature magnetization that generally exhibits a polarity stratigraphy and passes the available fold tests. We interpret the high-temperature component to be the primary magnetization acquired during chert deposition and early diagenesis. At Kandhia and Koliaky (Argolis), the primary declinations and previous results indicate clockwise vertical-axis rotations of 40° relative to “stable” Europe. Due to ambiguities in hemispheric origin (N or S) and thus paleomagnetic polarity, the observed declinations could indicate either clockwise (CW) or counterclockwise (CCW) vertical-axis rotations. Thus at Adriani (Koroni), the primary declinations indicate either CW or CCW rotations of 95° or 84°, depending on paleomagnetic polarity and age. The primary inclinations for all Peloponnesus sites indicate peri-equatorial paleolatitudes similar to those found for coeval radiolarian cherts exposed in other Mediterranean orogenic belts. Our new paleomagnetic data support the interpretation that Mesozoic radiolarites within the Tethys Ocean were originally deposited along peri-equatorial belts of divergence and high biologic productivity. 相似文献
13.
Geochemical Features and Formation of the Auriferous Cherts in the Mojiang Gold Deposit,Yunnan 总被引：1，自引：0，他引：1
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. 相似文献
14.
甘肃北山地区红山铁矿区硅质岩与铁矿体紧密伴生,且有些硅质岩本身就是铁矿石。常量元素分析表明:硅质岩普遍具有高Si、高Fe、低Al特征,K2O含量普遍高于Na2O,且Fe/Ti值为57.14～218.74,(Fe+Mn)/Ti值为57.54～224.16,Al/(Al+Fe+Mn)值为0.05～0.14,均符合热水沉积硅质岩的特征,但存在少量陆源物质介入。稀土微量元素分析表明:硅质岩大部分微量元素相对于克拉克值亏损,稀土元素总量低,经北美页岩标准化后,Ce异常明显或微明显,Eu呈现明显的正异常,重稀土相对轻稀土富集;δCe值为0.93～1.05,平均为0.99;La/Ce值为0.43～0.49,平均为0.44,更接近于大陆边缘硅质岩的特征。综合以上地球化学特征,硅质岩具有明显的热水沉积成因属性,同时有陆源组分的加入,进而得出红山铁矿为与热水沉积成因有关的铁矿床,当时形成环境为大陆边缘环境。 相似文献
15.
湖北蛇屋山金矿是亚洲最大的红土型金矿,矿区内硅质岩发育,覆于含矿红土层之上,并与硅化灰岩呈渐变过渡。硅质岩主要由微晶和隐晶组成,其SiO2含量为96.87%～97.90%,w(Si)/w(Al)=92.84～287.66,平均值为173,处于纯硅质岩的范畴。硅质岩样品的w(Al)/w(Al+Fe+Mn)为0.189～0.388,平均值为0.303,具热液成因硅质岩的特征。根据Al、Fe、Mn三角图的投点,认为本研究区硅质岩为热液成因硅质岩。微量元素w(Ni)/w(Co)、w(Fe)/w(Ti)、w(Ti)/w(V)、w(U)/w(Th)也说明了硅质岩具有热水沉积成因。w(Ce)/w(Ce*)为1.05～1.27,平均值为1.12,(La/Ce)N为0.84～1.12,平均值为0.98,微量元素V的含量(3.02～4.26μg/g)以及常量元素w(MnO)/w(TiO2)(1.0～2.0)都显示此研究区硅质岩具有大陆边缘硅质岩的特征。结合宏观特征,确定硅质岩有可能形成于卡林型金矿去碳酸盐化阶段,属于热液活动的产物。 相似文献
16.
T. J. BARRETT 《Sedimentology》1982,29(3):353-373
In the North Apennines of Italy, Upper Jurassic bedded chert stratigraphically overlies ophiolitic rocks and is overlain by Lower to Middle Cretaceous pelagic limestone and shale, and Upper Cretaceous flysch. The bedded chert, best exposed in East Liguria and on Elba, is typically 30–80 m thick, but occasionally reaches 150–200 m thickness. It consists of two main alternating lithologïes: siliceous mudstone (SM) and radiolarite (R). Chert sections commonly show characteristic stratigraphic changes. Lower cherts display a striking rhythmic alternation of R and ferruginous SM beds. In middle cherts, SM beds are much less ferruginous and shalier intercalations are locally present. In upper cherts, R beds are less frequent and SM beds are essentially non-ferruginous. R beds are generally 1–4 cm thick, and consist of 80–90% quartz, 5–15% clays and usually < 1% hematite. They are commonly parallel-laminated, and rarely size-graded. In size-graded beds, large radiolaria are more abundant near the bed base (commonly together with ophiolitic or SM clasts) and small radiolaria more abundant near the bed top. Sorting is poor throughout most R beds. R beds are interpreted as turbidites (cf. Nisbet & Price, 1974). Model calculations suggest that typical settling velocities of radiolaria during redeposition are < 1 cm sec?1, which is low and of restricted range relative to the 1–10 cm sec?1 settling velocities of clastic grains of comparable size range. Radiolaria therefore should have only a limited tendency to grade and sort during deposition from a turbulent current. SM beds are commonly 1–7 cm thick, although much thicker ones occur near the base of sections, and consist mainly of 50–70% quartz, 15–35% clays and 0–15% hematite. Microscopic clay-silica aggregates and highly corroded remnants of radiolaria are common. SM beds are interpreted as mainly ambient pelagic sediment which accumulated slowly in topographic lows, and which was modified by near-surface dissolution of biogenic silica. In SM beds which contain two texturally different layers, the lower one is interpreted as the top of the underlying radiolarian turbidite. North Apennine cherts represent the first sediment deposited on oceanic crust formed during the opening of the North Apennine part of the Tethys. The ophiolitic basement had a rugged topography which favoured the redeposition of siliceous sediment. Hematite and local Mn enrichments in SM beds in the lower chert sections represent hydrothermal precipitates inferred to have originated at a spreading axis. During seafloor spreading, accumulation of siliceous sediments progressively reduced the topography. Deposition of ophiolitic detritus within the sediments phased out during early chert sedimentation, and the hydrothermal contribution during early-middle chert sedimentation. As local basins filled, during late chert sedimentation, radiolarian turbidites became less frequent. The first limestones at the top of chert sections are calcareous ooze turbidites derived from above the CCD and deposited slightly below it. Gradual descent of the CCD to ocean floor depths at the end of the Jurassic (Bosellini & Winterer, 1975) led to the replacement of siliceous by carbonate sedimentation. 相似文献
17.
Hydrothermal origin of Late Proterozoic bedded chert at Gusui, Guangdong, China: petrological and geochemical evidence 总被引：2，自引：0，他引：2
YONGZHANG ZHOU†‡ E. H. CHOWN JAYANTA GUHA HUANGZHANG LU GUANGZHI TU† 《Sedimentology》1994,41(3):605-619
The Late Proterozoic bedded chert from Gusui in Guangdong Province, southern China, is characterized by bedded, laminated, massive and pseudobrecciated structures. The chert is depleted in TiO2, Al2O3 and most trace elements. However, it is enriched in Ba, As, Sb, Hg, and Se. In Al-Fe-Mn ternary diagrams, it falls into the ‘hydrothermal field’. Factor analysis shows that many elements show up in the principal trace element factor suggesting their enrichment results from leaching of the country rock by hydrothermal solutions. The chert has low REE concentrations and displays an REE pattern consistent with a hydrothermal origin. It may have formed in a Late Proterozoic rift or extension zone developed within the Yunkai continental margin back-arc basin, with a fault system linking it to an unknown heat source at depth. 相似文献
18.
西藏南部硅质岩的特征及相关问题探讨 总被引：1，自引：0，他引：1
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。 相似文献
19.
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. 相似文献
20.
Geochemical Characteristics and Tectonic Setting of the Laohushan Basalts, North Qilian Mountains 总被引：1，自引：0，他引：1
QIAN Qing ZHANG Qi Institute of Geology Geophysics CAS P. O. Box Beijing 《《地质学报》英文版》2001,75(4):452-457
The Ordovician Laohushan ophiolite, located in the eastern part of the North Qilian Mountains, is mainly composed of meta-peridotites, gabbros and basalts alternating with sediments. The sediments are mainly turbidites, including sandstones, siltstones, cherts etc. Major elements show that the basalts are subalkaline tholeiites and may be analogous to ocean-floor basalts. Except a few N-MORBs, most of the basalts are E-MORBs as indicated by incompatible element ratios such as (La/Ce)N, La/Sm, Ce/Zr, Zr/Y and Zr/Nb. Negative Nb anomaly is common but negative Zr, Hf and Ti anomalies are quite rare. Based on the geochemical characteristics, it is suggested that the Laohushan basalts were formed in a back-arc basin. εNd (t) of the basalts ranges between +3.0 and +8.9 and (87Sr/86Sr), ranges between 0.7030 and 0.7060, indicating a depleted mantle source which was mixed with more or less enriched mantle components. Furthermore, the petrography of the sandstones and geochemistry of the cherts suggest that the 相似文献