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1.
Thirty-six samples of well-preserved Precambrian sedimentary rocks have been analyzed for rare-earth elements (REE) as one test of the proposition that the present relative REE abundances at the Earth's surface have developed gradually with time. Except for Eu, the average relative abundances of the REE appear to be the same for the Precambrian sediments as in a composite of Paleozoic shales (taken as representative of younger sedimentary rocks). Abundances of Eu relative to the other REE in all except two of the Precambrian sedimente are significantly higher than in the younger sediments. The extent of enrichment of the Precambrian sediments in Eu over the composite of younger sediments ranges from 0.7 to 3.0. The average abundance of Eu relative to the other REE for all the Precambrian sedimente is the same as that in chondritic meteorites. Several of the oldest Precambrian sedimente have relative abundances of Eu that exceed that of chondrites. Large-scale separations of Eu from the other REE as a result of igneous, metamorphic, or sedimentary processes are considered. Present information does not convincingly support any of the proposed mechanisms. 相似文献
2.
中国北方元古宙沉积岩中自生稀土矿物特征及其意义--以北京、大连地区为例 总被引：4，自引：0，他引：4
大连震旦系十三里台组首次发现自生稀土元素矿物独居石后 ,在北京十三陵中元古代长城系常州沟组、串岭沟组和大红峪组又发现了自生的独居石以及其他磷酸盐和硅酸盐稀土矿物。自生稀土矿物的形成和岩石中稀土元素含量较高有关 ;电子探针背散射图像和 P、Th、L a、Ce、Nd、Y等元素面分布图像研究表明 ,沉积岩中的自生稀土矿物与岩浆岩、变质岩和碎屑成因的截然不同。本文首次报道了元古宙长城系常州沟组沉积岩中碎屑锆石边部成岩过程中形成的自生磷钇矿。中国北方元古宙泥砂质碎屑沉积岩普遍具有高稀土组合 ,许多地区都可能存在独居石等自生稀土矿物 ,如天津蓟县、辽西、辽南和宣化等地 ,为利用离子探针 (SHRIMP)确定其同位素地质年代提供了可能。此外 ,本文对比了大连震旦系自生独居石和内蒙白云鄂博矿区东矿的独居石晶形和化学成分的相似性 ,再一次提出中国北方元古宙富稀土地层可能是内蒙古白云鄂博巨型稀土元素矿床的矿源层问题。 相似文献
3.
The total rare-earth element values(ΣREE)of loess in the Xinjiang region vary over a range of 128-200 ppm ,with an average of 153ppm .The average REE content of loess lies between the earth‘s crust (155ppm) and sedimentary rocks(151ppm).The Xinjiang loess,with the REE distribu-tion patterns characterized by negative slopes ,is rich in the Ce-family elements, and has a distribu-tion pattern characteristic of sedimentary rocks.The North Xinjiang loess is relatively depleted in Tb,but rich in Yb and Lu.The South Xinjiang loess is relatively rich in light rare-earth elements.This is full proof that the Xinjiang loess comes partly from weathered materials(clay rock,sandstone)in the region studied.The REE distribution patterns in the Xinjiang loess are similar to those in the precipitated dust and Aeolian sand,indicating the same material source.The REE distribution pat-terns in the Xinjiang loess are also similar to those in loess from the middle Yellow River Valley,China and Taskent,the former USSR.This implies that loesses of the three locations(Xinjiang,the mid-dle Yellow River Valley and Taskent) come from a common material source.But the REE patterns in the Xinjiang loess are different from those in wall rocks (volcanic rock,K-bearing volcanic rock).Generally ,LREE/HREE,Eu/Eu* and Ce/Ce* ratios reflect the features of parent materials of loess,indicating that the parent rocks were probably in the early stage of alkaline weathering and the weathered materials existed in an oxidation environment with basic mediums under arid-climatic conditions before transport.As a result,the migration ability of the REE is weak. 相似文献
4.
We propose that the europium excess in Precambrian sedimentary rocks, relative to those of younger age is derived from volcanic rocks of ancient island arcs, which were the source materials for the sediments. Precambrian sedimentary rocks and present-day volcanic rocks of island arcs have similar REE patterns, total REE abundances and excess Eu, relative to the North American shale composite. The present upper crustal REE pattern, as exemplified by that of sediments, is depleted in Eu, relative to chondrites. This depletion is considered to be a consequence of development of a grandioritic upper crust by partial melting in the lower crust, which selectively retains europium. 相似文献
5.
There is a correlation between thorium and the light rare earth elements, indicated by La/Th ratios in fine grained sedimentary rocks of various ages from Australia and Greenland. The correlation between Th and the heavy rare earth elements (Th/Yb) is much less significant. Archean sedimentary rocks have a higher La/Th (3.6 ± 0.4) than post-Archean sedimentary rocks (La/Th = 2.7 ± 0.2).The cause of this correlation can be attributed to the coherent behaviour of these elements during most sedimentary processes (weathering, transport, diagenesis, etc.). Since the chondrite-normalized rare earth element distribution of clastic fine grained sedimentary rocks is accepted to be parallel to the distribution of REE in the upper continental crust, an estimate of upper crustal Th abundances can be made. Using reasonable assumptions of certain elemental ratios (K/U, Th/U, K/Rb) in the upper crust, minimum estimates of the abundances of K, U and Rb can also be made for the post-Archean and Archean upper crusts.The post-Archean values (K = 2.9%; Rb = 115 ppm; Th = 11.1 ppm; U = 2.9 ppm) compare favourably to some previous estimates made from direct sampling and theoretical considerations and help confirm a granodiorite present day upper continental crust. The Archean data (K = 0.92%; Rb = 30ppm; Th = 3.5 ppm; U = 0.92 ppm) support models which suggest a significantly more mafic exposed crust at that time. 相似文献
6.
7.
前寒武纪沉积岩中自生独居石的发现及其意义 总被引：5，自引：1，他引：5
由于受到葛家屯组中发现自生方铅矿的启发 ,首次于大连前寒武纪震旦系十三里台组泥岩中发现了自生独居石 ,这一发现为在中国北方前寒武纪沉积岩中寻找自生独居石提供了重要的线索。它为进一步探索 U、Th- Pb同位素测年拓展了新的研究领域和提供了可能性。本文列举了中国北方前寒武纪沉积岩中 Ce元素异常以及 REE较高的例证 ,认为上述地区都有可能发现自生独居石。同时介绍了最近又在北京十三陵中元古代的常州沟组和串岭沟组所发现自生独居石的新资料。研究表明 ,自生独居石的电子探针扫描形态 ,与岩浆岩、变质岩及砂矿中截然不同。在地质年代分布上 ,元古宙 REE相对丰度较高 ,特别是 L REE较高的泥质岩多数来自古陆壳上。资料对比结果显示 :在 L a+Ce+Nd、Yb+Y、Sm+Gd+Dy三角图中 ,北京十三陵元古宙泥质岩、大连震旦系十三里台组泥岩和辽南 -辽西中元古代泥质岩都属于近古陆的沉积类型。首次提出 ,中国北方元古宙沉积与南方震旦系磷块岩沉积环境存在明显的不同并反映在三角图中 ,前者离 L a+Ce+Nd端点近 ,而后者由于成因上属于洋流上升沉积物而远离该端点。按照大连震旦系十三里台组沉积环境特点 ,自生独居石应为生物成矿作用的产物 ,含矿物泥岩形成于总体氧化环境中的局部还原亚环境中。基于此 ,建立了 相似文献
8.
为了探讨华北克拉通南缘早前寒武纪形成演化,对华北克拉通南缘小秦岭地区早前寒武纪变质基底花岗质片麻岩开展了SHRIMP U-Pb锆石定年、Hf同位素组成及地球化学组成研究,获得了2个TTG片麻岩样品~2.50 Ga的岩浆锆石年龄和时代相近的变质锆石年龄,岩浆锆石εHf（t）值为0.71~4.37;1个花岗质片麻岩样品的岩浆锆石年龄为~2.30 Ga,εHf（t）值为－1.21~0.11.它们也记录了古元古代晚期变质锆石年龄或显示古元古代晚期构造热事件影响的存在.岩石以轻重稀土强烈分异为特征,大离子亲石元素相对富集,Nb、P、Ti相对亏损.在小秦岭地区首次发现新太古代晚期TTG岩石,并识别出新太古代晚期构造热事件.现有资料支持华北克拉通南缘与华北克拉通其他地区具有类似的早前寒武纪形成演化历史的认识.但是,华北克拉通南缘以2.3 Ga左右岩浆作用十分发育而显示出独特性. 相似文献
9.
《International Geology Review》2012,54(4):576-591
The total area of Precambrian rocks exposed at the surface or covered with a thin veneer of Quaternary deposits is estimated at 25 million km2, or 17 percent of the land surface. Recent development of test-hole drilling is yielding an increasing amount of information on the Precambrian crystalline basement of platforms covered with a thick sedimentary mantle. The picture of the Precambrian that has been built up is one of strongly altered rocks, injected with magmatic material, which have frequently been remelted and have herefore lost their original outlines. Petrographic study of Precambrian shields has been most widely employed and absolute dating has been concerned more with a particular intrusive phase than a particular stratigraphic complex — even though sedimentary-metamorphic rocks occur extensively and magmatic rocks merely breach them. The time has come to construct a stratigraphy of the Precambrian, not by distinguishing phases of rnagmatism and linking them up with stratified rocks, but on the basis of sequence of bedding as expressed in sedimentary-metamorphic complexes. Detailed mapping even in regions where strongly metamorphosed rocks occur and injections of magmatic material are considerable has established that the original sedimentary rocks are on the whole well preserved, that no remelting or displacement of the original material occurs to disturb boundaries between strata, and that the general character of the bedding has been retained. Aims of Precambrian geologic study are summarized and methods are enumerated. — W. D. Lowry. 相似文献
10.
西成矿田海底热水喷溢沉积成因硅质岩及其与矿化关系 总被引：2，自引：0，他引：2
硅质(化)岩与铅锌矿化关系十分密切是西成矿田显著特征之一,特别是毕家山型矿床,其主要矿体的容矿岩石为硅质岩,本文重点研究了该矿田硅质岩岩石学和地球化学特征及其与矿体(层)时空关系,认为该区硅质岩不是正常生物化学沉积成因的,而属热水沉积形成;并讨论了硅质岩与矿化关系以及硅质在成矿中的作用,为确定该类矿床的成因提供了重要证据. 相似文献
11.
Yusheng Wan ;Xianzheng Zhao ;Zejiu Wang ;Dunyi Liu ;Alfred KrOner ;Chunyan Dong ;Hangqian ;Yuansheng Geng ;Yuhai Zhang ;Runlong Fan ;HuiyiSun 《地学前缘(英文版)》2014,5(4):471-484
The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton. 相似文献
12.
A. D. Nozhkin A. V. Maslov N. V. Dmitrieva Yu. L. Ronkin 《Geochemistry International》2012,50(7):574-610
The paper reports data on metapelites (gneisses and crystalline schists) from various Early Precambrian lithostratigraphic units of the Yenisei Range (Kan and Yenisei complexes, Garevka Unit, and Teya Group). A representative selection of 57 samples from the examined vertical section were analyzed for major oxides, LILE, transition elements, HFSE, REE, radioactive elements, and other trace and minor elements. The most important distinctive geochemical feature of metapelites composing most of the vertical section is their elevated concentrations of Al, which are higher than in the Post-Archean Australian Shale (PAAS), K, Rb, Ba, Ga, Sc, Pb, Th, Nb, Y, and REE. These data and the systematics of elements in certain discriminant diagrams testify that the metasedimentary complexes in the sedimentation basin and the rocks associations composing the erosion area (crystalline massifs of the ancient basement of the Siberian craton) were strongly geochemically differentiated and mature. The composition of the eroded crustal material was close to the average composition of the post-Archean continental crust and PAAS. The metapelites of the Kuzeeva Unit in the Kan Complex and Penchenginskaya Formation in the Teya Group were the only ones that could have basic rocks as an additional source of material, as also follows from the elevated Cr concentrations of the rocks and their REE systematics. The three types of REE patterns of the metapelites largely correspond to the composition of the rocks that composed the ancient drainage areas and the degrees of the differentiation and averaging of their terrigenous material. The first predominant type corresponds to PAAS and the averaged composition of the upper continental crust. The second and third types of the REE patterns with high and low LaN/YbN ratios, respectively, and with or without negative Eu anomalies provide evidence of the contribution of tonalite-trondhjemite-granodiorite (TTG) or basite sources typical of granite-greenstone provinces. In certain diagrams, the compositional fields of Early Precambrian metapelites in the Yenisei Range almost exactly coincide with the compositional fields of regional fine-grained Riphean terrigenous rocks. This suggests that the Late Precambrian sedimentary rock sequences inherited their geochemical features from more ancient rocks. The reproduced ancient geodynamic environments in which the Early Precambrian metaterrigenous complexes of the Yenisei Range were accumulated correspond to ensialic continental marginal basins. The rocks of the Kan and Yenisei complexes and the Garevka Unit were formed mostly on active continental margins and, less frequently, on passive margins. The protoliths of metasedimentary rocks of the Teya Group were accumulated in a subplatform environment at a passive margin. 相似文献
13.
Insolation weathering and hygric dilatation: two competitive factors in stone degradation 总被引：2，自引：0，他引：2
In the natural environment, temperature and moisture are very important parameters in stone degradation. Therefore, on 28 samples from different lithologies (magmatic, metamorphic and sedimentary rocks) thermal and hygric expansion measurements were performed in order to constrain their behaviour related to environmental changes. It was shown that marbles, even those that are dolomitic, are sensitive to temperature impacts. For rocks with clay minerals the effect of temperature may be responsible for shrinking. Finally the amount of residual strain, i.e. the formation of microcracks due to temperature action controls the rate of stone degradation. For most sedimentary rocks no residual strain after heating was observed. In contrast to most magmatic and metamorphic low-porosity rocks, sedimentary rocks contain swelling minerals (like smectite and even phyllosilicates), which are very sensitive to hygric attacks. The cycles of wetting and drying, i.e. the natural environmental effects, may significantly control the deterioration velocity caused by hygric expansion. 相似文献
14.
Six graywackes from the Archean greenstone belt in the Wind River of Wyoming and 11 graywackes and three shales from the Archean Fig Tree Group, South Africa, were analyzed for the rare earth elements (REE). There are real deviations beyond analytical uncertainty among sediments from the same formation. The absolute abundance of the REE (∑REE) is somewhat lower than that of the North American shale composite (NASC) which is representative of younger sedimentary rocks. Still, the results confirm previous suggestions that the RE pattern in Precambrian sediments is the same as the NASC but that, relative to the heavy REE, graywackes are slightly enriched in the light REE, shales are depleted, and there is a slight enrichment in Eu relative to the NASC. The average abundance of Eu relative to the other REE for all the Precambrian sediments is the same as that in chondritic meteorites. Attesting to the complexity of graywacke genesis, there is no correlation between the variations in the REE parameters and the variations in major or minor element concentrations. In particular, there is no obvious correlation between the excess Eu and Sr abundance. The total amount of REE, the ratio, and the Eu enrichment factor, however, all increase in the graywackes with the amount granite-gneiss detritus in the rocks. In general, the REE distributions in Archean graywackes and shales appear to be related to the relative amounts of clastic feldspar, mica, and minor phases which concentrate the REE. 相似文献
15.
对大石寨组火山岩的形成时代和形成背景的认识存在较大的分歧。对采自建组剖面底部英安岩样品中的锆石进行阴极发光照相、微区原位LA-ICP-MS微量元素分析和U-Pb测年。锆石U-Pb同位素分析结果显示,锆石发育典型的岩浆震荡环带, 高Th/U(0.51~1.57),轻稀土元素亏损,重稀土元素富集,具有强烈的正Ce异常和强烈的负Eu异常等特征,表明锆石均属于岩浆成因。22个分析点获得的谐和年龄为(314±1)Ma(MSWD=0.88),代表了大石寨组火山岩的形成时代。结合前人的资料,认为大石寨组火山岩为裂陷早期的产物,而晚古生代的沉积岩石组合为在这一基础上发展起来的具有裂陷盆地性质的沉积组合。 相似文献
16.
QIN Tao GUO Rongrong ZANG Yanqing QIAN Cheng WANG Yan SI Qiuliang SUN Wei MA Yongfei 《《地质学报》英文版》2019,93(5):1434-1455
The Zhalantun terrane from the Xing'an massif, northeast China, was used to be considered as Proterozoic basements. However, amounts of detrital zircon ages from the meta-sedimentary rocks deny the existence of Precambrian basements recently. Notably, magmatic rocks were barely reported to limit the exact ages of the Zhalantun basements. In this study, we collected rhyolite, gabbro and quartz diorite for zircon in-situ U-Pb isotopic dating, which yield crystallization ages of ～505 Ma, ～447 Ma and ～125 Ma, respectively. Muscovite schist and siltstone define maximum depositional ages of ～499 Ma and ～489 Ma, respectively. Additionally, these dated supracrustal rocks and plutons also yield ancient detrital/xenocryst zircon ages of ～600–1000 Ma, ～1600–2220 Ma, ～2400 Ma, ～2600–2860 Ma. Based on the whole-rock major and trace element compositions, the ～505 Ma rhyolites display high SiO_2 and alkaline contents, low Fe_2O_3T, TiO_2 and Al_2O_3, and relatively high Mg O and Mg#, which exhibit calc-alkaline characteristics. These rhyolites yield fractionated REE patterns and negative Nb, Ta, Ti, Sr, P and Eu anomalies and positive Zr anomalies. The geochemistry, petrology and Lu-Hf isotopes imply that rhyolites were derived from the partial melting of continental basalt induced by upwelling of sub-arc mantle magmas, and then experienced fractional crystallization of plagioclase, which points to a continental arc regime. The ～447 Ma gabbros exhibit low Si O2 and alkaline contents, high Fe2 O3 T, Ti O2, Mg O and Mg#. They show minor depletions of La and Ce, flat MREE and HREE patterns, and negative Nb, Ta, Zr and Hf anomalies. Both sub-arc mantle and N-MORB-like mantle were involved in the formation of the gabbros, indicative of a probable back-arc basin tectonic setting. Given that, the previously believed Proterozoic supracrustal rocks and several plutons from the Zhalantun Precambrian basements were proved to be Paleozoic to Mesozoic rocks, among which these Paleozoic magmatic rocks were generally related to subduction regime. So far, none Proterozoic rocks have been identified from the Zhalantun Precambrian basement, though some ～600–3210 Ma ancient detrital/xenocryst zircons were reported. Combined with ancient zircon ages and newly reported ～2.5 Ga and ～1.8 Ga granites from the south of the Zhalantun, therefore, the Precambrian rocks probably once exposed in the Zhalantun while they were re-worked and consumed during later long tectonic evolutionary history, resulting in absence of Precambrian rocks in the Zhalantun. 相似文献
17.
Gold Deposits in Beishan Mountain, Northwestern China 总被引：2，自引：0，他引：2
Abstract. The Beishan Mountain spans three provinces ‐ Gansu, Xinjiang and Inner Mongolia, having an area of 120,000 km2 Tectonically, it transverses three different tectonic units, i.e. Siberia, Kazakhstan and Tarim plates, and is composed of nine ter‐rains with widely exposed Precambrian and Paleozoic strata, complex structures, intensive magmatic activities and widespread ore deposits. It is not only a main part of Tianshan‐Yinshan‐Great Hinggan metallogenic belt in China, but also a key to under‐stand the evolution of central‐Asian orogenic system. At present, more than 100 gold deposits and prospects have been discovered, explored and mined, among which Nanjinshan, Mazhuangshan, Liushashan, Jinwozi, Zhaobishan and Xiaoxigong are the most important ones. Based on the host rocks and the geological features, all these gold occurrences can be subdivided into three groups (or types): (1) hosted by Carboniferous or Permian volcanic or subvolcanic rocks; (2) hosted by or related to plutonic intrusions; and (3) hosted by Precambrian metamorphic rocks. The first group includes the Mazhuangshan gold deposit, which occurs in Hercynian quartz por‐phyry and rhyolite porphyry as gold‐bearing quartz veins. The second group is composed of the Liushashan, Nanjinshan Zhaobishan and Jinwozi gold deposits. Gold mineralization at these four deposits occurs within Hercynian granitoids intrusion: or late Paleozoic sedimentary rocks as quartz veins, veinlets and altered rocks. The Xiaoxigong gold deposit belongs to the third group, and is hosted by Precambrian schist, amphibolite and migmatite as quartz veins and altered rocks. Isotopic age dating data, geological and geochemical evidence suggest that most of the groups 1 and 2 gold deposits were generated during the emplacement of the Hercynian or partly Indosinian intrusions. These intrusions may provide both heat and metals for groups 1 and 2 deposits. In contrast, although the formation is closely related to the Hercynian magma‐tism, the ore‐forming materials of the group 3 deposits may not only come from the intrusions, but also from the Precambrian metamorphic rocks. 相似文献
18.
Abstract: The North granitic body of the Miyako pluton is located in the Northern Kitakami belt, Northeast Japan. The formation of the scheelite–chalcopyrite–magnetite–bearing aplitic veins and scheelite–chalcopyrite–magnetite–bearing Yamaguchi skarn deposit was closely associated with the formation of the Miyako plutons. Petrographic facies of the North granitic body vary from quartz diorite in marginal zone (zone A), to tonalite and granodiorite (zone B), and to granite (zone C) in the central. The large numbers of aplitic veins distributed around the Yamaguchi mining area are divided into two groups: barren and scheelite–mag–netite–chalcopyrite–bearing aplitic veins. The latter cut massive clinopyroxene skarns of the Yamaguchi deposit, and are composed of plagioclase, K‐feldspar and titanite. Some plagioclase crystals have dusty cores with irregularly shaped K‐feldspar flakes, and clear rims of albite. Textures of plagioclase in the mineralized aplitic veins are different from the idiomorphic textures with sharp plagioclase crystal boundaries that occur in the North granitic body and barren aplitic veins. These textural data suggest that the mineralized aplitic veins were formed from hydrothermal fluid. Changes in the contents of major and minor (Rb, Sr, Sc, Co, Th, U) elements in the North Miyako granitic body are similar to those of zoned plutons formed by typical magmatic differentiation processes. On the other hand, concentrations of REE, especially middle to heavy REE, of granitic rocks in zone C and barren aplitic veins are significantly lower than those of granitic rocks in zones A and B. The hypothetical chondrite‐normalized REE patterns, calculated assuming fractional crystallization from zone B granitic melt, suggest that REE concentrations of the residual melt increased with the degree of fractional crystallization, and changed into a pattern with enriched LREE and strongly negative Eu anomaly. However, the REE patterns of granitic rocks in zone C are different from the hypothetical patterns. Moreover, the REE patterns of magnetite–scheelite–chalcopyrite aplitic veins are quite different from those of granitic rocks. The Cu contents of granitic rocks in the North Miyako body increase from zone A (5–26 ppm) to zone B (10–26 ppm), and then clearly decrease to zone C (5–7 ppm) and drastically increase to the barren aplitic veins (39–235 ppm). Concentrations of Cu in the mineralized aplitic veins are also higher than those of the granitic rocks in zone C. The decrease in REE and Cu contents of granitic rocks from zone B to zone C is not a result of simple magmatic fractional differentiation. Fluid inclusions in quartz from mineralized aplitic veins contain 3.3 wt% NaCl equivalent and 5.8 wt% CO2. It was also demonstrated experimentally that the removal of MREE and HREE by fluid from melt enabled the formation of complexes of REE and ligands of OH‐ and CO32‐. Based on the possibility that the melt of the granitic rocks of zone C and the mineralized aplitic veins coexisted with CO2‐bearing fluid, it is thought that REE were extracted from the melt to the CO2‐bearing fluid, and that the REE in the mineralized aplitic veins were transported by the CO2‐bearing fluid. It is likely that the low HREE and Cu contents of the granitic rocks in zone C could have been caused by the removal of those elements from the granitic melt by the fluid coexisting with the melt. The expelled materials could have been the sources of scheelite–magnetite–chalcopyrite–bearing aplitic veins and copper mineralization of the Yamaguchi Cu‐W skarn deposit. 相似文献
19.
In order to study the characteristics of sea-floor exhalative sedimentary and magmatic hydrothermal superimposition on the Bainiuchang polymetallic deposit, the REE compositions of the granites, host-rocks and ores have been systematically analyzed by ICP-MS. As viewed from their REE compositions, the granites show obvious negative Eu anomalies and weak negative Ce anomalies. According to their REE characteristics, the host-rocks were derived partly from sea-floor exhalative sediments. In terms of their REE compositions, the ores can be divided into two groups: one group, of which the samples were collected from the Baiyang segment relatively far away from the Bozhushan granite batholith, possesses positive Eu anomalies or no Eu anomaly and negative Ce anomalies, indicating that ore-forming hydrothermal fluid was relatively reductive and its temperature was higher than 250 ℃. Furthermore, the coinstantaneous presence of positive Eu anomalies and negative Ce anomalies indicate that the convective mixing of a little amount of seawater with hydrothermal fluid had happened while ores were precipitated on ancient sea floor. The other group, of which the samples were mainly collected from the Chuanxindong and Duimenshan segments near the Bozhushan granite batholith, has similar chondrite-monalized REE distribution patterns to those of the magmatic rocks. But as a whole, the REE characteristics of both groups change gradually starting from the Bozhushan granite batholith. Based on the REE characteristics of the granites, host-rocks and ores, it is suggested that the ore-forming metals seem to have come from several different sources. 相似文献
20.
《Journal of Asian Earth Sciences》2000,18(1):109-121
The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(LaN/SmN)ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(LaN/SmN)ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent. 相似文献