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One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.
In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.
The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences. 相似文献
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本文通过对胶东地区早前寒武变质地层和花岗岩的研究，提出胶东绿岩带在变质地层、同位素年龄、变质程度、韧性剪切带和矿产等方面存在的证据，与世界典型绿岩带相比，胶东绿岩带具有５个方面的显著特征：①胶东绿岩带下部层位未发现完整科马提岩地层；②胶东绿岩带变质程度较高，一般可达角闪岩相，少数达麻粒岩相；③胶东绿岩带分布面积不大；④胶东绿岩带形成时间在２８．５８～２６亿ａ；⑤胶东绿岩带主要产出Ａｕ，Ａｇ，Ｚｎ，Ｃｕ等矿产。 相似文献
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O. G. Lazur 《Lithology and Mineral Resources》2009,44(1):36-42
Issue of the correlation between ferruginous-siliceous accumulation and volcanism at early stages of the continental lithosphere evolution is considered. Magnetite quartzites of the Early Precambrian volcanosedimentary greenstone and intensely metamorphosed complexes are subdivided into volcanosedimentary and sedimentary genetic types. Major features of the evolution of ferruginous-siliceous accumulation in the Early Precambrian are elucidated and a model of the evolution of volcanogenic iron accumulation is proposed. Principles of the prognosis of magnetite mineralization in the Early Precambrian metamorphic, primary volcanosedimentary, and greenstone complexes are elaborated. 相似文献
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The Zhaoanzhuang-type iron depositsoccur in the ultrabasic unit of the late Archaean greenstone belt. They are late magmatic apatite-titanomagnetite deposits, superimposed and enriched by hydrothermal solutions after their formation. Later they have also been subjected to regional metamorphism. These deposits are extremely complex in mineralogy, containing 87 kinds of mìnerals characteristic of ultrabasic-alkali complexes. Mineralization of iron, titanium and phosphorus took place mainly during the magmatic stage, while that of uranium and thorium during the pneumatolytic-hydrothermal stage. 相似文献
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简叙辽东地区太古宙绿岩带地质背景,并结合太古宙绿岩带铁-铀型铀矿成矿地质特征的阐述,提出太古宙绿岩带铀成矿作用具有明显的前寒武纪主期性。讨论太古宙富铀钾质花岗岩相关的太古宙残留体内构造-蚀变岩控制铀矿的观点：提出古韧性剪切-破碎断裂带、太古宙基底岩体顶上凸起带之凹洼中的＂突触＂与碱交代岩等三位一体的构造空间和构造-蚀变岩定位层间-不整合破裂带关联-碱交代热液型铀矿的成矿模式,并且太古宙绿岩带铁-铀型铀矿围岩蚀变清楚,铀成矿谱系反映为初贫后富-叠加再造成矿的矿物组合;以复杂-叠加、脉络清楚的太古宙绿岩带构造环境相关的铀矿聚集区为例,探讨太古宙绿岩带铀成矿作用的多期性。并认为太古宙绿岩带铁-铀型铀矿矿体埋藏深、隐伏性强、盲矿体多,潜在的找矿前景相当可观。 相似文献
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T. C. Devaraju R. P. Viljoen R. H. Sawkar T. L. Sudhakara 《Journal of the Geological Society of India》2009,73(1):73-100
Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the
West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts
and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in
nature, are preserved in the Sargur Group which is more than 3.3–3.4 Ga old, the youngest being manifested by 63–76 Ma old
mafic dyke magmatism, possibly related to Deccan volcanism.
In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties,
the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the
eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic
sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions
of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar
and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated
that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those
in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga.
Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8
and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central
part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined
to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga.
From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC
are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially
exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal
groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly
in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga
belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern
greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization
is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids,
derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures.
The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly
of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism
and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the
more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous
kimberlite/lamproite intrusions in the EDC. 相似文献
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Gravity anomaly patterns in the south-central Zimbabwe Archaean craton and their geological interpretation 总被引：1，自引：0，他引：1
The granite-greenstone terrain of south-central Zimbabwe, encompassing the Belingwe (Mberengwa) greenstone belt and sections of the Great Dyke, provides important constraints on models for the evolution of the Zimbabwe craton and the Archaean crust in general. In this paper we enhance and model existing and recently acquired gravity data from the region and correlate the anomalies and their derivatives with the known basement geology to evaluate models for greenstone belt development. We also study the spatial gneiss-granite-greenstone association in general, and the geologic implications of models of the anomaly patterns in particular. Although the Belingwe greenstone belt has been mapped, its subsurface geometry is poorly known. Similarly, the Great Dyke is well studied, but no systematic study of the extent and cross-cutting relations of other mafic dykes in the Archaean crust has been undertaken.The regional gravity field shows no evidence for crustal thickness variations in the area and the gravity anomalies can be explained by lateral density variations of the supracrustal rocks. Prominent gravity highs are observed over the high density ( 3000 kg/m3) volcano-sedimentary piles (greenstone belts) and ultramafic complexes. Well-defined elongate, sub-oval/elliptical gravity lows are associated with intrusive granitic plutons. The granite-greenstone contacts are marked by steep gravity gradients of up to 5 mGal/km that imply steeply dipping or near-vertical contacts for the anomalous bodies. This is tested and confirmed by 21/2D modelling of gravity profiles across the Belingwe and Fort Rixon greenstone belts, constrained by measured densities and observed geological data. The modelling also indicates that these belts, and possibly all the belts in the study area (based on comparable densities and anomaly amplitudes), have limited depth extents in the range of 3–5 km. This is comparable to thicknesses obtained elsewhere from deep seismic reflection data and geoelectrical studies, but mapped stratigraphic thicknesses give a maximum depth extent of about 9.5 km. Present studies and previous work support the idea that the volcanics were extruded within rift zones and laid on older granitic crust, followed by subsidence and rapid deposition of sediments that were sourced from the adjacent basement terrains. The volcano-sedimentary sequences were subsequently deformed by intruding younger plutons and affected by late-stage strike-slip activity producing cross-cutting structures. 相似文献
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中国绿岩带主要分布在华北地台。绿岩带产在古陆核之间或边缘，可能形成在古裂谷或古岛弧的构造环境。按原岩建造、地球化学特征和成矿作用，绿岩带可分夹皮沟型、清原型和小秦岭型。按后期的活化改造作用强度，可分基本稳定型、活化改造型和强烈活化改造型绿岩带。根据变质程度的不同，可分高级变质和中级变质为主的绿岩带。华北地台绿岩带的形成可分四个时期：中太古代、新太古代早期、新太古代晚期和早元古代。华北地台绿岩带同其他国家绿岩带相比较，其岩石类型、不一的变质程度、相似的花岗质岩石、形成在早前寒武纪和赋存丰富矿产资源等基本地质特征相似。但也具有独自的特色：１．分布面积小；２．科马提岩不甚发育；３．同构造晚期的浅成花岗岩侵入体尚未发现；４．变质程度较高；５．赋存的矿产类别有差异；６．受后期的活化改造作用强烈。 相似文献
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坦桑尼亚克拉通西北部的苏库马绿岩带是坦桑尼亚环维多利亚湖绿岩带重要的金矿聚集区。区内金矿化类型以含金石英脉为主,矿体的产出主要受剪切构造带控制。通过研究区域构造特征和岩石地球化学特征,认为苏库马绿岩带中玄武岩为弧后环境形成,流纹岩和花岗岩形成于陆弧环境,N--S向挤压作用和E--W向伸展作用是区域内主要的构造展布特征。建立了含金石英脉的成矿模式,表明具有金矿化作用的石英脉集中分布在EW、SEE、NE向剪切构造破碎带中,含金石英脉富矿体的形成经历了多期构造变形和成矿流体的叠加、富集作用。 相似文献
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Southern Cross was one of the earliest gold mining centres in Western Australia. Over 142 tonnes of gold have been produced
from the district, and, on a gold per hectare basis, the Southern Cross greenstone belt in the southwestern Yilgarn Craton
is the most productive of Western Australia's Archaean greenstone belts. The SW Yilgarn Craton is characterised by high-grade
(amphibolite- to granulite-facies) metamorphism, extensive granitoid magmatism and older greenstone volcanism ages, compared
to the well-known greenschist-facies metamorphism and younger (2.7 Ga) eruption ages which dominate in the Eastern Goldfields
Province. The Pb-isotope compositions of deep-seated granitoids in the SW Archaean Yilgarn Craton, which were emplaced coeval
with a craton-wide major orogenic lode-gold mineralization event at about 2.64–2.63 Ga, have been determined for 96 whole-rock
and 24 K-feldspar samples. The Pb isotope data of the granitoids are consistent with a crustal origin for their genesis, probably
by reworking (partial melting) of older continental crust. The Pb isotope composition of greenstones, which are the main host
rocks for gold mineralisation, and pyrites from the komatiite-hosted syngenetic Ni deposits in the amphibolite-facies Forrestania
greenstone belt, have also been determined, with initial Pb-isotope ratios higher than that for the Eastern Goldfields Province.
The Pb isotopic character of the orogenic lode-gold deposits in the region is intermediate between coeval granitoid and greenstone
Pb, indicating that the ore fluids contained metals from both reservoirs. The Pb in the ore fluid of the most deeply formed
deposit, Griffin's Find, overlaps the isotopic composition of coeval granitoids, indicating the deep-seated granitoid magmatism
was the primary source for Pb in the ore fluids.
Received: 8 October 1998 / Accepted 22 December 1998 相似文献
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地理信息系统在评价冀东地区绿岩型金矿资源中的应用 总被引：1，自引：1，他引：1
本文应用地理信息系统（ＧＩＳ）对冀东地区的地质、地球化学数据和地球物理、遥感解释资料（包括１∶５０万地质图，１∶２０万水系沉积物和１∶１００万地球物理、遥感）的推断成果，进行了综合分析。应用专家知识和统计规律建立起该区以地球化学为主要特征的绿岩型金矿的预测模型，并利用逻辑运算和贝叶斯规则把各种资料（反映绿岩型金矿的特征）有机地结合起来。采用地理信息系统空间分析有效手段，编制出了研究区绿岩型金矿预测图。以概率的形式表示金矿的潜在远景区，明确地证明了区内已知的大中型和小型矿区和高概率区是相符的，同时指示出一些有意义的靶区，为进一步勘探提供了证据 相似文献
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Shen Baofeng Peng Xiaoliang Luo Hui Mao DebaoTianjin Institute of Geology Ministry of Geology Mineral Resources Tianjin 《《地质学报》英文版》1994,68(1):15-29
Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparable to those in other countries but at the same time have unique features of their own. In view of their geochemistry, the Chinese greenstone belts may be grouped into three types: the Jiapigou type, Qingyuan type and Xiaoqinling type. The greenstone belts were formed possibly in a rift-type palaeo-tectonic setting, similar to that of the modern island are-continental margin mobile belts. 相似文献
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Regularities in the formation and evolution of volcanosedimentary complexes in the Early Precambrian
O. G. Lazur 《Lithology and Mineral Resources》2009,44(6):587-598
The main objective of this work was to define directions and principal features of the evolution of volcanism and sedimentation
in Early Precambrian active volcanic zones. Analysis of reconstructed type sections of intensely metamorphosed and greenstone
(sedimentary-volcanic) complexes revealed a general similarity of their structure and universal presence of two principal
(contrast and differentiated) volcanic associations that replace each other from the bottom to top (Lazur, 2006). Volcanic
complexes are crowned with significantly sedimentary sequences and the manifestation of bimodal (basalt-rhyolite) or acid
(dacite-rhyolite) volcanism (Luchitskii et al., 1982). Our work is based on specific rock complexes, for which the primary
nature of intensely metamorphosed Early Precambrian rocks has been reconstructed by geological, mineralogical, geochemical,
and other methods (Lazur, 1986; Rosen et al., 2005). 相似文献
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Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in the Archean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low-grade greenstone succession, dragged to mid-crustal depths (6 kbar) during greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low-strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery. Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–greenstone terranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history. 相似文献
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We combine a geological, geochemical and tectonic dataset from 118 ophiolite complexes of the major global Phanerozoic orogenic belts with similar datasets of ophiolites from 111 Precambrian greenstone belts to construct an overview of oceanic crust generation over 4 billion years. Geochemical discrimi- nation systematics built on immobile trace elements reveal that the basaltic units of the Phanerozoic ophiolites are dominantly subduction-related （75%）, linked to backarc processes and characterized by a strong MORB component, similar to ophiolites in Precambrian greenstone sequences （85%）. The remaining 25% Phanerozoic subduction-unrelated ophiolites are mainly （74%） of Mid-Ocean-Ridge type （MORB type）, in contrast to the equal proportion of RiftlContinental Margin, Plume, and MORB type ophiolites in the Precambrian greenstone belts. Throughout the Phanerozoic there are large geochemical variations in major and trace elements, but for average element values calculated in 5 bins of 100 million year intervals there are no obvious secular trends. By contrast, basaltic units in the ophiolites of the Precambrian greenstones （calculated in 12 bins of 250 million years intervals）, starting in late Paleo- to early Mesoproterozoic （ca. 2.0-1.8 Ga）, exhibit an apparent decrease in the average values of incom- patible elements such as Ti, P, Zr, Y and Nb, and an increase in the compatible elements Ni and Cr with deeper time to the end of the Archean and into the Hadean. These changes can be attributed to decreasing degrees of partial melting of the upper mantle from HadeanJArchean to Present. The onset of geochemical changes coincide with the timing of detectible changes in the structural architecture of the ophiolites such as greater volumes of gabbro and more common sheeted dyke complexes, and lesser occurrences of ocelli （varioles） in the pillow lavas in ophiolites younger than 2 Ga. The global data from the Precambrian ophiolites, representative of nearly 50% of all known worldwide greenston 相似文献
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广东绿玉的物质组成和谱学特征 总被引：1，自引：0，他引：1
本文主要通过化学分析、扫描电镜、Ｘ射线衍射，红外光谱和光吸收谱等测试手段对广东绿玉的物质组成，物化特征，呈色机理进行一些探讨，温试结果表明，广东绿玉的矿物成分主要是绢云母，其次为绿泥石，在不同颜色和质量的绿玉中，除了化学成分不同外，两种矿物的含量，矿物的结晶程度和了矣集状态也都不同，所呈现出的各种各样颜色主要与二阶铁离子、三价铁离子和钛离子的浓度及赋存状态有关。 相似文献