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1.
津巴布韦铁矿层分布普遍,是组成太古界津巴布韦克拉通绿岩带地层常见岩性单元,其岩石特征类似于“硫化物相”含铁建造层,实际上是一种伴有硅化和硫化物矿化的剪切带。过去这种构造成因的铁矿层被认为是一种原始沉积岩层,但从野外露头上可明显辨别出呈交织状分布的片理化带、褶皱变形带、石香肠构造和糜棱岩带;区域上铁矿层与层理或片理相交切,岩层呈交织状分布,同一岩层单元会重复出现。在花岗岩底辟上升作用导致岩石发生变形之前,一系列逆推断层使得原始火山沉积岩单元层产生水平伸展、叠瓦状构造推覆以及岩层重复出现。因此绿岩带地层中这种包含有构造型铁矿层的“馅饼状”岩层模式,愈来愈成为地质工作者研究的焦点。 相似文献
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UPb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma UPb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A UPb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma UPb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma UPb zircon age.Regional greenschist metamorphism has been dated directly, with a UPb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma UPb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones. 相似文献
4.
The lower portions of the volcanic sequence of some Archaean greenstone belts include members with crystallized from ultramafic liquids extruded at the earth's surface at 1600–1650°C. These liquids are interpreted as products of 60–80% melting of their mantle source composition which implies more catastrophic conditions of mantle melting than obtained in Palaeozoic, Mesozoic or Recent crust-mantle dynamics. Such conditions may be a consequence of major impacts on the surface of the primitive earth. It is suggested that the production of the lunar maria basins was accompanied by similar impacts on the earth and that such terrestrial maria played an important role in early stages of chemical differentiation of the crust and upper mantle. An hypothesis is presented in which some Archaean greenstone belts are interpreted as very large impact scars, initially filled with impact-triggered melts of ultramafic to mafic composition and thereafter evolving with further magmatism, deformation and metamorphism to the present Archaean greenstone belts. 相似文献
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C.J. Hawkesworth S. Moorbath R.K. O&#x;Nions J.F. Wilson 《Earth and Planetary Science Letters》1975,25(3):251-262
Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced2780 ± 30 m.y. ago.The initial87Sr/86Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement. 相似文献
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Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably date updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840–2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism. 相似文献
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Saeid Cheraghi Mostafa Naghizadeh David Snyder Rasmus Haugaard Thomas Gemmell 《Geophysical Prospecting》2020,68(1):62-81
In 2017, the Metal Earth multi-disciplinary exploration project acquired a total of 921 km of regional deep seismic reflection profiles and 184 km of high-resolution seismic reflection profiles in the Abitibi and Wabigoon greenstone belts of the Superior province of Canada. The Abitibi belt hosts several world-class mineral deposits, whereas the Wabigoon has sparse economic mineral deposits. Two high-resolution surveys in the Swayze area, a poorly endowed part of the western Abitibi greenstone belt, served as pioneer surveys with which to better understand subsurface geology and design a strategy to process other surveys in the near future. Swayze seismic data were acquired with crooked survey geometries along roads. Designing an effective seismic processing flow to address these geometries and complex geology required straight common midpoint lines along which both two-dimensional prestack dip-moveout correction and poststack migration processing were applied. The resulting seismic sections revealed steeply dipping and subhorizontal reflections; some correlate with folded surface rocks. An interpreted fault/deformation zone imaged in Swayze north would be a target for metal endowment if it extends the Porcupine–Destor structure. Because of the crooked line geometry of the surveys, two-dimensional /three-dimensional prestack time migration and swath three-dimensional processing were tested. The prestack time migration algorithm confirmed reflections at the interpreted base of the Abitibi greenstone belt. The swath three-dimensional images provided additional spatial details about the geometries of some reflections, but also had less resolution and did not detect many reflectors observed in two dimensions. Geological contacts between felsic, mafic and ultramafic greenstone rock layers are thought the main cause of reflectivity in the Swayze area. 相似文献
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R.T. Pidgeon 《Earth and Planetary Science Letters》1978,37(3):421-428
Zircon U-Pb systems from a single block sample of columnar dacite from the early Archaean Warrawoona Group from the layered “greenstone” succession of the Pilbara Craton, Western Australia, indicate an age of3452 ± 16 (2σ) m.y. This is interpreted as the age of the dacite, and is the oldest age so far determined from the Archaean of Australia. This confirms the antiquity of the Warrawoona Group and suggests a close age relationship between the Warrawoona Group in the Pilbara and the Onverwacht Group in South Africa.The Rb-Sr whole rock systems of the dacite have been disturbed since emplacement and indicate apparent ages that have no direct geological significance. This is in accord with previous published accounts of resetting of Rb-Sr whole rock ages determined on acid metavolcanics. 相似文献
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The discovery of the Neoarchean mafic dyke swarm in Hengshan and reinterpretation of the previous “Wutai greenstone belt” 总被引：2，自引：0，他引：2
The structural mapping and section study indicate that the “greenstone belts” in the southern to central parts of Hengshan
were intensively sheared and transposed mafic dyke swarm, which originally intruded into the Neoarchean grey gneiss and high-pressure
granulite terrain (HPGT). The HPGT is characterized by flat-dipping structures, to the south it became steep and was cut by
the Dianmen mafic dyke swarm. After high-pressure granulite-facies metamorphic event, the mafic dyke swarm occurred, and was
associated with the extensional setting and reworked by the late strike-slip shearing. The zircon age dating proves that the
Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma, followed by late tectonothermal reworking.
The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton
(NCC), providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other
cratonic blocks of the world. 相似文献
11.
The oldest rocks in the mainland southeastern Australian segment of the (Palaeozoic) Lachlan Fold Belt are Cambrian greenstones which outcrop in three narrow linear belts separated by Lower Palaeozoic marine troughs in which many thousands of meters of predominantly greywacke-shale sediments accumulated. The greenstone belt nearest the Australian craton, the Mt. Stavely Greenstone Belt, is composed of calc-alkalic meta-andesites, metadacites and intermediate and acid pyroclastics. The Heathcote Greenstone Belt, of central Victoria, consists of three segments; the northern and southern segments are very similar and their internal stratigraphy, petrology, and geochemistry suggest they represent an incomplete, disrupted ophiolite. However, the central segment of the Heathcote Greenstone Belt, which is more intensely deformed and metamorphosed than the northern and southern segments, is composed of a calc-alkalic volcanic suite dominated by meta-andesites. The Mt. Wellington Greenstone Belt of eastern Victoria shows remarkable overall similarities to the northern segment of the Heathcote Greenstone Belt and evidence, including the presence of fault slices of gabbro and peridotites, suggests that this belt too is a much disrupted ophiolite. We interpret the ophiolites to have been the crust of a marginal sea which developed by rifting of thin continental-type crust at the leading edge of a palaeo-Australian plate in the early Cambrian. Meta-andesites and associated rocks of the Mt. Stavely Greenstone Belt were probably erupted onto this thin continental crust above a Benioff zone, and a rifted-off fragment of this thin continental crust bearing a cover of calc-alkalic volcanic rocks has been preserved as the central segment of the Heathcote Greenstone Belt during later deformation events. 相似文献
12.
The discovery of the Neoarchean mafic dyke swarm in Hengshan and reinterpretation of the previous "Wutai greenstone belt" 总被引：2，自引：0，他引：2
The structural mapping and section study indicate that the "greenstone belts" in the southern to central parts of Hengshan were intensively sheared and transposed mafic dyke swarm,which originally intruded into the Neoarchean grey gneiss and high-pressure granulite terrain(HPGT).The HPGT is characterized by flat-dipping structures,to the south it became steep and was cut by the Dianmen mafic dyke swarm.After high-pressure granulite-facies metamorphic event,the mafic dyke swarm occurred,and was associated with the extensional setting and reworked by the late strike-slip shearing.The zircon age dating proves that the Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma,followed by late tectonothermal reworking.The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton(NCC),providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other cratonic blocks of the world. 相似文献
13.
《Earth and Planetary Science Letters》2007,253(1-2):1-16
Energy dispersive spectrometry (EDS), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) track analyses of chlorite-dominated quench-textured microkrystite spherules and LA-ICPMS spot analyses of intra-spherule Ni-rich skeletal quench chromites from the 3243 ± 4 Ma Barberton S3 impact fallout unit (lower part of the Mapepe Formation, Fig Tree Group, Barberton greenstone belt, Kaapvaal Craton, South Africa) reveal fractionated siderophile and PGE trace element patterns corresponding to chondrite-contaminated komatiite/basalt compositions. The chlorites, interpreted as altered glass, contain sharp siderophile elements and PGE spikes inherited from decomposed metal and Ni-rich chromite particles. LA-ICPMS spot analysis identifies PGE-rich micronuggets in Ni–chromites (Ir ∼ 12–100 ppm, Os ∼ 9–86 ppm, Ru ∼ 5–43 ppm) and lower levels of the volatile PGEs (Rh ∼ 1–11 ppm, Pd ∼ 0.68–0.96 ppm). Previously reported PGE anomalies in the order of hundreds of ppb in some Barberton microkrystite spherules are accounted for in terms of disintegration of PGE-rich micronuggets. Replacement of the Ni-chromites by sulphide masks primary chondritic patterns and condensation element distribution effects. High refractory/volatile PGE ratios pertain to both the chlorites and the Ni-rich chromites, consistent with similar compositional relations in microkrystite spherules from other impact fallout units in the Barberton greenstone belt and the Pilbara Craton, Western Australia. The near-consistent low Pt/Re and high V/Cr and V/Sc ratios in chlorite of the spherules, relative to komatiites, are suggestive of selective atmospheric condensation of the spherules which favored the relatively more refractory Re and V. Selective condensation may also be supported by depletion in the volatile Yb relative to Sm. Ni–Cr relationships allow estimates of the proportion of precursor crustal and meteoritic components of the spherules. Mass balance calculations based on the iridium flux allow estimates of the order of magnitude of the diameter of the chondritic projectile. 相似文献
14.
Frank K. Nyame 《Island Arc》2013,22(4):538-548
The Birimian of the West African Craton is an important repository of Paleoproterozoic mafic magmatism, described by many workers as petrologically dominated by basaltic to andesitic rocks. In spite of extensive work done, opinions still differ on the origin and other salient features in these globally significant igneous rocks. Even less well understood is the relationship that may exist between the mafic rocks and prolific metalliferous deposits such as manganese and gold that are hosted in these rocks. This review suggests that the frequently observed association between mafic rocks and important mineral deposits may have far more genetic implications than previously envisaged. 相似文献
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Drop calorimetry measurements between 900 and 1850 K are reported for amorphous anorthite, andesine, wollastonite, diopside, cordierite and pyrope. The isobaric heat capacities of the glassy and liquid phases of these materials, and the enthalpies of fusion of the minerals have been derived. The calorimetric entropies of fusion of these substances and of other oxide minerals are generally consistent with the calculated volume changes on melting and with the observed pressure dependences of the melting points. The thermodynamics of mixing of liquid plagioclases have been examined, and it is concluded that their enthalpies of mixing are markedly different from the values derived directly from solution calorimetry measurements at 985 K. 相似文献
17.
Michael O. Garcia Diane Hanano Ashton Flinders Dominique Weis Garrett Ito Mark D. Kurz 《Bulletin of Volcanology》2012,74(6):1445-1463
The size, shape, and magmatic history of the most recently discovered shield volcano in the Hawaiian Islands, Mahukona, have been controversial. Mahukona corresponds to what was thought to be a gap in the paired sequence (Loa and Kea trends) of younger Hawaiian volcanoes (<4?Ma). Here, we present the results of marine expeditions to Mahukona where new bathymetry, sidescan sonar, gravity data, and lava samples were collected to address these controversies. Modeling of bathymetric and gravity data indicate that Mahukona is one of the smallest Hawaiian volcanoes (～6,000?km3) and that its magmatic system was not focused in a long-lived central reservoir like most other Hawaiian volcanoes. This lack of a long-lived magmatic reservoir is reflected by the absence of a central residual gravity high and the random distribution of cones on Mahukona Volcano. Our reconstructed subsidence history for Mahukona suggests it grew to at least ～270?m below sea level but probably did not form an island. New 40Ar–39Ar plateau ages range from 350 to 654?ka providing temporal constraints for Mahukona’s post-shield and shield stages of volcanism, which ended prematurely. Mahukona post-shield lavas have high 3He/4He ratios (16–21?Ra), which have not been observed in post-shield lavas from other Hawaiian volcanoes. Lava compositions range widely at Mahukona, including Pb isotopic values that straddle the boundary between Kea and Loa sequences of volcanoes. The compositional diversity of Mahukona lavas may be related to its relatively small size (less extensive melting) and the absence of a central magma reservoir where magmas would have been homogenized. 相似文献
18.
The Mill Run watershed is a structurally-controlled synclinal basin on the eastern limb of the Massanutten Mountain complex of northwestern Virginia. Bedrock contacts are obscured by coarse sandstone debris from exposures near basin divides. Colluvium blankets more than half the basin, masking geomorphic surfaces, affecting vegetation patterns, and contributing to the convexity of the alluvial, terrace, pediment and erosion surfaces. Examination of the bedrock geology, geomorphology, soils and vegetation shows distinct distributional correspondences. Vegetation is strongly interdependent with geomorphology, bedrock geology, and soils. On convex colluvial slopes, mixed hardwood forests are most common. In concave coves and deep gorges, mixed hardwoods are replaced by conifers. In thin colluvium, in poorly developed soils, and on blockfields, chestnut oak is singularly prevalent. Conifers dominate shaley bedrock areas. Soils and surficial sediments have a major effect on near-surface hydrology. During wet seasons, cemented horizons in the subsurface cause temporary saturation in the superjacent horizons; lateral movement of soil-water effectively eliminates a vertical component of ground-water recharge. Vegetation is strongly dependent on water availability and thus reflects the distribution of subsurface barriers and sediment-soil fades changes. 相似文献
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Dynamics of dissolved major (Na,K, Ca,Mg, and Si) and trace (Al,Fe, Mn,Zn, Cu,and Cr) elements along the lower Orinoco River 
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下载免费PDF全文 Abrahan Mora Jürgen Mahlknecht Juan Carlos Baquero Alain Laraque Juan A. Alfonso Daniel Pisapia Laura Balza 《水文研究》2017,31(3):597-611
This study addresses the changes in dissolved major and trace element concentrations along the Orinoco River, including the mixing zone between the Orinoco and Apure Rivers. Water samples from the Apure and Orinoco Rivers were collected monthly in four sectors over a period of 15 months. Auxiliary parameters (pH, dissolved oxygen, conductivity, and temperature), total suspended sediments, dissolved organic carbon (DOC), and major (Na, K, Ca, Mg, and Si) and trace (Al, Fe, Mn, Zn, Cu, and Cr) element concentrations were measured in all sectors. The relative contribution of both rivers after the Apure–Orinoco confluence was determined using Ca as a tracer. Moreover, a mixing model was developed to determine whether dissolved species exhibit a conservative behavior during mixing. The results indicate that DOC is removed from waters during the Apure–Orinoco mixing, probably due to absorption of DOC on mineral phases supplied by the Apure River. Dissolved Na, Ca, and Mg behave conservatively during the mixing processes, and their concentrations are controlled by a dilution process. The anomaly in the temporal pattern of K in the Orinoco is caused by the input of biogenic K originating from the Apure River during the high‐water stage. The loss of dissolved Si during the low‐water stage can be explained by the uptake of Si by diatoms. Dissolved Mn, Zn, Al, and Fe showed a non‐conservative behavior during the Apure–Orinoco mixing. The removal of Mn and Zn from the dissolved phase can be explained by the formation of Mn‐oxyhydroxides and the scavenging of Zn onto Mn oxides. Dissolved Fe is controlled by redox processes, although the removals of Fe and Al due to the preferential adsorption of large organometallic complexes by mineral surfaces after the Apure–Orinoco confluence can affect the mobility of both elements during transport. The conservative behavior shown by Cu and Cr can be related to the tendency of both elements to be complexed with small organic colloids, which are not preferentially adsorbed by clays. 相似文献
20.
Geology and isotopic composition of helium,neon, xenon and metallogenic age of the Jinding and Baiyangping ore deposits,northwest Yunnan,China 总被引：23，自引：0，他引：23
TheJindingoredeposit,locatedinthenorthwestYunnanProvince,isthelargestPb-Zndeposit(Pb Zn>15Mt1),averagingZn=6.08%andPb/Zn=1/4.7)inChinaatpresent.Toitsnorth,alargeCu-Co-AgdeposithasbeenfoundatBaiyangpingrecently1).TheJindingPb-Zndepositishostedinsandstonesa… 相似文献