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Richard Herrington Valeriy Maslennikov Victor Zaykov Igor Seravkin Alexander Kosarev Bernd Buschmann Jean-Jacques Orgeval Nicola Holland Svetlana Tesalina Paolo Nimis Robin Armstrong 《Ore Geology Reviews》2005,27(1-4):203
VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host Cu–Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu–(Zn) and Cu–Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host Cu–Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn–Pb–Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu–Zn VMS deposits.The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis. 相似文献
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The aim of this paper is to evaluate current knowledge and uncertainties associated with the impact of increasing greenhouse
gas and aerosol concentrations on the West African monsoon. For this purpose, coupled and time-slice simulations are used.
A global measure of the monsoon changes is defined in order to avoid regional biases and to try and obtain significant results.
The position and width of the monsoon in latitude are the main focuses. There is almost no agreement between the Coupled General
Circulation models from the Coupled models Inter-Comparison project—Phase II in regard to the impact of climate change on
the monsoon. Moreover, very simple discriminations between the models seem inappropriate to get a better signal. The role
of the different forcings in time-slice simulations is then investigated. The sea surface temperature (SST) and particularly
the pattern of the SST are shown to be the most important forcing. This accounts for the diversity of the results either from
the coupled or the forced simulations with different SST changes. With a fixed SST, but of a smaller magnitude in AMJ, there
are still uncertainties, coming first from the Atmospheric General Circulation models and the way they balance greenhouse
gas and global SST increase. Finally the uncertainty due to the Land Surface models (LSMs) is not negligible. The greenhouse
gas and the LSMs are shown to have more impact in August, when the monsoon is at its highest latitude on the continent. 相似文献
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The aim of this article is to determine the main sources of the variability of large West African river discharges during the years 1951–2000. The analysis is based on river discharge simulations of ORCHIDEE Land-Surface Model forced by NCC (NCEP Corrected by CRU) over this period. In a first part, an analysis of the partitionning of precipitation in the different basins is given. It is first shown that total runoff is more variable and better correlated to precipitation than evapotranspiration over every basin on annual means. Then the more complex link between evapotranspiration and precipitation is investigated through correlation and regression analyses. Over the “dry” (soudano-sahelian) catchments, evapotranspiration is the most variable and mainly correlated to the annual precipitation. Over the “humid” (equatorial) catchment, it is mainly correlated to the season length and does not depend on other characteristics of the precipitation. Over the “intermediate” (guinean) catchments, annual precipitation and season length both play a role, along with the average intensity of rainfall events, and other characteristics such as the number of long breaks in the rainy season. A second part focuses on the way ORCHIDEE reproduces the variability of river discharges for the years 1951–2000. It is shown that relative anomalies are correctly simulated by ORCHIDEE forced by NCC over every catchment without taking into account any land-use change. Moreover, no significant difference is found in the accuracy with which ORCHIDEE simulates the humid (1953–1970) and the dry (1973–1990) periods over two selected catchments. This implies that the impact of land-use changes was much less important than the impact of precipitation changes over the years 1951–2000 in large West African catchments. 相似文献
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T. d’Orgeval Jean-Philippe Boulanger M. J. Capalbo E. Guevara O. Penalba S. Meira 《Climatic change》2010,98(3-4):565-580
The present article is a contribution to the CLARIS WorkPackage “Climate and Agriculture”, and aims at testing whether it is possible to predict yields and optimal sowing dates using seasonal climate information at three sites (Pergamino, Marcos Juarez and Anguil) which are representative of different climate and soil conditions in Argentina. Considering that we focus on the use of climate information only, and that official long time yield series are not always reliable and often influenced by both climate and technology changes, we decided to build a dataset with yields simulated by the DSSAT (Decision Support System for Agrotechnology Transfer) crop model, already calibrated in the selected three sites and for the two crops of interest (maize and soybean). We simulated yields for three different sowing dates for each crop in each of the three sites. Also considering that seasonal forecasts have a higher skill when using the 3-month average precipitation and temperature forecasts, and that regional climate change scenarios present less uncertainty at similar temporal scales, we decided to focus our analysis on the use of quarterly precipitation and temperature averages, measured at the three sites during the crop cycle. This type of information is used as input (predictand) for non-linear statistical methods (Multivariate Adaptive Regression Splines, MARS; and classification trees) in order to predict yields and their dependency to the chosen sowing date. MARS models show that the most valuable information to predict yield amplitude is the 3-month average precipitation around flowering. Classification trees are used to estimate whether climate information can be used to infer an optimal sowing date in order to optimize yields. In order to simplify the problem, we set a default sowing date (the most representative for the crop and the site) and compare the yield amplitudes between such a default date and possible alternative dates sometimes used by farmers. Above normal average temperatures at the beginning and the end of the crop cycle lead to respectively later and earlier optimal sowing. Using this classification, yields can be potentially improved by changing sowing date for maize but it is more limited for soybean. More generally, the sites and crops which have more variable yields are also the ones for which the proposed methodology is the most efficient. However, a full evaluation of the accuracy of seasonal forecasts should be the next step before confirming the reliability of this methodology under real conditions. 相似文献
5.
Les Malines Mine (Cévennes, France) provides an example of Pb-Zn ore bodies that underwent a polyphased evolution. On the Pb-Pb diagram, experimental points of the Les Malines ore types always plot in the central part of the field defined for the Cévennes metallogenic province. This homogeneity, the similarities with the Pb isotopic compositions of the surrounding rocks and the comparison with Pb isotopes evolution of whole rocks and minerals of the neighbouring continental lands, which emerged during Triassic and Liassic times, rule out the participation of extraneous Pb during the successive concentration stages and agree with an initial metal stock transformed and mobilized in a Pb closed system. Hercynian K-feldspars are the main source of the Pb preconcentrated during Triassic times. Most of the Cévennes Pb-Zn ore bodies could derive from this Triassic metal stock; nevertheless, local Pb isotopic anomalies suggest hydrothermal sources for some Pb-Zn ore bodies located along the Cévennes fault. 相似文献
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Ultrahigh-pressure garnet peridotites from the devolatilization of sea-floor hydrated ultramafic rocks 总被引：8，自引：0，他引：8
Garnet peridotites occur in quartzofeldspathic gneisses in the Northern Qaidam Mountains, western China. They are rich in Mg and Cr, with mineral compositions similar to those in mantle peridotites found in other orogenic belts and as xenoliths in kimberlite. Garnet‐bearing lherzolites interlayered with dunite display oriented ilmenite and chromite lamellae in olivine and pyroxene lamellae in garnet that have been interpreted to indicate pressures in excess of 6 GPa. However, some garnet porphyroblasts include hornblende, chlorite and spinel + orthopyroxene symplectite after garnet; some clinopyroxene porphyroblasts include abundant actinolite/edenite, calcite and lizardite in the lherzolite; some olivine porphyroblasts (Fo92) include an earlier generation Mg‐rich olivine (Fo95–99), F‐rich clinohumite, pyroxene, chromite, anthophyllite/cummingtonite, Cl‐rich lizardite, carbonates and a new type of brittle mica, here termed ‘Ca‐phlogopite’, in the associated dunite. The pyrope content of garnet increases from core to rim, reaching the pyrope content (72 mol.%) of garnet typically found in the xenoliths in kimberlite. The simplest interpretation of these observations is that the rock association was formerly mantle peridotite emplaced into the oceanic crust that was subjected to serpentinization by seawater‐derived fluids near the sea floor. Dehydration during subduction to 3.0–3.5 GPa and 700 °C transformed these serpentinites into garnet lherzolite and dunite, depending on their Al and Ca contents. Pseudosection modelling using thermocalc shows that dehydration of the serpentinites is progressive, and involved three stages for Al‐rich and two stages for Al‐poor serpentinites, corresponding to the breakdown of the key hydrous minerals. Static burial and exhumation make olivine a pressure vessel for the pre‐subduction mineral inclusions during ultrahigh‐pressure (UHP) metamorphism. The time span of the UHP event is constrained by the clear interface between the two generations of olivine to be very short, implying rapid subduction and exhumation. 相似文献
8.
Surface and gravity-cored sediments were collected from the Kaoping coastal area off southwestern Taiwan to determine particle size, organic carbon, trace metal concentration and enrichment factor (EF), 210Pb dating, and 206Pb/207Pb ratio for determining present and historical metal pollution. Surface distributions of trace metals ranged from 0.02 to 0.13 mg/kg for Cd (EF: 0.16-1.65), from 35 to 189 mg/kg for Cr (EF: 0.85-2.92), from 25 to 64 mg/kg for Ni (EF: 0.53-2.38), from 10 to 32 mg/kg for Pb (EF: 1.20-4.94), and from 29 to 129 mg/kg for Zn (EF: 1.18-3.50). Trace metal concentrations correlate closely with distributions of mud (<63 microm) and organic carbon which accumulate largely around river mouths and within the Kaoping Canyon. With the exception of Cd, Cr and Ni in certain areas with rather coarse sediments, metals were generally elevated above the baseline levels over the studied area. Metals were also relatively enriched in areas with high contents of mud and organic carbon. Sedimentation rates derived from the excess 210Pb data in core sediments sampled from the canyon illustrate metal pollution beginning around 1970 that is corresponding to the booming time of economic growth in Taiwan. Meanwhile, the status of Pb pollution in core sediments is verified by an inversed correlation between 206Pb/207Pb ratios and Pb concentrations. Anthropogenic Pb and other metals in the studied area were apparently derived primarily from the Kaoping River and accumulated around the river mouth and within the Kaoping Canyon. The Kaoping Canyon appears to act as a major sink for river borne trace metals. 相似文献
9.
R-P. Mnot A. Pêcher Y. Rolland J-J. Peucat A. Pelletier G. Duclaux S. Guillot 《Gondwana Research》2005,8(1):1-9
Geological maps of East Commonwealth Bay Unit (ECB), (Terre Adélie and Georges V Land, Antarctica) are presented with a summary of the main structural and metamorphic data for the region. The ECB unit was developed during Neoarchean_Paleoproterozoic event (at 2.5-2.42 Ga), with (i) granulite metamorphism at 9±1.5 kbar and 800±50°C in the lower crust section and amphibolite metamorphism (P=5 kbar, T=750°C) at the upper crustal levels; (ii) the lower crustal granulites were uplifted, and suffered local partial melting and retrogression to the amphibolite facies at 550±50°C_5 kbar. Granulites were extruded in the core of a crustal-scale anticlinal fold, but retrogressed only on the rims of the anticline. Crustal-scale folding, along with other structural features resulted from intense NE-SW shortening that prevailed during the Neoarchean orogenic cycle. Strike-slip and extensional motions were only minor components in that process; (iii) top-to-the-East thrusting and nappe piling had (at least locally) occurred under lower amphibolite to greenschist facies conditions. Finally, it seems that (iv) the Paleoproterozoic 1.7 Ga structural imprint may have only affected the rims of the Archean units. The tectonic context observed in the 1.7 Ga Cape Hunter phyllites features mainly an E-W shortening component and vertical extrusion. The eastern (Mertz) and western (Port Martin) parts of the Archean block were reactivated by localized dextral shearing. 相似文献
10.
The character of mountain building processes in Palaeoproterozoic times is subject to much debate. Based on the discovery of high-pressure granulites in the Man Rise (Côte d'Ivoire), several authors have argued that Eburnean (Palaeoproterozoic) reworking of the Archean basement was achieved by modern-style thrust-dominated tectonics.
A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P–T pseudosections calculated with thermocalc suggest granulite facies conditions of ∼ 13 kbar, 850 °C and <7 kbar, 700–800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm–Nd garnet – whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. 相似文献
A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P–T pseudosections calculated with thermocalc suggest granulite facies conditions of ∼ 13 kbar, 850 °C and <7 kbar, 700–800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm–Nd garnet – whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. 相似文献
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