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The Munni Munni Complex, Western Australia: Stratigraphy, Structure and Petrogenesis 总被引：3，自引：0，他引：3
The late Archaean Munni Munni Complex is a layered mafic-ultramaficintrusion emplaced into granitic rocks of the west Pilbara Block.It consists of a lower Ultramafic Zone with a maximum thicknessof 1850 m and an overlying Gabbroic Zone at least 3600 m thick.There are strong geometrical and stratigraphic similaritiesto the Great Dyke of Zimbabwe. The Ultramafic Zone comprises multiple macrorhythmic cyclesof olivine-clinopyroxene adcumulates and mesocumulates. Layeringdips towards the centre of the intrusion and trends laterallyinto a narrow and variably contaminated chilled margin. Higherlayers extend progressively further up the sloping floor ofthe intrusion. Cryptic layering is defined by rapid fluctuationsin Cr content of cumulus clinopyroxene, accompanied by relativelysmall variation in Fe/Mg ratio. The base of the Gabbroic Zone is marked by the first appearanceof cumulus plagioclase and the simultaneous appearance of pigeoniteas a persistent cumulus phase. Magnetite appears as a cumulusphase 400–600 m above this. Gabbroic Zone cumulates showa gradual linear upward increase in Fe/Mg and an absence ofcyclic layering, suggesting crystallization in a closed chamber. Chilled margin samples show evidence of in situ contamination,but indicate that the parent magma to the ultramafic portionof the intrusion was a high-Mg, low-Ti basalt with similaritiesto typical Archaean siliceous high-Mg basalts. Partial meltingof granitic wall rocks occurred along steep side walls but wasless extensive along the shallow-dipping floor. A pyroxenitedyke, the Cadgerina Dyke, intersects the floor of the intrusionat a level close to the top of the Ultramafic Zone, and appearsto have acted as a feeder conduit to the Gabbroic Zone and theuppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the GabbroicZone is a distinctive 30–50 m thick pyroxenite layer,the Porphyritic Websterite Layer, which also exlends laterallyup the side walls of the intrusion to form a 200 m thick marginalborder zone separating Gabbroic Zone cumulates from countryrock granites. A distinctive suite of bronzite-rich xenoliths,some containing Al-rich, Cr-poor spinel seams, occurs withinand just above the Porphyritic Websterite Layer in the centralpart of the intrusion. There is a steep gradient of decreasing Cr and increasing Fe/Mgin cumulus clinopyroxenes across the upper 100 m of the UltramaficZone. A sharp downward step in Cr occurs a few metres belowthe base of the Gabbroic Zone, immediately beneath a stronglyorthocumulate layer of augite cumulate containing disseminatedplatinum-group element (PGE)-rich sulphides. Lateral pyroxenecomposition trends within the Porphyritic Websterite Layer canbe accounted for by an increase in cumulus porosity as thislayer approaches the floor of the intrusion. Quantitative modelling of pyroxene composition trends indicatesthat Ultramafic Zone cumulates crystallized from relativelysmall volumes of magma, an order of magnitude less than thesize of the magma body inferred from trends in the GabbroicZone. This conclusion, together with the geometry of the PorphyriticWebsterite Layer, implies that the Porphyritic Websterite Layermarks a level at which the chamber expanded as a result of amajor new influx of magma. Pyroxene composition trends indicatethat this influx was of a distinetly different and more fractionatedcomposition than that parental to the Ultramafic Zone. Injection of fractionated tholeiitic magma into more primitivehigh-Mg basalt resident magma formed a turbulent fountain, whichentrained the resident magma and formed a cool, dense basalhybrid layer. Crystallization of the Porphyritic WebsteriteLayer occurred where the top of this hybrid layer impinged onthe sloping floor. Continuing injection of tholeiitic magmaexpanded the thickness of the hybrid layer, causing the PorphyriticWebsterite Layer to accrete progressively up the sloping floorand the walls. After the conclusion of the influx phase, thehybrid layer became homogenized to a final tholeiite-rich composition,which eventually crystallized to form the Gabbroic Zone. Thexenolithic rocks within and above the Porphyritic WebsteriteLayer were probably derived initially by crystallization ofa contaminated silica-enriched melt layer at the roof of theintrusion, followed by detachment and sinking or slumping tothe floor. Orthopyroxene phenocrysts within the PorphyriticWebsterite Layer may also have originated within this roof zone. 相似文献
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The quantitative relationships between stratigraphical variables from a succession of coal-bearing cycles deposited in a subsiding basin have been investigated using principal component- and factor analysis, in an attempt to reveal simple relationships undetected by conventional qualitative methods. Knowledge of such relationships will undoubtedly be useful in future computer simulation studies of cyclical sedimentation. Principal component analysis and factor analysis yield similar results, which confirm and amplify those of the trend surface analysis described in Read and Dean (1967). 相似文献
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Fe–Mg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaO–MgO–Al2O3–SiO2–FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·7–2·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about3–5°C. Relative to the solidus of model lherzolitein the CaO–MgO–Al2O3–SiO2 system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in P–T spaceto the comparable transition in the CaO–MgO–Al2O3–SiO2–Na2O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·5–2°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for Fe–Mgexchange between the coexisting crystalline phases and melt,KdFe–Mgxl–liq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na2O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting 相似文献
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Abstract Mathematical modelling of overbank inundation and flows faces many problems and is still in its infancy. Work to date has generally been restricted to small reaches. Large-scale models based on longer reaches of river channel are likely to be of greater value for engineering and flood plain management purposes, but the problems associated with the transition from small to large scales need to be assessed. A large-scale finite element model, RMA-2, has been applied to the flood plain of the lower reaches of the River Culm in southeast Devon, UK. Patterns of radiocaesium accumulation by overbank accretion during flood water inundation were used to assess the potential of using such models for explaining sedimentation rates and patterns. A strong correlation was found between values of the 137Cs inventory and surface concentration and the predicted flood water patterns derived using the RMA-2 model. Except where recession pondage occurs, an inverse relationship existed between 137Cs deposition and water depth. However, the discretization model developed cannot presently cope with large-scale compartmentalization of flows by barriers to flow and small-scale local features, such as ditches crossing the flood plain and the microtopography of the flood plain. This study appraises the potential for using the RMA-2 model to predict patterns of overbank deposition and represents an initial stage in the development of an integrated model of hydraulic and sediment dynamics. 相似文献
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Quantitative relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence in a Scottish Namurian basin 总被引：1，自引：0，他引：1
This detailed quantitative basin analysis of fluvial deposits in a subsiding Namurian structural basin is aimed at discovering underlying statistical relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence which could be used as a background to future sophisticated computer simulation experiments and would also facilitate comparison with other ancient basins. The succession studied lies between two widespread marine bands, one of Arnsbergian (E2) and one of Kinderscoutian (R1) age, and is dominated by upward-fining cycles: 94% of the semi-cycles containing sand, here termed grain-size cycles, are upward-fining and the ratio of fine members (mudstone+siltstone) to coarse members ranges from 0·23 to 5·0. Trend-surface analysis reveals basement structures, which influenced sedimentation, including a Caledonoid graben. Correlation coefficient values and results of principal component analysis demonstrate that the numbers of rooty horizons and grain-size cycles, together with the total thicknesses of sandstone and of mudstone+siltstone, all tend significantly towards a linear relationship with the total thickness of strata and hence net subsidence. The average thickness of grain-size cycles tends towards an inverse linear relationship with net subsidence. This probably reflects the presence of stacked, relatively thin, channel-fills within persistent channel belts. These belts tend to be localized in the areas of greatest subsidence within the basin and follow courses basically similar to channels already discovered in the underlying deltaic sediments. The highest concentrations of mudstone+siltstone tend to lie on the flanks of the basin, but high local sand concentrations found at points on the margin where channels entered the basin from the NW and NE and left to the SW, effectively disrupt any significant relationship between net subsidence and the proportion of sand. Coal has been selectively preserved on the flanks of the basin and the number of rooty horizons is greatest in an area of low subsidence which lay somewhat remote from the main channel belts. 相似文献
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Desmond E.WALLING 《国际泥沙研究》2005,20(1)
It was a great honour and privilege to be elected President of WASER during the first Council meeting, held in Beijing last Saturday. I hope that I will be able to fulfil the expectations of those that elected me and I look forward to serving the Association and promoting its development. As a hydrologist/geomorphologist who started working in the field of erosion and sedimentation some 35 years ago, I have long appreciated the need for, and importance of, interaction with those with simi… 相似文献
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The influence of river discharge on the thawing of sea ice, Mackenzie River Delta: albedo and temperature analyses 总被引：1，自引：0，他引：1
Multi-temporal satellite images, field observations and field measurements were used to investigate the mechanisms by which sea ice melts offshore from the Mackenzie River Delta. Satellite data recorded between April and August 1986 were corrected to a map projection and calibrated such that albedo and temperature values could be compared. Three stages in the melting of sea ice were identified: flooding (overflows), insolation and melting by warm river water. The albedo values of overflows were as much as 1/7 that of ice values while the albedo of ice decreased by 1/3 over the summer. Approximately two weeks after the overflows develop, sea surface temperatures rise as the river-discharge peaks and becomes the dominant source of energy. By this process, ice removal in the delta regime is initiated two months earlier than adjacent coasts with minimal runoff. However, the net result is only a two-week acceleration of ice removal in the delta region. 相似文献
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Electronic computers have enabled the quantitative relationships between stratigraphic variables to be calculated for a sequence of Namurian (E1 zone) cycles deposited in a differentially subsiding basin east of Stirling. Despite their limitations, coefficients of correlation and linear regression equations provide a relatively simple means of expressing the relationships between critical variables and so provide a useful way in which various cyclically deposited sequences can be compared. All possible pairs chosen from eleven variables are investigated, including the total number and average thickness of cycles, total number of coals and total thicknesses of coal, sandstone and shale. These six variables all tend to show a linear relationship with total thickness of strata and hence net subsidence. A second part of this paper, to be published later, will examine the relationships between variables in more detail, using the technique of factor analysis. 相似文献
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