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11.
High-Temperature Fluid-Rock Interaction and Scapolitization in an Extension-Related Hydrothermal System, Mary Kathleen, Australia 总被引：1，自引：0，他引：1
OLIVER NICHOLAS H. S.; RAWLING TIMOTHY J.; CARTWRIGHT IAN; PEARSON PAUL J. 《Journal of Petrology》1994,35(6):1455-1491
In the polymetamorphic Mary Kathleen Fold Belt, at the centreof the Proterozoic Mount Isa Inlier, Queensland, an early phaseof extension (1760–1730 Ma) resulted in intrusion of voluminousgranitic and doleritic magmas into the carbonate-evaporite-dominatedCorella Formation at 5–10 km depths. Widespread high-temperaturemetasomatism ensued, involving scapolitization in dolerite,formation of albite-scapolite shear zones in granite, exo- andendoskarn formation, and a zone of K–Na–Ca alterationin the lowermost Corella Formation. Granites and dolerites werealtered to an unusual Na–Ca-rich bulk composition, reflectinghigh-temperature infiltration of highly saline, chemically reactiveexternally derived fluid. The alteration products and theirdistribution suggest not only reaction of magmatic/aqueous fluidswith the country rocks but also extensive halite dissolutionand recirculation of saline fluids back into the intrusive bodies.The bulk of fluid flow occurred at high temperatures (500–700C), and major element and isotopic fronts were generally notsmoothed out by the effects of temperature gradients, with theexception of one part of the system which shows evidence forup-temperature fluid flow. Analysis of oxygen isotopic dataand the position of isotopic and geochemical fronts revealstime-integrated fluid fluxes of up to 2 104 m3/m2 for themetasomatism. Although very high salinities (up to 50 mol% NaCl)were attained by evaporite dissolution, 18O values of most alterationproducts are in the range 7–12%, reflecting a predominanceof oxygen derived from an igneous fluid. The position and interrelationshipsof metasomatic and isotopic fronts indicate an earlier stageof infiltration dominated by fluid released from crystallizinggranite (with 18O 10–12%), and a later stage (18O 7–9%)in which fluid had already interacted with halite and a mixedmafic-felsic igneous source or was repeatedly circulated betweenthese rock types during alteration. The data reflect only aminimal contribution from fluids produced by devolatilizationof the abundant carbonate-bearing rocks in the Corella Formation,and there are substantial areas of Corella Formation rocks thathave escaped metasomatism during this phase of intrusion-relatedhydrothermal activity and during the subsequent regional metamorphicoverprint. Along with the requirement that the fluids dissolvedlarge amounts of halite from the same sequence, and the structuralobservations, we favour a model where fluid was preferentiallychannelled along specific permeable conduits, including formerevaporite layers, before interaction with the now exposed alteredrocks. Fluid was probably driven by both convective circulationand dilatancy-related deformation accompanying emplacement ofmagmas into a major crustal extensional dcollement. 相似文献
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M. A. OLIVER R. WEBSTER 《International journal of geographical information science》2013,27(3):313-332
Geographical information systems could be improved by adding procedures for geostatistical spatial analysis to existing facilities. Most traditional methods of interpolation are based on mathematical as distinct from stochastic models of spatial variation. Spatially distributed data behave more like random variables, however, and regionalized variable theory provides a set of stochastic methods for analysing them. Kriging is the method of interpolation deriving from regionalized variable theory. It depends on expressing spatial variation of the property in terms of the variogram, and it minimizes the prediction errors which are themselves estimated. We describe the procedures and the way we link them using standard operating systems. We illustrate them using examples from case studies, one involving the mapping and control of soil salinity in the Jordan Valley of Israel, the other in semi-arid Botswana where the herbaceous cover was estimated and mapped from aerial photographic survey. 相似文献
14.
OLIVER DUKE-WILLIAMS PHILIP REES 《International journal of geographical information science》2013,27(6):579-605
The paper describes a problem faced by National Statistical Offices when publishing the results of decennial censuses for small geographical areas. If they publish statistical tables for two or more sets of areas, users can compare the tables and produce new statistics for the areas formed by differencing, which may have populations below confidentiality thresholds. To investigate the problem, the authors construct a software system and carry out a series of experiments using a large synthetic population base for Yorkshire and Humberside. The results indicate that publishing statistics for zones close in size to the primary areas is not safe unless the zones have been carefully designed. However, publishing statistics for sufficiently large areas such as 5 km grid squares or postal sectors alongside enumeration districts is safe. 相似文献
15.
A second natural occurrence of yoderite 总被引：3，自引：0，他引：3
A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg2 Al5.7 Fe0.3 Si4 O18 (OH)2 , similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg2 Al5.6 Fe0.4 Si4 O18 (OH)2 . Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13 kbar at 590 °C and 21 kbar at 650 °C. The yoderite-bearing whiteschist is associated with a 1.4 Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone. 相似文献
16.
The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south‐east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross‐bedded dune sandstone dominated by grain flow and translatent wind‐ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind‐ripple, wavy and horizontal planar‐laminated strata resulting from accumulation on a range of dry, damp or wet substrate‐types in the flats and hollows between migrating dunes; and (iii) extensive, near‐flat lying wind‐rippled sandsheet strata. Fluvial facies comprise channel‐fill sandstones, lag conglomerates and finer‐grained overbank sheet‐flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south‐east, whereas south‐westerly flowing fluvial systems periodically punctuated the dune fields from the north‐east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo‐water‐table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid‐plains that fed sediment to a coastline that lay to the south‐west and which ran approximately north‐west to south‐east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low‐relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non‐marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north‐eastward directed marine transgression into the basin prior to the onset of exclusively non‐marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small‐scale autocyclic behaviour of competing, coeval depositional systems and larger‐scale allocyclic changes that record system response to longer‐term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea‐level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio‐eustatic driving mechanism. 相似文献
17.
High fO2 Metasomatism During Whiteschist Metamorphism, Zambezi Belt, Northern Zimbabwe 总被引：1，自引：0，他引：1
The Kadunguri Whiteschists are a group of talc- and kyanite-bearinglithologies that occur in the Chewore Inliers from the ZambeziBelt of northern Zimbabwe. They crop out on the southern marginof the Chewore Ophiolite Terrane, a Mesoproterozoic ophioliteand island arc, as a 5 km x 1·5 km, southeasterly dipping,semi-continuous block, and contain the second known naturaloccurrence of yoderite. Major element analyses define the whiteschistswithin the relatively simple MFASH system. Major and trace elementanalyses indicate that the whiteschists originate from the metasomaticalteration of alkalic ocean-island-type metabasalts similarto those in the underlying Ophiolite Terrane. Synmetamorphicor metasomatic mineral parageneses indicate peak P–T conditionsof between 13 and 15 kbar at 550–600°C, and the highlyoxidizing nature of all reactions indicates the presence ofa high fO2 metasomatic fluid. The peak P–T conditionsrequire that this synmetamorphic, exotic metasomatic fluid wasavailable at depths near 55 km. The age of high-pressure metamorphismis constrained within the Pan African tectonothermal cycle at550–520 Ma. Tectonometamorphism in the Zambezi Belt isrelated to a period of extensive crustal thickening possiblyrelated to amalgamation of Gondwanaland. KEY WORDS: Congo Craton; high pressure; Kalahari Craton; metasomatism; whiteschist; Zambezi Belt 相似文献
18.
ABSTRACT. This article examines how peasant farmers build and maintain agrobiodiversity in home gardens found in two traditional peasant communities along the Marañón River in the Peruvian Amazon. Data were gathered through household and garden surveys as well as in‐depth interviews with garden tenders in an upland mixed agricultural village and a lowland fishing village. Substantial variations in cultivated plant diversity were encountered in gardens between and within the villages, which are found to be related to differential exchange of seeds, cuttings, suckers, and other planting material as well as to specific garden and household characteristics. Planting material flows along multiple pathways—from gift giving and purchase to inheritance and scavenging—to the gardens, reflecting a complex and often extensive network of exchange that enables the establishment and maintenance of home garden plant diversity in seemingly isolated and small communities. Further research is needed to identify broader geographical and sociocultural patterns of agrobiodiversity in Amazonia. 相似文献
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Mechanical controls on fluid flow during regional metamorphism: some numerical models 总被引：8，自引：0，他引：8
The control of fluid flow by plastic deformation during metamorphism is critical to our understanding of metamorphic processes. Various geological observations and field studies demonstrate the consequences of fluid flow control by deformation, so that the concept appears to be accepted, at least for small-scale PUBLIC (for example faults and vein PUBLIC). However, the concept appears to be less well recognized at regional scales. Considered here are examples of simple, conceptual models based on fully coupled mechanical–fluid flow concepts; they include deformation of a section of fluid-saturated crust containing a block or a layer of material of different properties from its surrounds. In particular, rheological and permeability contrasts between rock types during deformation associated with regional metamorphism are sufficient to control the form of fluid flow over the range of a few kilometres. Low contrasts and small strains allow pervasive fluid flow, whereas greater contrasts and increasing strains cause focusing of the flow. Such focusing is generally associated with localization of the deformation, especially for a strongly dilatant elastic–plastic material. However, a rate of fluid flow much greater than the rate of deformation may result in pervasive flow, although for most models pervasive flow is difficult to attain over regional distances. Furthermore, lateral and downward fluid flow may occur, demonstrated here by simple models for folding and for deformation of regions containing plutons. Therefore, such modelling may be used as a means of testing the various hypotheses concerning the volumes of fluid predicted to have passed through some rock volumes. Numerical models of the future will become increasingly complex and powerful, allowing greater coupling of thermal, mechanical, chemical and fluid flow effects, and based more on the physical processes involved. Combined field and laboratory studies will provide correspondingly greater understanding and will permit the determination of the timing of fluid flow and structural controls on fluid flow patterns. 相似文献
20.
Fluid migration and vein formation during deformation and greenschist facies metamorphism at Ormiston Gorge, central Australia 总被引：4，自引：0，他引：4
I. CARTWRIGHT W. L. POWER N. H. S. OLIVER R. K. VALENTA G. S. MCLATCHIE 《Journal of Metamorphic Geology》1994,12(4):373-386
During the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower- to middle-greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early-, syn-, and post-tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ18O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ18O(vein-wallrock) values of -0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ18O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ18O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ18O(vein-wallrock) values up to -4%o (average of -2.1%o). These systematic differences in δ18O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ13C values between some dolomite veins and wallrocks [δ13C(vein-wallrock) up to -1.9%o, average of -0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving large volumes of externally derived fluids. 相似文献