The evolution of the archaean crust of northeast Botswana     

Roger M. Key  Martin Litherland  John V. Hepworth 《Precambrian Research》1976,3(4):375-413

Some 1800 km² of Archaean terrain have been mapped (the Eastern Geotraverse of the Botswana Geological Survey, Geodynamics Project) including a part of the schist belt/granitic terrain of the Rhodesian Craton, and part of the Limpopo Mobile Belt in the south. The cratonic area includes the whole or part of four schist belts (greenstone belts) now referred to as ‘schist relics’ as they are shown to be mega-xenoliths surviving regional deformation and granitisation.The schist relics display a typical greenstone belt composition with basal ultra-mafic schist, extrusive meta-basalts, serpentinites, and meta-dacites, -andesites, -rhyolites and volcanoclastic rocks appearing up the succession. These are termed the Volcanic Group. They also contain marble, graphitic phyllite, meta-greywacke, banded ironstone and aluminous schist.The schist relics overlie thick sequences of granitised clastic sediments with intercalated volcanic and sedimentary layers which are regarded as an integral, lower part of the succession. The total sequence, including the Volcanic Group and underlying rocks is at least 30 km thick.The schist relics originally formed a more continuous and lithostratigraphically equivalent succession, which may have included some of the Rhodesian schist belts. Such a succession would be of the order of size of an island arc system in length and width.An early stage of regional folding (F1) is recognized, from preserved fabrics and structural analysis, which is tentatively proposed as nappe folding on a regional scale. This was accompanied by low-grade metamorphism. This first stage of deformation determined the regional pattern as it is still seen.Tonalite/monzonite plutons were emplaced within the Volcanic Group and probably protected the relics during regional granitisation.Regional granitisation, which isolated the schist relics, was accompanied by pyroxene hornfels metamorphism. This was followed by a regional, penetrative deformation (F2) with amphibolite facies (Barrovian type) metamorphism. This is the most prominent style of deformation which, although most intensely developed in the Limpopo Mobile Belt, is imposed throughout the whole area. The Limpopo Belt cuts across the area in the south as a large ductile shear zone and affects rocks largely similar to those of the craton, although anorthosites are not known outside it.The Limpopo Belt has its own style of deformation — interference folds, and several stages of folding related to transcurrent movement, and intense cataclasis.Metamorphism is similar throughout Limpopo Mobile Belt and Craton, each event being recognized in both domains and being of similar grades.In many respects the Botswana Eastern Geotraverse area resembles other Archaean terrains and analogies can be drawn. In attempting to find a geotectonic model into which it can be placed there is a considerable range of choices, and not enough constraints existing either within the imperfectly preserved geological record, nor within the expanding and elastic framework of Global tectonic theory.No direct evidence for subduction zones or plate boundaries has been found, nor is this surprising considering the subsequent history. On the other hand we see nothing in the rather detailed body of evidence that has been accumulated by mapping to preclude a place within Plate theory as outlined for the Phanerozoic. It allows a moderately uniformitarian interpretation rather than one supposing unique conditions preculiar to the Archaean, such as the concept of isolated volcanic depositaries, or of unique and peculiar crustal conditions.  相似文献   

Miocene sea-floor subsidence and later subaerial solution subsidence structures in the Maltese Islands     

John Newbery 《Proceedings of the Geologists' Association. Geologists' Association》1976,87(1):111

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²¹⁰Po and ²¹⁰Pb distributions in ocean water profiles from the Eastern South Pacific     

John Thomson  Karl K. Turekian 《Earth and Planetary Science Letters》1976,32(2):297-303

Two ocean profiles from the Peru Basin from regions with different surface productivities were analyzed for total²¹⁰Pb and²⁰¹Po to evaluate the influence of particulates in the water column on their distribution. Comparison with a published²²⁶Ra profile for the region was made. The profile closest to the coast, where upwelling and productivity are high, shows depletion of²¹⁰Pb relative to²²⁶Ra at all depths, with particularly marked excursions from radioactive equilibrium at the surface and in the bottom water.²¹⁰Po appears to be deficient relative to²¹⁰Pb at depth as well. Mean residence times in the deep water, relative to particulate removal from the water column to the sediments, of about 100 years for²¹⁰Pb and about two years for²¹⁰Po are indicated. The profile northwest of the upwelling region shows the²²⁶Ra²¹⁰Pb²¹⁰Po system close to equilibrium at all depths to 1500 m (except for the effect of atmospheric²¹⁰Pb input seen at the surface.  相似文献   

The Shaw chondrite,I. The case of the missing metal     

Ermanno R. Rambaldi  John W. Larimer 《Earth and Planetary Science Letters》1976,33(1):61-66

The mineralogy, elemental and isotopic composition of the Shaw meteorite indicate that it is a highly metamorphosed L-group chondrite which has lost a portion of its metal and sulfide. The metal which remains has an unusual composition relative to that in other L-group chondrites. It is enriched in Ga, Ge, Ir, Mo, Os, Pt, Re and Ru and depleted in As, Au, Cu and Sb. A comparison of the relative enrichments and depletions in Shaw to those observed in San Cristobal, the extreme end-member of group IAB iron meteorites, shows that the metal phases in these two meteorites have complementary compositions. This implies that the metal in Shaw represents the residual solid of a partial melting process while the missing metal, which drained away, may have gone to form an iron meteorite, like San Cristobal.  相似文献   

The isotopic composition of strontium and oxygen in lavas from St. Helena,South Atlantic     

Norman K. Grant  James L. Powell  Flora R. Burkholder  John V. Walther  Max L. Coleman 《Earth and Planetary Science Letters》1976,31(2):209-223

Strontium and oxygen isotope measurements on the alkali basalt-trachyte-phonolite suite of St. Helena show that some of the late-fractionated rocks are enriched in ⁸⁷Sr and depleted in ¹⁸O relative to the older basalts. The data rule out both the formation of the late-fractionated rocks by the partial melting of hydrothermally altered oceanic crust and the contamination of the volcanic rocks by oceanic sediment. It also appears to be incompatible with models based either on the melting of previously fractionated and crystallized liquids in the volcanic pile, or the long-term fractionation of lavas over several millions of years in a sub-volcanic magma chamber.It is concluded that hydrothermal interaction with meteoric water is the most important cause of the ¹⁸O depletion. If the interaction occurred at widely differing temperatures, and involved meteoric and seawaters, it might conceivably have caused both the oxygen and strontium isotope heterogeneities.  相似文献   

Long-term chemical effects of petroleum in south Louisiana wetlands—I. Organic carbon in sediments and waters     

Thomas Whelan  John T. Ishmael  W.S. Bishop 《Marine pollution bulletin》1976,7(8):150-155

The chemical effects of chronic petroleum input into a shallow water marsh were examined by measuring hydrocarbon levels and dissolved organic carbon content of sediments associated with two active oil fields in south Louisiana. Annual levels of total organic carbon in the surface waters of the oil fields were higher by 1 mg C/l. in the salt marsh and 5 mg C/l. in the fresh marsh than the respective controlsites. Average dissolved organic carbon concentrations in the interstitial waters of cores taken within the oil field environments were 105% higher than the control in the salt marsh and 43% higher than the control in the fresh marsh. Significantly lower ratios of C₁₇ to pristane occurred in both oil field sediments; however, average odd-even predominance values were not indicative of petroleum contaminated sediments. The results indicate that microbial processes are responsible for dissolution of petroleum into dissolved organic carbon and that dissolved organic carbon concentrations may be a more significant measure of chronic petroleum input than hydrocarbon distribution.  相似文献   

Effects of oil pollution on breeding Grey Seals     

John E Davis  Sheila S Anderson 《Marine pollution bulletin》1976,7(6):115-118

Marine mammals are probably frequently exposed to floating oil but little is known about the effect oil pollution has on them. Oil stranded on the shore in Pembrokeshire, West Wales in September 1974 coincided with the start of the Grey Seal breeding season there. Observations have been made on the effect the oil had on the newborn pups and their mothers.  相似文献   

The metamorphic history of LL-group ordinary chondrites     

John V. Heyse   《Earth and Planetary Science Letters》1978,40(3):365-381

A study of feldspar grain size and microprobe analysis of coexisting orthopyroxene, olivine, diopside, and metal in twenty equilibrated LL-group chondrites and clasts defines the conditions of metamorphism, indicates the relative position of samples within the parent body, and suggests an accretionary-metamorphic history.

Variation of mineral chemistry indicates a range of equilibrium temperature and parallels textural variation, providing two continuous relative temperature scales, based on mean feldspar grain size and the calcium content of orthopyroxene coexisting with diopside. Maximum equilibrium temperatures range from less than 600° up to 950°C. The total Tschermak's component in orthopyroxene indicates that maximum pressure was less than 1 kbar, and a pressure gradient correlates directly with temperature. A reduction sequence, evident from systematic variation of iron in ferromagnesian silicates and metal, correlates inversely with temperature.

The maximum permissible sampling depth for the formation of surface breccias (2/10 r) also limits pressure to less than 1 kbar. The LL parent body had a minimum radius of 200 km, determined using the permissible sampling depth and published cooling rates.

The data are broadly consistent with simple reheating of colder material within a completely accreted body. This model may have problems with volatile depletion, a required lack of impact sampling during metamorphism, and the timing of the event. The data are more consistent with accretion of colder material on a hot but cooling surface. Both models apparently require a recycling process during accretion to account for the reduction sequence.  相似文献   

Numerical weather prediction relevant to the monsoon problem     

T. N. Krishnamurti  John Molinari  Hua-Lu Pan  Vince Wong 《Pure and Applied Geophysics》1977,115(5-6):1357-1372

In this short paper we have identified some of the modelling groups that have the capability of simulating or carrying out short range numerical weather prediction over the monsoon belt. We have next outlined some of the important and desirable ingredients for a multilevel primitive equation model over the tropics, with most of the emphasis on the present version of Florida State University's Tropical Prediction Model. Finally, we present briefly some important results based on the present version of our prediction models that relate to the NWP efforts over the monsoon belt. Here we have identified the importance of mountains, convection, the radiative heating balance of the earth's surface, and the planetary boundary layer over the Arabian Sea.  相似文献   

Eruptive patterns for the volcano piton de la fournaise, Reunion Island     

John N. Ludden   《Journal of Volcanology and Geothermal Research》1977,2(4):385-395

A study of the historic record of activity of Piton de la Fournaise has revealed a cyclic pattern of eruption involving effusion of oceanite lava from major-flank centers every 20–40 years. Calculated volumes of the recent lava flows and pyroclastic ejecta have established an effusion rate of 3.9 m³ s⁻¹ since 1931 and 6.2 m³ s⁻¹ since 1951. Flank eruptions outside the present caldera define a distribution maximum which is expected to correlate with the depth range of a high-level magma reservoir.A model has been constructed which requires replenishment of a high-level magma chamber at a constant rate and regular eruption from summit and minor-flank centers, acting as “safety valves” to the magma chamber; when the magma chamber reaches its maximum expansion, a major-flank outburst of oceanitic lava occurs.The fact that calculated effusion rates are not consistent with radiometric dates implies an increase in effusion volume with time for the volcano.  相似文献