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JOHAN NILSSON 《地球，A辑:动力气象学与海洋学》1996,48(2):342-355
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Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet 总被引：9，自引：0，他引：9
The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock 87Sr/86Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial 87Sr/86Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An72 plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V2O5 of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO2 and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO2 and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation 相似文献
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JOHAN N. J. VISSER 《Sedimentology》1997,44(3):507-521
The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice-rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles. The massive diamictites formed as aprons and fans in front of the ice-grounding line, whereas the stratified diamictites represent more distal debris-flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes. Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter-term fluctuation of the ice margin is present within the major advance-retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea-level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono-eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer-term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills. 相似文献
4.
Major Addition of Magma at the Pyroxenite Marker in the Western Bushveld Complex, South Africa 总被引：3，自引：3，他引：3
A major, but gradual, reversal in the cryptic variation patternof the plagioclase and pyroxenes, of 13 mol% anorthite and 10mol% Mg/ (Mg + Fe) respectively, is documented in the Main Zoneof the western Bush veld Complex. These changes are accompaniedby a decrease in initial 87Sr/86Sr ratio from > 0.708 to< 0.707. The Pyroxenite Marker, a distinctive orthopyroxenitelayer, occurs close to the top of this reversed differentiationsequence. This is attributed to addition of less differentiatedmagma. On the basis of a mass balance calculation of the initial87Sr/ 86Sr ratios, it is estimated that the volume of magmaadded was comparable to that of the resident magma. Increases in the Fe2O3, TiO2, Al2O3, and Na2O contents of thepyroxenes above the level of magma addition indicate that thenew magma had a lower silica activity and higher fO2 than theresident magma. Quantification of the trace element and REEcontent of the two magmas is hampered by the very low proportionof trapped intercumulus component in these adcumulate rocks.However, semi-quantitative modelling indicates that the traceand REE signatures of the two magmas were similar, with moderateLREE enrichment and flat HREE profiles. The new magma had aslightly higher La/ Sm ratio than the resident magma, consistentwith its more alkaline nature. The new magma was probably added gradually, while 100–150m of cumulates formed. It probably intruded at an intermediatelevel within an existing stratified magma chamber, where itcooled and crystallized, and composite packets of liquid pluscrystals plunged to the base of the chamber. The cores of plagioclasegrains formed during this mixing interval show a wider rangeof compositions than in other sections, and plagioclase primocrystsfrom both magmas may be preserved within single samples. Therefore,although intimate physical mixing of packets of unknown sizeof the two magmas occurred, re-equilibration of the major oxidecomposition of the plagioclase primocrysts was not achieved.However, the data and calculations based on diffusion ratesindicate that partial Sr isotopic resetting of plagioclase mayhave occurred. 相似文献
5.
Deglaciation of western Central Norway 总被引：4，自引：0，他引：4
JOHAN LUDVIG SOLLID LEIF SØRBEL 《Boreas: An International Journal of Quaternary Research》1979,8(2):233-239
The glacier movements and corresponding ice margins in Central Norway during Younger Dryas and Preboreal are reconstructed. Scattered, older marginal deposits are difficult to correlate. Raised beach features indicate that the deep fjords became ice-free at an early stage due to calving. In Møre og Romsdal county the glacier front lay at the fjord heads during Younger Dryas, with extensive local glaciation in the intervening mountain areas, and a limit of glaciation 500–600 m lower than the present. In certain places local moraines older than Younger Dryas have been preserved. Autochthonous block fields are widespread in the mountains of Møre og Romsdal. The lower limit of block fields lies at c. 500 m above sea level on the outermost coast and rises to c. 1500 m above sea level in the interior fjord country. No erratics, striation or lateral moraines from the inland ice have been found above this limit. Its gradient, which in outer fjord districts is about 1%, seems to indicate the ice surface at the last maximum of Weichsel glaciation. 相似文献
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The presence of silt-rich, fine-grained material at the center of the well-developed sorted circles on Kvadehuksletta, Svalbard, is a precondition for their development. Field work, laboratory work, and data from published studies indicate that the fine-grained material is a dissolution product of the dolomitic bedrock. The silt is accumulated in situ and by slope wash in terrain depressions. Chemical weathering and other pedogenetic processes, such as the translocation of silt, are of great importance in Arctic regions and can create the sedimentological prerequisites for cryogenetic processes, such as frost sorting and cryoturbation. Therefore, the bedrock composition and the composition of surficial material are considered to be important control factors for these processes. 相似文献
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AFSOON M. KAZEROUNI METTE L. K. POULSEN HENRIK FRIIS JOHAN B. SVENDSEN JENS P. V. HANSEN 《Sedimentology》2013,60(3):679-692
Detrital glaucony in the Palaeogene glauconitic sandstones in Siri Canyon, Danish North Sea, has been analysed from 15 exploration wells by X‐ray diffraction, electron microprobe and scanning electron microscopy. These sandstones consist of mixed‐layer illite/smectite and have a large variability in chemical composition and structure. In the most shallow wells (ca 1700 m), the glaucony is rich in Fe and consists of mixed‐layer illite/smectite with random‐interstratification (R = 0). In the depth interval from 1700 to 2000 m, the composition changes as Si is incorporated. The structure changes to ordered R = 1. Further increase in burial leads to the loss of Fe. Ordered R = 3 mixed‐layer illite/smectite is recognized from burial depths of 2200 m. The proportion of illite in illite/smectite mixed layers increases only slightly with depth and temperature. Although the structural changes generally are associated with chemical changes, they can also take place isochemically when the detrital glaucony is tightly embedded in earlier cement, which prevented chemical exchange. The glaucony transformation in the Siri Canyon sandstones partly reflects a supply of Si and partly significant loss of Fe. Thus, the glaucony transformation relates to the general diagenesis of the host sandstone. These sandstones are cemented by microquartz at an early stage, followed by precipitation of Fe‐rich grain‐coating berthierine or chlorite. 相似文献
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Changes in aerosol size- and phase distributions due to physical and chemical processes in fog 总被引：4，自引：0，他引：4
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The polythcrmal valley glacier Erikbreen (79°40'N 12°30'E), northern Spitsbergen, was investigated in 1970 and 1990 using digital photogrammetry and digital elevation model (DEM) techniques. The bottom topography was derived from radio-echo soundings. Based on the DEM, mass balance and changes of surface slope, crevasse and flow pattern were evaluated, and internal ice deformation velocities were calculated. Calculations of the total mass balance show that Erikbreen has not been in equilibrium for the last 20 years. The average surface lowering was 0.38 m/a and the volume had decreased by 5% to 6% from 1970 to 1990 or on the average by 3.5 × 10−1 water. The glacier surface subsided over the whole glacier area except in minor areas with northfacing slopes in the accumulation area. The surface slope and the crevasse pattern, however, did not change significantly during the 20-year-period, except in areas below 100 ma.s.l. 相似文献