排序方式： 共有9条查询结果，搜索用时 46 毫秒
1
1.
Vocanic rocks of Karoo age which today cover more than 140,000 km2 of the southern African sub-continent occur as scattered outliers representing eroded remnants of an originally more extensive
volcanic province. The rocks are best preserved in central southern Africa including Lesotho, and the continental margin areas
of Namibia in the west and Mozambique. Zambabwe, Swaziland and South Africa in the east. Extensive lava fields (yet few volcanoes)
dykes, sills, layered intrusions and at least two major dyke swarms characterise the region. Volcano-stratigraphic and geochemical
mapping have been used to subdivide the volcanic successions found in the different areas and recently adopted nomenclature
is presented. Considerable more variability and complexity occurs in the volcanic succession than was previously recognised:
geochemical variations and stratigraphic relationships indicating that four major provinces can be recognised. Rocks from
the central Karoo areas are primarily of basaltic composition whereas those from the western and eastern marginal areas include
mafic basic, intermediate and acid types. Emplacement of rocks such as carbonatites, nephelinites, and picrite basalts enriched
in incompatible elements, indicate that derivation from a heterogeneously enriched source played a significant role in the
petrogenesis of a large proportion of the Karoo mafic and basic rocks. Age relationships of the volcanic rocks reveal that
vulcanicity extended over a period of 130 m.y. from mid-Triassic to early Cretaceous time. 相似文献
2.
During the early development of the Rhodesian craton the regional metamorphism of some of the oldest rocks of Bulawayan and Shamvalan age, was apparently progressive and is expressed in terms of a clearly defined metamorphic zonation established under conditions of relatively high geothermal gradients in which very low grades typical of the central craton pass into extremely high grades that define the cratonic margin particularly within the Zambezi and the Limpopo belts. The distribution of the associated early granitic rocks relates to the metamorphic pattern, thus implying that both metamorphism and granite development were due to thermal highs centred on the Limpopo mobile belt and possibly the Zambezi belt. 相似文献
3.
The preparation of the International Metamorphic Map of Africa has revealed a number of important trends in regional metamorphism across the orogenic belts of Africa. Relatively simple systematic increases in grade in some belts is in marked contrast to polycyclic metamorphism in others. Reworking of older sialic basement and the development of granulites are commonly recorded from belts. Archaean cratons are characterised by granitoids and high thermal metamorphism within a relatively low pressure environment. In late Precambrian belts regional metamorphism was more frequently of intermediate pressure type, the paired metamorphic belt regime not being recognised.
Zusammenfassung Die Herstellung der internationalen Karte der regionalmetamorphen Zonen Afrikas hat eine Anzahl wichtiger Trends in Bezug auf regionalen Metamorphismus, der sich über Afrika erstreckenden orogenen Gürtel, aufgezeigt. Verhältnismäßig unkomplizierter, systematischer Metamorphoseablauf in einigen Gürteln stehen in scharfem Kontrast zu polyzyklischen Metamorphismus in anderen. Die Weiterbeanspruchung eines älteren sialischen Grundstratums und die Entstehung von Granuliten werden im allgemeinen in vielen Gürtel beobachtet. Archaische Kratone zeigen granitartige Charakteristiken und hohen thermalen Metamorphismus in ihrer Umgebung unter verhältnismäßig niedrigem Druck. In spät-praekambrischen Gürteln war regionaler Metamorphismus öfters von mittleren Drucken begleitet.
Résumé La préparation de la carte internationale métamorphique de l'Afrique a révéle un certain nombre de tendances dans le métamorphisme régional des zones orogéniques de l'Afrique. Des accroissements systématiques relativement simples du degré métamorphique dans certains zones tranchent nettement avec le métamorphisme polycyclique dans d'autres. Le remainiement du vieux socle sialique et le développement de granulites sont couramment signalés dans nombreuses zones. Les cratons archéens se caractérisent par des granitoÏdes et par un métamorphisme thermique éllevé dans un environnement à pression relativement basse. Dans les zones précambriennes récentes le métamorphisme régional fut plus fréquemment du type à moyenne pression, la double Çeinture métamorphique n'étant pas connues.
, . . . . -- .相似文献
4.
E. P. Saggerson 《Bulletin of Volcanology》1970,34(1):38-76
In the Nairobi area and the adjacent region of the Gregory Rift Valley of Kenya two series of Cainozoic alkaline volcanic rocks, of mildly and strongly alkaline type respectively, are considered to have been derived from a single parental magma of alkali olivine basalt composition. Vulcanicity is genetically associated with tectonic movements attendant upon doming and rifting and distances from the rift margin decrease with crystal fractionation, the most acid differentiates being located at the maximum elevation of the rift floor. An early period of nepheline-bearing lava extrusion associated with central vulcanism is followed by a later period of welded tuff — trachyte — rhyolite fissure eruption. Caldera formation associated with central vulcanism within the Rift accompanies the later volcanic phase. 相似文献
5.
Alkaline olivine basalts and associated eclogite nodules from southern Kenya are described and attention drawn to the chemical and mineralogical similarities between these rocks and other continental, inclusion-bearing olivine basalts. The possibility of deriving the basalts from spinel pyroxenite and eclogite at high temperatures and pressures is discussed. 相似文献
6.
The rough pen shell Pinna rudis Linnaeus, 1758 (family Pinnidae), a mollusc with an Atlantic–Mediterranean distribution, is able to live in coarse sandy substrates. Considering its shell structure and ecological characteristics, P. rudis can enhance biodiversity by providing a substrate for settlement on its shell. For this reason, we compared the diversity of benthic taxa around P. rudis shells with the species diversity on P. rudis shells, at Matiota Beach, São Vicente Island, Cabo Verde. We sampled an area of 900 m2 and recorded data in situ to estimate the population size of P. rudis and the epibiotic and benthic community diversity. The average density of P. rudis estimated in the sampled area was 6.6 ind. 100 m–2 and the highest density was found at between 2 and 3 m depth, mostly in biogenic and sandy substrates. The epibiotic species diversity on P. rudis shells was significantly higher than the species diversity in the microhabitat around the shells. The P. rudis shell seems to play an important role in increasing the biodiversity of the ecosystem, with some species found only as epibionts on P. rudis. 相似文献
7.
Brine expulsion, fluid transport and metallization spanning 2.0 Gyr in basins of southern and central Africa 总被引：1，自引：0，他引：1
Mineral provinces in southern and central Africa are strongly controlled by major structural trends, the alignment of sedimentary basins and metamorphically induced thermal regimes deep in the crust. Ore deposits are preferentially located on cooler margins of orogenic belts and are ultimately end-products of fluid expulsion out of the deeper parts of orogenic axes. Metamorphic and structural vectors within orogenic belts adjacent to major cratons show a trend of high-grade thermal overprinting in the cores of axes and lower metamorphic regimes acting on the distal margins of orogens (e.g. foreland basins). This apparent pattern is considered to have importance in the expulsion of at least three generations of mineralizing fluids beginning with exhalative migration during diagenesis and culminating much later in thrust-controlled expulsion onto adjacent craton margins. Fluids within the hydrosphere that accumulate initially through topographic gradients in the sediments mixed with components of the mantle (CO2). After storage within the crust, migration, enforced by metamorphic processes, transferred fluids out of, and away from, high thermal regimes in the axes of belts, leading to their present preservation around the margins of the major cratonic nuclei. 相似文献
8.
A brief account is presented for the Lebombo volcanic succession which crops out in Natal, South Africa. The volcanic belt
is of late Karoo age and is composed of a thick sequence of basaltic lavas (Sabie River Formation) overlain by an equally
voluminous succession of acid-flows (Jozini Formation) erupted over a period of about 70 m.y. Field relationships indicate
that the Lebombo basalt pile consists of simple and compound flow units. The rhyolite succession consists of thick (80–284
m) flows units characterised by features found in both ignimbrites and rhyolitic lavas respectively. It is postulated that
they were extruded as high temperature, low volatile pyroclastic flows. The Bumbeni volcanic complex which crops out near
the southern termination of the Lebombo mountains, disconformably overlies the Jozini Formation and is characterised by a
suite of rocks that includes rhyolite lavas, air-fall and ash-flow tuffs, syenite intrusions and basic-intermediate lavas.
Dolerite dykes are ubiquitous throughout the succession and an extremely dense concentration of basic intrusions located along
the western margin of the belt gives rise to the Rooi Rand dyke swarm. Rare sill-forms are found associated with the mafic
volcanies. Acid intrusives are represented by simple and composite quartz-porphyry intrusions and rhyolite dykes. The structure
of the Lebombo is that of a faulted monocline, tilted to the east, developed prior to the fragmentation of eastern Gondwanaland.
The volcanic belt is located at the tectonic contact between two major Precambrian elements, the 3,000 m.y. Kaapvaal craton
to the west and the southerly extension of the 550 m.y. Mozambique belt to the east. It is bounded to the south by the 1,000
m.y. old Natal-Namaqua mobile belt. 相似文献
9.
Volcanic rocks of late Triassic to early Cretaceous age which today cover more than 140 000 km2 of the southern African sub-continent are discussed. The rocks occur as scattered outliers and represent small eroded remnants of an originally more extensive volcanic province. The rocks are best preserved in three main areas of southern Africa, in Namibia, in central South Africa, Transkei and Lesotho and in the southeastern areas of the sub-continent including South Africa, Swaziland, Mozambique and Zimbabwe. A history of previous work done on Karoo volcanism is presented and accounts of the geological relationships of the main, and some subsidiary, areas as well as events preceding the major phase of extrusive vulcanism are discussed. Relationships between dolerite dykes and basaltic lavas are noted. Volcano-stratigraphic and geochemical mapping have been used to subdivide the volcanic successions found in the different areas, and recently adopted nomenclature is presented. Considerable more variability has been recognised in the volcanic succession than was previously recognised, the flood basalt pile being more complex than was originally thought. Rocks from the central Karoo areas are primarily of basaltic composition whereas those from the western and eastern marginal areas include basic, and felsic types. Age relationships of the volcanics based on recent studies are outlined and geochemical variations within the entire province discussed. The classification system adopted for the Karoo volcanics is described briefly and chemical relationships of extrusive and intrusive rocks are included. In concluding some new ideas concerning the overall development of the Karoo volcanic province and related fragmentation of Gondwanaland are presented. 相似文献
1