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The explosion craters of Basotu, Tanganyika Territory

Bulletin of Volcanology (Impact Factor: 2.65). 24(1):389-420. DOI: 10.1007/BF02599357
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    ABSTRACT: In the Neogene volcanic province of Northern Tanganyika are surface limestones which cover large areas, despite variation and unsuitability of the bedrock. Field mapping has proved the existence of bedded carbonate tuffs dipping off Recent vents, and the field relationships and the trace element analyses prove the tuffs to be carbonatitic. Trace element analyses of some of the «surface limestones» show that they also have carbonatitic affinities, and the inference is that some, if not all, of the limestones are consolidated carbonatitic ashes. The presence of limestone in the area surrounding other carbonatite volcanoes is examined in the light of this evidence.
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    ABSTRACT: 49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies, associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite complexes probably fall into the second category.
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    ABSTRACT: Zusammenfassung Die Assimilation von Evaporiten und Solen durch basaltisches Magma in Zonen von kontinentalen Rifts dürfte für alkalischen Magmatismus und Metasomatismus verantwortlich sein. Die Spilite könnten das ozeanische Äquivalent der Alkalisyenite darstellen. Die Remobilisierung von alkalischem Material, ausgehend von Evaporiten und Solen, wäre auch für die Feldspatisation (z. B. die alpine Albitisierung) verantwortlich. Außerdem könnte dieser Vorgang eine wichtige Rolle bei der Bildung von Glaukophangesteinen spielen. Es ist von besonderer Bedeutung, daß die ältesten heute bekannten Lagerstätten von Evaporiten, Nephelinsyeniten und Glaukophanschiefern dem oberen Präkambrium (bzw. dem Eokambrium) angehören. Diese Gesteine scheinen mit abnehmendem Alter in ihrer Häufigkeit zuzunehmen. Dies könnte eine Evolution im Alkaligehalt des Meerwassers widerspiegeln, eine Erscheinung, welche ihrerseits einer zunehmenden Alkalisierung der Erdkruste entsprechen könnte.
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