Zusammenfassung
Natronreiche Effusiva — wie Phonolite und Trachyte — finden sich in Kenya in zahlreichen kleinen Vorkommen. Die Vulkanite zeigen trotz kennzeichnender Unterschiede gewisse Ähnlichkeiten mit einer Dazit-Rhyolith-Serie, die Strukturen aufweist, die man verschieden als ignimbritisch bezeichnet, oder als Aschestrom, Schmelztuffe und Tufflaven beschreiben kann. Die Vulkanite zeigen einen Übergang in wohl auskristallisierte Laven und können nicht mit derMarshallschen Ignimbrit-Hypothese erklärt werden; ebensowenig befriedigt die Hypothese vonRoss &Smith als Aschen-Glutfluß. Sie sollen hier als Schaum-Laven betrachtet werden.
Der primäre Prozeß, der zur heterogenen Ausbildung dieser Laven führt, beruht auf einer ungleichmäßigen Entgasung, die sowohl im Schlot als auch an der Oberfläche stattfindet. Die dabei entstehenden Strukturen — wie Brekziierung, Bänderung und Schmelzstrukturen — können alle innerhalb eines Lavastromes beobachtet werden und sollen hier unter dem Begriff „structurally composite lava flows“ zusammengefaßt werden.
Abstract
A field of volcanic rocks in Kenya includes numerous effusions of fragmental character and sodic alkaline composition — phonolites and trachytes. They show similarities to, and differences from, rhyolite / dacite suites referred to variously as ignimbrites, ash flows, welded tuffs and tuffolavas elsewhere. They are transitional into normal, well-crystallised lavas and cannot reasonably be explained by invoking eitherMarshall's classic ignimbrite hypothesis or its later modification, the ash flow hypothesis ofRoss andSmith. These are considered to be froth flows or foam lavas.
The primary process leading to heterogeneity is believed to have been one of very irregular volatiles segregation, due to the stirring effect of flow through conduits and over the surface. A varied range of fragmental, banded, eutaxitic and autobrecciated lava types has stemmed from effects of such segregation, and all these types of structure may be seen within indivual flows, termed “structurally composite lava flows”.
Résumé
Une association des coulées de lave renferment des fragments est décrit ici. Les rochers sont semblable aux des rochers rhyolite/dacite qu'on les appelle „ignimbrite“, „ash-flow“, „welded-tuff“ ou „tuffolava“. Ils sont transitionnel aux des laves d'une espèce normal et beaucoup crystallins; ainsi, une explication comme cette deMarshall (hypothèse „ignimbrite“) ou cette plus tard avancé parRoss etSmith (hypothèse „ash-flow“) est incroyable. L'auteur dit que les laves fragmentales de Kenya sont „froth-flows“ — des laves écumeux.
Le processus primaire qui faisait cette hétérogénéité fût le ségrégation des volatils par l'effet d'agitant la coulée en passant le conduit et en effusion a plein air. Une série des laves, fragmentales, rayées, eutaxitiques, et autobrecciées fut nées de cette ségregation, et toutes ces espèces se trouvent dedans cóulees individuelles, qu'on appelle „de structure composite“.
Краткое содержание
Исследовались вулка нические породы Кени и. Раньше их опнисывали, как игнимбриты, туфол авы или пепловый туф. П ричины разнородности этих пород следует ис кать в различных проц ессах, происходивших в труб ках взрыва и на поверхнос ти.
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McCall, G.J.H. Froth flows in Kenya. Geol Rundsch 54, 1148–1195 (1965). https://doi.org/10.1007/BF01820775
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DOI: https://doi.org/10.1007/BF01820775