Variolite
Variolites are
Varioles are millimeter- to centimeter-scale, light-colored, globular to spherical structures, that are conspicuously observable within aphanitic, mafic igneous rocks, such as basalt, komatiite, and
In the geologic literature, the usage of the term variole has been inconsistent and confusing. Initially, they were defined as spherical masses, which may or may not be spherulites, that are observed on the weathering surfaces of some basalts and diabases. In some modern literature, the term variole is defined as a type of spherulite that occurs in a mafic rock. However, because several different mechanisms can produce these small-scale, light-colored, globular to spherical structures, a specific set of varioles may or may not be spherulites that are composed of radiating crystals of either plagioclase or pyroxene. As a result, it is recommended that the term variole should be retained as originally defined. This definition is useful, not only because varioles may arise through several different mechanisms, but also because the alteration, specifically mineralization, and deformation associated with many Precambrian volcanic rocks, particularly Archean volcanic rocks, makes the determination of their origin difficult, if not impossible, without further laboratory analyses.[6][8] Phillips (1973)[9] provides a detailed review of the nomenclature of different types of varioles that have been proposed. Confusingly, a few Earth scientists use variolite as if it is synonymous with variole.[10]
Petrographic and geochemical analyses of varioles demonstrates that they can be the result of one of three possible processes. They are the blotchy alteration of a fine-grained igneous rock; the mingling of magma from two distinctly different sources; and the alteration and degradation of plagioclase spherulites. These analyses also found that their internal organization and geochemistry is incompatible with the hypothesis that they are quenched immiscible liquids, as has been suggested in the past by various authors.[6]
Varieties and occurrence
The variety of mafic igneous rocks that contain varioles are, with rare exceptions, no longer classified as variolites, which is not recommended for usage. Instead, they are designated using the modifier variolitic in conjunction with the major
References
- ^ ISBN 0-922152-76-4
- ^ ISBN 9780521619486
- ^ ISBN 0-444-41854-7
- ^ Aldrovandi, U. (1648) Bononiensis Musæi Metallici. Bologna, Ferronii. 992 pp.
- ^ Johannsen, A. (1938) A Descriptive Petrography of the Igneous Rocks. Volume 4, Part 1, The Feldspathoid Rocks and Part II, The Peridotites and Perknites. The University of Chicago Press, Chicago, Illinois. 523 pp.
- ^ ISBN 978-0444515063
- ^ a b Sandsta, N.R., B. Robins, H. Furnes, and M. de Wit (2011) The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa. Contributions to Mineralogy and Petrology. vol. 162, no. 2, pp. 365–377.
- ^ a b Fowler A.D., L.S. Jensen, and S.A. Peloquin (1987) Varioles in Archean basalts; products of spherulitic crystallization. Canadian Mineralogist. vol. 25, pp. 275–289
- ^ Phillips, W.J. (1973). Interpretation of crystalline spheroidal structures in igneous rocks. Lithos. vol. 6, pp. 235-244.
- ISBN 978-0-412-44640-5
- ^ Lewis, J.V. (1914) Origin of Pillow Lava. Bulletin of the Geological Society of America. vol. 25, pp. 15-25.
- ^ Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge University Press. p. 921.
- ^ Buffet-Croix-Blanche, G. (1989) Volcans fossiles dans la region du parc national des Ecrins; sept excursions pour une initiation volcanologique sur des lambeaux de volcans dates de 200 millions d'années. Parc National des Ecrins, France. 73 p.