Variolites aremafic,igneous, and typicallyvolcanicrocks, e.g.tholeiite,basalt orkomatiite, that contain centimeter-scale spherical or globular structures, calledvarioles, in a fine-grained matrix. These structures are lighter colored than the host rock and typically range in diameter from 0.05mm to over 5 cm.[1][2][3] In 1648, Aldrovandi created the termvariolite foraphanitic or fine-grained igneous rocks containing varioles.[4][5] Theweathering of varioles often cause variolites to have a pock-marked appearance. In allusion to the pock-marked appearance of weathered surfaces of variolite, this term is derived from the Latin word,variola, forsmallpox.[2][3]
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, andtachylite, that comprise eitherpillow lavas, subaeriallava flows, or volcanicdykes. Typically, they are less resistant to weathering than the enclosing aphanitic rock and, as a result, form pock-marks on the weathered surfaces of mafic rocks.[6][7]
In the geologic literature, the usage of the termvariole 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 anddiabases. In some modern literature, the termvariole is defined as a type ofspherulite 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 eitherplagioclase orpyroxene. As a result, it is recommended that the termvariole 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 manyPrecambrian volcanic rocks, particularlyArchean 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 usevariolite as if it is synonymous withvariole.[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]
The variety of mafic igneous rocks that contain varioles are, with rare exceptions, no longer classified asvariolites, which is not recommended for usage. Instead, they are designated using the modifiervariolitic in conjunction with the majorlithology. The major varieties ofvariolites are variolitic basalts, variolitic pillow lavas and variolitic komatiites.[1][2] Variolitic pillow lavas, that have been previously identified asvariolites and also classified asspilites, are found in the Durance,France; on Mont Genvre, France; inDevonian rocks ofGermany; and as cobbles on the beaches of theStrait of Juan de Fuca along the northern edge of theOlympic Peninsula.[11][12] Variolitic basalts and variolitic komatiites occur commonly asArchean lava flows in thegreenstone belts of South Africa and theCanadian Shield.[7][8] Finally, there is a group of spotted volcanic rocks formerly known toFrench petrographers as thevariolites du Drac from the locality in which they are found inParc National des Ecrins, France.[13] Additional research has found them to behydrothermally altered basalt volcanic rocks that containamygdules filled with whitecalcite and other secondary minerals.[14]