Variolite

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Beach pebble of variolitic pillow lava (varolite) from the Olympic Peninsula, Washington state Sample of Variolite, Olympic Peninsula, Washington USA.jpg
Beach pebble of variolitic pillow lava (varolite) from the Olympic Peninsula, Washington state

Variolites are mafic, igneous, and typically volcanic rocks, e.g. tholeiite, basalt or komatiite, that contain centimeter-scale spherical or globular structures, called varioles, 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 term variolite for aphanitic or fine-grained igneous rocks containing varioles. [4] [5] The weathering 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, for smallpox. [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, and tachylite, that comprise either pillow lavas, subaerial lava flows, or volcanic dykes. 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 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 lithology. The major varieties of variolites are variolitic basalts, variolitic pillow lavas and variolitic komatiites. [1] [2] Variolitic pillow lavas, that have been previously identified as variolites and also classified as spilites, are found in the Durance, France; on Mont Genvre, France; in Devonian rocks of Germany; and as cobbles on the beaches of the Strait of Juan de Fuca along the northern edge of the Olympic Peninsula. [11] [12] Variolitic basalts and variolitic komatiites occur commonly as Archean lava flows in the greenstone belts of South Africa and the Canadian Shield. [7] [8] Finally, there is a group of spotted volcanic rocks formerly known to French petrographers as the variolites du Drac from the locality in which they are found in Parc National des Ecrins, France. [13] Additional research has found them to be hydrothermally altered basalt volcanic rocks that contain amygdules filled with white calcite and other secondary minerals. [14]

Related Research Articles

In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava, and have low temperatures to keep the felsic minerals molten.

<span class="mw-page-title-main">Basalt</span> Magnesium- and iron-rich extrusive igneous rock

Basalt is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of Venus, which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows; and basalt is a common rock on the surface of Mars.

<span class="mw-page-title-main">Pillow lava</span> Lava containing characteristic pillow-shaped structures due to subaqueous extrusion

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. They form the upper part of Layer 2 of normal oceanic crust.

<span class="mw-page-title-main">Dacite</span> Volcanic rock intermediate in composition between andesite and rhyolite

Dacite is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.

<span class="mw-page-title-main">Volcanic rock</span> Rock formed from lava erupted from a volcano

Volcanic rocks are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

<span class="mw-page-title-main">Nephelinite</span> Igneous rock made up almost entirely of nepheline and clinopyroxene

Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene. If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase.

<span class="mw-page-title-main">Greenstone belt</span> Zone of variably metamorphosed rocks occurring in Archaean and Proterozoic cratons

Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.

<span class="mw-page-title-main">Diabase</span> Type of igneous rock

Diabase, also called dolerite or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained to aphanitic chilled margins which may contain tachylite.

<span class="mw-page-title-main">Ultramafic rock</span> Type of igneous and meta-igneous rock

Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

<span class="mw-page-title-main">Komatiite</span> Magnesium-rich igneous rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% magnesium oxide (MgO). It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

<span class="mw-page-title-main">Barberton Greenstone Belt</span> Ancient granite-greenstone terrane in South Africa

The Barberton Greenstone Belt is situated on the eastern edge of the Kaapvaal Craton in South Africa. It is known for its gold mineralisation and for its komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth are located in the Barberton Greenstone Belt of the Eswatini–Barberton areas and these contain some of the oldest traces of life on Earth, second only to the Isua Greenstone Belt of Western Greenland. The Makhonjwa Mountains make up 40% of the Baberton belt. It is named after the town Barberton, Mpumalanga.

<span class="mw-page-title-main">Komatii Formation</span> 3.475 billion year old rock formation in South Africa

The Komati Formation, also named as Komatii Formation, is a 3.475 billion year old Paleoarchean rock formation, named after the nearby Komati River in South Africa. It is the type locality for komatiite, a high temperature, magnesium-rich volcanic rock occurring in the Barberton Greenstone Belt, Mpumalanga.

<span class="mw-page-title-main">Winnipegosis komatiite belt</span>

The Winnipegosis komatiite belt is a 150 km (93 mi) long and 30 km (19 mi) wide greenstone belt located in the Lake Winnipegosis area of central Manitoba, Canada. It has no surface exposure and was identified based on geophysical signatures and drilling during mineral exploration by Cominco during the 1990s. The belt has an age of 1870 ± 7 million years and is predominantly composed of basaltic and komatiitic volcanic rocks with minor intrusive and sedimentary rocks. The belt is considered part of the larger Circum-Superior Belt and was likely generated by a mantle plume. The Winnipegosis Komatiite Belt is notable as one of the few examples of komatiite formed during the Proterozoic.

<span class="mw-page-title-main">Volcanism of Eastern Canada</span>

The volcanism of Eastern Canada includes the hundreds of volcanic areas and extensive lava formations in Eastern Canada. The region's different volcano and lava types originate from different tectonic settings and types of volcanic eruptions, ranging from passive lava eruptions to violent explosive eruptions. Eastern Canada has very large volumes of magmatic rock called large igneous provinces. They are represented by deep-level plumbing systems consisting of giant dike swarms, sill provinces and layered intrusions. The most capable large igneous provinces in Eastern Canada are Archean age greenstone belts containing a rare volcanic rock called komatiite.

<span class="mw-page-title-main">Geology of Zimbabwe</span>

The geology of Zimbabwe in southern Africa is centered on the Zimbabwe Craton, a core of Archean basement composed in the main of granitoids, schist and gneisses. It also incorporates greenstone belts comprising mafic, ultramafic and felsic volcanics which are associated with epiclastic sediments and iron formations. The craton is overlain in the north, northwest and east by Proterozoic and Phanerozoic sedimentary basins whilst to the northwest are the rocks of the Magondi Supergroup. Northwards is the Zambezi Belt and to the east the Mozambique Belt. South of the Zimbabwe Craton is the Kaapvaal Craton separated from it by the Limpopo Mobile Belt, a zone of deformation and metamorphism reflecting geological events from Archean to Mesoproterozoic times. The Zimbabwe Craton is intruded by an elongate ultramafic/mafic igneous complex known as the Great Dyke which runs for more than 500 km along a SSW/NNE oriented graben. It consists of peridotites, pyroxenites, norites and bands of chromitite.

<span class="mw-page-title-main">Tectonic evolution of the Barberton greenstone belt</span> Evolutionary history of ancient continental crust remnant located in southeastern Africa

The Barberton greenstone belt (BGB) is located in the Kapvaal craton of southeastern Africa. It characterizes one of the most well-preserved and oldest pieces of continental crust today by containing rocks in the Barberton Granite Greenstone Terrain (3.55–3.22 Ga). The BGB is a small, cusp-shaped succession of volcanic and sedimentary rocks, surrounded on all sides by granitoid plutons which range in age from >3547 to <3225 Ma. It is commonly known as the type locality of the ultramafic, extrusive volcanic rock, the komatiite. Greenstone belts are geologic regions generally composed of mafic to ultramafic volcanic sequences that have undergone metamorphism. These belts are associated with sedimentary rocks that occur within Archean and Proterozoic cratons between granitic bodies. Their name is derived from the green hue that comes from the metamorphic minerals associated with the mafic rocks. These regions are theorized to have formed at ancient oceanic spreading centers and island arcs. In simple terms, greenstone belts are described as metamorphosed volcanic belts. Being one of the few most well-preserved Archean portions of the crust, with Archean felsic volcanic rocks, the BGB is well studied. It provides present geologic evidence of Earth during the Archean (pre-3.0 Ga). Despite the BGB being a well studied area, its tectonic evolution has been the cause of much debate.

<span class="mw-page-title-main">Eastern Pilbara Craton</span> Carton in Western Australia

The Eastern Pilbara Craton is the eastern portion of the Pilbara Craton located in Western Australia. This region contains variably metamorphosed mafic and ultramafic greenstone belt rocks, intrusive granitic dome structures, and volcanic sedimentary rocks. These greenstone belts worldwide are thought to be the remnants of ancient volcanic belts, and are subject to much debate in today's scientific community. Areas such as Isua and Barberton which have similar lithologies and ages as Pilbara have been argued to be subduction accretion arcs, while others suggest that they are the result of vertical tectonics. This debate is crucial to investigating when/how plate tectonics began on Earth. The Pilbara Craton along with the Kaapvaal Craton are the only remaining areas of the Earth with pristine 3.6–2.5 Ga crust. The extremely old and rare nature of this crustal region makes it a valuable resource in the understanding of the evolution of the Archean Earth.

<span class="mw-page-title-main">Archean felsic volcanic rocks</span> Felsic volcanic rocks formed in the Archean Eon

Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon. The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ago, Archean felsic volcanic rocks provide clues on the Earth's first volcanic activities on the Earth's surface started 500 million years after the Earth's formation.

The geology of the Northwest Territories has been mapped in different quadrangles by the Canadian government. The region has some of the oldest rocks in the world and among the oldest in North America, formed from several sections of stable craton continental crust, including the Slave Craton, Rae Craton and Hearne Craton. These rocks form the Archean and Proterozoic Precambrian basement rock of the region and are the subject of extensive research to understand continental crust and tectonic conditions on the early Earth.

<span class="mw-page-title-main">Dharwar Craton</span> Part of the Indian Shield in south India

The Dharwar Craton is an Archean continental crust craton formed between 3.6-2.5 billion years ago (Ga), which is located in southern India and considered the oldest part of the Indian peninsula.

References

  1. 1 2 Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN   0-922152-76-4
  2. 1 2 3 Le Maitre, R.W., A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, and P. Bateman, eds., (2005) Igneous Rocks: A Classification and Glossary of Terms Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. United Kingdom, Cambridge University Press Cambridge. 256 pp. ISBN   9780521619486
  3. 1 2 Condie, K.C. (1981) Archean Greenstone Belts. Developments in Precambrian Geology, vol. 3. New york, New York, Elservier. 434 p. ISBN   0-444-41854-7
  4. ↑ Aldrovandi, U. (1648) Bononiensis Musæi Metallici. Bologna, Ferronii. 992 pp.
  5. ↑ 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.
  6. 1 2 3 Arndt, N., and Fowler A.D. (2004) Textures in komatiites and variolitic basalts. In: Erikson, K. ed., The Precambrian Earth: Tempos and Events. Elsevier, Amsterdam, pp 298–311. ISBN   978-0444515063
  7. 1 2 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.
  8. 1 2 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
  9. ↑ Phillips, W.J. (1973). Interpretation of crystalline spheroidal structures in igneous rocks. Lithos . vol. 6, pp. 235-244.
  10. ↑ Cas, R.A.F., and J.V. Wright (1987) Volcanic Successions, Modern and Ancient: A Geological Approach to Processes, Products, and Succession United Kingdom, London, Allen & Unwin. 528 p. ISBN   978-0-412-44640-5
  11. ↑ Lewis, J.V. (1914) Origin of Pillow Lava. Bulletin of the Geological Society of America. vol. 25, pp. 15-25.
  12. ↑ Snavely, P.D., Maceod, N.S., and Holly, C.W. (1973) Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau. Geological Society of America Bulletin. vol. 84, pp. 387–424
  13. ↑ Chisholm, Hugh, ed. (1911). "Variolites"  . Encyclopædia Britannica . Vol. 27 (11th ed.). Cambridge University Press. p. 921.
  14. ↑ 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.
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