Orbicular granite

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Orbicular granite
Igneous rock
Orbicular granite sample Mount Magnet Australia polished.jpg
Polished rock sample of orbicular granite from Mount Magnet, Western Australia.
Composition
Granitic
Outcrop of orbicular granite near Caldera, Chile. 2005.11.08 005 Granito Orbicular Caldera Chile.jpg
Outcrop of orbicular granite near Caldera, Chile.
Close-up of orbicular granite near Caldera, Chile. Close-up on orbicular granite, Caldera, Chile.JPG
Close-up of orbicular granite near Caldera, Chile.

Orbicular granite (also known as orbicular rock or orbiculite) is an uncommon plutonic rock type which is usually granitic in composition. These rocks have a unique appearance due to orbicules - concentrically layered, spheroidal structures, probably formed through nucleation around a grain in a cooling magma chamber due to rapid physical changes. [1] Almost one third of known orbicular rock occurrences are from Finland. [2] The occurrences are usually very small. [3]

Contents

Occurrences

Chile

South Africa

In the Namaqualand, South Africa, just west of the small town of Concordia, there is a rare occurrence of orbicular granite. The outcrop, known as Orbicule Hill or "wonderkoppie" (as it is locally known), is a provincial heritage site and one of just two known occurrences in South Africa. Another occurrence is on the farm Nigramoep just north west of Nababeep. This location has unfortunately been impacted by blasting for copper mining and has not been rehabilitated. When cut and polished, the granite has a very attractive pinkish colour with lighter and darker shades of grey oval shaped or orbicular inclusions. Orbiculite has been used to make jewellery and other decorative items in the past, but due to its rarity in South Africa, it is not commercially exploited and has become more a curiosity due to it being considered something of an enigma in geology. The geology of the surrounding area can be described as gneissic and granitic and is better known for its once rich copper deposits. The rocks of this region form part of the mid Proterozoic Eon and formed approximately one billion years ago. [7] [8]

Other localities

Related Research Articles

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar.

Nepheline syenite

Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.

Diorite Igneous rock type

Diorite is an intrusive igneous rock formed by the slow cooling underground of magma that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silica (mafic) gabbro and high-silica (felsic) granite.

Lamprophyre Ultrapotassic igneous rocks

Lamprophyres are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide, and high nickel and chromium.

Granodiorite Type of coarse grained intrusive igneous rock

Granodiorite is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar.

Monzonite Igneous intrusive rock with low quartz and equal plagioclase and alkali feldspar

Monzonite is an igneous intrusive rock, formed by slow cooling of underground magma that has a moderate silica content and is enriched in alkali metal oxides. Monzonite is composed mostly of plagioclase and alkali feldspar.

Restite is the residual material left at the site of melting during the in place production of granite through intense metamorphism.

Litchfieldite

Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite, sometimes called nepheline syenite gneiss or gneissic nepeheline syenite. Litchfieldite is composed of two varieties of feldspar, with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).

Monzogranite

Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly peraluminous (Al2O3/ (CaO + Na2O + K2O) = 0.98–1.11), and contain ilmenite, sphene, apatite and zircon as accessory minerals. Although the compositional range of the monzogranites is small, it defines a differentiation trend that is essentially controlled by biotite and plagioclase fractionation. (Fagiono, 2002). Monzogranites can be divided into two groups (magnesio-potassic monzogranite and ferro-potassic monzogranite) and are further categorized into rock types based on their macroscopic characteristics, melt characteristics, specific features, available isotopic data, and the locality in which they are found.

Quartz diorite Igneous, plutonic rock

Quartz diorite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less potassium feldspar. Quartz is present at between 5 and 20% of the rock. Biotite, amphiboles and pyroxenes are common dark accessory minerals.

Syenogranite is a fine to coarse grained intrusive igneous rock of the same general composition as granite. They are characteristically felsic.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Cathedral Peak Granodiorite Suite of intrusive rock in the Sierra Nevada

The Cathedral Peak Granodiorite (CPG) was named after its type locality, Cathedral Peak in Yosemite National Park, California. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada. It has been assigned radiometric ages between 88 and 87 million years and therefore reached its cooling stage in the Coniacian.

The Piégut-Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central in France. Its cooling age has been determined as 325 ± 14 million years BP.

Alkali feldspar granite Type of igneous rock rich in alkali feldspar

Alkali feldspar granite, some varieties of which are called 'red granite', is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar (K-spar). It is a dense rock with a phaneritic texture. The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals.

Boogardie quarry is a quarry on Boogardie Station, 35 km from Mount Magnet in the Mid West of Western Australia, that is a location of a rare deposit of orbicular granite.

Half Dome Granodiorite Half Dome Granodiorite is granodiorite (see also granite) found in Yosemite National Park

Half Dome Granodiorite is granodiorite found in a region on and near Half Dome, in Yosemite National Park, California, United States. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada.

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.

Siilinjärvi carbonatite

The Siilinjärvi carbonatite complex is located in central Finland close to the city of Kuopio. It is named after the nearby village of Siilinjärvi, located approximately 5 km west of the southern extension of the complex. Siilinjärvi is the second largest carbonatite complex in Finland after the Sokli formation, and one of the oldest carbonatites on Earth at 2610±4 Ma. The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss. The maximum width of the body is 1.5 km and the surface area is 14.7 km2. The complex was discovered in 1950 by the Geological Survey of Finland with help of local mineral collectors. The exploration drilling began in 1958 by Lohjan Kalkkitehdas Oy. Typpi Oy continued drilling between years 1964 and 1967, and Apatiitti Oy drilled from 1967 to 1968. After the drillings, the laboratory and pilot plant work were made. The mine was opened by Kemira Oyj in 1979 as an open pit. The operation was sold to Yara in 2007.

References

  1. 1 2 Lindh, Anders; Näsström, Helena (September 2006). "Crystallization of orbicular rocks exemplified by the Slättemossa occurrence, southeastern Sweden". Geological Magazine. 143 (5): 713–722. Bibcode:2006GeoM..143..713L. doi:10.1017/S001675680600210X. S2CID   129709727.
  2. Konopelko, D. (1 April 2006). "Lahti, S. (Editor) Orbicular Rocks in Finland 2005, with contributions by P. Raivio and I. Laitakari.: Espoo (Geological Survey of Finland) 2005. ISBN: 951-690-911-6. Price €30, 177 pp". Mineralogical Magazine. 70 (2): 238–239.
  3. Kristallin.de: Orbicular rocks vs. Rapakivis
  4. Aguirre, Luis L.; Hervé, Francisco A.; Campo, Mónica del (September 1976). "An orbicular tonalite from Caldera, Chile". Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and Mineralogy. 17 (2): 231–259. hdl:2115/36063.
  5. Niemeyer Rubilar, Hans (2018). "La granodiorita orbicular del Cordón de Lila, región de Antofagasta, Chile". Andean Geology (in Spanish). 45 (1): 104. doi: 10.5027/andgeoV45n1-3114 . Retrieved January 8, 2018.
  6. Soto, Aníbal (2019). La granodiorita orbicular del Cerro Recoba, Batolito Patagónico Norte, Chaitén (Thesis) (in Spanish). University of Chile.
  7. Norman, Nick; Whitfield, Gavin (2006). Geological Journeys: A Traveller's Guide to South Africa's Rocks and Landforms. Struik. ISBN   978-1770070622.
  8. Smalberger, John M. (1975). Aspects of the history of copper mining in Namaqualand, 1846-1931. Struik. ISBN   978-0869770436.
  9. "British Antarctic "Terra Nova" 1910-1913 Expedition Reports". 1914. Retrieved 29 March 2017.
  10. Te Ara Encyclopedia of New Zealand
  11. "Sites and Sections". Nelson Rock and Mineral Club. Retrieved 2019-06-30.
  12. Hultsfred municipality
  13. "Orbiculit von Slättemossa in Schweden". www.kristallin.de. Retrieved 2019-10-16.
  14. "Areas of Geological Interest in County Donegal" (PDF). Donegal County Council . Retrieved 29 March 2017.
  15. Simões, Margarida C.; "Ocorrência de granito orbicular em Couto do Osso, Serra da Peneda", in: Margarida C. Simões, Armando Moreira: Volume de homenagem ao Professor Doutor Carlos Teixeira / [ed. lit.] Sociedade Geológica de Portugal ; introdução Décio Thadeu & Fernando Real. - Lisboa : Sociedade Geológica de Portugal, 1981. - p. 125-128. Cota:7-294 NBP 30944
  16. Enz, Robert D.; Kudo, Albert M.; Brookins, Douglas G. (February 1979). "Igneous origin of the Orbicular Rocks of the Sandia Mountains, New Mexico". GSA Bulletin. 90 (2 Part II): 349–380. Bibcode:1979GSAB...90..349E. doi:10.1130/GSAB-P2-90-349.
  17. Basu, Adhir Kumar (2007). "Role of the Bundelkhand Granite Massif and the Son-Narmada megafault in precambrian crustal evolution and tectonism in Central and Western India". Journal of the Geological Society of India. 70 (5): 745–770.