Diabase

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Diabase

  1. 1. diabase, also called Dolerite, fine- to medium-grained, dark gray to black intrusive igneousrock. It is extremely hard and tough and is commonly quarried for crushed stone, under the nameof trap. Although not popular, it makes an excellent monumental stone and is one of the dark-coloured rocks commercially known as black granite. Diabase is widespread and occurs in dikes(tabular bodies inserted in fissures), sills (tabular bodies inserted while molten between otherrocks), and other relatively small, shallow bodies. Chemically and mineralogically, diabaseclosely resembles the volcanic rock basalt, but it is somewhat coarser and contains glass. Withincrease in grain size, diabase may pass into gabbro.About one-third to two-thirds of the rock is calcium-rich plagioclase feldspar; the remainder ismostly pyroxene or hornblende. In diabase, poorly formed pyroxene crystals wrap around ormold against long, rectangular plagioclase crystals to give it the characteristic texture known asdiabasic or ophitic. The larger pyroxene grains may completely enclose plagioclase; but as thequantity of the latter increases, pyroxene appears more interstitial.Certain flat tabular masses (thick sheets or sills) of diabase, such as that forming the Palisadesalong the Hudson River near New York City, show concentrations of heavy minerals (as olivineor pyroxene) in their lower portions. These concentrations are commonly believed to havedeveloped by the settling of early formed crystals in molten diabase.Diabase may show varying degrees of alteration: plagioclase is converted to sassurite; pyroxeneto hornblende, actinolite, or chlorite; and olivine to serpentine and magnetite. In British usage,such altered rock is called diabase. Some diabase masses have been subdivided by systematicfractures into rectangular blocks. Subsequent alteration and weathering along these fractureshave disintegrated and rounded off block corners and edges (spheroidal weathering), leavingregularly spaced, spherelike masses of fresh diabase enveloped by shells of progressively morealtered and disintegrated material.DiabaseFrom Wikipedia, the free encyclopediaJump to: navigation, searchDiabase
  2. 2. Diabase /ˈ daɪ.əbeɪs/ or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent tovolcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the freshrock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to alteredmaterial.[1][2] Diabase dikes and sills are typically shallow intrusive bodies and often exhibit finegrained to aphanitic chilled margins which may contain tachylite (dark mafic glass).Contents 1 Petrography 2 Diabase/dolerite 3 Locations 4 Use 5 See also 6 References 7 External linksPetrographyDiabase normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals(62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3%up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%).[3] Accessory and alterationminerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, andcalcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is alsotermed interstitial.[4] The feldspar is high in anorthite (as opposed to albite), the calciumendmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.Diabase/dolerite
  3. 3. The Candlestick, Tasman Peninsula, Tasmania, is composed of Jurassic Dolerite. Tasmania hasthe worlds largest areas of dolerite.In non-North American usage dolerite is preferred due to the various conflicting uses of diabase.Dolerite (Greek: doleros, meaning "deceptive") was the name given by Haüy in his 1822 Traitéde minéralogie. In continental Europe diabase was reserved by Brongniart for pre-Tertiary (pre-Cenozoic) material,[5] with dolerite used for more recent rock. The use of diabase in this sensewas abandoned in Britain in favor of dolerite for rocks of all ages by Allport (1874),[6] thoughsome British geologists continued to use diabase to describe slightly altered dolerite, in whichpyroxene has been altered to amphibole.[7]LocationsA diabase dike crosscutting horizontal limestone beds in Arizona
  4. 4. Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills.Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundredsof individual dikes or sills radiating from a single volcanic center.The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near NewYork City, is an example of a diabase sill. The dike complexes of the British Tertiary VolcanicProvince which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullionregion of Ireland, and extends across northern England contains many examples of diabase dikeswarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includesdolerite.[8] It is also abundant in large parts of Curaçao, an island off the coast of Venezuela.In Western Australia a 200 km long dolerite dike, the Norseman–Wiluna Belt[9] is associatedwith the non-alluvial gold mining area between Norseman and Kalgoolie, which includes thelargest gold mine in Australia,[10] the Super Pit gold mine. West of the Norseman–Wiluna Belt isthe Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclasticsediments.[11]The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup ofGondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dikeswarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica,and the largest of these, indeed the most extensive of all dolerite formations worldwide, arefound in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permianand Triassic rocks from multiple feeder sites, over a period of perhaps a million years, may haveexceeded 40,000 cubic kilometres.[12] In Tasmania alone dolerite dominates the landscape. Ringdikes are large, near vertical dikes showing above ground as circular outcrops up to 30 km indiameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are made upof plutonic rocks, rather than hypabyssal and are centred around deep intrusions.UseDiabase is used as crushed stone and as ornamental stone.

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