Diabase (/ˈdaɪ.əˌbeɪs/), also calleddolerite (/ˈdɒl.əˌraɪt/) ormicrogabbro,[1] is amafic,holocrystalline,subvolcanic rock equivalent tovolcanicbasalt orplutonicgabbro. Diabasedikes andsills are typically shallow intrusive bodies and often exhibit fine-grained toaphaniticchilled margins which may containtachylite (dark mafic glass).
Diabase is the preferred name in North America, whiledolerite is the preferred name in the rest of the English-speaking world, where sometimes the namediabase refers to altered dolerites and basalts. Some geologists prefer to avoid confusion by using the namemicrogabbro.
The namediabase comes from the Frenchdiabase, and ultimately from the Greekδιάβασιςdiábasis 'act of crossing over, transition',[2] whereas the namedolerite comes from the Frenchdolérite, from the Greekδολερόςdolerós 'deceitful, deceptive', because it was easily confused withdiorite.
Diabase normally has a fine but visibletexture ofeuhedrallath-shapedplagioclasecrystals (62%) set in a finer matrix ofclinopyroxene, typicallyaugite (20–29%), with minorolivine (3% up to 12% in olivine diabase),magnetite (2%), andilmenite (2%).[4] Accessory and alterationminerals includehornblende,biotite,apatite,pyrrhotite,chalcopyrite,serpentine,chlorite, andcalcite. The texture is termeddiabasic and is typical of diabases. This diabasic texture is also termedinterstitial.[5] Thefeldspar is high inanorthite (as opposed toalbite), thecalciumendmember of the plagioclase anorthite-albite solid solution series, most commonlylabradorite.
Diabase is usually found in smaller, relatively shallow intrusive bodies such asdikes andsills. Diabase dikes occur in regions ofcrustal extension and often occur indike swarms of hundreds of individual dikes or sills radiating from a singlevolcanic center.
ThePalisades Sill which makes up theNew Jersey Palisades on theHudson River, nearNew York City, New York, United States, is an example of a diabase sill. The dike complexes of theBritish Tertiary Volcanic Province includes Skye, Rum, Mull, and Arran of westernScotland, theSlieve Gullion region ofIreland, and dolerite dike swarms extending across northernEngland towards the Midlands, for exampleRowley Rag. Parts of theDeccan Traps of India, formed at the end of theCretaceous, also include dolerite.[6] It is also abundant in large parts ofCuraçao, an island off the coast ofVenezuela. Another example of diabase dikes has been recognized in theMongo area within theGuéra Massif ofChad in Central Africa.[7]
In theDeath Valley region of California,Precambrian diabase intrusions metamorphosed pre-existingdolomite into economically importanttalc deposits.[8]
In the Thuringian-Franconian-Vogtland Slate Mountains of centralGermany the diabase is entirely ofDevonian age.[9] They form typical domed landscapes, especially in theVogtland. Onegeotourist attraction is theSteinerne Rose nearSaalburg, a natural monument, whose present shape is due to the typical weathering of lava pillows.
Ageological event known as theOenpelli Dolerite intrusive event occurred about 1,720 million years ago in westernArnhem Land, in theNorthern Territory,[10] forming curved ridges of Oenpelli Dolerite stretching over 30,000 square kilometres (12,000 sq mi).[11] Further west, on the northern coast of Arnhem Land, a "subsurface radial dyke swarm" known asGaliwinku Dolerite, taking its name from theAboriginal name for Elcho Island, occurs on theGove Peninsula and continues under theArafura Sea and onWessel Islands, including Elcho andMilingimbi Islands.[12]
In theYilgarn craton ofWestern Australia, aProterozoic 200-kilometre (120 mi) long dolerite dike, theNorseman-Wiluna greenstone belt[13] is associated with the non-alluvialgold mining area betweenNorseman andKalgoorlie, which includes the largest gold mine in Australia,[14] theSuper Pit gold mine. West of the Norseman–Wiluna Belt is theYalgoo-Singleton greenstone belt, where complex dolerite dike swarms obscure the volcaniclastic sediments.[15] Large dolerite sills such as the Golden Mile Dolerite can exhibit coarse-grained texture, and show a large diversity in petrography and geochemistry across the width of the sill.[16]
The vast areas of mafic volcanism/plutonism associated with theJurassic breakup of theGondwana supercontinent in theSouthern Hemisphere include many large diabase/dolerite sills and dike swarms. These include theKaroo dolerites ofSouth Africa, theFerrar Dolerites ofAntarctica, and the largest of these, the most extensive of all dolerite formations worldwide, are found inTasmania. Here, the volume ofmagma which intruded into a thin veneer ofPermian andTriassic rocks from multiple feeder sites, over a period of perhaps a million years, may have exceeded 40,000 cubic kilometres.[17] In Tasmania, dolerite dominates much of the landscape, particularly alpine areas, with many examples ofcolumnar jointing.
Early Jurassic activity resulted in the formation ofdolerite intrusion in Prospect inSydney,[18] andquarrying ofbasalt forroadstone and other building materials has been an important activity there for over 180 years.[19][20]
Diabase is crushed and used as aconstruction aggregate for road beds, buildings, railroad beds (rail ballast), and within dams and levees.[21][22]
Diabase can be cut for use asheadstones and memorials; the base of theMarine Corps War Memorial is made of black diabase "granite" (a commercial term, not actual granite). Diabase can also be cut for use as ornamental stone for countertops, facing stone on buildings, and paving.[22] A form of dolerite, known asbluestone, is one of the materials used in the construction ofStonehenge.[23]
Diabase also serves as local building stone. In Tasmania, where it is one of the most common rocks found,[24] it is used for building, for landscaping and to erectdry-stone farm walls. In northernCounty Down, Northern Ireland, "dolerite" is used in buildings such asMount Stewart together with Scrabo Sandstone as both are quarried at Scrabo Hill.
Balls of diabase were used by the ancient Egyptians as pounding tools for working softer (but still hard) stones.[25]
Text was copied from this source, which is available under aAttribution 3.0 Australia (CC BY 3.0 AU) licence. Text was copied from this source, which is available under aAttribution 3.0 Australia (CC BY 3.0 AU) licence.Dolerite pounders are hand-held stone tools that were widely used in Egypt from the third to late first millennium BCE for quarrying and dressing granite and other hard rocks.
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