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Dolerite

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Subject: Mafic, Geochemistry, Freycinet National Park, Shiant Isles, Fair Head, Brown Clee Hill, Titterstone Clee Hill, Duck Reach Power Station, Rowley Hills, Quartz-dolerite
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Dolerite

Diabase /ˈd.əbs/ or 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 (dark mafic glass).

Petrography

Diabase 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%).[1] Accessory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is also termed interstitial.[2] The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.

Locations


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 hundreds of 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 New York City, is an example of a diabase sill. The dike complexes of the British Tertiary Volcanic Province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and extends across northern England contains many examples of diabase dike swarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includes dolerite.[3] 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[4] is associated with the non-alluvial gold mining area between Norseman and Kalgoolie, which includes the largest gold mine in Australia,[5] the Super Pit gold mine. West of the Norseman–Wiluna Belt is the Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclastic sediments.[6]

The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup of Gondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dike swarms. 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, are found in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permian and Triassic rocks from multiple feeder sites, over a period of perhaps a million years, may have exceeded 40,000 cubic kilometres.[7] In Tasmania alone dolerite dominates the landscape. Ring dikes are large, near vertical dikes showing above ground as circular outcrops up to 30 km in diameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are made up of plutonic rocks, rather than hypabyssal and are centred around deep intrusions.

Use

Diabase is used as crushed stone and as ornamental stone.

See also

References

External links

  • Collection of dikes in the Fish River Canyon, Namibia
cs:Dolerit

fr:Dolérite nl:Doleriet ja:輝緑岩

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