diabase

Diabase

Diabase (pronounced /ˈdaɪəbeɪs/) or Dolerite is a mafic, holocrystalline, intrusive igneous rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material.^[1][2] 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).

Petrology

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%).^[3] 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^[4]. The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.

Diabase/dolerite

The Candlestick, Tasman Peninsula, Tasmania, is composed of Jurassic Dolerite. Tasmania has the world's 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 Traité de 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 the this sense was abandoned in Britain in favor of dolerite for rocks of all ages by Allport (1874)^[6], though some British geologists continued to use diabase to describe slightly altered dolerite, in which pyroxene has been altered to amphibole.^[7]

Locations

A diabase dike crosscutting horizontal limestone beds in Arizona

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^[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 dyke, the Norseman–Wiluna Belt^[9] is associated with the non-alluvial gold mining area between Norseman and Kalgoolie, which includes the largest gold mine in Australia^[10], the Super Pit gold mine.

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 Jurassic rocks from multiple feeder sites, over a period of perhaps a million years, may have exceeded 40,000 cubic kilometres^[11]. In Tasmania alone dolerite dominates the landscape.

Ring dykes are large, near vertical dykes showing above ground as circular outcrops up to 30 km in diameter, with a depth from hundreds of metres to several kilometres. Thicker dykes are made up of plutonic rocks, rather than hypabyssal and are centred around deep intrusions. The central part may be a block sunken into underlying magma, the ring dykes forming in the fracture zone around the sunken block.

Inscription controversy

For seven centuries a diabase formation called Runamo was famous in Scandinavia as a runic inscription, until it became the object of a famous scientific controversy in the first half of the 19th century.

References

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