Everything about Dolerite totally explained
Diabase is a
mafic,
holocrystalline,
igneous rock equivalent to
volcanic basalt or
plutonic gabbro. Diabase is also called
dolerite in many references outside
North America. 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).
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%). 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. The
feldspar is high in
anorthite (as opposed to
albite), the
calcium end member 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
Hebridean 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. 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 is associated with the non-alluvial
gold mining area between
Norseman and
Kalgoolie, which includes the largest gold mine in
Australia, the
Fimiston Superpit.
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. In Tasmania alone dolerite dominates the landscape.
Diabase/dolerite
In non North American usage
dolerite is often preferred and
diabase is used to refer to an
altered dolerite. Dolerite (
Greek: doleros, meaning "deceptive") was the name given by
Haüy in his 1822
Traité de minéralogie. In current geologic usage diabase is preferred.
Inscription controversy
During 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.
Further Information
Get more info on 'Dolerite'.
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