This MedLibrary.org supplementary page on Diabase is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:
Related Sponsors
Diabase (pronounced /ˈdaɪəbeɪs/) or Dolerite is a mafic, holocrystalline, 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.12. 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).
Contents |
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 interstitial4. 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
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) material5, 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
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 dolerite8. 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 Belt9 is associated with the non-alluvial gold mining area between Norseman and Kalgoolie, which includes the largest gold mine in Australia10, 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 kilometres11. 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
- ^ Holmes, Arthur, 1974, Principles of Physical Geology, Halsted Press, 3rd ed., p. 70 ISBN 0-471-07251-6
- ^ Encyclopædia Britannica Eleventh Edition
- ^ Klein, Cornelus andCornelius S. Hurlbut, Jr., 1986, Manual of Mineralogy, Wiley, 20th ed., p. 483 ISBN 0-471-80580-7
- ^ Morehouse, W. W.,1959, The Study of Rocks in Thin Section, Harper & Row, p. 160
- ^ Harker, A. 1954 (8th Ed). Petrology for Students: An introduction to the study of rocks under the microscope, London & New York: Cambridge University Press. ISBN 0521052114
- ^ Allport, S. 1874, On the microscopic structure and composition of British Carboniferous dolerites. QJGS 30, pp 529-567
- ^ Whitten, D.G.A and Brooks, J.R.V, 1972. A Dictionary of Geology, Harmondsworth: Penguin Books. ISBN 0140510494
- ^ http://www.geokem.com/flood-basalts-1.html Continental Flood Basalts (and Layered Intrusions)
- ^ Hill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. Physical volcanology of komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia., Geological Society of Australia. ISBN 0-909869-55-3
- ^ O'Connor-Parsons, Tansy; Stanley, Clifford R. http://www.ingentaconnect.com/content/geol/geea/2007/00000007/00000002/art00003?crawler=true> Downhole lithogeochemical patterns relating to chemostratigraphy and igneous fractionation processes in the Golden Mile dolerite, Western Australia
- ^ Leaman, David 2002, "The Rock that Makes Tasmania", Leaman Geophysics, ISBN 0958119902 p117
Wikipedia content modification information:
- This page was last modified on 17 November 2008, at 11:22.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Diabase".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.