Diabase (pronounced /ˈdaɪəbeɪs/) or Dolerite is a mafic, holocrystalline, subvolcanic 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).
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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.
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]
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 Triassic 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.
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Redirecting to Diabase
DOLERITE (from Gr. boXepos, deceptive), in petrology, the name given by Haiiy to those basaltic rocks which are comparatively coarse grained and nearly, if not quite, holocrystalline. As may be inferred from their highly crystalline state they are very often intrusive, and occur as dikes and sills, but many of them form lava flows. Their essential minerals are those of basalt, viz. olivine, augite and plagioclase felspar, while hornblende, ilmenite, apatite and biotite are their commonest accessory ingredients. The chemical and microscopic features of these minerals agree generally with those presented in the basalts, and only their exceptional peculiarities need be mentioned here. Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these). Others, probably the majority, are non-porphyritic,and these are generally coarser grained than the ground-mass of the former group, though lacking their large conspicuous phenocrysts. The commonest type of structure in dolerite is the ophitic, which results from the felspar of the rock having crystallized before the augite; the latter mineral forms shapeless masses in which the idiomorphic felspars lie. The augite enclosing the felspars is well crystallized, though its continuity is interrupted more or less completely by the numerous crystals of felspar which it envelops, and in polarized light the former often behaves as a single individual over a considerable area, while the latter mineral consists of independent crystals. This structure may be so coarse as to be easily detected by the unaided eye, or so fine that it cannot be seen except in microscopic sections. Some of the porphyritic dolerites have ophitic ground-masses; in others this structure is imperfect (subophitic); while in many the augite, like the felspar, occurs as small and distinct individuals, which react differently on polarized light, and have the outlines of more or less perfectly shaped crystals. Ophitic structure is commonest in olivine-dolerites, though the olivine takes no part in it.
The quartz-dolerites are an important group, hardly less common than the olivine-dolerites. They contain a small amount of quartz, and often micropegmatite, as the last element to consolidate, filling up little angular interspaces between the felspars and pyroxenes, which had previously crystallized. They rarely contain olivine, but pleochroic hypersthene is by no means rare in them (hypersthene-dolerites). Some contain larger individuals of pale green, rather pleochroic augite (the so-called sahlite), and a little brown mica, and brownish-green hornblende may also be present.
Allied to these are olivine-free dolerites with more or less of interstitial glassy base (tholeites, &c.). In the rocks of this group ophitic structure is typically absent, and the presence of an interstitial finely crystalline or amorphous material gives rise to the structure which is known as "intersertal." Transitions to the porphyritic dolerites and basalts arise by increase in the proportion of this ground-mass. The edges of dolerite sills and dikes often contain much dark brown glass, and pass into tachylytes, in which this material preponderates.
Another interesting group of doleritic rocks contains analcite. They may be ophitic, though often they are not, and they usually contain olivine, while their augite has distinctly purple shades, and a feeble dichroism.
Their characteristic feature is the presence of a small amount of analcite, which never shows crystalline outlines but fills up the interspaces between the other minerals. Some writers held that this mineral has resulted from the decomposition of nepheline; others regard it as a primary mineral. Usually it can be clearly shown to be secondary to some extent, but there is reason to suppose that it is really a pneumatolytic deposit. These rocks are known as teschenites, and have a wide distribution in England, Scotland, on the continent and in America. Often they are comparatively rich in brown hornblende. This last-named mineral is not usually abundant in dolerites, but in a special group, the proterobases, it to a large extent replaces the customary augite. A few dolerites contain much brown mica (mica-dolerites). Nepheline may appear in these rocks, as in the basalts. Typical nepheline-dolerites are scarce, and consist of idiomorphic augite, surrounded by nepheline. Examples are known from the Tertiary volcanic districts of the Rhine.
Dolerites have a very wide distribution, as they are found wherever basalts occur in any number. It is superfluous to cite localities for them as they are among the commonest of igneous rocks. They are much employed for road-mending and for kerbstones, though their dark colour and the tendency they have to weather with a dingy brown crust make them unsuitable for the better classes of architectural work. (J. S. F.)
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