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Pyroxenite is an ultramafic igneous rock consisting essentially of of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into , , and the which contain both types of pyroxenes (see diagram below). Closely allied to this group are the , consisting essentially of hornblende and other .
They are essentially of Igneous rock origin, though some pyroxenites are included in the Metamorphism Lewisian complex of Scotland where the pyroxene-rich rocks result from the type of contact metamorphism known as pyroxene-hornfels facies, have siliceous sediment or basaltic protoliths, and are respectively metapelites and metabasites.
They are often very coarse-grained, containing individual which may be several inches in length. The principal accessory minerals, in addition to olivine and feldspar, are chromite and other , garnet, magnetite, rutile, and scapolite.
Pyroxenites can be formed as cumulates in ultramafic intrusions by accumulation of pyroxene crystals at the base of the magma chamber. Here they are generally associated with gabbro and anorthite cumulate layers and are typically high up in the intrusion. They may be accompanied by magnetite layers, ilmenite layers, but rarely chromite cumulates.
Pyroxenites are also found as layers within masses of peridotite. These layers most commonly have been interpreted as products of reaction between ascending magmas and peridotite of the upper mantle. The layers typically are a few centimeters to a meter or so in thickness. Pyroxenites that occur as in basalt and in kimberlite have been interpreted as fragments of such layers. Although some mantle pyroxenites contain garnet, they are not , as clinopyroxene in them is less sodic than omphacite and the pyroxenite compositions typically are unlike that of basalt. Pyroxenites might play an important role in basalt genesis (e.g., Lambart et al., 2016), either by contributing directly to the magma production, or indirectly as the result of reaction between peridotite and magma derived from partial melting of eclogite (e.g., Sobolev and others, 2007).
A type locality is the Gullewa Greenstone Belt, in the Murchison region of Western Australia, and the Duketon Belt near Laverton, where pyroxene spinifex lavas are closely associated with gold deposits.
The pyroxenites are often subject serpentinite under low temperature retrograde metamorphism and weathering. The rocks are often completely replaced by Serpentine group, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced. Occasionally rocks rich in pyroxene are found as basic facies of nepheline syenite; a good example is provided by the melanite pyroxenites associated with the borolanite variety found in the Loch Borralan igneous complex of Scotland.
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