Pyroxenite - Intrusive and Mantle Pyroxenites

Intrusive and Mantle Pyroxenites

The igneous pyroxenites are closely allied to the gabbros and norites, from which they differ by the absence of feldspar, and to the peridotites, which are distinguished from them by containing more than 40% olivine. This connection is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves.

They are often very coarse-grained, containing individual crystals which may be several inches in length. The principal accessory minerals, in addition to olivine and feldspar, are chromite and other spinels, garnet, magnetite, rutile, and scapolite.

Pyroxenites can be formed as cumulates in ultramafic intrusions by accumulation of pyroxene crystals at the base of the lava 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 xenoliths in basalt and in kimberlite have been interpreted as fragments of such layers. Although some mantle pyroxenites contain garnet, they are not eclogites, as clinopyroxene in them is less sodic than omphacite and the pyroxenite compositions typically are unlike that of basalt. It has been proposed that large volumes of pyroxenite form in the upper mantle as a result of reaction between peridotite and magma derived from partial melting of eclogite, and that such pyroxenite volumes are important sources of basalt magma (e.g., Sobolev and others, 2007).