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Diabase

Photographed by Michael P. Klimetz

Moose Lake

Granite County

MONTANA

Diabase [or Dolerite] is a mafic, holocrystalline, subvolcanic [hypabyssal] rock equivalent to volcanic basalt or plutonic gabbro. Based upon North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock as well as altered material. Diabase dikes and sills are typically shallow intrusive bodies,often exhibiting fine grained to aphanitic chilled margins which may contain tachylite, adark mafic glass. Diabase normally has a fine yet visible texture of euhedral lath-shaped plagioclase crystals (up to 62% by volume) set in a finer matrix of clinopyroxene, typically augite (between 20 and 29%), with minor olivine (from 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 characteristic. Diabasic texture is alternately referred to as interstitial. The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, and is most commonly labradorite. Diabase usually occurs in small, relatively shallow dikes and sills. Diabase dikes occur in regions of crustal extension and often occur in 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 western bank of Hudson River opposite New York City, is an example of a diabase sill.