Diabase

Diabase (ˈdaɪ.əˌbeɪs), also called dolerite (ˈdɒl.əˌraɪt) or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. 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). Diabase is the preferred name in North America, while dolerite is the preferred name in the rest of the English-speaking world, where sometimes the name diabase refers to altered dolerites and basalts. Some geologists prefer to avoid confusion by using the name microgabbro. The name diabase comes from the French diabase, and ultimately from the Greek διάβασις - meaning "act of crossing over, transition". 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%). 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. 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 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, New York, United States, is an example of a diabase sill. The dike complexes of the British Tertiary Volcanic Province includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and dolerite dike swarms extending across northern England towards the Midlands, for example Rowley Rag.