Diabase or dolerite 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, yet dolerite is the preferred name in most of the rest of the world, where sometimes the name diabase is applied to altered dolerites and basalts. Many petrologists prefer the name microgabbro to avoid this confusion.
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, 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 dolerite dike swarms, towards the Midlands other examples include Rowley Rag. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includes dolerite. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela. Another example of diabase dikes has been recognized in the Mongo area within the Guéra Massif of Chad in Central Africa.
In Western Australia a 200 km long dolerite dike, the Norseman-Wiluna Belt is associated with the non-alluvial gold mining area between Norseman and Kalgoorlie, which includes the largest gold mine in Australia, the Super Pit gold mine. West of the Norseman-Wiluna Belt is the Yalgoo-Singleton Belt, where complex dolerite dike swarms obscure the volcaniclastic sediments. Large dolerite sills such as the Golden Mile Dolerite can exhibit course grained texture, and show a large diversity in petrography and geochemistry across the width of the sill.
In the Thuringian-Franconian-Vogtland Slate Mountains of central Germany the diabase is entirely of Devonian age. They form typical domed landscapes, especially in the Vogtland. One geotourist attraction is the Steinerne Rose near Saalburg, a natural monument, whose present shape is due to the typical weathering of lava pillows.
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. In Tasmania, dolerite dominates much of the landscape, particularly alpine areas.
Ring dikes are large, near vertical dikes showing above ground as circular outcrops up to 30 km in diameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are made up of plutonic rocks, rather than hypabyssal and are centred on deep intrusions.
Diabase can also be cut for use as ornamental stone for countertops, facing stone on buildings, and paving. A form of dolerite, known as bluestone, is one of the materials used as in the construction of Stonehenge.
Diabase also serves as local building stone. In Tasmania, where it is one of the most common rocks found, it is used for building, for landscaping and to erect dry-stone farm walls. In northern County Down, Northern Ireland, "dolerite" is used in buildings such as Mount Stewart together with Scrabo Sandstone as both are quarried at Scrabo Hill.
The Blackbird (violin) is made of black diabase.