Diabase
Encyclopedia
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
, holocrystalline, subvolcanic rock
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and...
equivalent to volcanic basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
or pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
ic gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material. Diabase dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
s and sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
s are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margin
Chilled margin
A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid crystallization of the melt near the contact with the...
s which may contain tachylite (dark mafic glass).
Petrology
Diabase normally has a fine, but visible texture of euhedralEuhedral
Euhedral crystals are those that are well-formed with sharp, easily recognised faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma...
lath
Lath
A lath is a thin, narrow strip of some straight-grained wood or other material, including metal or gypsum. A lattice, or lattice-work, is a criss-crossed or interlaced arrangement of laths, or the pattern made by such an arrangement...
-shaped plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s (62%) set in a finer matrix of clinopyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, typically augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
(20–29%), with minor olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
(3% up to 12% in olivine diabase), magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
(2%), and ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
(2%). Accessory and alteration mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s include hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, pyrrhotite
Pyrrhotite
Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...
, chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
, serpentine, chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
, and calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
. The texture is termed diabasic and is typical of diabases. This diabasic texture is also termed interstitial. The feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
is high in anorthite
Anorthite
Anorthite is the calcium endmember of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8.-Mineralogy :...
(as opposed to albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
), the calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite
Labradorite
Labradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573....
.
Diabase/dolerite
René Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
in his 1822 Traité de minéralogie. In continental Europe diabase was reserved by Brongniart
Alexandre Brongniart
Alexandre Brongniart was a French chemist, mineralogist, and zoologist, who collaborated with Georges Cuvier on a study of the geology of the region around Paris...
for pre-Tertiary (pre-Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
) material, with dolerite used for more recent rock. The use of diabase in this sense was abandoned in Britain in favor of dolerite for rocks of all ages by Allport
Samuel Allport
Samuel Allport was an English petrologist. He was born in Birmingham and educated in that city.Although occupied in business during the greater portion of his life, his leisure was given to geological studies, and when residing for a short period in Bahia, South America, he made observations on...
(1874), though some British geologists continued to use diabase to describe slightly altered dolerite, in which pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
has been altered to amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
.
Locations
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
s. Diabase dikes occur in regions of crustal extension and often occur in dike swarm
Dike swarm
A dike swarm or dyke swarm is a large geological structure consisting of a major group of parallel, linear, or radially oriented dikes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event and are...
s of hundreds of individual dikes or sills radiating from a single volcanic center.
The Palisades Sill
Palisades Sill
The Palisades Sill is a Triassic, 200 Ma diabase intrusion. It extends through portions of New York and New Jersey. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the Hudson River...
which makes up the New Jersey Palisades
New Jersey Palisades
The Palisades, also called the New Jersey Palisades or the Hudson Palisades are a line of steep cliffs along the west side of the lower Hudson River in northeastern New Jersey and southern New York in the United States. The cliffs stretch north from Jersey City approximately 20 mi to near...
on the Hudson River
Hudson River
The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...
, near New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
, is an example of a diabase sill. The dike complexes of the British Tertiary Volcanic Province
British Tertiary Volcanic Province
The British Tertiary Volcanic Province was created 60-50 million years ago during the formation of the North Atlantic. As the Earth's crust stretched, volcanoes and lava flows erupted along a line from Ireland to the Hebrides. The Giant's Causeway and Staffa are the most famous natural monuments...
which includes Skye, Rum, Mull, and Arran of western Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, the Slieve Gullion
Slieve Gullion
Slieve Gullion is a mountain in the south of County Armagh in Northern Ireland. The mountain forms the core of the Ring of Gullion and stands as the highest point in the county with an elevation of 573 metres...
region of Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
, and extends across northern England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
contains many examples of diabase dike swarms. Parts of the Deccan Traps
Deccan Traps
The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than thick and cover an area of and a volume of...
of India, formed at the end of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
also includes dolerite. It is also abundant in large parts of Curaçao
Curaçao
Curaçao is an island in the southern Caribbean Sea, off the Venezuelan coast. The Country of Curaçao , which includes the main island plus the small, uninhabited island of Klein Curaçao , is a constituent country of the Kingdom of the Netherlands...
, an island off the coast of Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
.
In Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
a 200 km long dolerite dike, the Norseman–Wiluna Belt is associated with the non-alluvial gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
mining area between Norseman and Kalgoolie, which includes the largest gold mine in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, the Super Pit gold mine
Super Pit gold mine
The Fimiston Open Pit, colloquially known as the Super Pit, is Australia's largest open cut gold mine. The Super Pit is located off the Goldfields Highway on the south-east edge of Kalgoorlie-Boulder, Western Australia. The pit is oblong in shape and is approximately 3.5 kilometres long, 1.5...
. West of the Norseman–Wiluna Belt is the Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclastic sediments.
The vast areas of mafic volcanism/plutonism
Plutonism
Plutonism is the geologic theory that the rocks forming the Earth were formed in fire by volcanic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on the sea bed, re-formed into layers of sedimentary rock by heat and pressure, and...
associated with the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
breakup of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
land in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
include many large diabase/dolerite sills and dike swarms. These include the Karoo
Karoo
The Karoo is a semi-desert region of South Africa. It has two main sub-regions - the Great Karoo in the north and the Little Karoo in the south. The 'High' Karoo is one of the distinct physiographic provinces of the larger South African Platform division.-Great Karoo:The Great Karoo has an area of...
dolerites of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, the Ferrar Dolerites of Antarctica, and the largest of these, indeed the most extensive of all dolerite formations worldwide, are found in Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...
. Here, the volume of magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
which intruded into a thin veneer of Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "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 alone dolerite dominates the landscape.
Ring dike
Ring dike
A ring dike or ring dyke in geology refers to an intrusive igneous body. Their chemistry, petrology and field appearance precisely match those of dikes or sill, but their concentric or radial geometric distribution around a centre of volcanic activity indicates their subvolcanic origins.-Notable...
s 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 around deep intrusions. The central part may be a block sunken into underlying magma, the ring dikes forming in the fracture zone around the sunken block.