Microgabbro

[Hermalindo Lepicier]

Diabaseis 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.

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. Parts of the Deccan Traps of India, formed at the end of the Cretaceous, also include dolerite.[5] It is also abundant in large parts of Curaao, an island off the coast of Venezuela. Another example of diabase dikes has been recognized in the Mongo area within the Gura Massif of Chad in Central Africa.[6]

In the Thuringian-Franconian-Vogtland Slate Mountains of central Germany the diabase is entirely of Devonian age.[8] 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.

A geological event known as the Oenpelli Dolerite intrusive event occurred about 1,720 million years ago in western Arnhem Land, in the Northern Territory,[9] forming curved ridges of Oenpelli Dolerite stretching over 30,000 square kilometres (12,000 sq mi).[10] Further west, on the northern coast of Arnhem Land, a "subsurface radial dyke swarm" known as Galiwinku Dolerite, taking its name from the Aboriginal name for Elcho Island, occurs on the Gove Peninsula and continues under the Arafura Sea and on Wessel Islands, including Elcho and Milingimbi Islands.[11]

The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup of the Gondwana supercontinent 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, 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.[16] In Tasmania, dolerite dominates much of the landscape, particularly alpine areas, with many examples of columnar jointing.

Early Jurassic activity resulted in the formation of dolerite intrusion in Prospect in Sydney,[17] and quarrying of basalt for roadstone and other building materials has been an important activity there for over 180 years.[18][19]

Diabase can be cut for use as headstones and memorials; the base of the Marine Corps War Memorial is made of black diabase "granite" (a commercial term, not actual granite). Diabase can also be cut for use as ornamental stone for countertops, facing stone on buildings, and paving.[21] A form of dolerite, known as bluestone, is one of the materials used in the construction of Stonehenge.[22]

Diabase also serves as local building stone. In Tasmania, where it is one of the most common rocks found,[23] 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.

Diabase, dolerite, or microgabbro is a medium-grained, dark-colored, hypabyssal, or subvolcanic rock with a composition similar to plutonic gabbro or volcanic basalt. It commonly occurs in dikes, sills, and sometimes in plugs, sheeted dikes, laccoliths, or lopoliths.

However, diabase (preferred in the US and Canada) and dolerite (British usage) are accepted microgabbro synonyms. Why? Because they have long-established and widespread usage, notes Le Maitre (2002). But there are recommendations to end using the names dolerite or diabase.

Some authors indicate that dolerite is unaltered microgabbro while slightly altered or metamorphized as diabase. Some emphasis on diabase being a medium-grained equivalent of tholeiitic basalt. Others consider it an altered dolerite with plagioclase albitized or saussuritized and pyroxenes replaced by chlorite or amphibole.

You can define dolerite or diabase as a medium-grained, dark-colored subvolcanic rock with a composition similar to basalt or gabbro. It is a basic (low in silica 45-52 wt. %) or mafic rock rich in magnesium and iron and low in felsic minerals.

On the other hand, porphyritic dolerite will have large crystals in a finer-grained matrix or groundmass. It indicates two cooling stages: a slower one that grows the larger minerals and a faster one that forms the groundmass.

Dolerite will show mainly tubular or lath-shaped plagioclase on the thin section, usually white grains with dark strips due to crystal twinning. Also, you expect partly or fully enclosed by larger dark green to black anhedral augite, opaques (magnetite or ilmenite), olive green olivine, hornblende, etc.

Lastly, diabase rocks have more than 35% mafic content with a typical color index M > 35. Those with M 90% mafic content are ultramafic rocks like peridotites.

Dolerite or diabase is a mafic rock. It has primarily calcic plagioclase and augite with or without minor amounts of olivine, hornblende, enstatite, quartz, and feldspathoids (nepheline, analcime, etc.). Also, it may have a small amount of alkali feldspar.

On the other hand, augite is the dominant clinopyroxene and mafic mineral. However, some dolerite varieties may have calcium-poor pyroxenes. i.e., pigeonite (clinopyroxene) and enstatite (orthopyroxene).

Diabase may be classified into tholeiitic or alkali varieties, just like gabbro. This classification depends on the presence or absence of low-Ca pyroxenes, nepheline, analcite, quartz, and olivine (including amount). Also, some distinctive accessories may be an indicator.

Alkali diabase rocks are rich in alkali oxides (sodium and potassium oxides) and poorer in iron and calcium oxide than other diabases. These rocks often have olivine, interstitial feldspathoids nepheline, or analcite with no quartz or alkali feldspar.

Tholeiitic diabase is quartz-saturated or oversaturated relative to alkalis. Besides calcic plagioclase and augite, they will have interstitial quartz and calcium-poor pyroxenes (enstatite and pigeonite) with little or no olivine.

Trachydolerite is a medium-grained rock with a chemical composition between basaltic and trachyte. It has calcic plagioclase (labradorite), alkali feldspar (orthoclase), clinopyroxene, olivine, and sometimes analcime or leucite.

As Mindat.org notes, ophite is a term originated by Palasson in 1819. It describes diabase or dolerite that has retained its ophitic texture, but its pyroxene has been altered to uralite. This rock usually has a hypidiomorphic-granular texture.

The main difference between diabase and basalt is grain size. Diabase has a medium grain size, while basalt has a fine-grained or aphanitic texture. Thus, you can see individual diabase minerals without a magnifier, while you cannot see them in basalt. However, dolerite shows more mineral diversification than basalt.

This magma will intrude into weak zones, faults, or fractures in preexisting rock bodies, cool, and solidify within the crust, forming this rock. Afterward, erosion and uplift will eventually expose dolerite rocks to the surface.

Dolerite or diabase occurs mainly in dikes, sills, sometimes plugs, lopoliths, laccoliths, and very thick flood basalt. Dikes and sills can exist individually or in swarms associated with a single magma source, especially in flood basalts in large igneous provinces (LIPs).

Diabase is dense, hard, and durable rock. It makes construction aggregate and dimensional stones (building and architectural stone). Also, you can use it for landscaping, riprap (soil erosion control), paving, etc.

In the construction and dimension industries, dolerite and other rocks like basalt, gabbro, anorthosite, norite, and diorite are known as black granite. Also, it goes by various names like trap, traprock, whin, or whinstone.

Also, crushed dolerite stones are perfect for sewer filtration, riprap (soil erosion control), and landscaping. You can also use this gravel for unpaved roads, patios, walkways, stabilizing landscapes, etc.

The prehistoric (built in 3000 BC to 2000 BC) monumental Stonehenge at Salisbury Plain in Wiltshire, England, has about 80 pieces of spotted and non-spotted dolerite bluestone pillars. It remains a treasured architectural feature and was listed as a UNESCO Heritage Site in 1986.

An intrusive igneous rock with a columnar joint structure having a height of approximately 55 m cropped out in the Watugajah area, Gedangsari, Gunung Kidul Regency, Yogyakarta Special Province. This region geologically is in the western part of the Southern Mountains, part of the Baturagung Subzone, which belongs to the Kebo Formation. This study aims to characterize the igneous rock in the Watugajah area of the Southern Mountains. It suggests the petrogenetic information of this igneous rock and gives a new consideration of the geological history in the Kebo Formation.

Geological field mapping was managed in the research area to identify the correlation with other rock units. Rock samples were collected to characterize their composition by petrographical and geochemical analyses. The results suggest the igneous rocks intruded sedimentary rocks of the Kebo Formation, which consists of intercalations coarse sandstone and tuff, fine sandstone and siltstone, and tuff and fine sandstone units. The igneous rocks are classified as microgabbro and basalt in the form of the sill and dyke intrusions. The microgabbro and basalt consist of plagioclase, clinopyroxene, and magnetite, formed by the cooling of magma in the shallow depths. Based on geochemical studies, the igneous rocks of the Watugajah area were formed by subduction zone tectonic setting with the tholeiitic affinity.

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