Breccia

Breccia, derived from the Latin word for "broken," is typically a rock composed of angular fragments in a matrix that may be of a similar or a different material.

Sedimentary breccias

Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.

The other derivation of sedimentary breccia is as angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.

In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many SEDEX ore deposits.

A conglomerate by contrast is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccias and conglomerates are composed of fragments averaging greater than 2 mm in size. The angular shape of the fragments indicate that the material has not been transported far from its source. Breccias indicate accumulation in a juvenile stream channel or accumulations due to gravity erosion. Talus slopes may become buried and the talus cemented in a similar manner.

Breccia nomenclature

Breccias can be classified by their constituents, mode of occurrence and by their size, the types of clasts, source of clasts, and so forth. Several textural terms are used to describe the morphology and textural variations observed in breccias.

Milling
Breccias which are formed by injection of a slurry (be it as a hydrofracture breccia or, more usually, a volcanic or intrusive breccia) often show evidence of rounding of the clasts. With a sedimentary rock this may be called a conglomerate, except when the breccia is discordant with former lithology (see clastic dike). For an intrusive breccia, erosion and transport in a watercourse cannot be invoked to explain rounding. Breccias of this type which are rounded are said to be milled, a process by which the breccia matrix grinds the larger clasts and rounds them off. This has been observed to have occurred in some hydrothermal breccias.

Autobrecciation
Autobrecciation is the process by which a rock's mechanism of formation causes it to become broken and to include its broken fragments within itself. This is properly explained in the section on lava.

Fault breccias

Fault breccias result from the grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of mineral matter introduced by groundwater.

Igneous breccias

Igneous clastic rocks can be divided into two classes

Broken, fragmental rocks produced by intrusive processes, usually associated with plutons or porphyry stocks
Broken, fragmental rocks associated with volcanic eruptions, both of lava and pyroclastic type

Volcanic breccias

Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column. This may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge. Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia is uniform in rock type and chemical composition.

Lavas may also pick up foreign rock fragments, especially if flowing over unconsolidated rubble on the flanks of a volcano, and these form volcanic breccias.

The volcanic breccia environment is transitional into the plutonic breccia environment in the volcanic conduits of explosive volcanoes, where lava tends to solidify and may be repeatedly shattered by ensuing eruptions. This is typical of volcanic caldera settings.

Intrusive breccias

Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and sometimes kimberlite pipes, where they are transitional with volcanic breccias.

Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma is intruded into partly consolidated or solidified magma. This may be seen in many granite intrusions where later aplite veins form a late-stage stockwork through earlier phases of the granite mass. When particularly intense, the rock may appear as a chaotic breccia.

Clastic rocks in mafic and ultramafic intrusions are known and form via several processes;

consumption and melt-mingling with wall rocks, where the felsic wall rocks are softened and gradually invaded by the hotter ultramafic intrusion (termed taxitic texture by Russian authors
Accumulation of rocks which fall through the magma chamber from the roof, forming chaotic remnants
Autobrecciation of partly consolidated cumulate by fresh magma injections or by violent disturbances within the magma chamber (eg; postulated earthquakes)
Accumulation of xenoliths within a feeder conduit or vent conduit

Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking the earth, and are usually found at impact craters.

Impact breccias

A fairly rare form of breccia is formed during meteorite impact. This is composed primarily of ejecta; clasts of country rock, melted rock fragments, tektites (glass ejected from the impact crater) and exotic fragments, including fragments derived from the impactor itself.

Impact craters also form a basal breccia in the floor of the crater formed by a combination of shattering due to the force of the impact and the subsequent cooling and melting of the rocks, which may rise upwards to form a small mound in the centre of the crater.

Identifying a clastic rock as an impact breccia requires recognising shatter cones, tektites, spherulites, and the morphology of an impact crater, as well as potentially recognising particular chemical and trace element signatures, especially osmium and iridium.

Hydrothermal breccias

Hydrothermal breccias are usually formed by hydrofracture of rocks by highly pressured hydrothermal fluids. They are typical of the epithermal ore environment and are intimately associated with intrusive-related ore deposits such as skarns, greisens and porphyry-related mineralisation.

Breccia-hosted ore deposits are ubiquitous. The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias, or even some synsedimentary diatremes formed solely by the overpressure of pore fluid within sedimentary basins.

Ornamental uses of breccia

The striking visual appearance of breccias has for millennia made them a popular sculptural and architectural material. Breccia was used on a limited scale by the ancient Egyptians - one of the best-known examples is the statue of the goddess Tawaret in the British Museum). It was regarded by the Romans as an especially precious stone and was often used in high-profile public buildings. Many types of marble are brecciated, such as Breccia Oniciata or Breche Nouvelle.

It is most often used as an ornamental or facing material in walls and columns. A particularly striking example can be seen in the Pantheon in Rome, which features two gigantic columns of pavonazzetto, a breccia coming from Phrygia (in modern Turkey). Pavonazzetto obtains its name from its extremely colourful appearance, which is reminiscent of a peacock's feathers (pavone is "peacock" in Italian).

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