- Breccia
Breccia (pronEng|ˈbrɛtʃiə, ˈbrɛʃiə, _it. breach) is a rock composed of angular fragments of several
mineral s or rocks in a matrix, that is a cementing material, that may be similar or different in composition to the fragments. A breccia may have a variety of different origins, as indicated by the named types includingsedimentary breccia, tectonic breccia,igneous breccia, impact breccia and hydrothermal breccia.Nomenclature
Breccias can be classified by their constituents, mode of occurrence, constituent fragment size, the types of
clast s and source of clasts. 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 asedimentary rock this may be called a conglomerate, except when the breccia is discordant with formerlithology (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 onlava (Volcanic breccias).Types
Sedimentary
Sedimentary breccias are a type of clastic
sedimentary rock which are composed of angular to subangular, randomly orientedclasts of other sedimentary rocks. They are formed by either submarinedebris flow s,avalanches ,mud flow or mass flow in an aqueous medium. Technically,turbidite s 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 ingraben s.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.Sedimentary breccias can be described as 'arenaceous', from the
Latin word "harena" meaning 'sand', which aresand y or pebbly in nature.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 millimeters 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 because of gravity
erosion . Talus slopes might become buried and the talus cemented in a similar manner.Collapse
Collapse breccias form where there has been a collapse of rock, typically in a karst landscape. Collapse breccias form blankets in highly weathered
regolith due to the removal of rock components by dissolution.Tectonic
Tectonic breccias form where two
tectonic plate s create a crumbling of the interface, by their relative movements.Fault
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 ofmineral matter introduced bygroundwater .Igneous
Igneous clastic rocks can be divided into two classes
* Broken, fragmental rocks associated with volcanic eruptions, both oflava andpyroclastic type
* Broken, fragmental rocks produced by intrusive processes, usually associated with plutons or porphyry stocksVolcanic
Volcanic
pyroclastic rocks are formed by explosive eruption oflava and any rocks which are entrained within the eruptive column. This may include rocks plucked off the wall of themagma conduit, or physically picked up by the ensuingpyroclastic surge .Lava s, especiallyrhyolite anddacite 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 rock fragments, especially if flowing over unconsolidated rubble on the flanks of a volcano, and these form volcanic breccias, also called pillow 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
Clastic rocks are also commonly found in shallow subvolcanic
intrusion s such as porphyry stocks,granite s andkimberlite pipes, where they are transitional with volcanic breccias. [ [http://econgeol.geoscienceworld.org/cgi/content/abstract/69/3/412 T.W.Mitcham, "Origin of breccia pipes", American Journal of Science, volume 69, pages 412-413 (1974)] ]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-stagestockwork 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 geologists)
* 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 (e.g. postulatedearthquake s)
* Accumulation ofxenolith s within a feeder conduit or vent conduitImpact
Impact breccias are thought to be diagnostic of an
impact event such as anasteroid orcomet striking the Earth, and are usually found atimpact crater s. Impact breccia, a type ofimpactite , forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in theejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around a known impact crater, and/or an association with other products of impact cratering such as shatter cones, impact glass, shocked minerals, andchemical and isotopic evidence of contamination with extraterrestrial material (e.g. iridium andosmium anomalies).Hydrothermal
Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 to 350oC, when seismic activity (an
earthquake ) causes avoid to open along afault deep underground. The void draws in hotwater and as pressure in the cavity drops, the water violently boils - akin to an undergroundgeyser . In addition, the sudden opening of a cavity causes rock at sides of thefault to destabilise and implode inwards, the broken rock gets caught up in a churning mixture of rock,steam andboiling water. Rock fragments hit each other and sides of the fault, and attrition quickly rounds angular breccia fragments. Volatile gases are lost to the steam phase as boiling continues, in particular CO2. As a result, the chemistry of thefluid s change andore minerals rapidly precipitate.Breccia-hosted
ore deposits are ubiquitous. [ [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V90-3SWR7G6-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=9ce05bf112bf56acfa7962a963fbf123 Michel Jébrak, "Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution", Ore Geology Reviews, volume 12, pages 111-134 (1997)] ] The morphology of breccias associated with ore deposits varies from tabular sheeted veins andclastic dikes associated with overpressured sedimentary strata, to large-scale intrusivediatreme breccias, or even some synsedimentary diatremes formed solely by the overpressure of pore fluid withinsedimentary basin s. Hydrothermal breccias are usually formed by hydrofracturing of rocks by highly pressuredhydrothermal fluids. They are typical of theepithermal ore environment and are intimately associated with intrusive-related ore deposits such asskarn s,greisen s and porphyry-related mineralisation. Epithermal deposits are mined forcopper ,silver andgold .In the mesothermal regime, at much greater depths, over-pressured fluids under
lithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lowerhydrostatic pressure. On their journey, high-pressure fluids crack rock by hydrofracturing, forming an angular jigsaw breccia. Rounding of rock fragments less common in the mesothermal regime, as the formational event is brief. If boiling occurs,methane andhydrogen sulfide may be lost to the steam phase and ore may precipitate. Mesothermal deposits are often mined forgold .Ornamental uses
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 ofpavonazzetto , a breccia coming fromPhrygia (in modernTurkey ). Pavonazzetto obtains its name from its extremely colourful appearance, which is reminiscent of apeacock 's feathers ("pavone" is "peacock" in Italian).See also
*
Fault breccia
*Impact crater
* Hydrothermal circulation
*Vein (geology)
*Diatreme
*Kimberlite Line note references
Bibliography
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