Amphibolite (pronEng|æmˈfɪbəlaɪt) is the name given to a rock consisting mainly of hornblende
amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are usually crystal cumulates. Rocks with >90% amphibole which have a feldspar groundmass may be a lamprophyre.
Amphibolite is a grouping of rocks composed mainly of
amphibole(as hornblende) and plagioclase feldspars, with little or no quartz. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, certain 'dirty
marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomiteand sideritealso readily yield amphibolites (tremolite-schists, grunerite-schists, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalts create "ortho-amphibolites" and other chemically appropriate lithologies create "para-amphibolites".
Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed
ultramaficrocks, and thus tremolite-talc schists are not generally considered as 'amphibolites', because it is abundantly clear that one could just as easily say 'ultramafic schist'.
hornblende, as a mineral, is essentially a mineralogical 'garbage bin' and is stable across a very wide range of compositions and chemistries, as well as temperature and pressure conditions, it is suggested that the reader make use of the entries on amphibole chemistry.
Ortho-amphibolites vs. para-amphibolites
Metamorphic rocks composed primarily of
amphibole, albite, with subordinate epidote, zoisite, chlorite, quartz, sphene, and accessory leucoxene, ilmeniteand magnetitewhich have a protolithof an igneous rock are known as "Orthoamphibolites".
"Para-amphibolites" will generally have the same equilibrium mineral assemblage as orthoamphibolites, with more biotite, and may include more quartz, albite, and depending on the protolith, more
calcite/ aragoniteand wollastonite.
Often the easiest way to determine the true nature of an amphibolite is to inspect its field relationships; especially whether it is interfingered with other sediments, especially
greywackes and other poorly sorted sediments. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a dyke. Picking a sill and thin metamorphosed lavaflows may be more troublesome.
Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolites.
The word "metabasalt" was thus coined, largely to avoid the confusion between ortho-amphibolites and para-amphibolites. While not a true metamorphic rock name, as it infers an origin, it is a useful term.
Amphibolites define a particular set of temperature and pressure conditions known as the "Amphibolite Facies". However, caution must be applied here before embarking on metamorphic mapping based on amphibolites alone.
Firstly, for an (ortho)amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a
progrademetamorphic product, and not a retrograde metamorphic product. For instance, actinoliteamphibole is a common product of retrograde metamorphism of basalts at (upper) greenschistfacies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxeneis an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schists are often the result of hydrothermalalteration or metasomatism, and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation.
Secondly, the microstructure and crystal size of the rock must be appropriate. Amphibolite Facies conditions are experienced at temperatures in excess of 500 °C and pressures in excess of 1.2 GPa, well within the ductile deformation field. You should expect to find a gneissic texture somewhere nearby, if not
mylonitezones, foliations and ductile behaviour, including stretching lineations.
While it is not impossible to find remnant protolith mineralogy, this is rare. More common is to find
phenocrysts of pyroxene, olivine, plagioclaseand even magmatic amphibole such as pargasiterhombohedra, pseudomorphed by hornblendeamphibole. Original magmatic textures, especially crude magmatic layering in layered intrusions, is often preserved, though this may require imaginative and persistent study.
Amphibolite facies equilibrium mineral assemblages of various protolith rock types are laid out below;
* Basalt Ortho-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- accessories; often remnant
greenschistfacies assemblages including, notably, chlorite
* Sedimentary para-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- garnet (calcite +/- wollastonite)
* High-magnesia basalts; as ortho-amphibolite, but may contain
anthophyllite, a Mg-rich amphibole
* Ultramafic rocks;
tremolite, asbestiform amphibole, talc, pyroxene, wollastonite, prograde metamorphic olivine(rarely)
* Pelites; quartz,
orthoclase+/- albite, +/- biotite +/- actinolite +/- garnet +/- staurolite+/- sillimanite
Amphibolite facies is usually a product of "Barrovian Facies Sequence" or advanced "Abukuma Facies Sequence" metamorphic trajectories. Amphibolite facies is a result of continuing burial and thermal heating after
Greenschist faciesis exceeded.
Further burial and metamorphic compression (but little extra heat) will lead to
Granulite Faciesmetamorphism; it is rare to see much more advanced heating because the majority of rocks begin melting in excess of 650 to 700 degrees celsius in the presence of water. In dry rocks, however, additional heat (and burial) may result in Eclogite Faciesconditions.
Uralites are particular hydrothermally altered pyroxenites; during autogenic hydrothermal circulation their primary mineralogy of
pyroxeneand plagioclase, etc. has altered to actinoliteand saussurite (albite + epidote). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorphically after pyroxene, and saussuritised plagioclase.
The archaic term "epidiorite" is sometimes used to refer to a metamorphosed ortho-amphibolite with a
protolithof diorite, gabbroor other mafic intrusive rock. In epidiorite the original clinopyroxene(most often augite) has been replaced by the fibrous amphibole"uralite".
Basaltand other igneous rocktypes
List of rock types
* Mineralogy of
Amphibolite was a favourite material for the production of
adzes ( shoe-last-celts) in the central European early Neolithic( Linearbandkeramicand Rössencultures). In the VSG, it was used to produce bracelets as well.
Amphibolite is a common dimension stone used in construction, paving, facing of buildings, etcetera especially because of its attractive textures, dark colour, hardness and polishability and its ready availability.
Winter, John D., 2001. "An introduction to Igneous and Metamorphic Petrology", 695 pages, Prentice Hall, ISBN 0-13-240342-0
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Look at other dictionaries:
amphibolite — ● amphibolite nom féminin Roche métamorphique composée essentiellement d amphibole. ● amphibolite (expressions) nom féminin Faciès amphibolite, degré de métamorphisme moyen correspondant au domaine de stabilité de la hornblende … Encyclopédie Universelle
amphibolite — n. 1. a metamorphic rock composed chiefly of amphibole and plagioclase. [WordNet 1.5] … The Collaborative International Dictionary of English
amphibolite — [am fib′ə līt΄] n. [ AMPHIBOL(E) + ITE1] a metamorphic rock consisting largely of amphibole and plagioclase … English World dictionary
Amphibolite — Une amphibolite est une roche métamorphique à amphiboles et plagioclases du métamorphisme général (mésozone à catazone), à clivages médiocres et texture assez massive, vert sombre. Elle est essentiellement constituée de cristaux d amphibole, plus … Wikipédia en Français
amphibolite — amphibolitic /am fib euh lit ik/, adj. /am fib euh luyt /, n. Petrog. a metamorphic rock composed mainly of amphibole and plagioclase. [1825 35; AMPHIBOLE + ITE1] * * * Igneous or metamorphic rock composed largely or dominantly of amphibole… … Universalium
amphibolite — amfibolitas statusas T sritis chemija apibrėžtis Metamorfinė uoliena, sudaryta daugiausia iš amfibolo ir plagioklazo. atitikmenys: angl. amphibolite rus. амфиболит … Chemijos terminų aiškinamasis žodynas
Amphibolite — L amphibolite est une roche métamorphique à amphiboles et plagioclases du métamorphisme général, elle est constituée de cristaux d amphibole … L'Abécédaire du Vin
amphibolite — noun Date: 1826 a usually metamorphic rock consisting essentially of amphibole … New Collegiate Dictionary
amphibolite — noun Any of a class of metamorphic rock composed mainly of amphibole with some quartz etc. See Also: amphibolitic … Wiktionary
amphibolite — [am fɪbəlʌɪt] noun Geology a granular metamorphic rock consisting mainly of hornblende and plagioclase … English new terms dictionary