| Metamorphic rock | |
 An amphibolite boulder in the Botanical Gardens of Charles University, Prague, Czech Republic. | |
| Composition | |
|---|---|
| Amphiboles, such as hornblende and actinolite, often with plagioclase. | |
| Physical Characteristics | |
| Fabric | Schistose |
| Relationships | |
| Protoliths | Mafics, such asbasalt |
Amphibolite (/æmˈfɪbəlaɪt/) is ametamorphic rock that containsamphibole, especiallyhornblende andactinolite, as well asplagioclasefeldspar, but with little or noquartz. It is typically dark-colored and dense, with a weakly foliated orschistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Amphibolite frequently forms bymetamorphism ofmafic igneous rocks, such asbasalt. However, because metamorphism creates minerals entirely based upon the chemistry of theprotolith, certain 'dirtymarls' and volcanic sediments may also metamorphose to an amphibolite assemblage. Deposits containingdolomite andsiderite also readily yield amphibolite (tremolite-schist,grunerite-schist, and others) especially where there has been a certain amount ofcontact metamorphism by adjacentgranitic masses. Metamorphosed basalt (metabasalt) createsortho-amphibolite and other chemically appropriate lithologies createpara-amphibolite.
Although tremolite is a metamorphic amphibole, it is most commonly derived from highly metamorphosedultramafic rocks, and thus tremolite-talc schist is not generally considered a variety of amphibolite. Aholocrystallineplutonicigneous rock composed primarily of hornblende amphibole is called ahornblendite, which is usually a crystalcumulate rock. Igneous rocks with greater than 90% amphiboles, which have afeldspargroundmass, may belamprophyres.
Metamorphic rocks composed primarily ofamphibole,plagioclase, with subordinateepidote,zoisite,chlorite,quartz,titanite, and accessoryleucoxene,ilmenite andmagnetite which have aprotolith of an igneous rock are known asortho-amphibolite.
Para-amphibolite will generally have the same equilibrium mineral assemblage as ortho-amphibolite, with more biotite, and may include more quartz, plagioclase, and depending on the protolith, morecalcite/aragonite andwollastonite.
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 metasedimentary rocks, especiallygreywacke and other poorly sorted sedimentary rocks. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was adyke. Picking asill and thin metamorphosedlava flows may be more troublesome.
Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolite.
The wordmetabasalt was thus coined, largely to avoid the confusion between ortho-amphibolite and para-amphibolite. This term is recommended by theBritish Geological Survey when it is possible to determine the origin of the rock from its characteristics alone (and not from field relationships), particularly when the degree of metamorphism is low.[1]
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Amphibolite as a rock defines a particular set of temperature and pressure conditions known as theamphibolite facies. However, caution must be applied here before embarking on metamorphic mapping based on amphibolite alone.
First, for an ortho-amphibolite or amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is aprograde metamorphic product, and not a retrograde metamorphic product. For instance,actinolite amphibole is a common product of retrograde metamorphism of metabasalt at (upper)greenschist facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacingpyroxene is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schist is often the result ofhydrothermal alteration ormetasomatism, and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation.
Second, 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 less than 1.2 GPa, well within the ductile deformation field.Gneissic texture may occur nearby, if not thenmylonite zones,foliations and ductile behaviour, including stretching lineations may occur.
While it is not impossible to have remnant protolith mineralogy, this is rare. More common is to findphenocrysts of pyroxene,olivine,plagioclase and even magmatic amphibole such aspargasite rhombohedra, pseudomorphed byhornblende amphibole. Original magmatic textures, especially crude magmatic layering inlayered intrusions, is often preserved.
Amphibolite facies equilibrium mineral assemblages of various protolith rock types consist of:
Amphibolite facies is usually a product ofBarrovian Facies Sequence or advancedAbukuma Facies Sequence metamorphic trajectories. Amphibolite facies is a result of continuing burial and thermal heating aftergreenschist facies is exceeded. Further burial and metamorphic compression (but little extra heat) will lead toeclogite facies metamorphism; with more advanced heating the majority of rocks begin melting in excess of 650 to 700 °C in the presence of water. In dry rocks, however, additional heat (and burial) may result ingranulite facies conditions.
Uralite is a particularhydrothermally alteredpyroxenite; during autogenic hydrothermal circulation the primary mineralogy ofpyroxene andplagioclase, etc. has altered toactinolite andsaussurite (albite +epidote). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolitepseudomorphically after pyroxene, and saussuritised plagioclase.
The archaic termepidiorite is sometimes used, especially in Europe, to refer to a metamorphosed ortho-amphibolite with aprotolith ofdiorite,gabbro or other mafic intrusive rock. In epidiorite the originalclinopyroxene (most oftenaugite) has been replaced by the fibrousamphiboleuralite.
Amphibolite was a favourite material for the production ofadzes (shoe-last-celts) in the central European earlyNeolithic (Linearbandkeramic andRössen cultures).[citation needed]
Amphibolite is a commondimension stone used in construction, paving, facing of buildings, especially because of its attractive textures, dark color, hardness and polishability and its ready availability.[citation needed]
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