Amphibolite
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[[Image:Amphibolite-micro.jpg|thumb|Amphibolite]]
'''Amphibolite''' ({{pronEng|æmˈfɪbəlaɪt}}) is the name given to a [[Rock (geology)|rock]] consisting mainly of hornblende [[amphibole]], the use of the term being restricted, however, to [[metamorphic rock]]s. The modern terminology for a holocrystalline plutonic [[igneous rock]]s 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]] [[feldspar]]s, 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 [[marl]]s' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing [[dolomite]] and [[siderite]] also 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 [[ultramafic]] rocks, 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'.
Because [[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|amphibole chemistry]].
==Ortho-amphibolites vs. para-amphibolites==
Metamorphic rocks composed primarily of [[amphibole]], [[albite]], with subordinate [[epidote]], [[zoisite]], [[chlorite]], [[quartz]], [[sphene]], and accessory [[leucoxene]], [[ilmenite]] and [[magnetite]] which have a [[protolith]] of 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]]/[[aragonite]] and [[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 [[greywacke]]s 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 [[Dike (geology)|dyke]]. Picking a [[Sill (geology)|sill]] and thin metamorphosed [[lava]] flows 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.
==Amphibolite Facies==
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 [[prograde]] metamorphic product, and not a retrograde metamorphic product. For instance, [[actinolite]] amphibole is a common product of retrograde metamorphism of basalts at (upper) [[greenschist]] facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing [[pyroxene]] is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies.
<!-- Unsourced image removed: [[Image:Metamorphic Facies.JPG||right|thumb|300px|Amphibolite facies form at the high temperature and moderate to high pressure]] -->
Actinolite schists are often the result of [[hydrothermal]] alteration 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 [[gneiss|gneissic]] texture somewhere nearby, if not [[mylonite]] zones, [[foliation (geology)|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 [[phenocryst]]s of pyroxene, [[olivine]], [[plagioclase]] and even magmatic amphibole such as [[pargasite]] rhombohedra, pseudomorphed by [[hornblende]] amphibole. Original magmatic textures, especially crude magmatic layering in [[ultramafic to mafic layered intrusions|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 [[greenschist]] facies 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]], [[asbestos|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 facies]] is exceeded. <br>
Further burial and metamorphic compression (but little extra heat) will lead to [[Granulite Facies]] metamorphism; 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 Facies]] conditions.
===Uralite===
Uralites are particular hydrothermally altered pyroxenites; during autogenic hydrothermal circulation their primary mineralogy of [[pyroxene]] and [[plagioclase]], etc. has altered to [[actinolite]] and saussurite (albite + epidote). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorphically after pyroxene, and saussuritised plagioclase.
===Epidiorite===
The archaic term ''epidiorite'' is sometimes used to refer to a metamorphosed ortho-amphibolite with a [[protolith]] of [[diorite]], [[gabbro]] or other mafic intrusive rock. In epidiorite the original [[clinopyroxene]] (most often [[augite]]) has been replaced by the fibrous [[amphibole]] ''uralite''.
==See also==
* [[Basalt]] and other [[igneous rock]] types
* [[List of rock types]]
* [[Metamorphism]]
* Mineralogy of [[amphibole]]
* [[Pyroxene]]
==Uses==
Amphibolite was a favourite material for the production of [[adze]]s ([[shoe-last-celt]]s) in the central European early [[Neolithic]] ([[Linearbandkeramic]] and [[Rössen]] cultures). 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.
==References==
Winter, John D., 2001. ''An introduction to Igneous and Metamorphic Petrology'', 695 pages, Prentice Hall, ISBN 0-13-240342-0
{{Commonscat|Amphibolite}}
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[[Category:Petrology]]
[[Category:Igneous rocks]]
[[Category:Metamorphic rocks]]
[[Category:Structural geology]]
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