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 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 chemistry.
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 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 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.
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. 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 gneissic texture somewhere nearby, if not mylonite zones, 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, plagioclase and even magmatic amphibole such as pargasite rhombohedra, pseudomorphed by hornblende amphibole. 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;
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.
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.
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.