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Easton Rivera
Easton Rivera


Amphibolite frequently forms by metamorphism of mafic igneous rocks, such as basalt. However, because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may also metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt (metabasalt) creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite.


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Although tremolite is a metamorphic amphibole, it is most commonly derived from highly metamorphosed ultramafic rocks, and thus tremolite-talc schist is not generally considered a variety of amphibolite. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole is called a hornblendite, which is usually a crystal cumulate rock. Igneous rocks with greater than 90% amphiboles, which have a feldspar groundmass, may be lamprophyres.

Metamorphic rocks composed primarily of amphibole, plagioclase, with subordinate epidote, zoisite, chlorite, quartz, titanite, and accessory leucoxene, ilmenite and magnetite which have a protolith of an igneous rock are known as ortho-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, 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 metasedimentary rocks, especially greywacke 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 a dyke. Picking a sill and thin metamorphosed lava flows may be more troublesome.

The word metabasalt was thus coined, largely to avoid the confusion between ortho-amphibolite and para-amphibolite. This term is recommended by the British 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]

Amphibolite as a rock defines 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 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 a prograde 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 replacing pyroxene is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schist is often the result of hydrothermal alteration or metasomatism, and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation.

The archaic term epidiorite is sometimes used, especially in Europe, 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.

Higher quality stone is quarried, cut, and polished for architectural use. It is used as facing stone on the exterior of buildings, and used as floor tile and panels indoors. Some of the most attractive pieces are cut for use as countertops. In these architectural uses, amphibolite is one of the many types of stone sold as "black granite."

Some amphibolite deposits, such as the one at Gore Mountain in the Adirondacks of New York, contain significant amounts of garnet. If enough garnet is present and of proper quality, the amphibolite can be mined and the garnet recovered for use as an abrasive.

Tucked away in the northwestern corner of the state near Boone and the Tennessee line is a mountain range with an unusual name and a stunning landscape that attracts birders, hikers, and mountain lovers. A mineral-rich rock type called amphibolite gneiss underpins the peaks in the Amphibolite Mountains, yielding a more neutral soil than typically found in the mountains and feeding a rich diversity of plant life. The Amphibolites, which include Three Top Mountain, Elk Knob, Mount Jefferson, and Bluff Mountain, average about 5,000 feet, with 5,566-foot Snake Mountain reaching the highest elevation. The mountain range is oriented in a generally northeast to southwest direction and separates the north and south forks of the New River (another IBA, and one of the oldest rivers in the world). This 90,557-acre IBA is located in Ashe and Watauga Counties.

Amphibolite is a coarse-grained metamorphic rock, predominantly composed of mineral amphibole and plagioclase feldspar. It can also contain minor amounts of other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite. Amphibolite is found around metamorphic and igneous rock intrusions that solidify between other rocks that are located within the Earth. Also, amphibolite has significant components found in both volcanic and plutonic rocks that range in composition from granitic to gabbroic. The formation of amphibolite took place millions of years ago and is found in various countries around the world today.

1) The modal compositions of amphibolites show that most ofthem contain more than 50% of amphibole, but those with 50 to 30% are notunusual. The content of amphibole and plagioclase together is mostly higherthan 90%, and may be as low as 75%.

5) The presence of other major mineral constituents (>5%)is expressed by the corresponding prefix according to general SCMR rules (e.g.garnet amphibolite, pyroxene amphibolite, quartz amphibolite, etc.).

6) The amphibolite is characterised by the presence ofhydroxyl-bearing minerals (amphibole, biotite), which prevail over thehydroxyl-free ones (garnet, diopside). The boundary with the higher grade,granulite-facies metamorphic rocks, is determined by the appearance oforthopyroxene.

Amphibolites define a particular set of temperature andpressure conditions known as the amphibolite facies, with temperature of 500 to750 C and pressures of 8-7 kbar. Changes in mineralogy depends very much onprotolith, however, production of abundant garnet and hornblende are mostcharacteristic. Sodic feldspars are oligoclase rather than the albite thatdominates at lower T. Biotite and muscovite are both abundant in pelitic rocksof amphibolite facies. Kyanite and sillimanite are often produced by reactionof muscovite and quartz.

Metamorphic rocks composed primarily of amphibole, albite,with subordinate epidote, zoisite, chlorite, quartz, sphene, and accessoryleucoxene, ilmenite and magnetite which have a protolith of an igneous rock areknown as Orthoamphibolites.

Para-amphiboliteswill generally have the same equilibrium mineral assemblage asorthoamphibolites, with more biotite, and may include more quartz, albite, anddepending on the protolith, more calcite/aragonite and wollastonite.

The archaic term epidiorite is sometimes used to refer to ametamorphosed ortho-amphibolite with a protolith of diorite, gabbro or othermafic intrusive rock. In epidiorite the original clinopyroxene (most oftenaugite) has been replaced by the fibrous amphibole uralite.

Gemologists andlapidary workers have discovered that some amphibolite rock produces a shimmereffect when it is polished. They use rounded and polished pieces of amphibolitefor various pieces of jewelry.

There are many optionsto amphibolite as dimension stone. Marble, granite, and quartzite, forinstance, can all be polished and used as facing on the interior and exteriorof buildings. In some environments even sandstone can be used for buildingconstruction. In the end, amphibolite is chosen for the particular color,texture and overall look it gives to a building. Substitutes that provide asimilar look include plastics and some varieties of other dark rock like darkgranite.

Figure 1. Simplified map of the Fennoscandian Shield showing the locations of the TTG-amphibolite terrains of Arctic Fennoscandia. (1) Lake Inari Complex of the Kola Province, (2) Rommaeno Complex of the Norrbotten Province, and (3) the West Troms Complex (West Troms Basement Complex).

Figure 6. Transition from amphibolite (right boudin) to nebulite (left boudin) in a coherent boudinage structure. Laassaniemi, NW Finland, Rommaeno Complex, Norrbotten Province. Length of scale 50 cm.

Figure 9. Raft diatexite, Lewisian Complex of the North Atlantic Craton, Outer Hebrides, NW Scotland. At the start of the metatexite-diatexite transition, leuco- or mesocratic flow bands of diatexite disaggregates the original rock and isolates meter-scale blocks of amphibolite that often show rounded ends. Smaller rafts (schollen) are visible on the left. Photo: Pekka Kivimäki.

Figure 10. Weathering enhances the appearance of a flow band wrapping around the amphibolite raft. Lake Inari, Kola Province, NE Finland. Length of scale 20 cm. Photo: Pekka Kivimäki.

Figure 11. Folded raft diatexite, Näätämö, Lake Inari Complex of the Kola Craton, NE Finland. Deformation and folding separates and stretches rafts of amphibolite. Photo: Pekka Kivimäki.

It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process. 041b061a72


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