Amphibolite

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Amphibolite
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
FabricSchistose
Relationships
ProtolithsMafics, such as basalt
Amphibolite from Cape Cod, Massachusetts
Garnet bearing amphibolite from Val di Fleres, Italy

Amphibolite (

schistose
(flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

Amphibolite frequently forms by

contact metamorphism by adjacent granitic
masses. Metamorphosed basalt (metabasalt) creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite.

Although tremolite is a metamorphic amphibole, it is most commonly derived from highly metamorphosed

groundmass, may be lamprophyres
.

Ortho-amphibolite vs. para-amphibolite

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.

Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolite.

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 facies

Garnet amphibolite, sold as "Nordic Sunset Granite", reportedly from Murmansk area
Erratic boulder of dark-colored amphibolite mingled with light-colored granitic bands, near the Trift Glacier, Switzerland

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.

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 then mylonite 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 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.

Amphibolite facies equilibrium mineral assemblages of various protolith rock types consist of:

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

eclogite 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 in granulite facies
conditions.

Uralite

Uralite is a particular

hydrothermally altered pyroxenite; during autogenic hydrothermal circulation the 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, especially in Europe, to refer to a metamorphosed ortho-amphibolite with a

clinopyroxene (most often augite) has been replaced by the fibrous amphibole
uralite.

Uses

Amphibolite was a favourite material for the production of

Rössen cultures).[citation needed
]

Amphibolite is a common dimension 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]

References

  1. ^ Robertson, S. (1999). "BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks" (PDF). British Geological Survey Research Report. RR 99-02. Retrieved 27 February 2021.
  • Winter, John D., 2001. An introduction to Igneous and Metamorphic Petrology, 695 pages, Prentice Hall,
    ISBN 0-13-240342-0
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