- Ultramafic rock
Ultramafic (also referred to as ultrabasic) rocks are igneous and meta-igneous rocks with very low
silica content (less than 45%), generally >18% MgO, high FeO, lowpotassium , and are composed of usually greater than 90%mafic mineral s (dark colored, highmagnesium andiron content). TheEarth's mantle is considered to be composed of ultramafic rocks.Intrusive ultramafic rocks
Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers [Ballhaus, C.G. & Glikson, A.Y., 1995,
Petrology of layered mafic-ultramafic intrusions of the Giles Complex, westernMusgrave Block ,central Australia . AGSO Journal, 16/1&2: 69-90.] . Such cumulate rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include thedunite s,peridotite s andpyroxenite s. Other rare varieties includetroctolite which has a greater percentage of calcic plagioclase. These grade into theanorthosite s.Gabbro andnorite often occur in the upper portions of the layered ultramafic sequences.Hornblendite and, rarelyphlogopitite are also found.Volcanic ultramafic rocks
Volcanic ultramafic rocks are rare outside of theArchaean and are essentially restricted to theNeoproterozoic or earlier, although someboninite lava s currently erupted withinback-arc basin s (Manus Trough ,Philippines ) verge on being ultramafic. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on Io may be ultramafic, as evidenced by theirtemperature s which are higher than terrestrialmafic eruptions.Examples include
komatiite [Hill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. Physicalvolcanology of komatiites; A field guide to the komatiites of the Norseman-WilunaGreenstone Belt , Eastern Goldfields Province,Yilgarn Block ,Western Australia .,Geological Society of Australia . ISBN 0-909869-55-3] and picritic basalt. Komatiites can be host toore deposits ofnickel [Lesher, C.M., Arndt, N.T., and Groves, D.I., 1984, Genesis of komatiite-associated nickelsulfide deposits atKambalda , Western Australia: A distal volcanic model, in Buchanan, D.L., and Jones, M.J. (Editors), Sulphide Deposits in Mafic and Ultramafic Rocks,Institution of Mining and Metallurgy ,London , p. 70-80.] .Ultrapotassic ultramafic rocks
Technically ultrapotassic rocks and melilitic rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO. which can be considered "ultramafic".
Ultrapotassic, ultramafic igneous rocks such as
lamprophyre ,lamproite andkimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved.Most of these rocks occur as dykes,
diatreme s,lopolith s orlaccolith s, and very rarely, intrusions. Most kimberlite and lampproite occurrences occur asvolcanic and subvolcanic diatremes andmaar s; lavas are virtually unknown.Vents of
Proterozoic lamproite (Argyle diamond mine ), andCenozoic lamproite (Gaussberg ,Antarctica ) are known, as are vents ofDevonian lamprophyre (Scotland ). Kimberlite pipes inCanada ,Russia andSouth Africa have incompletely-preservedtephra andagglomerate facies .These are generally
diatreme events and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramaficchemistry via a different process to typical ultramafic rocks.Carbonatite s are rare high-carbonate , low-silica igneous rocks.Metamorphic ultramafic rocks
Metamorphism of ultramafic rocks in the presence ofwater and/orcarbon dioxide results in two main classes of metamorphic ultramafic rock;talc carbonate andserpentinite .Talc carbonation reactions occur in ultramafic rocks at lower
greenschist through togranulite facies metamorphism when the rock in question is subjected to metamorphism and the metamorphic fluid has more than 10% molar proportion ofcarbon dioxide .When the metamorphic fluids in contact with the ultramafic rock have less than 10% CO2 the metamorphic reactions favor serpentinisation reactions, resulting in
chlorite -serpentine -amphibole type assemblages.Distribution in space and time
The majority of ultramafic rocks are exposed in
orogenic belts, and predominate inArchaean andProterozoic terranes. Ultramafic magmas in thePhanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.Many surface exposures of ultramafic rocks occur in
ophiolite complexes where deep mantle-derived rocks have been obducted ontocontinental crust along and abovesubduction zones.Ultramafic rocks and the regolith
Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as
calcium ,potassium andphosphorus ) are exposed on the surface, the highmetal content of the rocks creates uniquevegetation . Examples are the ultramaficwoodland s and ultramafic barrens of theAppalachian mountain s and piedmont, the "wetmaquis " of theNew Caledonia rain forests , and the ultramaficforest s ofMount Kinabalu and other peaks inSabah ,Malaysia . Vegetation is typically stunted, and is sometimes home to endemic species adapted to the metallicsoil s.Often thick,
magnesite -calcrete caprock ,clay eylaterite andduricrust forms over ultramafic rocks intropical andsubtropical environments. Particularfloral assemblages associated with highly nickeliferous ultramafic rocks are indicative tools formineral exploration .Weathered ultramafic rocks may form
lateritic nickel ore deposits [Golightly, J.P. (1981): Nickeliferous Laterite Deposits.Economic Geology 75, 710-735] [Schellmann, W. (1983):Geochemical principles of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes,Sao Paulo , 119-135]See also
* Ultramafic rock types:
Peridotite ,dunite ,norite ,essexite ,komatiite .
*Cumulate rocks and rock types:chromitite ,magnetite ,anorthosite
* Ultramafic-associated ore deposits:Lateritic nickel ore deposits ,kambalda type komatiitic nickel ore deposits ,diamond
*Kimberlite ,lamproite ,lamprophyre
*Ophiolite
*Ultramafic to mafic layered intrusions
*Igneous differentiation , fractional crystallisationReferences
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