- Ultramafic rock
Ultramafic (also referred to as ultrabasic) rocks are igneous and meta-igneous rocks with very low
silicacontent (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesiumand ironcontent). The Earth's mantleis 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,
Petrologyof layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave 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 the dunites, peridotites and pyroxenites. Other rare varieties include troctolitewhich has a greater percentage of calcic plagioclase. These grade into the anorthosites. Gabbroand noriteoften occur in the upper portions of the layered ultramafic sequences. Hornblenditeand, rarely phlogopititeare also found.
Volcanic ultramafic rocks
Volcanicultramafic rocks are rare outside of the Archaeanand are essentially restricted to the Neoproterozoicor earlier, although some boninite lavas currently erupted within back-arc basins ( 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 their temperatures which are higher than terrestrial maficeruptions.
komatiite[Hill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. Physical volcanologyof komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia., Geological Society of Australia. ISBN 0-909869-55-3] and picritic basalt. Komatiites can be host to oredeposits of nickel[Lesher, C.M., Arndt, N.T., and Groves, D.I., 1984, Genesis of komatiite-associated nickel sulfidedeposits at Kambalda, 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, lamproiteand kimberliteare 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,
diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lampproite occurrences occur as volcanicand subvolcanic diatremes and maars; lavas are virtually unknown.
Proterozoiclamproite ( Argyle diamond mine), and Cenozoiclamproite ( Gaussberg, Antarctica) are known, as are vents of Devonianlamprophyre ( Scotland). Kimberlite pipes in Canada, Russiaand South Africahave incompletely-preserved tephraand agglomerate facies.
These are generally
diatremeevents and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low- volumevolatile melts and attain their ultramafic chemistryvia a different process to typical ultramafic rocks. Carbonatites are rare high- carbonate, low-silica igneous rocks.
Metamorphic ultramafic rocks
Metamorphismof ultramafic rocks in the presence of waterand/or carbon dioxideresults in two main classes of metamorphic ultramafic rock; talc carbonateand serpentinite.
Talc carbonation reactions occur in ultramafic rocks at lower
greenschistthrough to granulitefacies metamorphism when the rock in question is subjected to metamorphism and the metamorphic fluid has more than 10% molar proportion of carbon 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- amphiboletype assemblages.
Distribution in space and time
The majority of ultramafic rocks are exposed in
orogenicbelts, and predominate in Archaeanand Proterozoicterranes. Ultramafic magmas in the Phanerozoicare rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.
Many surface exposures of ultramafic rocks occur in
ophiolitecomplexes where deep mantle-derived rocks have been obducted onto continental crustalong and above subductionzones.
Ultramafic rocks and the regolith
Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as
calcium, potassiumand phosphorus) are exposed on the surface, the high metalcontent of the rocks creates unique vegetation. Examples are the ultramafic woodlands and ultramafic barrens of the Appalachian mountains and piedmont, the "wet maquis" of the New Caledonia rain forests, and the ultramafic forests of Mount Kinabaluand other peaks in Sabah, Malaysia. Vegetation is typically stunted, and is sometimes home to endemic species adapted to the metallic soils.
magnesite- calcrete caprock, clayey lateriteand duricrustforms over ultramafic rocks in tropicaland subtropicalenvironments. Particular floralassemblages associated with highly nickeliferous ultramafic rocks are indicative tools for mineral exploration.
Weathered ultramafic rocks may form
lateritic nickel ore deposits[Golightly, J.P. (1981): Nickeliferous Laterite Deposits. Economic Geology75, 710-735] [Schellmann, W. (1983): Geochemicalprinciples of lateritic nickel ore formation. Proceedings of the 2. International Seminar on Lateritisation Processes, Sao Paulo, 119-135]
* Ultramafic rock types:
Peridotite, dunite, norite, essexite, komatiite.
Cumulate rocksand rock types: chromitite, magnetite, anorthosite
* Ultramafic-associated ore deposits:
Lateritic nickel ore deposits, kambalda type komatiitic nickel ore deposits, diamond
Kimberlite, lamproite, lamprophyre
Ultramafic to mafic layered intrusions
Igneous differentiation, fractional crystallisation
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