Komatiites Omer M. Ahmed University of Kerala, India 2015

o Named after the Komati River in South Africa. o first described by Morris and Richard (twins) for ultramafic units in the Barberton Greenstone belt of South Africa. Komatiites

o Mostly of komatiite are Archean age o distributed in the Archaean shield areas. o Also a few are Proterozoic and Phanerozoic. o In all ages komatiites are highly magnesium. o Mostly a volcanic rock; occasionally intrusive. o Mafic rocks were identified as extrusive because of their volcanic textures and structures, and they seem to have been accepted as a normal component of Archean volcanic successions, Abitibi in Canada. o The ultramafic rocks were interpreted as intrusive which are founded as sills and dykes, Barberton in South Africa. Komatiites

S pinifex texture-typical of Komatiites The name Spinifex refer to a spiky grass in Australian. Restricted in komatiite and most distinctive feature in the field. Formed due to rapid cooling of rich magnesium lavas. The olivine grains are less closely packed, randomly distributed and poorly sorted. Pyroxene shows zoning. Recognized in well preserved Archaean greenstone belts.

(a)Spinifex grass (b)Weathered surface of Komatiite shows Spinifex texture. (Nicholas Arndt, 2008). (a)

Olivine pyroxene Spinifex texture in PPL. shows partial Serpentinization and iron oxide. (Source- Robin Gill, 2010 ). In XPL

Spinifex texture with large zoned Pyroxene ( Kambalda, Nicholas Arndt, 2008 ).

 Abundance of olivine, pyroxene rich in chromite and glassy material.  Some silicates confined in matrix between olivine grains like augite, plagioclase or Ni–Fe sulfides and in some case quartz.  Most komatiites are altered- olivine to chlorite or serpentine; Pyroxene is partially replaced by tremolite and chlorite; and glass to secondary hydrous minerals. Mineralogy

ElementsPercentage SiO 2 46 TiO Al 2 O Cr 2 O 3 0 Σ FeO11.3 MnO0 MgO20.6 NiO0 CaO9.3 Na 2 O0.84 K 2 O 0.03 Total99.7 Robin Gill, 2010 Geochemistry  ultramafic mantle-derived volcanic rocks.  chemically distinctive in being highly magnesium (18% < MgO < 32% ).  low SiO 2, low K 2 O, low Al 2 O 3.

Association Komatiites occur with other mafic and ultramafic high- magnesium rocks such as: Pillow basalts. Peridotite. Picrites Calc–alkaline felsic volcanics rocks in subduction zone.

Komatiites have been recognized in many areas around the world such as:  The Greenstone Belt of Barberton South Africa.  The Pilbara Block and the Yilgarn Craton, Western Australia.  Munro Township, Ontario.  Abitibi Greenstone Belt in Canada.  Lion Hills Vetreny belt, Russia.  The Belingwe belt in Zimbabwe. Occurrence of Komatiite in the world

Komatiite in India The Archaean Greenstone Belt of Kolar Schist, Karnataka.  Komatiite reported from Dharwar craton - shows spinifex texture.  Associated with metamorphosed Pyroxenite, Norite gabbro, anorthosite, altered dunite, harzburgite, Chromite and schist.  In east Dharwar craton it surrounded and intruded by granodioritic to quartzo monozonitic gneisses.  Dharwar Craton has metabasaltic rocks from both high and low grade schist belt (Drury,1984).

Mineral deposits associated with Komatiite Komatiites contain valuable primary and secondary ore deposits such as:  primary magmatic Sulfide deposits which mined for their nickel (about 18%of global sulfide nickel ) content with minor product of Cu and PGEs mineralization.  secondary laterite deposits enriched in nickel and cobalt, developed over thick olivine cumulate units.  At Dachine in French, komatiites (ultramafic) are the host rocks of a large deposit of micro diamonds.

Conclusion komatiite is mafic / ultramafic Shows that lavas of ultramafic composition have erupted on the Earth’s surface, at early times of history. Spinifex texture is a product of extremely rapid crystallization of high MgO minerals. Many komatiites are host for massive sulphide deposits. Komatiites associated with high magnesium rocks

References  Gill, R (2010), Igneous Rocks and Processes A Practical Guide, John Wiley & Sons Ltd, 428 P.  Gupta A K.Alok (1998),Igneous rocks, Allied Publishers Limited 690 P.  Srivastava R K (2011), Dyke Swarms: Keys for Geodynamic Interpretation, Springer-Verlag Berlin Heidelberg, 605 P.

Thanks So Much