Komatiite

Komatiite

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  Komatiites Omer M. Ahmed Universityof Kerala, India 2015
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  o Named afterthe 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
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  o Mostly ofkomatiite 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
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  Spinifex texture-typical ofKomatiites  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.
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  (a) Spinifex grass (b)Weathered surface of Komatiite shows Spinifex texture. (Nicholas Arndt, 2008). (a)
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  Spinifex texture inPPL. shows partial Serpentinization and iron oxide. (Source-Robin Gill, 2010). In XPL
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  Spinifex texture withlarge zoned Pyroxene (Kambalda, Nicholas Arndt, 2008).
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   Abundance ofolivine, 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
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  Elements Percentage SiO2 46 TiO20.53 Al2O3 11.1 Cr2O3 0 Σ FeO 11.3 MnO 0 MgO 20.6 NiO 0 CaO 9.3 Na2O 0.84 K2O 0.03 Total 99.7 Robin Gill, 2010 Geochemistry  ultramafic mantle-derived volcanic rocks.  chemically distinctive in being highly magnesium (18% < MgO < 32% ).  low SiO2, low K2O, low Al2O3.
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  Association Komatiites occur withother mafic and ultramafic high- magnesium rocks such as: • Pillow basalts. • Peridotite. • Picrites • Calc–alkaline felsic volcanics rocks in subduction zone.
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  Komatiites have beenrecognized 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
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  Komatiite in India TheArchaean 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).
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  Mineral deposits associatedwith 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.
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  Conclusion • komatiite ismafic / 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
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  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.
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  Thanks So Much