Chapter 19: Continental Alkaline Magmatism - Faculty web pages

Chapter 19: Continental Alkaline Magmatism Alkaline rocks have more alkalis than can be takenup by feldspars alone. Excess alkalis appear in feldspathoids, and Na-richpyroxenes-amphiboles Alkaline rocks are depleted in SiO 2with respect toNa 2O, K 2O, and CaO

Table 19-1.1. Nomenclature of some alkaline igneousrocks (mostly volcanic/hypabyssalhypabyssal)*Basanite- feldspathoid-bearing basalt. Usually contains nepheline, butmay have leucite + olivine*Tephrite- olivine-free basanite*Leucitite- a volcanic rock that contains leucite + clinopyroxene ±olivine. It typically lacks feldspar*Nephelinite- a volcanic rock that contains nepheline + clinopyroxene± olivine. It typically lacks feldspar.Urtite - plutonic nepheline-pyroxene pyroxene (aegirine-augite) augite) rock with over70% nepheline and no feldsparIjolite - plutonic nepheline-pyroxene pyroxene rock with 30-70% nephelineMelilitite - a predominantly melilite - clinopyroxene volcanicShoshonite - K-rich basalt with K-feldspar K± leucite*Phonolite- felsic alkaline volcanic with alkali feldspar + nepheline.See Fig. 14-2. (plutonic = nepheline syenite)Comendite - peralkaline rhyolite with molar (Na O+K 2 2 O)/Al 2 O 3slightly > 1. May contain Na-pyroxene or amphibole

Chapter 19:Continental AlkalineMagmatism.The East African RiftFigure 19-2. Map of the EastAfrican Rift system (afterKampunzu and Mohr, 1991),Magmatic evolution andpetrogenesis in the East AfricanRift system. In A. B. Kampunzu and R. T. Lubala(eds.), Magmatism in Extensional Settings, thePhanerozoic African Plate. Springer-Verlag, Berlin, pp.85-136. Winter (2001) An Introduction to Igneous andMetamorphic Petrology. Prentice Hall.

Chapter 19: Continental Alkaline Magmatism.The East African RiftFigure 19-5. ChondritenormalizedREE variationdiagram for examples ofthe four magmatic seriesof the East African Rift(after Kampunzu and Mohr, 1991),Magmatic evolution and petrogenesis in theEast African Rift system. In A. B.Kampunzu and R. T. Lubala (eds.),Magmatism in Extensional Settings, thePhanerozoic African Plate. Springer-Verlag,Berlin, pp. 85-136. Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

Figure 19-7. Phase diagram for the system SiO 2-NaAlSiO 4-KAlSiO 4-H 2O at 1 atm.pressure. Insert shows a T-X section from the silica-undersaturated thermal minimum(M u) to the silica-oversaturated thermal minimum (M s). that crosses the lowest point (M)on the binary Ab-Or thermal barrier that separates the undersaturated and oversaturatedzones. After Schairer and Bowen (1935) Trans. Amer. Geophys. Union, 16 th Ann. Meeting, and Schairer (1950), J. Geol., 58, 512-517.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 19-9. Hypotheticalcross sections (same verticaland horizontal scales)showing a proposed modelfor the progressivedevelopment of the EastAfrican Rift System. a. Preriftstage, an asthenosphericmantle diapir rises into thelithosphere. Decompressionmelting (cross-hatch-greenindicate areas undergoingpartial melting) producesvariably alkaline melts. Somepartial melting of themetasomatized SCLM mayalso occur. Reverseddecollements (D 1) provideroom for the diapir.. In A. B.Kampunzu and R. T. Lubala (eds.),Magmatism in Extensional Settings, thePhanerozoic African Plate. Springer-Verlag,Berlin, pp. 85-136 and P. Mohr (personalcommunication). Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

Figure 19-9. b. Rift stage:development ofcontinental rifting,eruption of alkalinemagmas (red) mostly froma deep asthenosphericsource. Rise of hotasthenosphere inducessome crustal anatexis. Riftvalleys accumulatevolcanics andvolcaniclastic material..

Figure 19-9. c. Afar stage,in which asthenosphericascent reaches crustallevels. This is transitionalto the development ofoceanic crust. Successivelyhigher reverseddecollements (D 2and D 3)accommodate space forthe rising diapir.After Kampunzu and Mohr (1991),Magmatic evolution and petrogenesis in theEast African Rift system. In A. B.Kampunzu and R. T. Lubala (eds.),Magmatism in Extensional Settings, thePhanerozoic African Plate. Springer-Verlag,Berlin, pp. 85-136 and P. Mohr (personalcommunication). Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

Chapter 19: Continental Alkaline Magmatism.CarbonatitesTable 19-4. Some Minerals in Carbonatites.Table 19-3. Carbonatite NomenclatureNameAlternativeCoarse Med.-FineCalcite-carbonatite sövite alvikiteDolomite-carbonatite rauhaugite* beforsiteFerrocarbonatiteNatrocarbonatite* Rarely used, beforsite may be applied to any grain size.CarbonatesSulfidesCalcitePyrrhotiteDolomitePyriteAnkeriteGalenaSideriteSphaleriteStrontaniteOxides-HydroxidesBastnäsite (Ce,La)FCO 3 ) Magnetite* Nyerereite ((Na,K) 2 Ca(CO 3 ) 2 ) Pyrochlore* Gregoryite ((Na,K) 2 CO 3 ) PerovskiteSilicatesHematitePyroxeneIlmeniteAegirine-augiteRutileDiopsideBaddeleyiteAugitePyrolusiteOlivineHalidesMonticelliteFluoriteAlkali amphibolePhosphatesAllaniteApatiteAndraditeMonazitePhlogopiteZirconSource: Heinrich (1966), Hogarth (1989) * only in natrocarbonatite

CarbonatitesFigure 19-10. Africancarbonatite occurrencesand approximate ages inMa. OL = OldoinyoLengai natrocarbonatitevolcano. After Woolley (1989) Thespatial and temporal distribution ofcarbonatites. In K. Bell (ed.), Carbonatites:Genesis and Evolution. Unwin Hyman,London, pp. 15-37. Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

CarbonatitesOldoinyo Lengai -natrocarbonatitevolcano

Oldoinyo Lengai - natrocarbonatite volcanoFire in hornito

Oldoinyo Lengai - natrocarbonatite volcanoNaCO 3 frothNaCO 3 Lava

Figure 19-11. Idealizedcross section of acarbonatite-alkalinesilicate complex withearly ijolite cut by moreevolved urtite.Carbonatite intrudes thesilicate plutons, and isitself cut by later dikes orcone sheets of carbonatiteand ferrocarbonatite. Thelast events in manycomplexes are late podsof Fe and REE-richcarbonatites. A feniteaureole surrounds thecarbonatite phases andperhaps also the alkalinesilicate magmas. After Le Bas(1987) Carbonatite magmas. Mineral.Mag., 44, 133-40. Winter (2001) AnIntroduction to Igneous andMetamorphic Petrology. Prentice Hall.Carbonatites

Figure 19-15. Silicate-carbonateliquid immiscibility in thesystem Na 2O-CaO-SiO 2-Al 2O 3-CO 2(modified by Freestone andHamilton, 1980, to incorporateK 2O, MgO, FeO, and TiO 2). Thesystem is projected from CO 2forCO 2-saturated conditions. Thedark shaded liquids enclose themiscibility gap of Kjarsgaardand Hamilton (1988, 1989) at 0.5GPa, that extends to the alkalifreeside (A-A). The lightershaded liquids enclose thesmaller gap (B) of Lee andWyllie (1994) at 2.5 GPa. C-C isthe revised gap of Kjarsgaardand Hamilton. Dashed tie-linesconnect some of the conjugatesilicate-carbonate liquid pairsfound to coexist in the system.After Lee and Wyllie (1996)International Geology Review, 36, 797-819. Winter (2001) An Introduction toIgneous and Metamorphic Petrology.Prentice Hall.Carbonatites

Chapter 19:Continental AlkalineMagmatism.KimberlitesFigure 19-19. Model of an idealizedkimberlite system, illustrating thehypabyssal dike-sill complex leading toa diatreme and tuff ring explosivecrater. This model is not to scale, as thediatreme portion is expanded toillustrate it better. From Mitchell (1986) Kimberlites:Mineralogy, Geochemistry, and Petrology. Plenum. New York.Winter (2001) An Introduction to Igneous and MetamorphicPetrology. Prentice Hall.

Chapter 19:Continental AlkalineMagmatism.KimberlitesFigure 19-20a. Chondrite-normalized REE diagramfor kimberlites, unevolved orangeites, andphlogopite lamproites (with typical OIB andMORB). After Mitchell (1995) Kimberlites,Orangeites, and Related Rocks. Plenum. New York.and Mitchell and Bergman (1991) Petrology ofLamproites. Plenum. New York. Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

KimberlitesFigure 19-20b. Chondrite-normalized spiderdiagram for kimberlites, unevolved orangeites, andphlogopite lamproites (with typical OIB andMORB). After Mitchell (1995) Kimberlites,Orangeites, and Related Rocks. Plenum. New York.and Mitchell and Bergman (1991) Petrology ofLamproites. Plenum. New York. Winter (2001) AnIntroduction to Igneous and MetamorphicPetrology. Prentice Hall.

KimberlitesFigure 19-20b. Hypothetical cross section of an Archean craton with an extinct ancient mobile belt(once associated with subduction) and a young rift. The low cratonal geotherm causes the graphitediamondtransition to rise in the central portion. Lithospheric diamonds therefore occur only in theperidotites and eclogites of the deep cratonal root, where they are then incorporated by rising magmas(mostly kimberlitic- “K”). After Mitchell (1995) Kimberlites, Orangeites, and Related Rocks. Plenum. New York. Winter (2001)An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.