Chapter 19: Continental Chapter 19: Continental Alkaline MagmatismAlkaline Magmatism

Ol Doinyo Lengai volcano

Alkaline rocks generally have more alkalis than can be Alkaline rocks generally have more alkalis than can be accommodated by feldspars alone. The excess alkalis accommodated by feldspars alone. The excess alkalis appear in feldspathoids, sodic pyroxenes and appear in feldspathoids, sodic pyroxenes and amphiboles, or other alkali-rich phasesamphiboles, or other alkali-rich phases

In the most restricted sense, In the most restricted sense, alkalinealkaline rocks are deficient rocks are deficient in SiOin SiO22 with respect to Na with respect to Na22O, KO, K22O, and CaO to the extent O, and CaO to the extent

that they become “critically undersaturated” in SiOthat they become “critically undersaturated” in SiO22, ,

and Nepheline or Acmite appears in the normand Nepheline or Acmite appears in the norm

Alternatively, some rocks may be deficient in AlAlternatively, some rocks may be deficient in Al22OO33 (and not necessarily (and not necessarily

SiOSiO22) so that Al) so that Al22OO33 may not be able to accommodate the alkalis in may not be able to accommodate the alkalis in

normative feldspars. Such rocks are normative feldspars. Such rocks are peralkalineperalkaline (see Fig. 18-2) and may be (see Fig. 18-2) and may be either silica undersaturated or oversaturatedeither silica undersaturated or oversaturated

Table 19-1.Table 19-1. Nomenclature of some alkaline igneous rocksNomenclature of some alkaline igneous rocks (mostly volcanic/hypabyssal)(mostly volcanic/hypabyssal)

BasaniteBasanite feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine

TephriteTephrite olivine-free basanite olivine-free basanite

LeucititeLeucitite a volcanic rock that contains leucite + clinopyroxene a volcanic rock that contains leucite + clinopyroxene olivine. It typically lacks feldspar olivine. It typically lacks feldspar

NepheliniteNephelinite a volcanic rock that contains nepheline + clinopyroxene a volcanic rock that contains nepheline + clinopyroxene olivine. It typically lacks feldspar. Fig. 14-2 olivine. It typically lacks feldspar. Fig. 14-2

UrtiteUrtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar

IjoliteIjolite plutonic nepheline-pyroxene rock with 30-70% nepheline plutonic nepheline-pyroxene rock with 30-70% nepheline

MelilititeMelilitite a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites) a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites)

ShoshoniteShoshonite K-rich basalt with K-feldspar ± leucite K-rich basalt with K-feldspar ± leucite

PhonolitePhonolite felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite)nepheline syenite)

ComenditeComendite peralkaline rhyolite with molar (Naperalkaline rhyolite with molar (Na22O+KO+K

22O)/AlO)/Al22OO33 slightly > 1. May contain Na-pyroxene or amphibole slightly > 1. May contain Na-pyroxene or amphibole

PantelleritePantellerite peralkaline rhyolite with molar (Naperalkaline rhyolite with molar (Na22O+KO+K

22O)/AlO)/Al22OO33 = 1.6 - 1.8. Contains Na-pyroxene or amphibole = 1.6 - 1.8. Contains Na-pyroxene or amphibole

LamproiteLamproite a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is variable, but most contain phenocrysts of olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine. variable, but most contain phenocrysts of olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine. Table 19-6 Table 19-6

LamprophyreLamprophyre a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic), commonly a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic), commonly highly potassic (K>Al) rocks. They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotite-phlogopite and/or highly potassic (K>Al) rocks. They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotite-phlogopite and/or amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks. Table 19-7 amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks. Table 19-7

KimberliteKimberlite a complex group of hybrid volatile-rich (dominantly COa complex group of hybrid volatile-rich (dominantly CO22), potassic, ultramafic rocks with a fine-grained ), potassic, ultramafic rocks with a fine-grained

matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocrysts and xenoliths are also common chromite. Xenocrysts and xenoliths are also common

Group I kimberliteGroup I kimberlite is typically CO is typically CO22-rich and less potassic than Group 2 kimberlite -rich and less potassic than Group 2 kimberlite

Group II kimberlite (orangeite)Group II kimberlite (orangeite) is typically H is typically H22O-rich and has a mica-rich matrix (also with calcite, diopside, apatite) O-rich and has a mica-rich matrix (also with calcite, diopside, apatite)

CarbonatiteCarbonatite an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite), an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite), and often with any of clinopyroxene alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich carbonatites and often with any of clinopyroxene alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich carbonatites are technically not alkaline, but are commonly associated with, and thus included with, the alkaline rocks. Table 19-3are technically not alkaline, but are commonly associated with, and thus included with, the alkaline rocks. Table 19-3

For more details, see Sørensen (1974), Streckeisen (1978), and Woolley For more details, see Sørensen (1974), Streckeisen (1978), and Woolley et alet al. (1996) . (1996)

Mt Erebus a basanite in Antarctica

Basanite a feldspathoid-bearing basalt

Example of Alkali Magma: Nepheline SyeniteMostly Orthoclase, no quartz, excess alkali to Nepheline

Figure 19-1. Variations in alkali ratios (wt. %) for oceanic (a) and continental (b) alkaline series. The heavy dashed lines distinguish the alkaline magma subdivisions from Figure 8-14 and the shaded area represents the range for the more common oceanic intraplate series. After McBirney (1993). Igneous Petrology (2nd ed.), Jones and Bartlett. Boston. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Continental alkaline series are much more varied than OIAs

Alkaline Alkaline Magmatism 1 Magmatism 1 - East African - East African

Rift Rift Figure 19-2. Map of the East African Rift system (after Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Alkaline Magmatism.Alkaline Magmatism.The East African Rift REEs The East African Rift REEs

Figure 19-5. Chondrite-normalized REE variation diagram for examples of the four magmatic series of the East African Rift (after Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

These authors compared isotope ratios (not shown) and incompatible frequencies and, again, found no correlation. This was taken to mean that enrichment in incompatibles occurs just before magma generation.

High LREEs

Alkaline Magmatism:Alkaline Magmatism:The East African Rift The East African Rift

Figure 19-3. 143Nd/144Nd vs. 87Sr/86Sr for East African Rift lavas (solid outline) and xenoliths (dashed). The “cross-hair” intersects at Bulk Earth (after Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136.

East Africa Rift lavas are enriched in Rubidium and Nd incompatibles, as expected in alkaline rocks*. These, over time, should decay, resulting in high 87Sr/86Sr and 143Nd/144Nd ratios.

This is not true, suggesting that the incompatibles enrichment occurs just before magma generation, and the Rb in the magma just got there in these young lavas.Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.

•*For example, 87Rb is a LIL, so is expected in late fractionation/ lower temperature solids. 87Sr is its daughterhttp://en.wikipedia.org/wiki/Rubidium-strontium_dating

Alkaline Alkaline Magmatism in Magmatism in

The East African The East African Rift Rift

Figure 19-4. 208Pb/204Pb vs. 206Pb/204Pb (a) and 207Pb/204Pb vs. 206Pb/204Pb (b) diagrams for some lavas (solid outline) and mantle xenoliths (dashed) from the East African Rift . The two distinct Virunga trends in (a) reflect heterogeneity between two different samples. After Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Pb data are similar to OIBs

OIBs are thought to be plume generated

Figure 19-6a. Ta vs. Tb for rocks of the Red Sea, Afar, and the Ethiopian Plateau. (after Treuil and Varet, 1973; Ferrara and Treuil, 1974).

Rocks from a particular area show nearly constant ratios of the two excluded elements, consistent with fractional crystallization of magmas with distinct Ta/Tb ratios produced either by variable degrees of partial melting of a single source, or varied sources

Figure 19-7. Phase diagram for the system SiO2-NaAlSiO4-KAlSiO4-H2O at 1 atm. pressure After Schairer and Bowen (1935) Trans.

Amer. Geophys. Union, 16th Ann. Meeting, and Schairer (1950), J. Geol., 58, 512-517. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

. Insert shows a T-X section from the silica-undersaturated thermal minimum (Mu) to the

silica-oversaturated thermal minimum (Ms). that crosses the lowest point (M) on the binary

Ab-Or thermal barrier that separates the undersaturated and oversaturated zones.

You either get Tridymite or Nepheline, not both.

Figure 19-9.. a. Pre-rift stage asthenospheric mantle diapir rises (forcefully or passively) into the lithosphere. Decompression melting (cross-hatch-green indicate areas undergoing partial melting) produces variably alkaline melts. Some partial melting of the metasomatized sub-continental lithospheric mantle (SCLM) may also occur. Reversed decollements (D1) provide room for

the diapir.

b. Rift stage: development of continental rifting, eruption of alkaline magmas (red) mostly from a deep asthenospheric source. Rise of hot asthenosphere induces some crustal anatexis. Rift valleys accumulate volcanics and volcaniclastic material

. c. Afar stage, in which asthenospheric ascent reaches crustal levels. This is transitional to the development of oceanic crust.

2 - Carbonatites2 - Carbonatites

Rare, mantle-derived igneous rock dominated by Calcite and Dolomite with associated silicates

Ol Doinyo Lengai volcano

Continental Alkaline Continental Alkaline Magmatism:CarbonatitesMagmatism:Carbonatites

Coarse Med.-Fine Calcite-carbonatite sövite alvikite Dolomite-carbonatite rauhaugite* beforsite Ferrocarbonatite Natrocarbonatite* Rarely used, beforsite may be applied to any grain size.

Table 19-3. Carbonatite Nomenclature

AlternativeName

Carbonates SulfidesCalcite PyrrhotiteDolomite PyriteAnkerite GalenaSiderite SphaleriteStrontanite Oxides-Hydroxides

Bastnäsite (Ce,La)FCO3) Magnetite

* Nyerereite ((Na,K)2Ca(CO3)2) Pyrochlore

* Gregoryite ((Na,K)2CO3) Perovskite

Silicates HematitePyroxene Ilmenite Aegirine-augite Rutile Diopside Baddeleyite Augite PyrolusiteOlivine HalidesMonticellite FluoriteAlkali amphibole PhosphatesAllanite ApatiteAndradite MonazitePhlogopiteZircon

Source: Heinrich (1966), Hogarth (1989) * only in natrocarbonatite

Table 19-4. Some Minerals in Carbonatites.

Carbonatites Carbonatites Figure 19-10. African carbonatite occurrences and approximate ages in Ma. OL = Oldoinyo Lengai natrocarbonatite volcano. After Woolley (1989) The spatial and temporal distribution of carbonatites. In K. Bell (ed.), Carbonatites: Genesis and Evolution. Unwin Hyman, London, pp. 15-37. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

CarbonatitCarbonatiteses

Figure 19-11. Idealized cross section of a carbonatite-alkaline silicate complex with early ijolite cut by more evolved urtite. Carbonatite (most commonly calcitic Sovite) intrudes the silicate plutons, and is itself cut by later dikes or cone sheets of carbonatite and ferrocarbonatite. The last events in many complexes are late pods of Fe and REE-rich carbonatites. A fenite aureole surrounds the carbonatite phases and perhaps also the alkaline silicate magmas. After Le Bas (1987) Carbonatite magmas. Mineral. Mag., 44, 133-40. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Ijolite plutonic nepheline-pyroxene rock with 30-70% nephelineUrtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar

CarbonatiteCarbonatiteOrigins Origins

Figure 19-12. Initial 143Nd/144Nd vs. 87Sr/86Sr diagrams for young carbonatites (dark shaded), and the East African Carbonatite Line (EACL), plus the HIMU and EMI mantle reservoirs.

Ratios plot along lines from HIMU to EMI

Isotopic signatures of carbonatites and associated silicates indicates they are genetically related

Figure 19-13. Solidus curve (purple) for lherzolite-CO2-H2O with a defined ratio of CO2

: H2O = 0.8. Red curves = H2O-saturated and

volatile-free peridotite solidi. Approximate shield geotherm in dashed green.

CarbonatitesCarbonatites as primary magmasas primary magmas

At about 70 km depth, the presence of CO2 begins to convert silicates to carbonates:CaMgSi2O6 +2 Mg2SiO4 + 2 CO2

CPx Ol = CaMg(CO3)2 + 4 MgSiO3

dolomite + Opx

Making a Hbl + Dol region V= vapor M= meltAs much as 45 wt. % CO2 is dissolved in the melt

The presence of H2O brings the melting pt. of Calcite down to 600 C

If sufficient CO2 and H2O in rising aesthenosphere plume, melting will occur as rising Lherzolite passes 2. Rise to solidus at 3, then solidfy

3 - Lamproites3 - LamproitesUltramafic rock with uniquely high alkali (especially K) content that exceeds the alumina on a molar basis, so they are called peralkaline.

Figure 19-17. Chondrite-normalized rare earth element diagram showing the range of patterns for olivine-, phlogopite-, and madupitic-lamproites from Mitchell and Bergman (1991) Petrology of Lamproites. Plenum. New York. Typical MORB and OIB from Figure 10-13 for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3- Lamproites 3- Lamproites Peralkaline, volatile rich, ultra-potassic rocksTi and K-rich amphibole, OlivineDiopside, leucite and sanadine.

No plagioclase, nepheline, or Sodalite

Little differentiation, strictly in thick continental settings, on craton margins over extinct subduction zones.

Figure 19-18a. Initial 87Sr/86Sr vs. 143Nd/144Nd for lamproites (red-brown) and kimberlites (red). MORB and the Mantle Array are included for reference. After Mitchell and Bergman (1991) Petrology of Lamproites. Plenum. New York. Typical MORB and OIB from Figure 10-13 for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Lamproites Lamproites

Enriched wrt bulk earth low Nd/Nd and high Sr/Sr

Lamproites are thought nevertheless to be from a Mantle source, the Sub Continental Lithospheric Mantle SCLM, not crust contamination

LamprophyresLamprophyresPorphyritic dike rocks with large phenocrysts of mafic minerals

Many types with different origins

LamprophyresLamprophyres

biotite, hornblende, Na- Ti- amphib., melilite, biotite,feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite

(± olivine) (± olivine) olivine, biotite ± olivine ± calciteor > pl -- minette vogesitepl > or -- kersantite spessartiteor > pl feld > foid sannaitepl > or feld > foid camptonite

-- glass or foid monchiquite polzenite-- -- alnöite

Lamprophyre branch: Alkaline MeliliticAfter Le Maitre (1989), Table B.3, p. 11.

Calc-alkaline

constituents

Table 19-7. Lamprophyre Nomenclature

Light-colored Predominant mafic minerals

Only common feature is hydrated mineralogy amphiboles and micasPolygenetic

Figure 19-19. Model of an idealized kimberlite system, illustrating the hypabyssal dike-sill complex leading to a diatreme and tuff ring explosive crater. This model is not to scale, as the diatreme portion is expanded to illustrate it better. From Mitchell (1986) Kimberlites: Mineralogy, Geochemistry, and Petrology. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Kimberlites Kimberlites

Kimberlites

Kimberlite Pipe

Kimberlite Sample rich in Olivine

Figure 19-20a. Chondrite-normalized REE diagram for kimberlites, unevolved orangeites, and phlogopite lamproites (with typical OIB and MORB). After Mitchell (1995) Kimberlites, Orangeites, and Related Rocks. Plenum. New York. and Mitchell and Bergman (1991) Petrology of Lamproites. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Kimberlites Kimberlites

Differentiation of HREE suggests a deep Garnet Lherzolite, and the greatest known LREE enrichment suggest enrichment from a subduction zone during ascent

Figure 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 graphite-diamond transition to rise in the central portion. Lithospheric diamonds therefore occur only in the peridotites and eclogites of the deep cratonal root, where they are then incorporated by rising magmas (mostly kimberlitic- “K”). Lithospheric orangeites (“O”) and some lamproites (“L”) may also scavenge diamonds. Melilitites (“M”) are generated by more extensive partial melting of the asthenosphere. Depending on the depth of segregation they may contain diamonds. Nephelinites (“N”) and associated carbonatites develop from extensive partial melting at shallow depths in rift areas. After Mitchell (1995) Kimberlites, Orangeites, and Related Rocks. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Kimberlites Kimberlites