Chapter 19: Continental Alkaline Magmatism Ol Doinyo Lengai volcano

Alkaline rocks generally have more alkalis than can be accommodated by feldspars alone. The excess alkalis appear in feldspathoids, sodic pyroxenes and amphiboles, or other alkali-rich phases In the most restricted sense, alkaline rocks are deficient in SiO2 with respect to Na2O, K2O, and CaO to the extent that they become “critically undersaturated” in SiO2, and Nepheline or Acmite appears in the norm Alternatively, some rocks may be deficient in Al2O3 (and not necessarily SiO2) so that Al2O3 may not be able to accommodate the alkalis in normative feldspars. Such rocks are peralkaline (see Fig. 18-2) and may be either silica undersaturated or oversaturated

Tephrite olivine-free basanite Table 19-1. Nomenclature of some alkaline igneous rocks (mostly volcanic/hypabyssal) Basanite feldspathoid-bearing basalt. Usually contains nepheline, but may 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. Fig. 14-2 Urtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar Ijolite plutonic nepheline-pyroxene rock with 30-70% nepheline Melilitite a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites) Shoshonite K-rich basalt with K-feldspar ± leucite Phonolite felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite) Comendite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 slightly > 1. May contain Na-pyroxene or amphibole Pantellerite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 = 1.6 - 1.8. Contains Na-pyroxene or amphibole Lamproite 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. Table 19-6 Lamprophyre 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 amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks. Table 19-7 Kimberlite a complex group of hybrid volatile-rich (dominantly CO2), potassic, ultramafic rocks with a fine-grained matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocrysts and xenoliths are also common Group I kimberlite is typically CO2-rich and less potassic than Group 2 kimberlite Group II kimberlite (orangeite) is typically H2O-rich and has a mica-rich matrix (also with calcite, diopside, apatite) Carbonatite 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 are 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 et al. (1996) The mildly alkaline series (e.g. Hawaii): Ankaramite (alkali picrite), Alkali Basalt, Hawaiite, Mugearite, Benmoreite, Trachyte is discussed in Section 14.3 (see Fig. 14-2).

Mt Erebus a basanite in Antarctica Basanite a feldspathoid-bearing basalt

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

Continental alkaline series are much more varied than OIAs 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.

Alkaline Magmatism 1 - East African 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. The East African Rift REEs 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 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.

Alkaline Magmatism: The East African Rift *For example, 87Rb is a LIL, so is expected in late fractionation/ lower temperature solids. 87Sr is its daughter http://en.wikipedia.org/wiki/Rubidium-strontium_dating 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. 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.

Alkaline Magmatism in The East African Rift Pb data are similar to OIBs OIBs are thought to be plume generated 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.

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

You either get Tridymite or Nepheline, not both. . 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. 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.

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 - Carbonatites Rare, mantle-derived igneous rock dominated by Calcite and Dolomite with associated silicates Ol Doinyo Lengai volcano

Continental Alkaline Magmatism: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.

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

Carbonatite Origins Ratios plot along lines from HIMU to EMI 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

Carbonatites as 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= melt As 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 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.

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

3- Lamproites Peralkaline, volatile rich, ultra-potassic rocks Ti and K-rich amphibole, Olivine Diopside, leucite and sanadine. No plagioclase, nepheline, or Sodalite Little differentiation, strictly in thick continental settings, on craton margins over extinct subduction zones. 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.

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 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.

Lamprophyres Porphyritic dike rocks with large phenocrysts of mafic minerals Many types with different origins

Lamprophyres Only common feature is hydrated mineralogy amphiboles and micas Polygenetic

Kimberlites 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 Kimberlite Pipe Kimberlite Sample rich in Olivine

Kimberlites Differentiation of HREE suggests a deep Garnet Lherzolite, and the greatest known LREE enrichment suggest enrichment from a subduction zone during ascent Figure 19-20a. 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 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.