Chapter 19: Continental Alkaline Magmatism

1. Chapter 19: Continental Alkaline Magmatism Alkaline rocks occur in all tectonic environments, including the ocean basins Conversely, Chapters 12, 15, 17, and 18 have shown us that magmatism on the continents can be highly varied, including tholeiitic and calc-alkaline varieties Now focus on the alkaline rocks that compose an extremely diverse spectrum of magmas occurring predominantly in the anorogenic portions of continental terranes

2. Chapter 19: Continental Alkaline Magmatism Alkaline rocks generally have more alkalis than can be accommodated by feldspars alone. The excess alkalis appear in feldspathoids, sodic pyroxenes-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

3. 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 Urtiteplutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar Ijoliteplutonic 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)

4. Chapter 19: Continental Alkaline Magmatism 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.

5. Chapter 19: Continental Alkaline Magmatism.The 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.

7. Chapter 19: Continental Alkaline Magmatism.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. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

8. Chapter 19: Continental Alkaline Magmatism.The East African 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.

9. Chapter 19: Continental Alkaline Magmatism.The East African Rift 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.

10. Chapter 19: Continental Alkaline Magmatism Figure 19.6a. Ta vs. Tb for rocks of the Red Sea, Afar, and the Ethiopian Plateau. 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 (after Treuil and Varet, 1973; Ferrara and Treuil, 1974).

11. Chapter 19: Continental Alkaline Magmatism Figure 19.7. Phase diagram for the system SiO2-NaAlSiO4-KAlSiO4-H2O at 1 atm. pressure. 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. 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.

12. Chapter 19: Continental Alkaline Magmatism Figure 19.8. Part of the Ne-Ks-SiO2-H2O system at 1 atm, 0.1 GPa, and 0.2 GPa, illustrating the reduction in the leucite field with increasing PH2O. At 0.2 GPa the Lc-liquid field crosses the Ab-Or join, and the system goes from peritectic to eutectic behavior. Also shown are contours for analyses of 122 undersaturated volcanics. After Gittins, (1979), The feldspathoidal alkaline rocks. In H. S. Yoder Jr. (ed.), The Evolution of Igneous Rocks Fiftieth Anniversary Perspectives. Princeton University Press. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

13. Figure 19.9. Hypothetical cross sections (same vertical and horizontal scales) showing a proposed model for the progressive development of the East African Rift System. a. Pre-rift stage, in which an 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. Successively higher reversed decollements (D2 and D3) accommodate space for the rising diapir. 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 and P. Mohr (personal communication). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

14. Chapter 19: Continental Alkaline Magmatism.Carbonatites

15. Chapter 19: Continental Alkaline Magmatism.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.

16. 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) 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.

17. Chapter 19: Continental Alkaline Magmatism.Carbonatites

18. Chapter 19: Continental Alkaline Magmatism.Carbonatites 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. From Bell and Blenkinsop (1987, Geology, 15, 99-102), (1989, in K. Bell (ed.), Carbonatites: Genesis and Evolution. Unwin Hyman, London, pp. 278-300 ). Also included are the data for Oldoinyo Lengai natrocarbonatites and alkali silicate rocks (from Bell and Dawson, 1995, in Bell, K. and J. Keller (eds.), (1995). Carbonatite Volcanism: Oldoinyo Lengai and the Petrogenesis of Natrocarbonatites. Springer-Verlag. Berlin, pp. 100-112 ). MORB values and the Mantle Array are from Figure 10-15. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

19. Chapter 19: Continental Alkaline Magmatism.Carbonatites 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. After Wyllie (1989) Origin of carbonatites: Evidence from phase equilibrium studies. In K. Bell (ed.), Carbonatites: Genesis and Evolution. Unwin Hyman, London. pp. 500-545. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

20. Chapter 19: Continental Alkaline Magmatism Figure 19.14. Grid showing the melting products as a function of pressure and % partial melting of model pyrolite mantle with 0.1% H2O. Dashed curves are the stability limits of the minerals indicated. After Green (1970), Phys. Earth Planet. Inter., 3, 221-235. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

21. Chapter 19: Continental Alkaline Magmatism.Carbonatites Figure 19.15. Silicate-carbonate liquid immiscibility in the system Na2O-CaO-SiO2-Al2O3-CO2 (modified by Freestone and Hamilton, 1980, to incorporate K2O, MgO, FeO, and TiO2). The system is projected from CO2 for CO2-saturated conditions. The dark shaded liquids enclose the miscibility gap of Kjarsgaard and Hamilton (1988, 1989) at 0.5 GPa, that extends to the alkali-free side (A-A). The lighter shaded liquids enclose the smaller gap (B) of Lee and Wyllie (1994) at 2.5 GPa. C-C is the revised gap of Kjarsgaard and Hamilton. Dashed tie-lines connect some of the conjugate silicate-carbonate liquid pairs found to coexist in the system. After Lee and Wyllie (1996) International Geology Review, 36, 797-819. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

22. Chapter 19: Continental Alkaline Magmatism.Carbonatites Figure 19.16. Schematic cross section of an asthenospheric mantle plume beneath a continental rift environment, and the genesis of nephelinite-carbonatites and kimberlite-carbonatites. Numbers correspond to Figure 19-13. After Wyllie (1989, Origin of carbonatites: Evidence from phase equilibrium studies. In K. Bell (ed.), Carbonatites: Genesis and Evolution. Unwin Hyman, London. pp. 500-545) and Wyllie et al., (1990, Lithos, 26, 3-19). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

23. Chapter 19: Continental Alkaline Magmatism.Lamproites 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.

24. Chapter 19: Continental Alkaline Magmatism.Lamproites

25. Chapter 19: Continental Alkaline Magmatism.Lamproites 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.

26. Chapter 19: Continental Alkaline Magmatism.Lamproites Figure 19.18b. 207Pb/204Pb vs. 206Pb/204Pb for lamproites and kimberlites. After Mitchell and Bergman (1991). 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.

27. Chapter 19: Continental Alkaline Magmatism.Lamprophyres

28. Chapter 19: Continental Alkaline Magmatism.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.

29. Chapter 19: Continental Alkaline Magmatism.Kimberlites

30. Chapter 19: Continental Alkaline Magmatism.Kimberlites 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.

31. Chapter 19: Continental Alkaline Magmatism.Kimberlites Figure 19.20b. Chondrite-normalized spider 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.

32. Chapter 19: Continental Alkaline Magmatism.Kimberlites Figure 19.21. 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.