Intraplate magmatism

1. Intraplate magmatism • Hotspots • Rift zones (often associated with hotspots) • Intra-oceanic plate: Tholeitic to alkaline series; mostly basalts (OIB = Oceanic Islands Basalts), some differenciated alkaline terms • Intra-continental plate: • either large tholeitic basaltic provinces (CFB = Continental Flood Basalts), occasionally bimodal (ass. with rhyolites) • or smaller, alkaline to hyper-alkaline, differenciated intrusions/volcanoes (syenites/phonolites; carbonatites; kimberlites; and more…)

2. Continental alkaline series Alkali volcanoes – basaltic strombolian cone in front, trachytic pelean dome behind– in the West European rift

3. The West-european rift sytem

4. 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.

5. Continental alkaline series • Rift (or hotspot) related • Large diversity (possibly > 80% of the rock names, for <1% volume !) • Strange rocks (carbonatites…)

6. Common features of continental alkali series • Alkaline (!) • Undersaturated to just oversaturated • Peralkaline

7. Alkaline series Mildly alkaline Strongly alkaline

8. Q Quartzolite 90 90 Quartz-rich Granitoid 60 60 Saturated Grano- Tonalite Granite Alkali Feldspar Granite diorite Alkali Fs. Qtz. Diorite/ 20 20 Quartz Syenite Qtz. Gabbro Quartz Quartz Quartz Monzonite Syenite Monzodiorite Alkali Fs. 5 Diorite/Gabbro/ 5 Syenite Syenite Monzodiorite Monzonite Anorthosite 90 35 10 65 A P (Foid)-bearing (Foid)-bearing (Foid)-bearing Syenite Monzonite Monzodiorite (Foid)-bearing 10 10 Diorite/Gabbro (Foid)-bearing Alkali Fs. Syenite (Foid) Syenite (Foid) (Foid) Monzosyenite Monzodiorite (Foid) Gabbro Under-saturated 60 60 (Foid)olites F

9. The alkali eutectic 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.

10. Figure 18-2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks. Chapman Hall.

11. Trace elements enriched 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.

12. Enriched mantle source 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.

13. Generated from low to very low melt fractions 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.

14. Diversity of alkaline continental magmas – some examples • Saturated alkaline series • Undersaturated alkaline series • Carbonatites • Lamprophyres, kimberlites & co. Series with a true geological importance Oddities and curiosities – but economic importance!

15. 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.

16. 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.

17. East African rift (Afar) – mildly alkaline

18. Central African Rift – Strongly alkaline

19. Two main series • Basalts-Trachydandesites-Trachydacites-Rhyolites (stronly bimodal): (just) saturated alkali series • A-type granites can be formed there • Role of the preexisting crust? • Basanite-Foidite (nephelinite)-Phonolite: strongly undersaturated alkali series

20. 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.

21. Bimodal associations(the « Daly gap ») • Mantle vs. Crustal sources? • Remelting of underplated basalts? • Simply an effect of the different eutectics?

22. The oddities… • Carbonatites • Lamproites, kimberlites, etc.

23. Chapter 19: Continental Alkaline Magmatism.Carbonatites

24. 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.

25. 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.

26. Chapter 19: Continental Alkaline Magmatism.Carbonatites Figure 19-15. 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.

27. Lamproites and kimberlites • … many, many, many rock types • … many, many different names – mostly purely local and after the one known occurrence of that rock type (Vosgesite, Wyomingite, Orangeite …)

28. Chapter 19: Continental Alkaline Magmatism.Kimberlites

29. 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.

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 19-19. 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.

32. Chapter 19: Continental Alkaline Magmatism.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.