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Intraplate magmatism

Intraplate magmatism

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Intraplate magmatism

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  1. Intraplate magmatism

  2. 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…)

  3. Ocean islands and seamounts Commonly associated with hot spots Figure 14-1. After Crough (1983)Ann. Rev. Earth Planet. Sci., 11, 165-193.

  4. Oceanic islands

  5. Hotspots

  6. Mantle convection and mantle plumes

  7. Types of OIB Magmas Two principal magma series • Tholeiitic series (dominant type) • Parental ocean island tholeiitic basalt, or OIT • Similar to MORB, but some distinct chemical and mineralogical differences • Alkaline series (subordinate) • Parental ocean island alkaline basalt, or OIA • Two principal alkaline sub-series • silica undersaturated • slightly silica oversaturated (less common series)

  8. Hawaiian Scenario Cyclic, pattern to the eruptive history 1. Pre-shield-building stage somewhat alkaline and variable 2. Shield-building stage begins with tremendous outpourings of tholeiitic basalts

  9. Hawaiian Scenario 3. Waning activity more alkaline, episodic, and violent (Mauna Kea, Hualalai, and Kohala). Lavas are also more diverse, with a larger proportion of differentiated liquids 4. A long period of dormancy, followed by a late, post-erosional stage. Characterized by highly alkaline and silica-undersaturated magmas, including alkali basalts, nephelinites, melilite basalts, and basanites

  10. Evolution in the Series Tholeiitic, alkaline, and highly alkaline Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

  11. Trace Elements • The LIL trace elements (K, Rb, Cs, Ba, Pb2+ and Sr) are incompatible and are all enriched in OIB magmas with respect to MORBs • The ratios of incompatible elements have been employed to distinguish between source reservoirs • N-MORB: the K/Ba ratio is high (usually > 100) • E-MORB: the K/Ba ratio is in the mid 30’s • OITs range from 25-40, and OIAs in the upper 20’s Thus all appear to have distinctive sources

  12. Trace Elements • HFS elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are also incompatible, and are enriched in OIBs > MORBs • Ratios of these elements are also used to distinguish mantle sources • The Zr/Nb ratio • N-MORBgenerally quite high (>30) • OIBs are low (<10)

  13. Trace Elements: REEs Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

  14. MORB-normalized Spider Diagrams Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  15. Generation of tholeiitic and alkaline basalts from a chemically uniform mantle Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

  16. Ne Volatile-free E 3GPa E 2Gpa E 1GPa Ab Highly undesaturated (nepheline - bearing) alkali basalts E 1atm Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Pressure effects: Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634.

  17. Tholeiites favored by shallower melting • 25% melting at <30 km ® tholeiite • 25% melting at 60 km ® olivine basalt • Tholeiites favored by greater % partial melting • 20 % melting at 60 km ® alkaline basalt • incompatibles (alkalis) ® initial melts • 30 % melting at 60 km ® tholeiite

  18. Isotope Geochemistry • Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the source • OIBs, which sample a great expanse of oceanic mantle in places where crustal contamination is minimal, provide incomparable evidence as to the nature of the mantle

  19. Simple Mixing Models Ternary All analyses fall within triangle determined by three reservoirs Binary All analyses fall between two reservoirs as magmas mix Figure 14-5. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  20. Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  21. Mantle Reservoirs 1.DM (Depleted Mantle) = N-MORB source Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  22. 2.BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  23. 3.EMI = enriched mantle type I has lower 87Sr/86Sr (near primordial) 4.EMII = enriched mantle type II has higher 87Sr/86Sr (> 0.720, well above any reasonable mantle sources Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  24. 5.PREMA (PREvalent MAntle) Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  25. Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  26. Pb Isotopes • Pb produced by radioactive decay of U & Th 238U 234U 206Pb 235U 207Pb 232Th 208Pb • Pb isotopes also characterize the different reservoirs (see paper presentation Hart 1984)

  27. Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).

  28. Kellogg et al. (1999)

  29. A Model for Oceanic Magmatism Continental Reservoirs DM OIB EM and HIMU from crustal sources (subducted OC + CC seds) Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).

  30. “Marble cake” model for mantle convection & mixing

  31. Continental Flood Basalts Large Igneous Provinces (LIPs) Oceanic plateaus Some rifts Continental flood basalts (CFBs) Figure 15-1. Columbia River Basalts at Hat Point, Snake River area. Cover of Geol. Soc. Amer Special Paper 239. Photo courtesy Steve Reidel.

  32. Trapp volcanism

  33. LIPs (Large Igneous Provinces)

  34. CFB’s • Associated to major continental break-up • … or/and to plume head impact

  35. Figure 15-2. Flood basalt provinces of Gondwanaland prior to break-up and separation. After Cox (1978) Nature, 274, 47-49.

  36. Figure 15-3. Relationship of the Etendeka and Paraná plateau provinces to the Tristan hot spot. After Wilson (1989), Igneous Petrogenesis. Kluwer.

  37. Geochemistry • Deccan traps basalts

  38. Bimodal magmas • Basalts and rhyolites • Secondary melting? • Effect of the two eutectics?

  39. Figure 15-7. Condrite-normalized rare earth element patterns of some typical CRBG samples. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol., 34, 1203-1246.

  40. Figure 15-13. A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett., 162, 63-80.

  41. LIPs and mass extinctions

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

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

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

  45. Alkaline series Mildly alkaline Strongly alkaline

  46. 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.

  47. 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.

  48. 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.