Flood Basalts Francis, 2013

1. Flood Basalts Francis, 2013 Baffin Picrites North Atlantic Igneous Province 60 mys

3. Age No particular time association, flood basalts appear to have erupted throughout the Earth’s history. Many flood basalt provinces appear to be associated with continental break up and/or OIB hot-spot traces.

4. Characteristic features of Flood Basalts: Flood basalt provinces occur both as 1 – 5 km thick sequences of effusive basalts on continents and as plateau of anomalously thick oceanic crust (20 – 40 km) capped by 5 – 10 km of basaltic lavas (oceanic plateau). Flood basalt provinces are characterized by thick sheet-like flows, an absence of central volcanic constructs, and very large volumes ( 106 km3). In comparison, the volume of Mauna Loa, the largest volcano on Earth, is only 75,000 km3. Single flows often reach thicknesses of 100m, for example the La Roza flow of Columbia River Flood Province is 100+m thick and has a volume of  160 km3). It is commonly difficult to identify actual eruption vents. Dominated by remarkably uniform aphyric quartz tholeiites and basaltic andesites that are too evolved in composition to equilibrate with a pyrolitic mantle. Commonly associated with significant late-stage rhyolitic volcanism in continental settings (Karroo, Parana, Yellowstone), but there is a marked absence of silicic andesites and dacites, ie. flood basalt provinces are distinctly bimodal. Commonly minor associated picrite and alkaline lavas occur low in the volcanic succession. Picrites associated with flood basalts share the major element characteristics of OIB picrites, in terms of high Fe, and low Al, most falling in the harzburgite residue field of the Al - Si diagram.

5. Duration Individual flood basalt provinces have remarkably short histories considering the large volumes of erupted lava. The majority of well-dated flood basalt provinces have main stage durations on the order of only 1 - 4 ma. The better the dating, the shorter the time interval. These short eruption durations, combined with the large volumes, correspond to eruption rates that are 10 - 100 times those seen in OIB suites such as Hawaii. Flood basalt provinces thus appear to represent catastrophic events, with their associated hot-spot tracks representing a long waning period.

6. Columbia River Flood Basalts ~15 ± 1 mys

8. North Atlantic Igneous Province 60 ± 1mys

9. Deccan Traps 66 ± 1mys

10. Curacao –Carribean 88-90 mys

11. Ontong Java Ocean Plateau Mid-Cretaceous 122 & 90 mys Anomalously thick oceanic crust (20 – 40 km) capped by 5 – 10 km of basaltic lavas..

12. Siberian Traps 248 ± 2mys

13. Parana – Etendeka 120 ±5mys

14. Karoo Flood Basalts 183 ± 1mys

15. Despite their relatively small major element compositional ranges, the variations in relatively incompatible elements such as Ti indicate that most flood basalt suites have experienced a relatively large degree of crystal fractionation along a tholeiitic gabbroic cotectic. This implies the existence of voluminous gabbroic cumulates (Xcumulate/Xlava 0.5), most likely in sills at the base of the crust. These gabbroic cumulates may represent an important contribution to the growth of continental crust. Different Mantle Sources or Extensive Fractionation?

19. Trace Element Characteristics of Flood Basalts Unlike OIB and MORB basalts and picrites, continental flood basalts are commonly characterized by negative high field strength element anomalies (eg. Nb) and positive Pb anomalies, and are characteristically enriched in LIL elements (K, Rb, & Ba). Continental alkaline basalts associated with flood basalt provinces, however, are virtually indistinguishable from those of OIB suites, exhibiting neither Nb nor Pb anomalies, and relative depletions in LIL elements. Oceanic plateau, in contrast, have relatively flat, unfractionated trace element profiles, with relative depletions in LIL elements, and lack Nb and Pb anomalies. This may in fact be an observational problem, because we do not have much in the way of samples from the lower parts of ocean plateau successions.

20. Individual continental flood basalt provinces commonly exhibit chemically and isotopically polarised stratigraphic successions, with picrites and basalts that are trace element and isotopically enriched near the base of the succession, but with trace element and isotopic enrichment, and Nb and Pb anomalies, decreasing up section to late-stage basalts that have MORB-like trace element and isotopic characteristics (Deccan, Karroo, Coppermine). This pattern suggests that the effects of crustal contamination (or lithospheric mantle?) are most common in the early stages of flood basalt volcanism.

21. Continental Flood Basalts Ocean Plateau Continental flood tholeiites have relatively high 87Sr/86Sr and low 143Nd/144Nd ratios, that typically extend from near Bulk Earth into the lower right quadrant of the mantle array. Some continental tholeiites actually have lower Pb isotopic ratios than MORB, and thus appear to contain very old Pb (Yellowstone), derived from continental crust or lithospheric mantle. Ocean plateau provinces, in contrast, are characterized by relatively low 87Sr/86Sr and high 143Nd/144Nd isotopic ratios, commonly plotting in the upper left hand quadrant of the mantle array, similar to many apparently plume-related OIB tholeiites.

23. Rhyolites in continental flood basalt provinces are typically even more isotopically enriched in terms of Sr and Nd, and clearly distinct from their associated basalts. Such rhyolites appear to be melts of the crust.

24. Some of the Columbia River flood basalts and associated Yellowstone basalts and rhyolites contain very old lead, presumably derived from the North American Proterozoic crust. 1 Ga

26. Age No particular time association, flood basalts appear to have erupted throughout the Earth’s history. Many flood basalt provinces appear to be associated with continental break up and/or OIB hot-spot traces.

27. A Role for Eclogite? Large Volume of Monotonous Si-rich Compositions Plume Head

28. Olivines in MORB, OIB, and Continental Flood Basalts

29. Olivine Compositions

31. Multi-Stage Melting Model: Lithosphere

32. Lithosphere

33. Lithosphere Melting of peridotite containing eclogite or pyroxenite pods