Chapter 15: Continental Flood Chapter 15: Continental Flood BasaltsBasalts

Large Igneous Provinces (LIPs)Large Igneous Provinces (LIPs) Oceanic plateausOceanic plateaus Some riftsSome rifts Continental flood basalts Continental flood basalts

(CFBs)(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.

Table 15-1. Major Flood Basalt Provinces

Name Volume Age Locality

CRB (1.7x105 km3) Miocene NW US

Keeweenawan (4x105 km3) Precambrian Superior area

Deccan (106 km3) Cret.-Eocene India

Parana (area > 106 km2) early Cret. Brazil

Karroo (2x106 km3?) early Jurassic S. Africa

Tectonic Setting of CFBsTectonic Setting of CFBs Continental hot spotsContinental hot spots Continental rifting may be associated with hot Continental rifting may be associated with hot

spotsspots Successful riftsSuccessful rifts Failed rifts (aulacogens)Failed rifts (aulacogens)

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

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

Present setting of the Columbia River Basalt Group in the Northwestern United States. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Table 15-2. Stratigraphy of the Columbia RiverBasalt Group.

Magnetic K/ArFormation Member Polarity* Dates

Lower Monumental N 6 MaIce Harbor N,R 8.5

Saddle Buford RElephant Mountain R,T 10.5

Mountains Pomona R 12Esquatzel N

Basalt Weissenfels Ridge NAsotin N 13Wilbur Creek NUmatilla NPriest Rapids R 14.5

Wanapum Roza T,RBasalt Frenchman Springs N 15.3

Eckler Mountain N Grande See Reidel et al . N2 15.6

Ronde Picture (1989) for R2

Basalt Gorge Grande N1

Ronde Units R1 16.5See Hooper et al . R1

Imnaha (1984) for Imnaha TBasalt Units N0

R0 17.5Data from Reidel et al . (1989); Hooper and Hawkesworth (1993).

* N = normal, R = reversed, T = transitional

C l

a r

k s

t o

n

Figure 15-5. Time-averaged extrusion rate of CRBG basalts as a function of time, showing cumulative volume. After Hooper (1988a) The Columbia River Basalt. In J. D. Macdougall (ed.), Continental Flood Basalts. Kluwer. 1-34.

Figure 15-6. Variation in wt.% of selected major element oxides vs. Mg# for units of the Columbia River Basalt Group. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from BVTP (Table 1.2.3.3), Hooper (1988a), Hooper and Hawkesworth (1993).

Imhana first, Grande Ronde second. Flows increase in SiO2, and K with time; Imhana first, Grande Ronde second. Flows increase in SiO2, and K with time; decrease in Aldecrease in Al++++++, Ca++ as Mg# decreases. , Ca++ as Mg# decreases. Compatible element depletion Compatible element depletion

consistent with fractional crystallization of Plagioclase plus Orthopyroxene andconsistent with fractional crystallization of Plagioclase plus Orthopyroxene and//or or Olivine.Olivine.

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.

Imhana 1st, Grande Ronde 2nd, Wanapum 3rd, Saddle Mts. 4Imhana 1st, Grande Ronde 2nd, Wanapum 3rd, Saddle Mts. 4 thth

Note LREE enrichment. Tholeiitic Continental Flood Basalts (CFBs) therefore Note LREE enrichment. Tholeiitic Continental Flood Basalts (CFBs) therefore show compatible element depletion and incompatible element enrichment, which show compatible element depletion and incompatible element enrichment, which distinguishes them from N-MORBS. They are considerably more fractionated distinguishes them from N-MORBS. They are considerably more fractionated “since separating from their peridotitic mantle source” Winter p. 282.“since separating from their peridotitic mantle source” Winter p. 282.

Most incompatible Most incompatible on lefton left

Figure 15-8. N-MORB-normalized spider diagram for some representative analyses from the CRBG. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol., 34, 1203-1246. Picture Gorge from Bailey (1989) Geol. Soc. Amer. Special Paper, 239, 67-84.

Late magmas enriched in incompatible LILs such as Late magmas enriched in incompatible LILs such as K, Rb, Ba and Th compared to N-MORBsK, Rb, Ba and Th compared to N-MORBs

Most incompatible Most incompatible in centerin center

Figure 15-9. OIB-normalized spider diagram for some representative CRBG analyses. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. (data as in Figure 15-8).

CFB magmas compared to similar OIBsCFB magmas compared to similar OIBs

Most incompatible Most incompatible on lefton left

Figure 15-11. 208Pb/204Pb vs. 206Pb/204Pb for the basalts of the CRBG. Included for reference are EMI, EMII, the DUPAL group, the MORB array, and the NRHL (northern hemisphere reference line) connecting DM and HIMU mantle reservoirs from Figure 14-6. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper (1988a), Carlson et al. (1981), Carlson (1984), McDougall (1976), Brandon et al. (1993), Hooper and Hawkesworth (1993).

Imhana, Grande Ronde,and Picture Gorge plot on Imhana, Grande Ronde,and Picture Gorge plot on Northern Hemisphere Reference Line, suggesting a Northern Hemisphere Reference Line, suggesting a HIMU component with Radiogenic Lead. Pb 206HIMU component with Radiogenic Lead. Pb 206

The later Saddle The later Saddle Mountains floods are Mountains floods are closer to DUPAL (Dupre closer to DUPAL (Dupre and Allegre), suggesting and Allegre), suggesting ( to Winter ) a mixture of ( to Winter ) a mixture of EMI and EMII also. I EMI and EMII also. I think they just get most of think they just get most of the Thorium, and thus the Thorium, and thus radiogenic Pb 208radiogenic Pb 208

Recall the OIB case, and note Recall the OIB case, and note the thinning of the plume with timethe thinning of the plume with time

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.

• Melting within a heterogeneous plume head (initial stages of the Yellowstone hot spot). Melting within a heterogeneous plume head (initial stages of the Yellowstone hot spot). •The plume head contains recycled stringers of recycled oceanic crust that melts before the Peridotite, The plume head contains recycled stringers of recycled oceanic crust that melts before the Peridotite, yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet-yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet-clinopyroxene residue. clinopyroxene residue. •The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite. ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite. •Basalt escapes along a northward trending rift system to feed the CRBG.Basalt escapes along a northward trending rift system to feed the CRBG.

Figure 15-14. Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir. Winter (20010 An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.