Chapter 15: Continental Flood Basalts

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

Tectonic Setting of CFBs l Continental hot spots l Continental rifting may be associated with hot spots F Successful rifts F Failed rifts (aulacogens)

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

Figure 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.

Figure 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

Figure 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 ), Hooper (1988a), Hooper and Hawkesworth (1993). Imhana first, Grande Ronde second. Flows increase in SiO2, and K with time; decrease in Al +++, Ca++ as Mg# decreases. Compatible element depletion consistent with fractional crystallization of Plagioclase plus Orthopyroxene and / or Olivine.

Figure 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, Imhana 1st, Grande Ronde 2nd, Wanapum 3rd, Saddle Mts. 4 th Note LREE enrichment. Tholeiitic Continental Flood Basalts (CFBs) therefore show compatible element depletion and incompatible element enrichment, which distinguishes them from N-MORBS. They are considerably more fractionated “since separating from their peridotitic mantle source” Winter p Most incompatible on left

Figure 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, Picture Gorge from Bailey (1989) Geol. Soc. Amer. Special Paper, 239, Late magmas enriched in incompatible LILs such as K, Rb, Ba and Th compared to N-MORBs Most incompatible in center

Figure 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 OIBs Most incompatible on left

Figure Pb/ 204 Pb vs. 206 Pb/ 204 Pb 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 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 Northern Hemisphere Reference Line, suggesting a HIMU component with Radiogenic Lead. Pb 206 The later Saddle Mountains floods are closer to DUPAL (Dupre and Allegre), suggesting ( to Winter ) a mixture of EMI and EMII also. I think they just get most of the Thorium, and thus radiogenic Pb 208 radiogenic Pb 208

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

Figure A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett., 162, 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, yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet- clinopyroxene residue.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- clinopyroxene residue. 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.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. 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 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.