Flood Basalts

Continental Flood Basalts

(Chapter 15)

last update:10/09/06

Large Igneous Provinces (LIPs)

Oceanic plateaus

Some rifts

Continental flood basalts (CFBs)

	The flows are unusually fluid and rapidly erupted and cover extensive areas.
	They are also fissure-fed at rifts

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 - rifting environs

Continental hot spots

Continental rifting may be associated with hot spots

Successful rifts - e.g. Parana, Karroo, Antarctic and North Atlantic provinces

Failed rifts (aulacogens) - e.g. Keweenawan, Siberian traps

Common precursors to rifted margins

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

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

Example: Columbia River Basalts

Present setting of the Columbia River Basalt Group in the Northwestern United States. Period of calc-alkaline volcanism with subduction followed by basaltic volcanism Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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.

	The Columbia River basalts are tholeiitic or basaltic andesite, and chemically similar to MORBs and OIT, but more evolved
	Restricted compositional range - slightly silica saturated
	Fractional crystallization of plagioclase plus cpx and/or olivine

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

Trace elements

low Ni and Cr imply these are not primary magmas

LREE and incompatible element enrichment, unlike N-MORBs

Chondrite-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.

Ce/Zr vs. Ce/Nb (unnormalized) for the basalts of the Columbia River Basalt Group. The different trends imply distinct mantle sources. After Hooper and Hawkesworth (1993) J. Petrol., 34, 1203-1246.

Present setting of the Columbia River Basalt Group in the Northwestern United States. Also shown is the Snake River Plain (SRP) basalt-rhyolite province and proposed trace of the Snake River-Yellowstone hot spot by Geist and Richards (1993) Geology, 21, 789-792. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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

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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.