Continental Flood Basalts (Hot Spots)

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L20-CFB.html

A. Def: voluminous terrestrial lava flows

-Fissure eruptions: low viscosity, high rate of effusion => vast horizontal sheets (also called plateau basalts)

-Examples:

-Cretaceous Deccan Traps, India - 500,000 km2, 600 m thick
-Miocene Columbia River basalts, US - 200,000 km2, 1500 m thick
-Compare: 1783 Iceland eruption of 12 km3 is largest in historic time

-Individual flows usually meters thick, up to 100 m

-Often show columnar jointing = regular fractures due to cooling > 5 or 6-sided pillars

B. Location

-CFB are associated with continental rifting => most found along passive margins formed when new ocean basins open

-Some are located within continents along failed rifts: precambrian Keweenawan in upper Michigan; triassic Siberian Traps in north central Asia

-Many CFB produced during breakup of Pangaea: Deccan; Cretaceous South Atlantic; Eocene North Atlantic

-May be associated with dike swarms

-Erupted into sedimentary basins

II. Composition

A. Rock type: most are quartz tholeiite

-Minor olivine tholeiite, alkaline basalts

-Aphyric to slightly porphyritic: phenocrysts of olivine, plagioclase, augite

-Flows are mostly homogeneous, even over long distances :Xenoliths ;rarely vesicular;eruptions often episodic

B. Trace elements and isotopes

-LREE-enriched pattern

-Sr and Nd ratios variable but near BSE

-Experiments: CFB at low P are saturated (or nearly) in olivine and plagioclase : fractional crystallization of these phases; however, can''t be much plag, because there is only a small Eu anomaly in REE diagram

III. History

A. Origin
-Must identify : primary magma; where fractionation occurred; what processes were involved; how large volumes were produced

-Uniformity of lavas suggests a single primary magma: most primitive (and most common) rock type is quartz tholeiite: parental but not primary; some CFB provinces have picrite basalts with Mb # > 70 => could be primary.

-Enriched component also necessary: sedimentary contribution; or enriched mantle; sub-continental mantle lithosphere

C. Differentiation

-F.C. of olivine + plagioclase (+/- augite) in shallow sills prior to eruption.

-Problem: tholeiitic trend => Fe enrichment: magmas would be denser than sediments, so how could they erupt? Plus, dikes ~ lavas cut through sills and sediments

D. Model

-Picritic magma rises to base of crust, spreads laterally there : some may erupt, but most is blocked by crust; crustal contamination may occur at this stage

-Differentiation > lower density

-Calculations show density is minimum at Mg # ~60

-Why are flows aphyric? Sills are commonly more porphyritic than flows. Visualize a rising magma. Near surface, at low P, hot flow may be above liquidus => phenocrysts resorbed

-Implications: decreasing % of crystals > lower density > faster rise > more melting; melting > ~10% volume increase > faster rise ; thus, positive feedbacks may contribute to high extrusion rates

-Liquid composition changes in equilibrium with melting crystals => difficult to separate this effect from F.C.

E. Modified model

-CFB triggered by "head" of a new mantle plume impinging on base of crust

-Magma accumulates at base of crust, fractionates

-Erupts when: density decreases; extension fractures tap still-dense magma

-3 possible tectonic settings: plume + rifting (right side); plume alone, beneath continent

-Back-arc setting