Flood Basalts and Continental Rifts Wilson Ch 10, p. 287-323 and Ch 11, p. 325-374 • In this lecture: Flood basalts Flood basalts • What? • Where? • When? – Geochemistry – Isotopic composition – Origin of Flood basalt Continental rifting Continental rifting • What? • Where? • When? – Geochemistry – Isotopic composition of magmas – Origin Columbia River Columbia River Plateau Plateau Flood Basalt – Massive eruptions Massive eruptions – Short Short-lived lived – Intracratonic – Intitiation of rifting – Opening of ocean basins – Plume-related – Relation to mass extinctions ~ 1 ~ 1 45 45 o N? N? >2.0 >2.0 249 ± 1 249 ± 1 Siberian Siberian ~ 0.6 30 o N >1.0? 201 ± 1 Newark 0.5 - 1 45 o S >2.0 183 ± 1 Karoo ~ 1? 50-6 o S >0.5 176 ± 1 Antarctica ~ 1 or ~ 5? 40 o S >1.0 132 ± 1 Serra Geral/ Etendeka ~ 2 50 o S ? 116 ± 1 Rajmahal ~ 6? 45 o S ? 88 ± 1 Madagascar ~ 1 ~ 1 20 20 o S >2.0 >2.0 66 ± 1 66 ± 1 Deccan Deccan ~ 1 65 o N >1.0 57 ± 1 North Atlantic ~ 1 10 o N ~ 1.0 31 ± 1 Ethiopia ~ 1 (for 90%) ~ 1 (for 90%) 45 45 o N 0.25 0.25 16 ± 1 16 ± 1 Columbia River Columbia River Duration (Myr) Paleolatitude Volume(10 6 km 3 ) Age (Myr) Province Siberian Traps Siberian Traps DeccanTraps DeccanTraps Columbia River Columbia River Plateau Plateau DeccanTraps DeccanTraps
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Flood Basalts and Continental RiftsWilson Ch 10, p. 287-323 and Ch 11, p. 325-374
• In this lecture:Flood basaltsFlood basalts
• What?• Where?• When?
– Geochemistry– Isotopic composition– Origin of Flood basalt
– Massive eruptions– Short-lived– Intracratonic– Intitiation of rifting– Opening of ocean
basins–– PlumePlume--relatedrelated– Relation to mass
extinctions
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Flood BasaltsThe cause of mass extinctions?The cause of mass extinctions?
poisoning atmosphere with CO2 and SO4
Table 2 compares the LIP ages given in Table 1 with the estimated ages of stratigraphicboundaries involving significant biotic changes, dated according to the most recent geological time scale. In at least three cases (the Deccan, Newark, and Siberian flood basalts), a direct measure of correlation with major extinction events is possible. The probability that three major volcanic events that typically last ~1 Myr should occur within 1 Myr of major extinction events during the last 250 Myr (of which there are ~12) is about 10-4. Thinking about the ways in which these two types of global event might be causally linked is a worthy scientific challenge.
16.416.4Early/MidEarly/Mid--MioceneMiocene16 ± 116 ± 1Columbia RiverColumbia River
AgeStratigraphic BoundaryAgeFlood Basalt Episode
Flood Basalt
Columbia River Basalt•• Youngest flood basalt (16 Ma)Youngest flood basalt (16 Ma)•• 250,000 km250,000 km33 of of tholeiitictholeiitic basaltbasalt• Back arc basin?• Relation to Yellowstone hot spot?
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Flood Basalt
Columbia River Basalt•• 170,000 km170,000 km33 erupted in < 2 erupted in < 2 myrmyr•• EnsialicEnsialic Back arc basin?Back arc basin?• Relation to Yellowstone hot spot?
Flood Basalt
Chemical composition• Subalkaline, tholeiitic• Scatter in ME compositions• TE enrichments relative to MORB, OIB
Isotopic compositionNd-Sr variationsMantle is isotopically heterogenous
Some flood basalts in “mantle array”Others more “crustal”
Columbia River basaltMixing array
MORB source mantle + upper crust
Parana-Etendeka basaltMixing
OIB mantle + crustal components
Flood Basalt
Genesis is complex1. crustal contamination of MORB-like melts2. melting of enriched lithospheric mantle3. mixing depeleted+enriched mantle4. mixing enriched mantle melts and crust
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Continental rifts
Regions of localized lithospheric extension10-100 km wide100-1000 km longCentral depressionUplifted flanksCrustal thinningSites of incipient continental fragmentation
Precedes ocean basin development
Continental rifts
Classic examplesEast African riftRio Grande rift
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Continental rifts
Fundamentally basaltic volcanismSmaller volumes of highly potassic or evolved
magmas
Continental rifts
Lithospheric extension+thinningCompression of isothermsUpwelling asthenosphereHigh heat flow
Active vs. Passive rifting?
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Continental rifts
Lithospheric extension+thinningCompression of isothermsUpwelling asthenosphereHigh heat flow
Active vs. Passive rifting?
Continental rifts
Subalkaline to alkaline magmasFractional crystallization control on chemical variations
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Continental rifts
Trace element patternsSimilar to flood basalts
Continental rifts
Isotopic compositionsNd-Sr variations
Most basaltsMORB-OIB field
Some basaltsEnriched lithospheric
mantle?
Or
Crustal contamination?
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Continental rifts
Petrogenesis complexCompare to processes at oceanic hot spots
Much larger possibility for magmas to be generated within or interact with lithospheric mantle or old, evolved continental crust