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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

Continental riftingContinental rifting• What?• Where?• When?

– Geochemistry– Isotopic composition of magmas– Origin

Columbia River Columbia River PlateauPlateau

Flood Basalt

–– Massive eruptionsMassive eruptions–– ShortShort--livedlived– Intracratonic– Intitiation of rifting– Opening of ocean

basins– Plume-related– Relation to mass

extinctions

~ 1~ 14545ooN?N?>2.0>2.0249 ± 1249 ± 1SiberianSiberian

~ 0.630oN>1.0?201 ± 1Newark

0.5 - 145oS>2.0183 ± 1Karoo

~ 1?50-6oS>0.5176 ± 1Antarctica

~ 1 or ~ 5?40oS>1.0132 ± 1Serra Geral/ Etendeka

~ 250oS?116 ± 1Rajmahal

~ 6?45oS?88 ± 1Madagascar

~ 1~ 12020ooSS>2.0>2.066 ± 166 ± 1DeccanDeccan

~ 165oN>1.057 ± 1North Atlantic

~ 110oN~ 1.031 ± 1Ethiopia

~ 1 (for 90%)~ 1 (for 90%)4545ooNN0.250.2516 ± 116 ± 1Columbia RiverColumbia River

Duration(Myr)PaleolatitudeVolume(106 km3)Age

(Myr)Province

Siberian TrapsSiberian TrapsDeccanTrapsDeccanTrapsColumbia River Columbia River

PlateauPlateau

DeccanTrapsDeccanTraps

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Flood Basalt Flood Basalt

– Massive eruptions– Short-lived–– IntracratonicIntracratonic riftsrifts

•• KeweenawanKeweenawan–– IntitiationIntitiation of riftingof rifting–– Opening of ocean basinsOpening of ocean basins

•• Atlantic & Indian oceansAtlantic & Indian oceans•• ParanaParana--EtendekaEtendeka--KarooKaroo--

AntarcticaAntarctica–– BreakBreak--up of up of GondwnanalandGondwnanaland

supercontinentsupercontinent–– Jurassic Jurassic -- CretaceousCretaceous

– Plume-related– Relation to mass extinctions

Flood Basalt 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.

248.2 ± 4.8248.2 ± 4.8Permian/TriassicPermian/Triassic249 ± 1249 ± 1SiberianSiberian

205.7 ± 4Triassic/Jurassic201 ± 1Newark

180.1 ± 4Early/Middle Jurassic183 ± 1Karoo

(176.5 ± 4)(Aalenian/Bajocian)176 ± 1 or 183 ± 1Antarctica

142 ± 2.6(132 ± 1.9)

Jurassic/Cretaceous(Hauterivian/Valanginian)132 ± 1Serra Geral/Etendeka

112.2 ± 1.1Aptian/Albian116 ± 1Rajmahal

93.5 ± 0.2(89 ± 0.5)

Cenomanian/Turonian(Turonian/Coniacian)88 ± 1Madagascar

65.0 ± 0.165.0 ± 0.1Cretaceous/TertiaryCretaceous/Tertiary66 ± 166 ± 1DeccanDeccan

54.8(57.9)

Paleocene/Eocene(Thanetian/Selandian)57 ± 1North Atlantic

30Early/Late Oligocene31 ± 1Ethiopia

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

Suggest:• Polybaric crystallization?• Mantle source heterogeneity?

– Subcontinental lithosphere?• Crustal contamination?

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Flood Basalt

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