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LARGE IGNEOUS PROVINCES: Results of Delamination?

Don L. Anderson

Caltech

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A new GSA book

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Delamination:The Eclogite Engine

• Kay, R.W. & Kay, S.M., Delamination and delamination magmatism, Tectonophysics, 219, 177-189, 1993.

• mechanism can explain some long-standing geophysical problems, e.g.– subsidence prior to LIP emplacement– short duration– bottoming of seismic tomography

anomalies beneath “hot spots”

• but what happens to this lower crust?

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Summary of model

• When crust thickens to > 50 km:– converts to dense eclogite– delaminates– sinks– heats up– rises

• eclogites have low Vs for their density - may be confused with high T

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Rocks and minerals arranged by density: crust & upper mantle

eclogite: here used as a general term for garnet & pyroxene-rich rock

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Rocks and minerals arranged by density: crust & upper mantle

• delaminates when crust > 50 km thick

• warmer than MORB

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Rocks and minerals arranged by density: upper mantle

Where does delaminate reach neutral buoyancy?Vs

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Delaminated roots warm quickly

• will start to melt before reaching same T as surrounding mantle

• already in TBL, so starts off warm

• when 30% melt, garnet mostly gone & will start to rise

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• pink eclogite is only temporarily stable at these depths

• “arclogites” less SiO2 than MORB eclogite – do not sink so far

• Vs of eclogite low at depth

• low melting point• as it warms, it

rises

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Mantle stratification

• irregular chemical discontinuities expected

• difficult to see in tomography

• can be seen in receiver functions

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Underside reflections 0 – 1,000 km depth

• 410 & 660-km discontinuities clear

• ~ 10 others

• may be chemical

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• dense roots– fall off– warm up in

ambient mantle– rise

• possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly

ROOT FORMATION

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DELAMINATION ridge

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SPREADING

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UPWELLING

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SPREADING

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Delamination cycle

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Many ways for eclogite to get into the mantle

• collision belts, arcs

• can fuel melting anomalies at normal T

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LIPs are associated with continental breakup

• reconstruction at ~ 30 Ma

• dual volcanism– on breakup– ~ 30-40 Myr later

• oceanic plateaus form ~ 1,000 km offshore

• = rising of delaminated root?

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Eclogite 70% molten before peridotite starts to melt

• eclogite sinkers warmed by conduction

• rise before T has risen to that of ambient mantle

• eclogite 70% molten at peridotite solidus

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• delamination controls crustal thickness

• very sharp cut-off at 50 km

• interpreted as eclogite phase change

from Mooney et al., 1998

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Example 1: Rio Grande riftAre LVZs delaminated roots?

from Gao et al., 2004

hot?

eclogite?

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Example 2: Sierra Nevada

P-wave slowness attenuation

anisotropyVp/Vs

garnet peridotite

garnet pyroxenite

from Boyd et al., 2004

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Example 3: Iceland

Ritsema et al., 1999

• Restricted LVZ• possibly Caledonian

arc roots delaminated on breakup

• Cold, dense, sinking eclogite can be LVZ

• warmed, melted, rising eclogite can also be buoyant if ~ 1/2 garnet eliminated

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Summary

• Dense, mafic cumulates may be twice the thickness of arc crust

• Delamination accompanied by upwelling & adiabatic decompression of the asthenosphere; a whole cycle may take 30-40 Myr

• The global recycling flux of arcologite is ~ 10% that of oceanic crust, i.e. ~ hotspot volume rate

• It starts out hotter & by-passes normal subduction zone processing

• Delaminated arclogites preferentially melt & form a unique component of hotspot & ridge magmas (e.g. suggested DUPAL = Gondwana crust).

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Resources

Please visit:

www.mantleplumes.org/Eclogite.html

www.mantleplumes.org/LowerCrust.html

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