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LARGE IGNEOUS PROVINCES: Results of Delamination?

LARGE IGNEOUS PROVINCES: Results of Delamination? . Don L. Anderson Caltech. A new GSA book. Delamination: The Eclogite Engine. Kay, R.W. & Kay, S.M., Delamination and delamination magmatism, Tectonophysics, 219, 177-189, 1993.

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LARGE IGNEOUS PROVINCES: Results of Delamination?

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  1. LARGE IGNEOUS PROVINCES: Results of Delamination? Don L. Anderson Caltech

  2. A new GSA book

  3. 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?

  4. 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

  5. Rocks and minerals arranged by density: crust & upper mantle eclogite: here used as a general term for garnet & pyroxene-rich rock

  6. Rocks and minerals arranged by density: crust & upper mantle • delaminates when crust > 50 km thick • warmer than MORB

  7. Rocks and minerals arranged by density: upper mantle Where does delaminate reach neutral buoyancy? r Vs

  8. 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

  9. 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

  10. Mantle stratification • irregular chemical discontinuities expected • difficult to see in tomography • can be seen in receiver functions

  11. Underside reflections 0 – 1,000 km depth • 410 & 660-km discontinuities clear • ~ 10 others • may be chemical

  12. Delamination cycle • dense roots • fall off • warm up in ambient mantle • rise • possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly

  13. Many ways for eclogite to get into the mantle • collision belts, arcs • can fuel melting anomalies at normal T

  14. 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?

  15. 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

  16. delamination controls crustal thickness • very sharp cut-off at 50 km • interpreted as eclogite phase change from Mooney et al., 1998

  17. Example 1: Rio Grande riftAre LVZs delaminated roots? hot? eclogite? from Gao et al., 2004

  18. garnet peridotite garnet pyroxenite Example 2: Sierra Nevada attenuation P-wave slowness Vp/Vs anisotropy from Boyd et al., 2004

  19. Example 3: Iceland • 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 Ritsema et al., 1999

  20. 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).

  21. Resources Please visit: www.mantleplumes.org/Eclogite.html www.mantleplumes.org/LowerCrust.html End

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