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, , 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? 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 SiO 2 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 dense roots –fall off –warm up in ambient mantle –rise possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly Delamination cycle

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 –~ 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 rift Are LVZs delaminated roots? from Gao et al., 2004 hot? eclogite?

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

19 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

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 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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