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1 Queens College and the Graduate Center, City University of New York, Queens College, Flushing, New York 11367 and Lamont-Doherty Earth Observatory, Palisades, New York 10964
Garnet- (and probably spinel-) bearing ultramafic rocks that form tectonic lenses within high-metamorphic-grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge. This model is most credible for garnet peridotites associated with ultrahigh-pressure metamorphic terranes in mountains formed by continent-continent collision. The extremely high pressures implied by ultrahigh-pressure metamorphism indicate that continental crust was subducted to mantle depths >100–120 km, where the mantle is in the garnet peridotite facies. Exchange between mantle and crust seems likely, particularly the transfer of mantle from the hanging wall into the crustal footwall. The mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable. If the interface becomes hot or wet enough so that peridotite can deform by ductile flow, it would sink as mantle blobs into the underlying crustal rocks. If not, the exchange presumably occurs through a brittle mechanism. The intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface. This mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh-pressure Western Gneiss region of the Norwegian Caledonides during the Caledonian orogeny.
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