A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet-bearing peridotite lenses are minor but significant components of mo st metamorphic terranes characterized by high-temperature eclogite facies a ssemblages. Most peridotite intrudes when slabs of continental crust are su bducted deeply (60-120 km) into the mantle, usually by following oceanic li thosphere down an established subduction zone. Peridotite is transferred fr om the resulting mantle wedge into the crustal footwall through brittle and /or ductile mechanisms. These 'mantle' peridotites vary petrographically, c hemically, isotopically, chronologically and thermobarometrically from orog en to orogen, within orogens and even within individual terranes. The varia tions reflect: (1) derivation from different mantle sources (oceanic or con tinental lithosphere, asthenosphere); (2) perturbations while the mantle we dges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to rec rystallize and be contaminated by fluids derived from the subducting oceani c crust. These 'subduction zone peridotites' intrude during the subsequent subduction of continental crust. Low-pressure protoliths introduced at shal low (serpentinite, plagioclase peridotite) and intermediate (spinel peridot ite) mantle depths (20-50 km) may be carried to deeper levels within the ho st slab and undergo high-pressure metamorphism along with the enclosing roc ks. If subducted deeply enough, the peridotites will develop garnet-bearing assemblages that are isofacial with, and give the same recrystallization a ges as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50-120 km) may already contain garnet when they intrude and will n ot necessarily be isofacial or isochronous with the enclosing crustal rocks . Some garnet peridotites recrystallize from spinel peridotite precursors a t very high temperatures (c. 1200 degrees C) and may derive ultimately from the asthenosphere. Other peridotites are from old (> 1 Ga), cold (c. 850 d egrees C), subcontinental mantle ('relict peridotites') and seem to require the development of major intra-cratonic faults to effect their intrusion.