Garnet peridotites in Eurasian high-pressure and ultrahigh-pressure terranes: A diversity of origins and thermal histories

Garnet peridotites and pyroxenites have been reported from 11 of the 15 or so high-pressure/ultrahigh-pressure (HP/UHP) terranes in Eurasia. Most of t hese ultramafic rocks are Mg-Cr types, derived from depleted upper mantle, but some are more Fe-rich and originated by crystallization in ultramafic-m afic igneous complexes. The peridotites are polymetamorphic, with HP/UHP ga rnet-bearing assemblages being followed by a succession of retrograde assem blages related to exhumation and cooling; some also contain evidence for a pre-HP/UHP stage, such as spinel inclusions in garnet or the presence of Ti -clinohumite. Equilibration conditions have been calculated from all availa ble analyses of garnetiferous assemblages, by application of the olivine-ga rnet Fe-Mg exchange thermometer and the Al-in-orthopyroxene barometer, resu lting in two distinct P-T regimes for garnet peridotites--one at high P/T i n the coesite and diamond fields, and another at low P/T in the vicinity of the spinel-to-garnet transition. Garnet peridotites are thought to have evolved in at least four different t ectonothermal settings, including: (1) emplacement of peridotites into ocea nic or continental crust, followed by transport of peridotites and associat ed crust to UHP conditions by a subducting plate; (2) transfer of peridotit es from a mantle wedge to the crust of an underlying, subducting plate; (3) origination from upwelling asthenosphere that passed through a high-temper ature spinel peridotite stage, followed by cooling into the garnet peridoti te field: and (4) extraction of garnet peridotites from ancient subcontinen tal lithosphere, perhaps by deep-seated faulting within a continental plate .