Garnet Peridotites in Eurasian High-Pressure and Ultrahigh-Pressure Terranes: A Diversity of Origins and Thermal Histories

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Abstract
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 these ultramafic rocks are Mg-Cr types, derived from depleted upper mantle, but some are more Fe-rich and originated by crystallization in ultramafic-mafic igneous complexes. The peridotites are polymetamorphic, with HP/UHP garnet-bearing assemblages being followed by a succession of retrograde assemblages 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 available analyses of garnetiferous assemblages, by application of the olivine-garnet Fe-Mg exchange thermometer and the Alin-orthopyroxene barometer, resulting in two distinct P-T regimes for garnet peridotites—one at high P/T in 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 tectonothermal settings, including: (1) emplacement of peridotites into oceanic or continental crust, followed by transport of peridotites and associated crust to UHP conditions by a subducting plate; (2) transfer of peridotites from a mantle wedge to the crust of an underlying, subducting plate; (3) origination from upwelling asthenosphere that passed through a high-temperature spinel peridotite stage, followed by cooling into the garnet peridotite field; and (4) extraction of garnet peridotites from ancient subcontinental lithosphere, perhaps by deep-seated faulting within a continental plate.