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

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 gar net-bearing assemblages being followed by a succession of retrograde assemblages related to exhu mation 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 Al-in-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.

Non‐silicate inclusions in garnet from an ultra‐deep orogenic peridotite

“…1000 C). This independent temperature estimate is, however, at least 200 C higher than that derived by standard thermobarometric methods using garnet, olivine and pyroxene mineral compositions (Medaris and Carswell 1990;Krogh and Carswell 1995;Brueckner and Medaris 1998;Medaris 1999). The latter methods consistently yield P±T estimates around 750±820 C and 28±32 kbar, which were interpreted by these authors to represent the peak-metamorphic P±T conditions for the formation of protogranular, porphyroclastic and equigranular garnets (box II in Figure 1).…”

“…The WGR (Mg-Cr type) garnet peridotites have been treated as a uniform group (Medaris and Carswell 1990;Krogh and Carswell 1995;Medaris 1998, 2000). The P±T conditions of the garnet±olivine assemblages, derived by standard thermobarometric methods, are characterized by: (a) upper mantle re-equilibration conditions (Medaris and Carswell 1990;Krogh and Carswell 1995;Van Roermund and Drury 1998;Medaris 1999); and (b) a P±T array that is consistent with a geothermal gradient of a cold continental lithosphere (Medaris and Carswell 1990;Krogh and Carswell 1995;Brueckner and Medaris 1998;Medaris 1999). In addition, Carswell (1986), Medaris and Carswell (1990) and Krogh and Carswell (1995) established a seven-stage tectonometamorphic evolution for the WGR Mg-Cr type garnet peridotites.…”

Hydrogen Incorporation in Natural Mantle Olivines

“…The IR spectrum of sample GRR949, originally published by Miller et al [1987], shows very weak Group I bands and stronger Group Ia bands, but the other two olivines exhibit distinctly different IR spectra (Figure 3), with strong Group I bands at 3613, 3598 and 3579 cm· 1 . These crystals may have experienced peak pressures of 3.8 GPa [Medaris, 1999] or even higher [van Roermund et al, 2000]. However, they obviously had a more protracted history of exhumation to the surface than the xenoliths discussed above and have likely been affected by fluid infiltration events at lower pressures [Kostenko et al, 2002], reflected by the ubiquitous presence of serpentine in these samples.…”

Structural and petrological constraints on the tectonic evolution of the garnet‐lherzolite facies Higashi‐akaishi peridotite body, Sanbagawa belt, SW Japan

“…[31] Garnet-orthopyroxene geothermobarometry is a useful petrological method to estimate the equilibrium P-T conditions of unhydrated garnet-bearing ultramafic rocks [e.g., Harley and Green, 1982;Medaris, 1999]. Enami et al [2004] use this approach to estimate peak pressures of 2.9-3.8 GPa.…”

“…[2] Garnet-bearing peridotite bodies occur as important components of continental collision zones where they are associated with high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks [e.g., Medaris, 1999;Brueckener and Medaris, 2000]. Studies of such bodies are important for investigating processes occurring deep within convergent plate boundaries.…”