Metamorphic conditions of late Archean high-grade gneisses, Minnesota River valley, U.S.A.
A detailed, comparative geothermobarometric analysis for the Minnesota River valley (MRV) Archean gneiss and migmatite terrane yields temperature and pressure ranges of 650–750 °C and 4.5–7.5 kbar (450–750 MPa), based on garnet–biotite, two-feldspar, garnet–clinopyroxene, garnet–cordierite, orthopyroxene–clinopyroxene, and magnetite–ilmenite thermometry and garnet–cordierite–sillimanite–quartz and garnet–orthopyroxene–plagioclase–quartz barometry. The temperature variation observed is interpreted to be primarily a result of varying degrees of re-equilibration of assemblages with falling temperature but may include real temperature variations between areas exhibiting granulite facies versus upper amphibolite facies–migmatitic lithologies. The large pressure range is primarily a result of differences among the pressure calibrations applied but may also record variations in pressure within the terrane. Lower pressures are consistent with the occurrence of cordierite-bearing assemblages at Granite Falls and D...