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 Delhi. Based on the compositional zoning patterns determined in garnet adjacent to clinopyroxene, orthopyroxene, cordierite, and biotite and in orthopyroxene adjacent to garnet, the granulites are interpreted to have experienced nearly isobaric cooling, suggesting a magmatically influenced thermal regime may have been responsible for the high-grade conditions and relatively high transient metamorphic gradient (32 °C/km) attained in the MRV.