Metamorphic conditions of late Archaean high-grade gneisses, Minnesota River Valley, U.S.A.

A detailed, comparative geothermobarometric analysis for the Minnesota River Valley Archaean gneiss and migmatite terrain yields T and P ranges of 650-750oC and 4.5-7.5 kbar (450-750 MPa), based on garnet-biotite, two-feldspar, garnet-clinopyroxene, garnet-cordierite, two pyroxene and magnetite-ilmenite thermometry and garnet-cordierite-sillimanite-quartz and garnet-orthopyroxene-plagioclase-quartz barometry. The T variation observed is interpreted to be the result of varying degrees of re-equilibration of assemblages with falling T, but may include real T, variations between areas exhibiting granulite facies vs upper amphibolite facies-migmatite lithologies. The large P range may record variations in P within the terrain. Lower P 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. This suggests that a magmatically influenced thermal regime may have been responsible for the high-grade conditions and relatively high transient metamorphic gradient (32oC/km) attained in the Minnesota River Valley. (Author's abstract)-J.M.H.