Metamorphic Conditions of an Archean Core Complex in the Northern Wind River Range, Wyoming

Abstract

The Archean granulite-facies rocks of the northern Wind River Range consist of extensive granitic orthogneisses and migtnatites hosting banded iron formations, amphibolites, metapelites, metabasites, ultramafites and quartzites. Quantitative pressure and temperature estimates from inclusions within garnet porphyroblasts are 815±5O°C and 8±1 kb using equilibria buffered by the assemblages spinel-quartz-garnet-sillimanite and garnet-rutile-ilmenite-sillimanite-quartz. Pressure-temperature estimates from the groundmass core assemblages of the banded iron formations and hornblende granulites are 750 ± 50 °C and 5-5 + 1 kb using garnet-clinopyroxene, garnet-orthopyroxene, and twopyroxene thermometry, and geobarometers based on the assemblages garnet-quartz-plagioclaseorthopyroxene and orthopyroxene-olivine-quartz. Rim compositions of the matrix minerals indicate nearly isobaric cooling from the conditions of 750 °C and 5-5 kb to < 600°C at 5 kb. Taken together, the P-T estimates from both the garnet inclusions and matrix assemblages are consistent with a clockwise P-T-t path for this terrane.Temperature estimates based on oxygen isotope thermometry in the banded iron formations vary systematically with the degree of visible late-stage deformation. There is no correlation between the isotopic temperature estimates and those from cation-based thermometers. The highest pressures and temperatures for the Wind River terrane are preserved by the inclusions in garnet porphyroblasts. The ability of these inclusions to preserve chemistries corresponding to higher pressures and temperatures is attributed to the combined effects of inclusion isolation and fixed inclusion volume within the garnet porphyroblasts. Cation-based thermometers in the groundmass preserve lower temperatures as a result of diffusional partial resetting. Isotopic thermometry will yield the lowest temperatures if there is even minor retrograde deformation.Geothermobarometry for the northern Wind River Archean terrane is consistent with a tectonic regime of doubly thickened crust. Peak metamorphic conditions preserved in the cores of the garnets are compatible with deep burial during the early stages of tectonism. Rapid to intermediate uplift due to erosion of the upper plate could explain the nearly isothermal decompression from 80 to 5-5 kb. The later, nearly isobaric, cooling path indicated by the rim compositions of the matrix minerals is consistent with relaxation of the elevated geotherm.

A metamorphic history from micron-scale 207Pb/206Pb chronometry of Archean monazite

“…2.85 Ga zircon cores from a charnockite analysed by SIMS [11]. If the original growth occurred at 2.77-2.82 Ga, this time may be correlated with the peak conditions (800 + 50°C; 8 ± 1 kb) determined by Sharp and Essene [1] using mineral inclusions in pelitic garnets.…”

“…They are metapelites from the Crescent Lake region from where Sharp and Essene [1] estimated peak metamorphic conditions of 800+50°C and 8_+ 1 kb. Previous geochronology in the Wind River Range has focused upon the late Archean granitoids for which whole-rock ages between 2670 and 2550 Ma have been reported [10,20].…”

“…They are petrographically indistinguishable and have an assemblage of biotitecordierite-garnet-K-feldspar-plagioclase-quartz with inclusions of sillimanite in the garnet. Inclusion assemblages in SW 121 were used by Sharp and Essene [1] to infer peak metamorphic conditions of 800_+50 °C and 8_+1 Kb. Monazite occurs in this sample either as inclusions in garnet, or as part of the groundmass in association with quartz, plagioclase and biotite.…”

“…[uc] * Sharp and Essene, 1991. uplift of metamorphic rocks during orogenesis. This development has been made possible by improved techniques for isotopic chronometry of metamorphic minerals applied in conjunction with thermobarometry of metamorphic assemblages.…”

“…This development has been made possible by improved techniques for isotopic chronometry of metamorphic minerals applied in conjunction with thermobarometry of metamorphic assemblages. Several recent studies have used Sm-Nd [1,2], Rb-Sr [2,3] and U-Pb [2,5] isotope systematics of garnets to date their growth, thereby allowing calculation of burial, heating, uplift and cooling rates along metamorphic P-T paths. These studies have amply demonstrated that the key information may be preserved only on the sub-millimetre scale, but so far little use has been made of spatially resolved techniques of isotope analysis such as secondary ion mass spectrometry (SIMS) for minerals other than zircon.…”