P-T PATH OF SHORT-LIVED NEOARCHEAN GRANULITE FACIES METAMORPHISM OF THE WYOMING CRATON

Partial melting of the continental crust is an important process for crustal stabilization, driven by tectonic processes. Understanding partial melting of Archean continental crust is thus paramount to understanding Archean tectonics and how stable cratons form. The Beartooth Mountains in MT/WY, USA expose Archean rocks of the Wyoming Craton and are dominantly comprised of a ~2.8 Ga granitoid batholith known as the Long Lake Magmatic Complex (LLMC). Contained within the LLMC are roof pendants of metasedimentary granulites, which have previously been interpreted to result from contact heating with the LLMC. However, field evidence and Sm-Nd grt dates suggest that high temperature (HT) metamorphism of the roof pendants post-dated LLMC emplacement. Diffusion modeling of major element zoning in grt suggests that residence at peak T was brief (probably <2 Myrs).

Phase equilibria modeling accounting for an evolving effective bulk composition (EBC) reveals the P-T path of this metamorphism. Grt from a residual pelite shows inclusion rich cores with relatively inclusion-poor rims, coincident with a sharp, step-like increase in Ca. These high-Ca grt overgrowths are interpreted to record peritectic grt growth during heating via biotite and plagioclase breakdown melting, based on petrographic relationships. Pseudosections accounting for fractionation of material into grt crystal cores suggest formation of these high-Ca overgrowths at 6.5 kbar, 760-800 ˚C. A pseudosection for the matrix composition (i.e. excluding the composition of garnet crystals) constrains P-T conditions for the end of grt growth at 5.5 kbar, 780-800 ˚C, suggesting that the preserved Ca zoning in grt rims records continued grt growth during ~1 kbar of near-isothermal decompression. P-X_melt pseudosections show that: 1) this grt growth can occur during near-isothermal decompression if melt is extracted, and 2) this melt loss has little effect on the Ca content of grt. Grt rim Fe and Mg contents are consistent with exchange during relatively rapid subsequent cooling. Together, these results define a relatively tight, clockwise P-T path marked by rapid HT grt growth during both heating and subsequent near-isothermal decompression.