Use of Phase Diagram Sections to Constrain the P-T of Garnet Porphyroblast Growth in Amphibolite: An Example from the Nason Ridge Migmatitic Gneiss, WA

Phase diagram sections combined with mineral compositions provide a powerful tool for estimating the pressure and temperature conditions of garnet growth. Isopleths for almandine (Alm), grossular (Grs), and pyrope (Prp) are contoured over the range of garnet stability predicted by thermodynamic modeling. This technique has been widely applied to garnet growth in pelitic rocks. In this example, Theriak Domino models are used to estimate P-T conditions for garnet growth in two amphibolites from the Nason Ridge Migmatitic Gneiss (NRMG). Both samples [07NC247 & 04NC106] contain Bt + Grt + Hbl + Pl + Qtz + Zo + Rt, with minor secondary chlorite after biotite. Samples were collected near the contact of the tonalitic Wenatchee Ridge ortho-Gneiss (WRG). Preliminary model [NaCaKFeMgAlSiH – H₂O in excess] results predict that the observed ‘peak' mineral assemblage should be stable between 550-670 °C, 8-15 kbar [07NC247] and 550-680 °C, 7-14.5 kbar [04NC106]. Garnet core compositions in 07NC247 (Alm=0.632, Grs=0.283, Prp=0.080) constrain temperature for garnet core growth between 570-590 °C and plot within the stability field of the observed assemblage. 07NC247 garnet rim compositions (Alm = 0.563, Grs = 0.287) also plot within the predicted stability field of this assemblage. Rim Prp compositions fall at T greater than the peak assemblage field, this problem may relate to fractionation of bulk composition during porphyroblast growth. Anorthite in plagioclase isopleths plot in the peak assemblage field and constrain the peak pressure. Peak P-T results agree with previous estimates for ca. 88 Ma regional metamorphism in the surrounding Nason terrane. However, P estimates are lower than those for epidote group mineral growth in veins within the WRG. This discrepancy suggests possible epidote growth, and WRG crystallization, at greater depth than the surrounding NRMG. This would require either preservation of two different ages of mineral growth or fault juxtaposition.