P-T-t Evolution of the Valhalla Complex, British Columbia, Canada

Abstract

The main assemblage in the petitic paragneiss is garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz + ilmenite ruffle. Garnet is feebly zoned with rimward increases in Ca, Mn, Fe and Fe/(Fe + Mg), and a decrease in Mg. Zoning in Fe, Mg and Mn is smooth as a result of diffusion whereas Ca zoning is discontinuous as a result of garnet production by reaction (2) below and slower diffusion. Peak pressures from GASP, GPBQ and GRAIL barometry provide consistent values of 8 1 kbar at 800~ (Fig. 1). A peak temperature of 820 20~ is constrained by dehydration and vapor-absent melting reactions (T quoted at 8 kbar): (1) Ms + P1 + Qtz = Sil + K.fs + Liquid (constrains T > 700~ (2) Bt + Sil + P1 + Qtz= Grt + Kfs + Liquid (constrains T > 7500C) (3) Ms = Kfs + Cm + H20 (constrains T > 800~ (4) at + Qtz = Opx + Kfs + Liquid (constrains T 5 wt.%) are found as inclusions in garnet and yield apparent temperatures of 830 to 655~ which are closure temperatures resulting from Fe-Mg exchange between garnet and biotite. The initial apparent temperature for these inclusions in garnet would have likely been T ~ 870~ and the smaller inclusion re-equilibrated to lower temperature. Following the metamorphic peak at T ~ 820~ minor back reaction of (2) or the similar reaction (5) Bt + Sil + Pl + Qtz = Grt + Kfs + H20 produced garnet rims and matrix biotites that have higher Fe / (Fe+Mg) than at the peak, and produced the observed garnet zoning and matrix biotite in_homogeneity in Fe/(Fe+Mg) and Ti. Garnet core + matrix biotite temperatures range from 800~ which may reflect near peak equilibration, to 1050~ which reflects biotites with higher Fe /Mg than ' peak ' bioti tes. Thermodynamic modeling suggests that reaction (2) or (5) took place over a 25-100~ interval (depending on H20 availability) with the net transfer reaction shutting down at approximately 700~ consistent with the matrix biotite + garnet rim temperatures of 700-750~ The maximum amount of water required is approximately 2 % volume, which may have exsolved from melts produced by (2) during crystallization.