MODELLING CLINOPYROXENE-GARNET AMPHIBOLITE ASSEMBLAGES AT MICA CREEK, BRITISH COLUMBIA: THE ROLE OF THE FLUID PHASE

Orthopyroxene-free clinopyroxene (Cpx) + garnet (Grt) + plagioclase (Pl) + quartz (Qtz) assemblages occur in the upper amphibolite facies at Mica Creek, British Columbia. This assemblage in the upper amphibolite facies is inconsistent with predictions from recent thermodynamic datasets assuming that a pure H2O fluid was present. We have modelled this assemblage with a reduced activity of H2O in a fluid saturated rock and with a variable amount of bulk rock H2O content.

Phase diagram sections constructed in the chemical system TiO2-Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O illustrate the effect of H2O activity and H2O bulk rock content on predicted equilibria for a Grt- Cpx- hornblende (Hb)- Pl- titanite (Ttn)- ilmenite (Ilm)- Qtz amphibolite bulk rock composition. The activity of H2O has to be < 0.5 to obtain the assemblage at amphibolite facies conditions. Evidence for a very reduced activity of H2O in a fluid phase is lacking. Isobaric temperature-composition sections (bulk-rock H2O) indicate that at low bulk-rock H2O contents (fluid phase absent), a Grt- Cpx- Hb- Pl- Ilm- Qtz assemblage can develop at amphibolite facies conditions. The models do not predict a stability field for Ttn + Cpx as observed in the rock. A reduced H2O activity section at an H2O activity of 0.5 predicts the stable univariant equilibrium 52 Amp + 48 Ilm + 75 Qtz + 148 Ttn=25 Grt + 10 Pl + 242 Cpx + 196 rutile + 52 H2O from 8.7 - 8.9 kilobars, 700 - 690 C, along which Cpx reacts into and Ttn reacts out of the assemblage for this bulk rock composition. The bulk-rock H2O sections require pressures > 9 kilobars for Ttn stability. The observed modes compared to those predicted by the reduced H2O activity and bulk-rock H2O sections suggest the sections overestimate modal garnet and underestimate modal clinopyroxene.