MINERAL EQUILIBRIA IN QUARTZ LEUCOAMPHIBOLITES

Pods and lenses of leucoamphibolite a few meters in maximum dimension contain quartz (Qtz) – plagioclase (Pl) – horn-blende (Hbl) – garnet (Grt) – epidote (Ep), with or without biotite (Bt). They occur within Late Precambrian clastic metasedimentary rocks of the Esplanade Range of southeastern British Columbia, and range in metamorphic grade from upper biotite zone to kyanite (Ky) zone (defined in pelitic rocks). Mineral equilibria using the database of Berman provide estimates of metamorphic pressure (P), temperature (T), and fluid compositions. These estimates can be compared to those from coexisting pelitic rocks to provide a check on consistency. Grt–Bt Mg–Fe exchange geothermometry suggests temperatures ranging from 530 to 580°C at 7 kbar. The lack of kyanite in leucoamphibolites suggests a maximum P of about 7–8 kbar. Pl–Qtz–Ep–Grt assemblages without calcite suggest H2O-rich fluid compositions [generally X(H2O)> 0.8]. For a(H2O) = 1.0, Grt–Ep–Pl–Qtz– H2O equilibria produce calculated temperatures near 565°C at 7 kbar. Although there is considerable overlap, estimates of P and T from the leucoamphibolites do not agree very well with estimates from nearby pelites. On the basis of natural occurrences, other investigators have suggested a miscibility gap and near-constant activity of anorthite over the range An40–90. Electron-microprobe spot analyses in our study indicate that plagioclase compositions near ~An40 and ~An90 occur within single grains and within single samples. Other samples have compositions concentrated near either An40 or An90 or compositions with An≤40. These results are consistent with petrological studies reported by other researchers