The metamorphic environment of the Foyers Granitic Complex

Abstract

SynopsisIn rocks assigned to the Central Highland Division north-east of the Foyers Complex, a Cordierite-K feldspar Zone is developed at the highest grades of contact metamorphism. Outside this zone, the muscovite + quartz/sillimanite + K feldspar transition is ill-defined because muscovite tends to have been consumed by a lower-grade garnet-resorbing reaction, in response to reduced pressure. Within the Cordierite-K feldspar Zone garnet was equilibrated with cordierite and biotite at c. 650–680°C. Cordierite + K feldspar coexistence at these temperatures requires water activities far below the water-saturated condition aH2O = 1. The Hold-away-Lee treatment of biotite + quartz + cordierite + garnet + K feldspar + sillimanite gives aH2O ≈ 0·15, and a pressure of approximately 3·9 kbar. The unusually low aH2O permits cordierite + K feldspar coexistence without melting, which is consistent with the scarcity of contact-migmatitic phenomena other than intrusive veining. Low aH2O is attributed to a combination of protracted cooling from the regional metamorphic climax and general rarity of muscovite. In comparison with the Strontian complex, the (drier) metamorphic environment at Foyers supports geochronological arguments that the two complexes are distinct. The difference in aH2O may bear on structural differences between the two areas.