Polythermal metamorphism of limestones at Kilchoan, Ardnamurchan

Abstract

Summary Jurassic limestones occurring in a screen between two ring dykes are shown to have undergone two periods of thermal metamorphism; the first characterized by a high ratio of CO 2 :H 2 O and the second by a high ratio of H 2 O:CO 2 in the vapour phase. The first metamorphism produced minerals characteristic of Bowen's decarbonation series. Some of these minerals became unstable in the second metamorphism and (OH, F) bearing minerals were produced. A stage of fracture-controlled replacement veins with mineral assemblages which indicate a varying CO 2 :H 2 O ratio in the vapour and a general falling temperature sequence terminate the metamorphic history. Three new minerals, rustumite (Ca 3 Si 2 O 7 .Caterminate (OH) 2 ), dellaite (Ca 12 Si 6 O 22 (OH) 4 ), and kilchoanite (Ca 3 Si 2 O 7 ), of which the last has previously been described, are characteristic of the second episode of metamorphism.