Petrography and carbonate chemistry of some Dalradian dolomitic metasediments: preservation of diagenetic textures

The Bonahaven Formation of northern Islay, Scotland, despite having undergone high-pressure greenschist facies metamorphism, contains many relics of its diagenetic history. Early diagenetic dolomite textures, calcite and clay mineral cements are well preserved locally. Burial diagenetic compactional and pressure solution features are recognizable together with quartz and feldspar overgrowths and variable styles of replacement by and recrystallization of dolomite, calcite, pyrite, quartz and albite. The imprint of deformation and metamorphism is variable, probably as a result of differences in the degree of pressure solution and the CO₂, content of pore fluid. Typically, some solution-transfer phenomena occur, accompanied by growth of cleavage-defining phengite and sporadic phlogopite. Locally, abundant phlogopite or biotite is present in lithologies with fully developed metamorphic textures. Dolomite shows good preservation of pre-metamorphic textures and minor element zonation, but calcite does not. Metamorphic re-equilibration was insufficient to allow the derivation of maximum temperatures by calcite–dolomite geo-thermometry.