Metamorphic and post‐metamorphic fluid flow in the low‐grade rocks of the Harlech Dome, north Wales

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low‐grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H₂O‐rich and CH₄‐rich fluids coexisted. Late‐stage inclusions are of calcium‐rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz‐oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end‐Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of ¹⁸O‐enriched calcites and micas. In the case of vein minerals it is possible that these crystallized directly from late‐stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re‐equilibrated with later ¹⁸O‐enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re‐equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late‐stage fluids observed as inclusions.