Proterozoic metasediments at Glen Strathfarrar, Inverness-shire, host a deposit of vein-graphite that is unique within the Moinian stratigraphic sequence. Carbon isotopic analysis, Raman microspectroscopy, fluid inclusion studies and volatile gas analysis were used to constrain the origin of the vein-graphite. Carbon isotopic analysis shows that the carbon is heavier than that expected for sedimentary material (δ 13C value of -14.35‰), suggesting that fractionation has taken place since sedimentation. Graphitization of sedimentary carbon in the Glenfinnan Group pelitic gneiss occurred in response to changes in both temperature and pressure arising from a series of regional metamorphic events. Scavenging of carbon by metamorphic fluids generated during amphibolite-grade metamorphism associated with the youngest, possibly Caledonian, event is the most probable source of the vein-graphite at Glen Strathfarrar. Similarities in gas composition with quartz veins occurring in Dalradian rocks show that carbonaceous sedimentary sequences consistently generate fluids rich in CO 2 and N 2, making volatile gas analysis a valuable tool in determining the history of metamorphic fluids. The occurrence of crystalline graphite in the pelitic gneiss at Glen Strathfarrar makes it highly likely that the carbon-rich fluids have been generated from within the Glenfinnan Group sediments rather than from Lewisian rocks at depth. Despite the assertion that graphite is uncommon in Moine rocks, the high ammonium content of biotite in these metasediments and the consistently high N 2 values of fluid inclusions within the siliceous schist and quartz veins suggest that, prior to metamorphism, the sediments contained more organic matter than their present composition suggests.
This work was supported by the EPSRC as grant EP/P501253/1. Dr Emrys Phillips from BGS Edinburgh kindly provided limestone samples from their Scottish archive. Dr Martin Feely from the Geofluids Research Group at the National University of Ireland, Galway, is thanked for assistance with petrography and microthermometric analysis of fluid inclusion wafers. Dr Adrian Boyce from SUERC is thanked for providing carbon isotopic data. Dr Richard Wells is thanked for access to the Raman spectrometer at the University of Aberdeen, which is funded by the BBSRC. John and Sue Thomson (Kilmorack Heritage Association), Alex du Toit (Highland Library Archive) and the late Giles Foster (Lovat Estates) kindly assisted with historical research. Two anonymous reviewers are thanked for their comments which significantly improved an earlier draft of this paper.