The stable isotope (C,O,S) record of Palaeoproterozoic marbles, Scotland

Abstract

Palaeoproterozoic marbles occur widely in North West Scotland. The isotopically heavy carbonate carbon (δ 13 C >3 ‰) in marbles that characterizes the worldwide Lomagundi-Jatuli Event (2.3 to 2.05 Ga) is recognized in each of the Laurentian Foreland, the Moine Nappe and the Sgurr Beag Nappe, over a 150 km transect across the Caledonian thrust belt. A light oxygen isotope composition distinguishes marbles which have been sheared and retrogressed by ingress of meteoric water, possibly during both Laxfordian and Caledonian orogenesis. The shearing of marbles also contributed to graphite formation (mean δ 13 C -7.2 ‰). Pyrite in the marbles contains isotopically heavy sulphur, typical of Palaeoproterozoic diagenetic sulphides precipitated from low-sulphate seawater. These data show that the ∼2 Ga marbles in Scotland are a high-quality archive of information on their depositional and post-depositional history. The data emphasize a continuum of a Palaeoproterozoic marble-graphite-sulphide bearing assemblage from Eastern Canada and Greenland through Scotland to Scandinavia.