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

Abstract

Paleoproterozoic marbles occur widely in NW Scotland. The isotopically heavy carbonate carbon ( δ 13 C >3‰) in marbles that characterizes the worldwide Lomagundi–Jatuli Event (2.3–2.05 Ga) is recognized in 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 that 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 sulfur, typical of Paleoproterozoic diagenetic sulfides precipitated from low-sulfate seawater. These data show that the c. 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 Paleoproterozoic marble–graphite–sulfide-bearing assemblage from eastern Canada and Greenland through Scotland to Scandinavia.