Seawater signatures in the supracrustal Lewisian Complex, Scotland

Marble in the supracrustal rocks of the Lewisian Complex, Tiree, includes chlorine-bearing amphiboles, chlorine-rich apatite, sulphur-rich scapolite, albite and phlogopite, all of which are regarded as evidence for evaporites in other metamorphosed sequences. Titanite yields U-Pb ages of similar to 1.6 Ga, i.e. late Laxfordian, which excludes a younger imprint of sodium metasomatism. Traces of anhydrite, and isotopically heavy pyrite, also indicate deposition from seawater. Elsewhere in the Hebrides, tourmaline in Lewisian Complex marbles may represent seafloor exhalative deposits. Combined, the evidence suggests Lewisian Complex supracrustal marbles formed in an evaporative environment, like other Palaeoproterozoic successions across the North Atlantic region.

Automated summary

Marble in the supracrustal rocks of the Lewisian Complex in Tiree contains chlorine-bearing amphiboles, chlorine-rich apatite, sulphur-rich scapolite, albite, and phlogopite, indicating the presence of evaporites in other metamorphosed sequences. The U-Pb ages of titanite point to a late Laxfordian age of approximately 1.6 billion years, ruling out a younger imprint of sodium metasomatism. The presence of anhydrite and isotopically heavy pyrite suggests that the marbles were deposited from seawater. Tourmaline in Lewisian Complex marbles elsewhere in the Hebrides may represent seafloor exhalative deposits. Taken together, the evidence suggests that the supracrustal marbles of the Lewisian Complex formed in an evaporative environment, similar to other Paleoproterozoic successions across the North Atlantic region.