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.
Bibliographical noteJ. Bowie, C. Brolly, J. Armstrong and J. Johnston provided skilled technical support. Electron microscopy was performed with the help of J. Still in the ACEMAC Facility at the University of Aberdeen. Scapolite was analysed on a sample from the Hunterian Museum, Glasgow (no. 134720), loaned courtesy of J. Faithfull. The work was supported in part by UK Natural Environment Research Council grant NE/M010953/1. Careful review by A.A. Cabral helped to improve the manuscript.
Data Availability StatementSupplementary material
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- RICH SCAPOLITE
- FLUID INCLUSION