The geochemistry of Lewisian marbles

Abstract

Summary To rectify the dearth of published petrographical and analytical data for Lewisian marbles, 307 samples, from nearly all outcrops known, have been examined in thin section, and 114 samples analysed for 35 elements. Lewisian marbles can be divided texturally into: 1) macrocrystalline (generally white, pure to silicate-rich) eg from Glenelg; 2) microcrystalline (often coloured, ornamental, pure to silicate-rich), eg from Iona; 3) microbrecciated (grey, crushed, fine-grained, silicate-opaque-rich), eg from the Loch Maree Group (LMG). Lewisian marbles are geochemically typical of ancient carbonates, tending to have lower contents of trace elements (notably Sr) than younger ( eg Scottish Dalradian) marbles. Whereas all analyzed marbles from the South Harris and Coll foreland outcrops and from the Glenelg and Kintail inliers are dolostones (%MgO 15–20, %SiO 2 5–15), those of the eastern inliers (Glen Strathfarrar, Shinness) are pure limestones (%MgO < 10, %SiO 2 < 10). Elsewhere, limestones and dolostones are intimately associated or interbanded ( eg Scardroy inlier). Microbrecciated LMG marbles (Flowerdale belt of Gairloch; Furnace belt of Loch Maree) have notably higher chalcophile and siderophile element contents than macrocrystalline marbles in adjacent belts (Cloiche of Gairloch; Gleann Tulacha of Loch Maree). Aside from such local differences, however, limestones and dolostones of the inliers, LMG and remaining foreland reveal few non-trivial differences in composition. Moreover, Lewisian marbles collectively show strong Pearson and Spearman correlations between elements concentrated in accessory phases ( eg Y-La-Ce, K-Rb-Al, Fe-Co-V). Given established secular variations in the chemistry of global carbonate rocks, these relationships between both marble groups and chemical variables are difficult to explain if some Lewisian marbles ( eg S. Harris) represent late Scourian, and others ( eg LMG) early Proterozoic sediments. They may provide a tentative hint that most Lewisian marbles were deposited contemporaneously.