The metamorphosed mafic rocks of the ophiolitic Highland Border Suite on Arran and Bute are of 2 types: greenschist facies pillow lavas and epidote-amphibolite facies hornblende schists, respectively. Both types have been affected by post-magmatic alteration processes, including spilitization (i.e. Na-enrichment coupled with Ca-depletion). On the basis of 'immobile element' chemistry and other criteria, the mafic rocks are split into 2 (magmatic) groups: a tholeiitic one which shows the effects of extensive low-pressure fractionation (e.g. Fe- and Ti-enrichment), and a less-fractionated alkaline group that contains relict Al- and Ti-rich clinopyroxenes. The tholeiitic rocks are similar to modem ocean-floor or marginal-basin basalts, and the alkaline rocks show affinities with 'within-plate' basalts. This 'magma-type' and 'tectonic-setting' dichotomy is found at other Highland Border localities and is present in both the pillow lavas and hornblende schists. The Highland Border Suite mafic rocks may have formed as tholeiitic ocean-basin crust upon which alkaline ocean-island lavas had been erupted, or alternatively, as is suggested in this paper, they may have formed during the early stages of development of a pre-Grampian marginal basin of Cambrian (or Cambro-Ordovician) age.

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