Abstract

About 5 km of probable lower Cambrian metamorphosed doleritic and gabbroic sills and overlying submarine volcanics occur in the Dalradian Series of the SW. Scottish Highlands.

An altered, soda-rich group of spilitic affinity, often accompanied by epidotic segregations, is readily distinguished from unaltered basaltic compositions using chemical and modal criteria.

The metabasite suite, of strongly tholeiitic affinities, has a complex differentiation history; evolution towards extreme enrichment in Fe, Ti and P, and depletion in Ni and Cr, is achieved in part by fractionation at greater depth, and partly by differentiation within the thicker sills. Excepting rare quartz kerato-phyres, salic and felsic differentiates are lacking. Consideration of the behaviour of Ti, P, Zr and Y indicates a transitional nature between continental and oceanic tholeiites, and also derivation of the lowest lavas by tapping of the upper portions of underlying differentiating ferrogabbro sills.

Development of this thick sill-flow complex, possibly correlative with the broadly contemporaneous tholeiitic flood basalts and dyke swarms of NW. Newfoundland and Labrador, may reflect continental distension and incipient opening of a Proto-Atlantic ocean in lower Cambrian times.

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