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Thirty-six rock units sampled from a 980 m borehole drilled into the island of Sao Miguel, Azores were analysed for major and trace elements. Twenty-four basalts, two trachybasalts, four trachytes, two intrusives, and four pyroclastic units are represented. Electron microprobe analyses of olivine, clinopyroxene, and plagioclase phenocrysts were made.

The chemical and microprobe studies indicate that the rocks are of alkaline affinity. A lack of rocks of intermediate chemical compositions is demonstrated and is considered to represent a Daly Gap for the volcanic flow units. However, the pyroclastic units appear to have chemical compositions intermediate to the trachytes and trachybasalts.

Projections in the CMAS system show a probable equilibration of the basalts at one atmosphere pressure with an enrichment in plagioclase phenocrysts.

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Pages:195
Supervisors: Barrie Clarke