Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.
New chemical data from quartz syenite xenoliths and their silica-oversaturated host rocks from the Azores and the Canaries suggest that the nodules are cognate. Silica-over-saturated volcanic rocks on Atlantic islands may be derived by differentiation from basaltic melts. The Canaries, Madeira, and Azores show no simple systematic geochemical differences with respect to distance from the Mid-Atlantic Ridge. Each group contains highly over-saturated rocks and shows major inter-island differences.
The relation between geochemical trends in a volcanic island and its tectonic setting is complex and should be evaluated by a) using several geochemical parameters, such as degree of silica saturation, alkali content, alkali ratios and others; b) recognizing the state of magmatic evolution of an individual island, rather than treating an island group as a homogeneous unit; c) specifying type of tectonic setting that may have caused or influenced generation of a particular magma type, such as melting spot, fracture, and subsidiary rift, apart from the Mid-Atlantic Ridge; d) combining mantle parameters such as depth of generation (pressure) and degree of partial melting with others such as zone refining, rate of ascent and processes in shallow magma chambers.
A brief attempt to interpret the three island groups is hampered by inadequate knowledge of the tectonics and details of stratigraphy, petrology, and age data. Integrated studies are needed to obtain data from islands that have had historic eruptions and are located at different distances from a spreading center or differ in tectonics. The Azores appear most favorable for such a study.