23 May 1973 Volcanic Studies Group Meeting

The geochemistry of volcanic rocks relative to their tectonic setting

Trace elenlent geochemistry related to the tectonic environment of basalt eruption J. R. Cann & J. A. Pearce, University of East Anglia.

Recent investigations have shown that there is a good correspondence between the trace element geochemistry of basic and intermediate volcanic rocks and the tectonic environment of their eruption. Major elements, with the possible exception of Mg, show less discrimination between tectonic settings (Ti and K are counted here as trace elements). The following tectonic-cure-chemical classes of basalts and basaltic andesites can be distinguished from one another: ocean-floor basalts (characteristic of relatively fast-spreading plate boundaries); low-potassium tholeiites, calc-alkaline volcanics and shoshonites of island arcs; and within-plate basalts, of which alkalic and tholeiitic varieties may be distinguished.

There is some overlap for almost all elements yet investigated between some ocean-floor basalts and some low-potassium tholeiites of island arcs, but the other classes are reasonably distinct except where transitional varieties of volcanic rock are found. The tectonic classification of environment of eruption cannot be considered final, especially as within-plate basalts also appear too to be associated with slowly-spreading plate boundaries, and because of the possible tectonic importance of plumes rising from the lower mantle.

The petrogenetic processes leading to these geochemical differences are far from clear but it seems that the variation observed can be explained by a model involving migration of a hydrous, incompatible-rich fluid in the mantle, with consequent changes in phases on the basalt liquidus.

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