Late Caenozoic Shoshonitic Lavas in North-western Viti Levu, Fiji

Abstract

MOST petrologists recognize two fundamental types of basaltic magma—tholeiite and alkaline olivine basalt¹. The ratio of alkalis to silica is consistently higher in alkaline olivine basalts than in tholeiitic (sub-alkaline²) basalts^3,4. Kuno⁵ recognized a third type of magma, high-alumina basalt characterized by high contents of alumina and intermediate alkali, and Hamilton⁶ noted its apparent genetic relationship to the calc–alkaline intermediate magmas of island arcs. Joplin⁷ recognized a fourth type of basaltic magma characterized by high potash content, and proposed the name shoshonite⁸ for the magma series derived from a high-potash parent magma. The potash-rich shoshonites are as alkalic as the soda-rich alkaline olivine basalts, but the ratio of potash to soda in shoshonites is near or in excess of 1.0, whereas the same ratio is 0–5 or less in typical alkaline olivine basalts and their derivatives. Joplin noted that shoshonitic lavas seem to be especially characteristic of newly stabilized or just consolidated orogenic regions. The possibility that shoshonitic lavas might be characteristic of the late Caenozoic volcanic sequence of Fiji was called to our attention by G. A. Joplin and A. J. R. White on the basis of an analysis obtained by V. lyengar and quoted by Rickard⁹. The estimated volume of late Caenozoic shoshonite in Fiji is about 10⁴ km³; therefore the origin of the magma presents a significant challenge to petrologic theory.