PETROCHEMISTRY AND MINERALOCY OF A K₂O-RICH MAFIC DIKE IN SHODO-SHIMA, KAGAWA PREFECTURE

Abstract

One of the so-called ‘lamprophyre dikes’ of Late Cretaceous age on Shodo-shima Island shows as high as 2.7 wt.% K₂O content and 2.6 K₂O/Na₂O ratio. The major-element composition corresponds to absarokite of the shoshonitic association. Discriminating the magma-type using the chemistry of ‘immobile elements’ and the mineralogy of plagioclase and clinopyroxene phenocrysts, however, revealed that the dike was not originally absarokitic (alkalic), but probably tholeiitic like the other ‘lamprophyre dikes’ nearby. The dike seems to have been altered chemically by alteration.