PETROCHEMICAL EFFECT OF SIMULTANEOUS COOLING OF ADJOINING BASALTIC AND RHYOLITIC MAGMAS

The characteristics of the Stretishorn composite dike of eastern Iceland are described in terms of variation in the major, minor, and trace element chemistry. The dike is 90 ft wide, consisting of a central rhyolitic core separated from basaltic flanks by narrow transition zones of homogenized hybrid rock comparable to andesite, dacite and rhyodacite. Variations within the marginal basalt suggest a contamination by up to 10 per cent of rhyolite prior to emplacement. This hybrid magma series yields linear variation diagrams for all elements except Na, K, Rb and Th which appear to have been fractionated, Pb and Zn which appear to have been concentrated in the transition zones hydrothermally, and Al, Sr in some rhyolites apparently due to relocation of those elements during the deuteric formation of zeolites. Hybridized xenoliths within the rhyolite have abnormally high Ca and Mn contents associated with the introduction of deuterio calcite and rhodochrosite. The trend of the variation diagrams are similar to those of some calc-alkaline series from orogenic regions. The concentration of thorium in the rhyolite is similar to that in orogenic rhyolites, suggesting that the rhyolitic core is more likely to have resulted from fractionation of a basalt parent magma than by assimilation of marine sediments which usually have very low Th contents. © 1969.