Contributions to the mineral chemistry of Hawaiian rocks

Detailed examination of rhyodacite from a new locality on Kauaopuu Ridge, Oahu, Hawaii (the second occurrence of rhyodacite in the Hawaiian volcanic province) yields the following results and conclusions: i) Bulk composition, mineral content, texture, and geographic location indicate that the newly found rhyodacite is part of the Mauna Kuwale rhyodacite flow described by Macdonald (1940); ii) Mineral phases include plagioclase (An_53–23), biotite, orthopyroxene, Ti-rich hornblende, titaniferous magnetite, and ilmenite; iii) Groundmass glass, about 75% by volume, is similar in bulk composition to oceanic rhyolite such as that of Easter Island and Iceland; iv) Similarity in composition between rhyodacite groundmass glass, described here, and bulk composition of oceanic rhyolite from Easter Island and Iceland, suggests that the latter could conceivably have formed by crystal fractionation of a rhyodacitic magma.