Spherulitic rhyolite dike from Goat Island, southeastern Alaska

Spherulites, as much as 4 mm in diameter, locally constitute almost 50 percent by volume of a dark-red rhyolite dike that intrudes porphyritic pyroxene gabbro along the northeast coast of Goat Island, 3.2 km northwest of Hydaburg, southeastern Alaska. The largest spherulites are composed mainly of o-quartz, whereas the smallest consist of a microcrystalline intergrowth of tridymite and alkali feldspar. Textural relations indicate that the radial and concentrically zoned structures are the products.of devitrification of glassy lava. The mineralogy of the dike is similar to that of other spherulitic rhyolites except that quartz occurs to the exclusion of cristobalite. The posteruptive formation of the spherulites has also resulted in alkali transfer, the groundmass of the dike being enriched in both sodium and potassium relative to the spherulites.