Dikes and the petrology of Waianae Volcano, Oahu

Abstract

We summarize the distribution, orientation, and composition of the dikes of Waianae Volcano and attempt to relate this summary to the evolution of the volcano. Dikes are located in the caldera, in a well‐defined northwest rift zone, and in less well‐defined south and northeast rift zones. The active axis of the south rift zone may have shifted with time. The dikes generally are parallel to their rift zones and are oriented in many directions in the caldera area. They commonly dip about 70° to 80° and are generally less than a meter thick. Most are virtually identical in texture, mineralogy, and composition to nearby extrusive rocks. A few have unusual low‐magnesium silicic compositions not yet observed in flows; these may be classified as icelandites. Trends defined by the compositions of tholeiitic dikes are similar to, but extend further than, crystal fractionation trends of tholeiitic flows of Waianae and other Hawaiian volcanoes. The basalts can be divided into a more silica saturated strongly tholeiitic subgroup and a less saturated mildly tholeiitic subgroup. The icelandites lie on trend with the second group. Most of the analyzed rift zone dikes are strongly tholeiitic; most of the analyzed caldera dikes are mildly tholeiitic; all of the differentiated high‐silica low‐magnesium dikes found are inside the caldera near the eruptive center.