The
Hauraki Volcanic Region of the North Island developed between 18 and 1.9 Ma.
Initial andesitic volcanism from 18 to 12 Ma was followed by 12 to 1.9 Ma
volcanic activity that included rhyolitic, dacitic, andesitic, and basaltic
compositions, with a gradual migration of activity to the SSE. Early andesitic
volcanism evolved over the course of 8 m.y. to more silicic compositions; this
trend was reset and repeated by post-10 Ma andesitic volcanism. Early and later
andesites are otherwise remarkably similar in trace element composition.
Rhyolites have the most diverse compositions, suggesting a major role for
crustal input in their formation.