UNRAVELING 100MA OF VOLCANIC ACTIVITY IN NORTHLAND NEW ZEALAND

The purpose of this study is to compare the composition of the Miocene to Late Cretaceous mafic volcanic rocks on the North Island of New Zealand. It is known that there are continental arc, intraplate, and island arc volcanic rocks present on the North Island, yet there are unclear field relations between all of these different events. Samples were mostly taken from quarries, ranging from just north of Auckland to the far north tip of the island, near Cape Reinga.

• Late Cretaceous continental arc samples were taken from Kerikeri and Whatuwhiwhi. In general the continental arc basalts are porphyritic with plagioclase and olivine crystals. The sample taken from Whatuwhiwhi was cryptocrystalline pillow basalt taken on the west side of Parakerake Bay.

• Miocene intraplate samples were taken from Te Hapua and Picadilly. These samples varied in grain size, but both contained plagioclase. The Te Hapua sample was coarse and contained veining and mafics, while the Picadilly sample was fine grained and contained vesicles and olivine.

• Late Cretaceous island arc samples were taken from Pukepoto, Lamar Road, Te Paki, and Te Kao. Generally the island arc basalts are fine to very fine grained, with plagioclase crystals present and chlorite in the groundmass. Most of the samples had either small white veins or zeolites present in veins or along joint surfaces.

• Miocene volcanic arc samples were taken from Waipua and a dike at Pukepoto. The Miocene samples are pyroxene basalt, with pyroxene and plagioclase present.

Hopefully, ongoing petrographic and geochemical analysis will help further differentiate between the different volcanic events.