Abstract
The occurrence of alkaline volcanism can be found in various environments ranging from rift zones to intraplate settings but is generally missing in subduction zones. However, in the North Island of New Zealand, alkaline and arc-related magmas can be found temporally and spatially associated. Active volcanism in the North Island is represented by the calc-alkaline Taupo Volcanic Zone and the intraplate alkaline Auckland Volcanic Field (AVF) to the northwest. However, during the Plio-Pleistocene, intraplate and arc-related volcanism overlapped within the back-arc, forming both arc-related stratovolcanoes and a dispersed, alkaline monogenetic volcanic field (Okete Volcanic Formation; OVF) that constitute the Alexandra Volcanic Group (AVG). This intercalation suggests a petrogenetic relationship between the formation of intraplate and arc-related magmas, and raises questions about the source of alkaline magmas in the OVF as well as the active AVF to the north.
It is suggested that the AVG lineament, as well as the coevality of intraplate/arc-related eruptions could be the result of a slab-tear allowing asthenospheric flow to rise above the slab providing the source for alkaline magmas. To extend this work and detail the petrogenesis of alkaline magmas in the OVF, we provide additional elemental and isotopic data on basalts and mantle/crustal xenoliths from the OVF and compare their compositions to other alkaline volcanic fields in the North Island.
We aim to evaluate: (1) the mechanisms responsible for the emplacement of alkaline magmas (OVF) within the AVG; and (2) the possible sources of alkaline magmas and the processes leading to time-migration of alkaline magmatism throughout North Island.