THE MAGMATIC HISTORY OF THE MT. HERBERT VOLCANIC GROUP AND THE AKAROA VOLCANIC COMPLEX, BANKS PENINSULA, NEW ZEALAND

Banks Peninsula, situated southeast of Christchurch, New Zealand, is comprised of a sequence of volcanic rocks erupted 5.8-12 million years ago in the center of the Zealandia tectonic plate. From oldest to youngest the rocks include: Lyttelton Volcanic Complex (VC), Mt. Herbert Volcanic Group (VG), Akaroa VC, and Diamond Harbour VG. Twenty samples were collected from the co-erupted Mt. Herbert VG and Akaroa VC and detailed petrologic and geochemical studies of these rocks, along with a compilation of existing data, are used to interpret their origins.

Samples analyzed fall on the mafic end of the spectrum for rocks of the peninsula on a standard alkali-silica diagram, classifying as tephrite, alkali basalt, and hawaiite. Mg#s range from 60-38 and samples are all LREE enriched relative to primitive mantle abundances. Slight positive Nb, Ta, and Sr anomalies and a small negative K anomaly are revealed on extended REE plots. Phenocrysts include plagioclase (An_50-70 ), olivine (Fo_50-70), clinopyroxene, titanomagnetite and ilmenite. These minerals also occur as groundmass phases, along with rare alkali feldspar.

This new data has been integrated with previously published geochemical data to produce a comprehensive database for the entire peninsula. Trends within the Mt. Herbert VG and the Akaroa VC suggest fractional crystallization is primarily responsible for the geochemical differences between the volcanic products observed. An evaluation of whole-rock geochemical trends for the four volcanic groups show patterns of magmatic evolution that are nearly identical for Mt. Herbert and Akaroa, but different for the older Lyttelton VC and the younger Diamond Harbour VG. From this comparison, it appears that three distinct magma generation events must be accounted for in any future model for the origin and evolution of volcanism at Banks Peninsula.

Trace element data from each of the four groups is consistent with an intraplate setting for volcanism. Uniformly LREE enriched patterns are consistent with partial melting of an enriched mantle source. The details of this mantle source remain unclear, as a progression of a pyroxenite-type source for the older Lyttelton VC to a peridotite-type source for the late stage Akaroa VC suggested by previous workers requires re-evaluation when a more comprehensive data set is considered.