The K2O : Na2O ratios of ancient volcanic arcs in New Zealand

Abstract

Ratios of K₂O : Na₂O in Permian and Cretaceous basic igneous rocks of New Zealand show that two distinct belts of volcanic rocks can be defined in both the Permian and Cretaceous. In the Permian, a western belt, characterised by high K₂O : Na₂O ratios, can be traced along the western margin of the Southland — Key Summit and Nelson synclines, and an eastern belt, with low K₂O : Na₂O ratios occupies the eastern margin of these synclines. Close agreement of ratios between the Brook Street Volcanics and Eglinton Volcanics of the western belt, and the Lee River Group and Livingstone Volcanics of the eastern belt, supports correlation of these groups across the Alpine Fault. In the Cretaceous, a western belt, with high K₂O : Na₂O ratios can be traced from Mount Somers — Malvern Hills and Mandamus, to the Awatere Valley and Inland Kaikoura Range. An eastern belt extends from the Kaikoura coast through the eastern Wairarapa to East Cape. Comparison with present-day volcanic arcs shows similarities in the increase in K₂O : Na₂O ratios in going from the oceanic to the continental side of the arc. This supports the concept of a continental land mass to the west of New Zealand during the Permian and Cretaceous, and the existence of oceanic troughs to the east.