Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage

Abstract

The occurrence of volcanic basanite cobbles in Pleistocene terraces at Galloway and in the upper Clutha valley in Central Otago, in an area devoid of known volcanic edifices, has implications for Cenozoic intraplate volcanism and Pleistocene drainage in the region. We present petrographic, mineralogical, whole rock geochemical and Sr-Nd-Pb isotopic data, as well as an 39Ar-40Ar date of 24.4 ± 0.06 Ma, together with mineral analyses of enclosed peridotite xenoliths. We conclude from this dataset that these alkaline basanite cobbles are most likely derived from the Alpine Dike Swarm in northwest Otago, probably in the Lake Wanaka-Hawea-Luggate area, which extends the volcanic field east of its known extent. The pilotaxitic textures and presence of intersertal glass, along with the volume of basanitic material, suggests that the source rocks were likely effusive, which would make this the first such known component of the Alpine Dike Swarm and requires emergent land at that time. The occurrence of the Galloway basanite cobbles implies that Pleistocene Clutha River flowed through or over what is now the Dunstan Range. The young (late Pleistocene) age of all the basanite-bearing gravels means that the basanite cobbles have likely been reworked several times before final deposition.