The petrology and alteration of tertiary basalts of the coalgate area, Northwest Canterbury

Abstract

Basaltic volcanism in the Coalgate area spans a period from lowermost Eocene to Pliocene times. The oldest basalt rocks emplaced are olivine dolerite dykes of probable Paleocene age. The main period of activity commenced in the lower Miocene with hyaloclastic eruptions of olivine-augite basalt with strong alkaline affinities (Burnt Hill Ash and Sand Pit Tuff). Subsequently intrusives, extrusives, and breccias rich in labradorite ,and of high alumina character were emplaced (Bluff Basalt). The youngest igneous rocks in the area are three tholeiitic flows — the Harper Hills Basalt. Alteration of tuffs to iron-rich montmorillonite under freshwater and heavy leaching conditions supports arguments previously advanced for the basaltic nature of the parent material of the Coalgate Bentonite and the processes involved in its montmorillonitisation.