Abstract
Volcanogenic sediments of the Lower Permian Takitimu Group into the Gladstone Peak area are mapped and sub-divided into seven lithofacies. Some of the lithofacies indicate a shallow marine near shore environment, others indicate subaqueous and possibly subaerial pyroclastic flows. These volcanic sediments are cut by dykes, sills and plugs of predominantly basaltic composition and by a small diorite intrusion. The origin of the diorite intrusion is attributed to fractional crystallisation and flow differentiation. Six rocks have been chemically analysed and their geochemistry is examined. Some of the component oxides (SiO2, Fe2O3, CaO, Na2O) in the volcanic rocks have been remobilized, redistributed and deposited elsewhere as authigenic minerals.
The Takitimu Group rocks and intrusives have been metamorphosed to Zeolite facies and probably Prehnite-Pumpellyite facies in a low pressure type II terrain. There is a contact metamorphic aureole around the diorite intrusion. Some of the zeolites present would appear to require alkaline, mildly saline ground waters, relatively low activity of H2O, low chemical potential of CO2 and a relatively high temperature (ca. 250°C) for their formation. The metamorphism is similar to that which prevails in the Tanzawa Mountains, Japan where the geothermal gradient is inferred to have been between 40-60°C/km.
The area is part of the Princhester Fault Block (new name) which is itself block faulted. It is likely that the dykes have intruded these Permian block faults. The strata in the Gladstone Peak area were later tilted. They characteristically dip gently toward the South-East and are interrupted by minor faults and cross-cutting veins.