Eglinton Volcanics—stratigraphy, petrography, and metamorphism

Abstract

The Eglinton Volcanics are mapped as two lithologically distinct terranes separated by the Mistake Diorite of the Mackay Intrusives. The eastern (Plato) terrane which is divided into four formations consists largely of marine deposits, including an Atomodesma limestone member. It is a steeply dipping east-younging sequence of basic to intermediate volcanogenic sediments intruded by abundant basaltic dikes. Turbidites of volcaniclastic detritus are common but pillow lavas and flow rocks are not recognised. Fossils from two new localities include brachiopods and bivalves which confirm the Early to Middle Permian age of these rocks established by Landis & Waterhouse. Rocks of the western (Largs) terrane are mainly of andesitic to dacitic composition and predominantly volcaniclastic, although massive andesite bodies are mapped at either end of the terrane. Fossils, limestones, and turbidites are unknown whereas strongly hematitic rocks, restricted to minor occurrences in only the lowermost strata of the Plato terrane, are common. Andesitic dikes are widespread throughout the terrane, but several distinctive Plato dike lithologies are entirely absent in Largs rocks. The Largs terrane may be partly terrestrial in 'Origin. Because of the absence of fossils its age is unknown. It is structurally more complex than the Plato terrane. Formal stratigraphic subdivision is not proposed and its stratigraphic relations to other rocks are unknown. Metamorphic grade increases essentially from east to west across the Eglinton Volcanics and three metamorphic zones are mapped. The lowest grade zone (zone I) includes sporadic prehnite and pumpellyite, zone II is characterised by epidote and actinolite, and authigenic biotite occurs in zone III. The Eglinton Volcanics are considered to have undergone burial metamorphism during Permian times and regional metamorphism during the Rangitata orogeny. However, the relative effects of these two events are unknown, apart from in zone III where the relatively high metamorphic grade reflects proximity to Mackay plutons.