Petrology, structure and geochronology of some high grade metamorphic rocks at Fishery Bay and Cape Carnot, southern Eyre Peninsula

Abstract

Two coastal sections were mapped in detail at Fishery Bay and Cape Carnot, southern Eyre Peninsula, in an attempt to delineate characteristics of the Hutchison Group (Johns 1960),

Structurally, the rocks in both areas have undergone three phases of deformation, resulting in a folded layering, which has also been crenulated. The lithologies are believed to represent a series of meta-sediments which have been intruded by basaltic dykes (quartz tholeiites in composition). At Cape Carnot an intrusion of an anatectic granite probably took place prior to the basaltic intrusions. A younger intrusion of pegmatites has subsequently occurred, but prior to this intrusion the sequence had been metamorphosed to the pyroxene granulite sub facies (Turner 1968). It is proposed that the intrusion of pegmatites was accompanied by retrogression to the hornblende-pyroxene subfacies.

A comparison of the mineralogy observed with a compiled pressure-temperature plot of experimentally determined curves placed the temperature of the prograde granulite facies metamorphism at 800 to 1000°C and the pressure at 8 to 10 Kb.

A detailed Rb-Sr whole rock geochronological study of the augen gneiss, layered gneiss and leuco-gneiss at Cape Carnot failed to produce a satisfactory isochron. However the age of these gneisses does appear to lie in the range 2100 to 2850 Ma and several alternative explanations of the data have been proposed.