THE GEOLOGY AND PETROLOGY OF THE MURRUMBIDGEE BATHOLITH

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Abstract
The Murrumbidgee batholith is a composite body cropping out over an area of 550 square miles to the immediate south-west of Canberra, Australia. It intrudes Lower Palaeozoic deposits of the Tasman geosyncline and is probably late Silurian or early Devonian in age. The batholith is an elongated body concordant on a large scale with the regional meridional trend. Nine separate componentshave been mapped, the rock types ranging from tonalite to granite sensu stricto ; the dominant rocks are biotite granodiorites. The rock types can be divided into three groups, namely: contaminated granites sensu lato, uncontaminated granites sensu lato, and leucogranites, probably in that order of intrusion. The uncontaminated granites are believed to represent the parental magma from which the contaminated granites were derived by the assimilation of semi-pelitic and pelitie sediments and, in one component, basic igneous rock. The contaminated granites appear also to have been enriched in some of the minerals which would have been precipitated during the early stages of crystallization. The composition of the leucogranites places the min the residual system of petrogenesis and they presumably represent the residual liquids resulting from the crystallization of the other rock types, the amount of these liquids being enhanced by the salie and less refractory material “sweated out” of the assimilated sediments. The place of the Murrumbidgee batholith in the plutonie history of the Tasman geosyncline is briefly discussed.

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