Geology and field relations of the Wilsons Promontory batholith, Victoria: multiple, shallow-dipping, S-type, granitic sheets

ABSTRACT

The 1200 km², Early Devonian (395 Ma) Wilsons Promontory batholith is a post-tectonic, high-level, composite body of S-type granites exposed on Wilsons Promontory and its offshore islands. Four plutons and six members are mapped and described. The rocks commonly contain magmatic garnet and cordierite, in addition to biotite, and biotite–quartz pseudomorphs after orthopyroxene. Planar fabrics abound in the batholith, which is characterised by emplacement of shallow-dipping granitic sheets, on a variety of scales. Particle size and density separation occurred during magma flow, and produced a wide variety of structures including layering, pipes and whorls rich in mafic minerals, K-feldspar phenocryst alignments and a notable swarm of enclaves. Local filter pressing may have played a role in the production of accumulations of K-feldspar crystals and the formation of late, tourmaline-bearing leucogranites and quartz veins. Batholith zonation and the distribution of component plutons are inferred to have been formed through sequential intrusion of separate magma batches rather than in situ differentiation. Overall, the batholith appears to consist of saucer-shaped plutons, and it is tilted gently to the east.

KEYWORDS:

• Wilsons Promontory
• batholith
• episodic intrusion
• sheet-like intrusion
• enclave swarm
• mineral segregations
• magmatic structures
• geological map

Acknowledgements

This paper is based on the observations and mapping carried out by GLW, in connection with his MSc thesis at Monash University; he wishes to make the following statement:

‘This paper is dedicated to Joan Margaret Wallis (née Hindle). The original research basis of this paper would not have been possible without the long-term and selfless support of my late wife Joan. I will forever be indebted to her perseverance as a field assistant and proof reader but also for her enthusiastic support and sacrifices made for my studies over several years.’

JDC acknowledges financial support from the South African National Research Foundation's former scheme of Incentive Funds for Rated Researchers and a travel grant from the University of Stellenbosch Division of Research and Development. Nick Petford provided counsel on issues relating to the physics of magma flow and Arthur Day suggested some improvements to figures. GLW acknowledges the assistance of the National Parks Service of Victoria for provision of a research grant, rangers and other staff of the Wilsons Promontory National Park for valuable assistance, and the National Parks Service, Melbourne University Department of Botany, and the Department of Transport for accommodation and provision a helicopter for aerial photography. GLW's travel to the islands was facilitated by several local people, and the Technical Division of the Victorian Education Department provided part-time study leave (from his then employment). GLW's MSc project was supervised jointly by the late Dr Vic Wall and by Dr Ian Nicholls. The geological results presented here will be followed by a paper (in preparation) that deals with the petrology and petrogenesis of the Wilsons Promontory granitic rocks. These further research activities are being facilitated through the grant of a research permit from the Victorian Department of Environment, Land, Water and Planning.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary paper

Appendix 1: Detailed descriptions of the plutons and members within the Wilsons Promontory batholith.