Cambrian greenstone on Phillip Island, Victoria

Abstract

Outcrops of altered mafic volcanic rocks (greenstone) occur near Watt Point on the south coast of Phillip Island, Victoria. They are unconformably overlain by Tertiary agglomerate and basalt belonging to the Older Volcanics. The greenstone ranges from metabasalt to metadolerite, with less common metacumulate rocks. All show typical greenschist facies alteration assemblages of albite, amphibole, chlorite, epidote and prehnite. The original mafic minerals have not survived alteration. Geochemical data for total Fe, Mg, Ti and trace elements Cr, Ni, Zr, Y, Sc, and V indicate that the Phillip Island greenstone belongs to the low‐K tholeiitic suite which dominates the Cambrian Heathcote and Mt Wellington Greenstone Belts in Victoria. However the Phillip Island greenstone appears to represent a more primitive magma on the fractionation path for the Victorian Cambrian tholeiite suite. The occurrence lends support to a model involving detachment in a near‐continuous basal layer of Cambrian metavolcanic rocks during Middle Devonian deformation of the Lachlan Fold Belt in central Victoria.