The Woolamai Granite of Phillip Island: S-, I- or A-type, stock or batholith?

Abstract

The Woolamai Granite forms the rugged southeastern cape on Phillip Island in Victoria. Another small exposure off the coast, at Pyramid Rock, to the west, suggests that the pluton has batholithic proportions. The Woolamai Granite is a highly fractionated, high-K syenogranite with a deep, negative Eu anomaly, and its Na2O/K2O plots close to the discrimination line between S- and I-type rocks. Various chemical indices show that the rocks do not have A-type characteristics. Other factors, such as the peraluminous chemistry, the abundance of inherited zircon and the presence of secondary muscovite, are non-diagnostic of magmatic affiliation. On the other hand, the occurrence of brown-coloured biotite in thin-section, the absence of large euhedral, β-habit quartz and the pink colouration of the K-feldspar all suggest that this is an I-type. Previous research, by other authors, yielded a weighted mean U–Pb age for small zircon populations of 360 ± 3 Ma, indicating that this is probably the youngest Late Devonian granitic rock in Victoria. This relatively young age is also consistent with I-type affiliation, since the I-types in central Victoria are systematically younger than the S-types.