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.

KEYPOINTS

1. The Woolamai Granite is a highly fractionated, high-K syenogranite and is probably the youngest Late Devonian granitic rock in Victoria.

2. Geophysical interpretations and the occurrence of granite on Pyramid Rock suggest that the intrusion may have batholithic proportions.

3. The occurrence of brown biotite and pink K-feldspar suggests that the Woolamai Granite is an I-type rock, an inference consistent with its young age within the region.

Keywords:

• granite
• Cape Woolamai
• Woolamai Granite
• Phillip Island
• petrology
• S-type
• I-type
• mineralogy; stock; batholith

Acknowledgements

Sample E3219 was provided by Museums Victoria, and thanks also go to Ben Healley of Museums Victoria, for producing the macrophotograph in Figure 6. David Moore clarified the method by which Moore and Armit (2024) arrived at the extent of the Woolamai Granite in the map shown here as Figure 3, and also provided an edited version of Moore and Armit (2024, figure 1), which forms the basis for Figure 3. We thank Fritz Finger and an anonymous reviewer for their valuable comments and suggestions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

All data used here are contained within the paper, its tables and figures.