Petrology and origin of a Devonian volcanic complex on Arthurs Seat, Mornington Peninsula, Victoria, Australia: multum in parvo

The Arthurs Seat complex on the Mornington Peninsula is central Victoria’s most compact occurrence of Late Devonian silicic volcanic rocks. Nevertheless, this small area contains a remarkable diversity of rock types and is the only known occurrence of I-type ignimbrites in the Melbourne tectonometamorphic zone. The main volcanic rock types are biotite- and hornblende-phyric rhyolite ignimbrites, some of which contain clasts of at least one earlier ignimbrite deposit. The main plutonic rocks are the Dromana Granite and a substantial body of hypabyssal biotite rhyolite. The granite intruded and contact metamorphosed the hypabyssal biotite rhyolite and the ignimbrites. Minor intrusions also occur within the area mapped as the ignimbrites. These include an A-type hornblende–biotite rhyolite that was originally vitreous and distinct from the coarser-grained hypabyssal biotite rhyolite, as well as a hypabyssal biotite–hornblende rhyodacite and a biotite–hornblende dacite. These latter rocks are not recrystallised and probably postdate the I-type Dromana Granite. The most surprising discovery, among specimens held by Museums Victoria, is an S-type sekaninaite-bearing rhyolite ignimbrite. This is only the second known occurrence of sekaninaite phenocrysts in a silicic volcanic rock from Victoria, the other being in the ignimbrites of the Rubicon Rhyolite and the stratigraphically lowermost parts of the Lake Mountain Rhyodacite in the Marysville Igneous Complex. Although there are various mineralogical and petrographic signs of magma mingling, none of the Arthurs Seat magmatic rocks appear to be related to each other by differentiation or magma mixing and, in contrast to all other I-type rocks in the region, the Arthurs Seat I-type volcanic rocks are distinctly sodic and represent an oxidised series with accessory magnetite. This remarkable variability, with I-, S- and A-type rocks present in such a small area, speaks to deep crustal heterogeneity on a variety of scales, probably with the main variations being vertical in orientation. This small area contains a remarkable diversity of rock types, and the magmas were not related to each other by fractionation or mixing processes. It is the only known occurrence of Devonian I-type ignimbrites in the Melbourne Zone. Unlike I-type plutonic rocks in the Melbourne Zone, these volcanic I-types are distinctly sodic and oxidised rocks with magnetite. The area also contains an S-type ignimbrite with sekaninaite phenocrysts and a devitrified A-type rhyolite.