Zircon U–Pb data for the Arthurs Seat Igneous Complex (Mornington Peninsula, Victoria): implications for Devonian tectonics and the Selwyn Block

The Arthurs Seat Igneous Complex, on the Mornington Peninsula, has been assumed to be Late Devonian in age, in common with the vast majority of Devonian silicic igneous rocks in central Victoria. New zircon U–Pb isotope data show that the volcanic part of the complex is Middle Devonian, with a sample of a biotite–hornblende rhyolite ignimbrite (AS2) yielding a model 1 discordia upper intercept age of 389.7 ± 4.3 Ma (2σ, MSWD = 1.1). Compared with other I-type rocks in the region, the zircon population contains an unusually low proportion of inherited grains—just 15%. This inherited zircon is not derived from the Paleozoic metasedimentary rocks in the Melbourne Zone and is inferred to have been inherited from the source rocks of the rhyolitic magma. Given that previous work has shown that the initial Sr and Nd isotope ratios indicate magma derivation through partial melting of relatively unevolved rocks, the most likely source of the Arthurs Seat magmas is Cambrian andesitic to dacitic metavolcanic rocks in the Selwyn Block, similar to those that crop out on Phillip Island and at Cape Liptrap. The high-K chemistry of some Arthurs Seat rocks, most particularly the Dromana Granite, indicates the presence of metasedimentary rocks accompanying the meta-igneous source materials. In common with all the Devonian silicic igneous rocks of the region, the main population of inherited zircon in sample AS2 has an age of ca 400 Ma. This shows that there was a widespread episode of deep-crustal, high-grade metamorphism and partial melting during the Tabberabberan Orogeny. However, the absence of Early Devonian silicic magmatic rocks in the Melbourne Zone suggests that magma ascent to upper-crustal levels was hampered by the overall compressive tectonic regime at this time.