Assembly of a zoned volcanic magma chamber from multiple magma batches: The Cerberean Cauldron, Marysville Igneous Complex, Australia

The Late Devonian (374 Ma) Cerberean Cauldron forms the northern part of the Marysville Igneous Complex, in Central Victoria, Australia, filled with around 900 km(3) of intra-caldera ignimbrites. The basal volcanic formation is the rhyolitic high-Al Rubicon Ignimbrite, overlain by a larger volume of crystal-rich rhyolitic low-Al Rubicon Ignimbrite. which grades upward into the voluminous. rhyodacitic Lake Mountain lgnimbrite. The rocks are S-type in character, with initial Sr-87/Sr-86 around 0.709 to 0.710 and epsilon(Nd)t varying from -4.7 to -6.0. suggesting metagreywacke protoliths. The chemistry of the volcanic rocks is incompatible with formation by a differentiation mechanism. Experimentally determined phase relations of a low-Al Rubicon lgnimbrite and a Lake Mountain Ignimbrite show that early crystallisation of the Lake Mountain magma began at >450 MPa and at >875 degrees C (possibly up to 940 degrees C), with an initial magma H2O content of 4.1 to 53 wt.%. In the pre-eruption magma chamber, the Rubicon Ignimbrite magma had a temperature of 78 degrees CC and contained wt.% H2O. Each formation, and indeed smaller volumes of rock, appears to have been produced by partial melting of slightly contrasting greywackes in a protolith with spatial variations in its chemistry and mineralogy, with the magma delivered in batches to a high-level chamber. The Rubicon Ignimbrite magmas underwent some internal differentiation, probably by crystal settling, prior to eruption, and variations in the Lake Mountain lgnimbrite are most probably due to small but variable degrees of peritectic phase entrainment. The limited gradation between the Rubicon Ignimbrite and Lake Mountain Ignimbrite is due to minor, pre-eruption mixing across the magma interface. Such limited mixing between individual magma batches appears typical of anatectic granitic magmas. (C) 2012 Elsevier B.V. All rights reserved.