Late Triassic silicic volcanism in the southeast Queensland segment of the New England Fold. Belt is preserved within a complex of co-magmatic granites, rhyolites and ignimbrite within a collapse cauldron, and an adjacent rhyolite dome complex that may be a sag caldera. The collapse cauldron is a 50km by 35km ovoid structure oriented transverse to the regional NNW structural fabric. Basement to the cauldron comprises Palaeozoic meta-sediments of the Coastal Block, and meta-volcanics of the Gympie Group. The cauldron is traversed by the Perry Lineament, which offsets the eastern third of the cauldron 8 km sinistrally. This lineament is a complex series of upright faults that are aligned with the eastern margin of the Middle Triassic Esk Trough, to the south. The cauldron thus occupies a unique location at the intersection of four basement terranes and situated across a regional fault structure.
The rhyolite dome complex predates the collapse cauldron, and comprises three cryptodome complexes. One of these complexes intrudes an ignimbrite sheet, and is interpreted to be a vent area for that ignimbrite. Extrusive rock types within the collapse cauldron include intra-caldera quartz ignimbrite and associated bedded rhyolite breccias, and pre-eruption rhyolite and dacite domes and lavas. The eastern section of the cauldron exposes successively deeper levels within the roof zone of the cauldron into the granite magma chamber that fed eruption. Granite comprises a heterogeneous complex of biotite granite, porphyritic biotite granite, rhyolite and aplite, and peripheral intrusive rhyolite domes. A texturally homogeneous pluton of weakly porphyritic biotite granite is interpreted to be a resurgent granite emplaced soon after eruption. The ring structure of the cauldron is marked by a dyke of porphyritic biotite granite or rhyolite up to 200m in width. A stock of alkali granite straddles the northeastern segment of the ring fracture, and intrudes at least 400m into intracaldera quartz ignimbrite.
Trace element variations within the granite types show the effects of, and constrain possible, petrogenetic processes. Granites, rhyolites and ignimbrites display distinct calc-alkaline isotope and trace element geochemistry, including high Ba/La and Th/Ta and depletions in HFS elements. Strontium isotope data give a calculated age of 221.8±0.6Ma with lsr=0.70357. Chondrite-normalized REE plots show steep LREE and flat HREE profiles and moderate europium anomalies. LaN, ranges from 85-110 for granite and rhyolite, and LaN/YbN from 5-10. δ18O averages 8.0 ±0.5 per mill, and εNd, falls within a narrow range averaging +4.2. The alkali granite displays A-type characteristics and identical isotopic compositions to biotite granite and rhyolite. Incompatible element contents are, however, too high to be produced by fractional crystallisation from any observed biotite granite composition. Ignimbrite and granite intruding the ring fracture display ternary minimum mineralogies, but are enriched in Ba, Eu and alkalies relative to typical granite and rhyolite. These characteristics cannot be explained by simple crystal fractionation, and require enrichment of alkali feldspar in the melt prior to eruption.
The isotope data plot close to the mantle array and within the field of island arc basalts and sub-continental mantle. ISr is more primitive than Late Permian l-type granites of the New England Batholith, but similar to the isotope compositions of Early Triassic andesites which occur throughout the central New England Fold Belt. Batch partial melting models for biotite granites support the proposal that the granites were produced by 20% partial melting of an andesitic crust. The origin of the alkali granite is less clear, but partial melting calculations do not support an origin by second-stage partial melting of the residual andesitic crust. An origin by fractional crystallisation from basaltic magma contaminated by andesitic crust is proposed. Evidence that contamination of basaltic magmas occurred is found within coeval mafic lavas that are exposed outside the boundary of the cauldron, and which erupted between the silicic eruptions of the Gayndah Centre and Mungore Cauldron.
Late Triassic silicic volcanism in the New England Fold Belt, including that preserved within the Mungore Cauldron and adjacent Gayndah Centre, is interpreted to have developed during the transition of the Mesozoic eastern Australian margin from a compressional destructive plate margin to an extensional margin. This period heralded continental fragmentation that culminated in the Cretaceous opening of the Tasman Sea.