Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms

Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea D acite, a widespread (12,000 km(2)), thick (>200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia . Throughout its vast extent, the Yardea Dacite shows typical lava-like fea tures, in that it is massive, columnar jointed and evenly porphyritic with 30-40% crystals in a spherulitic and granophyric groundmass. In addition, n ow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clas ts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of be ing confined to a particular stratigraphic level in the dacite. The most ab undant and largest megablocks are granitoids derived from older basement an d from early-crystallised plutons of the Hiltaba Suite, which is broadly co eval and comagmatic with the GRV. The granitoid megablocks have been partia lly melted, most Likely prior to eruption when resident in the thermal aure ole of the Yardea Dacite magma chamber. The lithic megablock occurrences ar e unlike coarse pyro-elastic breccias but are similar in distribution and a bundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma den sity and magma viscosity, we show that the rise rate of the dacitic magma e xceeded the settling velocity of the megablocks, implying that they could h ave been entrained and erupted effusively. All but the largest and least-me lted megablocks would have remained suspended or else settled very slowly i n the dacitic lava during outflow. The rapid rate of magma withdrawal requi red to produce such an extensive felsic sheet could have also triggered dis integration of the thermally stressed wallrock surrounding the magma chambe r, dislodging megablocks that were later entrained and effusively erupted.