Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms |
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Authors: | A Garner J McPhie |
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Institution: | (1) Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia e-mail: J.McPhie@utas.edu.au Fax: +61-3-62267662, AU |
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Abstract: | Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea Dacite, a widespread (12,000 km2), 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 features, in that it is massive, columnar jointed and evenly porphyritic
with 30–40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations.
The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered
throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence
of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids
derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic
with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal
aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are
similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using
reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma
exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All
but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava
during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered
disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained
and effusively erupted.
Received: 11 November 1998 / Accepted: 18 April 1999 |
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Keywords: | Megablock Silicic lava High temperature Viscosity Effusive eruption Gawler Range Volcanics Proterozoic |
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