Megabreccia Blocks and Calderas in the Silurian Goobarragandra Volcanics, Southeastern NSW

Kelsie A. Dadd

Key Centre for Geochemical Evolution and Metallogeny of Continents, School of Earth Sciences, Macquarie University, NSW 2109 and Department of Applied Geology, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007

Megabreccias made up of blocks of limestone, interbedded limestone and chert, and siltstone which are embedded in tuff have recently been identified in the Silurian Goobarragandra Volcanics. The Volcanics dominantly comprise an extensive uniform succession of crystal-rich dacitic tuffs but include some epiclastic rocks, minor lava flows and mafic and felsic intrusives. The crystal-rich nature of the tuffs, rarely preserved shards, welding textures, perlitic cracking, and a uniform nature over large areas indicate they were emplaced as ash-flow tuffs and were originally substantially welded and glassy and hence the depositional environment was most likely subaerial.

Blocks within the megabreccia have discordant contacts with the surrounding tuff and lack any indication of gradational relationships between blocks and tuff. Thus, large outcrops of massive fossiliferous limestone are surrounded by tuff without sign of mixed rocks or, for example, adjoining reef-flank facies. The age of most blocks is uncertain, but some siltstone-dominant blocks contain Late Ordovician graptolites; an age significantly older than the surrounding tuffs. Although contacts are rarely exposed they indicate that siltstone and dacite do not form conformable sequences, that the siltstone was lithified and that the dacite was hot when it contacted the siltstone. Siltstone is locally baked at the contact with the tuff and in places is brecciated and intruded by veins of dacite and the tuff is locally chilled against the siltstone.

The blocks within the Goobarragandra Volcanics were previously considered to be conformably interstratified with the tuff and were used to indicate the structure of the volcanic sequence and to place constraints on its environment of deposition. The presence of limestone was taken to indicate a marine depositional environment for at least the adjacent tuff although there is little or no other corroborative evidence.

The term megabreccia is used in volcanology to describe a unit "in which many clasts are larger than 1 m in diameter, and the clastic nature of the deposit is obscure in many individual outcrops" (Lipman, 1976, p. 1398). Lipman suggested that megabreccia is dominant in the lower part of caldera-fill sequences, is especially thick adjacent to caldera walls, and accumulates during intense stages of ash-flow eruptions. The presence of megabreccia in the Goobarragandra Volcanics suggests that the host tuffs are the fill of a large, eroded caldera, consistent with the inferred great thickness of the ash-flow sheets.

The wide distribution of the ash flow tuffs and the comagmatic Young Granodiorite suggests that the eruptive complex consisted of several calderas and that the Goobarragandra Volcanics include both caldera-fill and outflow deposits. The tuff and the granodiorite, however, are uniform in composition and texture over large areas and these features cannot be used to separate the product of one caldera from another. However megabreccia blocks of a particular lithology tend to be concentrated in discrete areas and are likely derived from a proximal source. Mapping the distribution of block lithologies may lead to delineation of separate caldera structures.

The graptolite-bearing blocks in some megabreccia units indicate that the Goobarragandra Volcanics erupted through a basement which included a Late Ordovician sequence. These blocks are tentatively correlated with the Warbisco Shale of eastern Victoria and southeastern NSW which has a mid-Gisbornian to mid-Bolindian range. This unit probably extended beneath the present outcrop of the Goobarragandra Volcanics and was exposed in the steep caldera walls early in the development of the caldera complex.

REFERENCES

Lipman, P.W., 1976. Caldera-collapse breccias in the western San Juan Mountains, Colorado. Geological Society of America Bulletin, 87, 1397-1410.

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