Origin and significance of megabreccia blocks in the Silurian Goobarragandra Volcanics, southeastern New South Wales

Abstract

Blocks of limestone, bedded carbonate and chert, graded siltstone and sandstone sequences, and graptolite‐bearing siltstone within crystal‐rich dacitic tuff of the Silurian Goobarragandra Volcanics form the framework to a megabreccia. The crystal‐rich nature of the dacitic rocks, rarely preserved shards, welding textures, perlitic cracking, and their uniform nature over large areas indicate they were likely erupted as subaerial ash‐flow tuffs and were originally welded and glassy. Blocks within the megabreccia have sharp discordant contacts with the surrounding tuff. The siltstone and dacite do not form conformable sequences. Some blocks of siltstone and minor interbedded sandstone contain graptolites of Ordovician age and are clearly older than the Goobarragandra Volcanics. The siltstone was lithified and the dacite was hot when it contacted the siltstone. Outcrops of limestone and bedded carbonate and chert are surrounded by outcrops of dacitic tuff. There are no facies indicative of gradation from limestone to tuff suggesting the limestone forms blocks in a matrix of tuff. The presence of megabreccia suggests that the host dacitic tuff is in part the fill of a large, deeply eroded caldera. The extensive distribution of the Goobarragandra Volcanics and the comag‐matic Young Granodiorite suggests that the complex consisted of several overlapping calderas. The identification of Late Ordovician siltstone blocks in the megabreccia indicates that a Late Ordovician sequence was exposed in the caldera walls. This sequence is no longer exposed in the Goobarragandra Block.

Key words:

• caldera
• Goobarragandra Volcanics
• megabreccia
• Silurian