Ring‐structures of the Mt. Alford district south‐east Queensland

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Abstract
Two ring‐structures, one a ring‐dyke complex, the other a solitary ring‐dyke, have intruded gently‐dipping Mesozoic sedimentary rocks at Mt. Alford, S.S.W. of Brisbane. In the ring‐dyke complex, a central stock of porphyritic microdiorite with closely associated granophyre has dragged up the surrounding sandstones and tilted them to a vertical position around its northern margin. Narrow, steeply‐dipping ring‐dykes of rhyolite and trachyte have intruded the upturned sediments and andesite has invaded the stock, probably as a result of cauldron subsidence. Further dyke intrusions of rhyolite and trachyte and explosive activity were followed by a basaltic dyke swarm. The complex is adjoined on the south and east by stocks of rhyolite, a neck of bedded breccia with centroclinal dips and a ring‐dyke of alkaline rhyolite. It is suggested that the alternation of acid and basic magma is due to tapping of a magma reservoir which rises and falls in the crust, deriving acid magma from the sial. The present highest point in the complex is thought to be below the original surface at the time of intrusion (probably early Tertiary).

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