Abstract

The Murrawong Creek and Pipeclay Creek Formations of the New England Fold Belt of eastern Australia provide one of the few records of Middle Cambrian-(?) Early Ordovician sedimentation along the eastern Gondwana margin. They comprise conglomerate, sandstone, siltstone, and laminated argillite that accumulated in an inner-submarine-fan environment. Sandstones are quartz-poor volcanolithic varieties. Conglomerate clasts are mainly andesite but range from basalt to rhyolite; all these rocks are porphyritic with phenocrysts of feldspar (albite after calcic plagioclase), augite, and, uncommonly, titanomagnetite. The augite is a low-TiO 2 variety, and titanomagnetite has only a modest TiO 2 content. Alteration has modified the magmatic composition of the clasts, but the less mobile elements indicate they were derived from a low-K orogenic suite, in consonance with petrographic and mineralogical data. A western source and the presence of lower Paleozoic subduction-complex rocks to the east of the Cambrian-(?) Ordovician units indicates that the latter accumulated in a forearc setting. The distributive province may have been the precursor of the Macquarie Volcanic Belt of the eastern Lachlan Fold Belt in N.S.W., or one of the Cambrian oceanic island arcs preserved in the fragmented Gondwana margin sequences in southeastern Australia, northern Victoria Land (Antarctica), and New Zealand.

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