The emplacement of acid magma in the epizone,and the relationship with ignimbrites,in North Queensland,Australia

There is an aggregate outcrop of 12,000 square miles of Permian-Triassic acid igneous rocks inland from Cairns and Townsville, North Queensland. The rocks consist of ignimbrite and rhyolite, which are structurally and magmatically related to three high-level intrusions the Herbert River, Esmeralda, and Elizabeth Creek Granites. Most of the igneous rocks intrude the Precambrian Georgetown Inlier, but some of them intrude along a fractured zone on the junction of the Inlier and the shelf zone of the adjacent Palaeozoic Tasman Geosyncline. The Upper Palaeozoic — Triassic igneous period consists of two main epochs, both consisting of granite, ignimbrite, and rhyolite. In both epochs granite intrudes the comagmatic and coeval ignimbrite and rhyolite. Rapid horizontal movement of granitic magma through the epizone and major fracturing of the crust are postulated to explain the widespread intrusion of the granite. The granitic magma was probably initially generated 5 miles below the surface of the crust by partial melting of the sediments at the base of the Tasman Geosyncline. Epeirogenic movement in the Precambrian Inlier area formed sheet-like fractures, which provided channels for rapid horizontal movement of the granitic magma. This magma was emplaced along the fractured marginal zone of the Inlier to form a thick sill-like body of granite — the Herbert River Granite — in the first epoch. Magma for the second epoch was derived from melting of the lower part of the granite of the first epoch. Renewed fracturing of the Inlier area formed cauldron subsidence areas and rift, which were quicly filled with rhyolite and ignimbrite. In these collapsed areas the granitic magma crystallized as the Elizabeth Creek and Esmeralda Granites under an insulating cover of about 1,000 feet of rhyolite and ignimbrite.