The barnard metamorphics and their relation to the Barron river metamorphics and the Hodgkinson formation, North Queensland

Abstract

During the Middle Palaeozoic, the Tasman Geosyncline extended to northeastern Queensland, including the area between Cape York Peninsula and Ingham. The geosyncline was here represented by the Hodgkinson Basin, in which the Hodgkinson Formation forms the main geosynclinal deposit. Meta‐morphic rocks of doubtful age, exposed along the coast within the geographical limits of the Hodgkinson Basin, are called Barron River Metamorphics (green‐schist facies of metamorphism) and Barnard Metamorphics (amphibolite facies). They have hitherto been thought to have formed tectonic land during the Middle Palaeozoic geosynclinal sedimentation—hence to be older than the Hodgkinson Formation—and their ages were estimated to be Early Palaeozoic (Barron River Metamorphics) and Precambrian (Barnard Metamorphics). Recent field work suggests that the two units may both be the metamorphic equivalents of the Hodgkinson Formation: it is concluded that they formed an integral part of the Tasman Geosyncline during the Middle Palaeozoic, and were metamorphosed during the Late Palaeozoic orogeny under conditions of high temperature and relatively low pressure. This idea is supported by geological examples from other places of the world, and by the latest views on the relationships between metamorphosed and unmetamorphosed rock sequences in orogenic belts. The picture thus obtained appears to harmonize with the circum‐Pacific area in general, where this type of metamorphism (andalusite‐sillimanite association) is rather common.