Tectonic Implications of Early Paleozoic Metamorphism in the Anakie Inlier, Central Queensland, Australia

Abstract

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure–high temperature metamorphism is associated with D1, medium pressure–high temperature metamorphism with D2, and retrograde, low pressure–low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64–0.79 GPa and 580°–640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana.