Structure of the Early Palaeozoic Cape River Metamorphics, Tasmanides of north Queensland: evaluation of the roles of convergent and extensional tectonics

Abstract

The Early Palaeozoic Cape River Metamorphics consist mainly of psammitic gneiss and schist and occur as an extensive linear belt at the western margin of the Charters Towers Province 200 km southwest of Townsville in the northern Tasmanides. A prominent foliation (S₂) is the main structure in the belt and is associated with tight to isoclinal folds, subparallel mineral and intersection lineations, and boudinaged pods of vein quartz and pegmatite. In the southwest, the main foliation is a crenulation cleavage (S₂) related to D₂ deformation. It overprints steeply dipping foliation (S₁) formed in a D₁ deformation but no associated folds have been found. Gently plunging, upright, open folds (D₃ deformation) with axial planar S₃ crenulation cleavage have affected the main foliation (S₂). These deformations were associated with upper greenschist to lower amphibolite facies metamorphism. Amphibolite-grade orthogneiss containing S₂ and S₃, deformed granite and migmatite of the Fat Hen Creek Complex occurs in the northeast. In the southwest, the main foliation (S₂) is folded around a map-scale, gently plunging synclinorium indicating that S₂ formed with a subhorizontal orientation. In metamorphic rocks, the origin of widespread, intense subhorizontal foliation, usually associated with recumbent folds, has been considered problematic and in many cases is attributed to crustal extension. We relate the origin of D₂ structures to subvertical shortening (i.e. extension) resulting in orientations that are strikingly divergent to those of upright D₁ and D₃ structures that were induced by compression. The proposed extensional event is poorly constrained in timing but it affected much of the Fat Hen Creek Complex, the oldest known phase of which is 493 Ma, and occurred prior to ⁴⁰Ar/³⁹Ar cooling ages at 423 – 409 Ma that also post-dated the D₃ deformation.

Keywords:

• argon – argon dating
• contractional deformation
• extensional tectonics
• foliation
• metamorphic rocks
• Palaeozoic
• Queensland
• Tasmanides

ACKNOWLEDGEMENTS

The Australian Research Council (grant number A00103036) funded this work with additional support from James Cook University, the University of Wollongong and the Geological Survey of Queensland. We are grateful to the landowners in the Cape River region for allowing us access to their properties. We thank Laurie Hutton for comments on a draft of the manuscript. David Carrie made many excellent thin-sections. Peter Johnson computer drafted the figures. Stan Szczepanski is thanked for assistance with the ⁴⁰Ar/³⁹Ar analyses. Major revisions suggested by Peter Cawood and an anonymous reviewer has resulted in substantial changes to the manuscript but we remain responsible for any deficiencies that remain.