The use of porphyroblasts to resolve the history of macro-scale structures: an example from the Robertson River Metamorphics, North-Eastern Australia

Abstract

The successions of inclusion trail asymmetries defining foliation intersection/inflection axes (FIAs) preserved in porphyroblasts document the geometry of deformation associated with folding and fabric development during discrete episodes of bulk shortening and suggest that deformation histories are commonly more complex than previously considered. A succession of four FIAs trending ENE–WSW, E–W, N–S and NE–SW has been distinguished based upon relative timing plus inclusion texture and orientation in the Proterozoic Robertson River Metamorphics (Georgetown Inlier, Qld, Australia). The successions of asymmetries formed around these FIAs bear no relationship to the geometry of macroscale folds present in the area suggesting that these folds predate porphyroblast growth, the widespread metamorphism and matrix fabric development. The onset of regional macro-scale folding may have begun soon after the deposition at around 1655 Ma in the Georgetown Inlier as opposed to the previously suggested date of 1570 Ma. These folds were then amplified, overturned and refolded during NNW–SSE, N–S, E–W and NW–SE directed regional bulk shortening. Earlier deformations were erased from the matrix because of bedding-induced shearing (reactivation) on the limbs of pre-existing macro-scale folds. Four foliations, S 1– S 4, identified in the matrix provided information about the youngest deformations preserved in these rocks. These results also suggest that existing tectonothermal models regarding the NE Australian Craton should be reconsidered.