Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW Queensland, Australia): the preservation of early W-E-trending foliations in porphyroblasts revealed by independent 3D measurement techniques

3D microstructural analyses of porphyroblast inclusion trails using both the 'asymmetry switch' method for determining foliation intersection axes preserved in porphyroblasts (FIAs) and the recently developed 'FitPitch' method, reveal W-E- and N-S-trending FIA sets in the White Blow Formation of the Mount Isa Inlier. Each method reveals two subsets of FIAs centered on each of these major trends. These were distinguished based on the relative timing, trend, and orientation of inclusion trail patterns. Thirty-six samples were analyzed using both techniques and produced very similar results. Pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin-sections and trends in horizontal sections yield distinct near-orthogonal modal orientations from all the analyzed samples. This indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes. W-E- and N-S-trending FIAs have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages. Garnet and staurolite growth during bulk north-south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle-amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain. This period of bulk shortening and associated metamorphism formed during a period of orogenesis called O-1. W-E shortening formed N-S striking foliations that preserve a period of orogenesis (O-2), and another succession of metamorphism involving more phases of porphyroblast growth preserving N-S-trending FIAs. Overprinting of successive FIA trends (WSW-ENE, WNW-ESE, NNW-SSE, and SSW-NNE) suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from N-S to W-E overall, with a major 'tectonic break' or decompression between O-1 and O-2. The porphyroblast inclusion trail patterns preserving W-E-trending FIAs provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations. (C) 2005 Elsevier Ltd. All rights reserved.