N-S shortening during orogenesis in the Mt Isa Inlier: the preservation of W-E structures and their tectonic and metamorphic significance
Sayab, Mohammad (2005) N-S shortening during orogenesis in the Mt Isa Inlier: the preservation of W-E structures and their tectonic and metamorphic significance. PhD thesis, James Cook University.
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Mesoproterozoic Mt Isa inlier of NW Queensland exhibits complex tectonometamorphic history that is generally considered to result from low-pressure/high-temperature (LP/HT) metamorphism with an anticlockwise pressure-temperature-deformation path. Yet studies regarding the nature of the P-T history and tectonic regime that led to such a LP/HT signature have been quite limited. A detailed FIA (Foliation Intersection/Inflection Axes preserved in the porphyroblasts) analysis combined with textural relationships and P-T pseudosections, using key localities across the Eastern Fold Belt of the Mt Isa Inlier, has resolved the cause of the LP/HT signature. Measurement of FIAs in the Eastern Fold Belt has revealed phases of deformation and metamorphism that could not previously be distinguished from one another. Both the ‘asymmetry switch’ and ‘FitPitch’ FIA measurement techniques have been applied to key localities of polymetamorphosed and multiply deformed Eastern Fold Belt, and they yielded the same result. These independent techniques have revealed (1) W-E trending structures that formed during N-S bulk shortening (O1) and N-S oriented structures that formed during W-E bulk shortening (O2) in the Eastern Fold Belt, (2) the presence of separate periods of metamorphism associated with each direction of bulk shortening, and (3) the crustal scale tectonic processes associated with polymetamorphism. The structural overprinting relationships do not support previously suggested non-coaxial west vergent, nappe-style folding in the region. A progressive succession of overprinted FIA trends reveals a clockwise rotation of the principal direction of bulk shortening with time. This requires a radical shift of relative plate movement from N-S to W-E during development of the north Australian craton in the Mesoproterozoic (ca 1.60 and 1.50Ga). Significantly, O1 porphyroblasts preserving W-E FIAs exhibit mineral textures of Barrovian style, whereas O2 formed porphyroblasts preserving N-S FIAs are Buchan in style. This supports the emplacement of the Williams/Naraku Batholiths after O1 around the onset of O2. Higher-pressure garnet cores, modeled in MnNCKFMASH P-T pseudosections preserved early W-E FIAs and formed during O1. This was followed by decompression and then low pressure – high temperature (LP/HT) metamorphism when N-S FIAs were preserved within porphyroblasts. This is further supported by the presences of at least two distinctive generations of staurolite and kyanite that grew both before and after andalusite/cordierite. Middle to upper amphibolite facies metamorphic conditions occurred during O1 with crustal thickening followed by fast erosion and near-isothermal decompression leading to LP/HT conditions. This was followed by O2 and a second period of middle- to upper- amphibolite facies metamorphism that obliterated and/or obscured the tectonometamorphic signature of primitive O1 in the matrix of most rocks. This history appears to correlate better with that observed in the southwest United States, which may have been located against the NE of the Australia at this period in time.
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