Sayab, Mohammad (2006) Decompression through clockwise P-T path: implications for early N-S shortening orogenesis in the Mesoproterozoic Mt Isa Inlier (NE Australia). Journal of Metamorphic Geology, 24 (2). pp. 89-105.

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Abstract

Mesoproterozoic terranes of the Australian craton exhibit complex tectonometamorphic histories that are generally considered to result from low-pressure/high-temperature (LPHT) metamorphism with an anticlockwise pressure (P)–temperature (T) path. Yet studies regarding the nature of the P–T history and tectonic regime that led to such a LPHT signature have been quite limited. A foliation intersection/inflection axes preserved in the porphyroblast (FIA) analysis combined with textural relationships and P–T pseudosections, using a key locality of the Eastern Fold Belt of the Mt Isa Inlier, has resolved the cause of the LPHT signature in this region. Two periods of porphyroblast growth have been distinguished using a change in FIA trends with time; the first formed during N–S shortening and the second during W–E shortening orogenesis (D1 & D2, respectively). Significantly, D1 porphyroblasts preserving W–E FIAs are minerals of the Barrovian style, whereas D2 formed porphyroblasts preserving N–S FIAs are Buchan in style. This is consistent with the emplacement of the Williams/Naraku Batholiths after D1 around the onset of D2. Higher-pressure garnet cores, that can be modelled in MnNCKFMASH P–T pseudosections, preserve early W–E FIA and formed during D1. This was followed by decompression and then LPHT metamorphism and deformation during which N–S FIAs were preserved within porphyroblasts. This is supported by the presence of at least two generations of staurolite that crystallized before and after andalusite/cordierite. Middle to upper amphibolite facies metamorphic conditions occurred during D1 with crustal thickening followed by near-isothermal decompression leading to LPHT conditions. This was followed by D2 and a second period of middle to upper amphibolite facies metamorphism that obliterated and/or obscured the tectonometamorphic signature of D1 in the matrix of most rocks.

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