Palaeoproterozoic high-T, low-P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

A sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional-scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite-atidalusite-K-feldspar assemblages and cordierite-bearing leucosomes with biotite-andalusite selvages, reflecting P-T conditions of c. 3 kbar and c. 650-680 degreesC. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P-T evolution, or a later independent higher-P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al2SiO5 polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz-absent sillimanite-spinel-cordierite melanosomes. whilst in semipelitic migmatites. discontinuous leucosomes enclose cordierite-spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet-orthopyroxene-cordierite-biotite-quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near-isobaric heating-cooling path, with peak metamorphism occurring at 2.6-4.0 kbar and c. 750-800 degreesC. SHRIMP U-Pb dating of metamorphic zircon rims in a cordierite-orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 +/- 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 +/- 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high-T, low-P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age or deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low-pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near-isobaric Palaeoproterozoic P-T-t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.