The retrograde P-T-t path for low-pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low-P/high-T metamorphism

I. S. Buick, I. Cartwright, S. L. Harley

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Several aspects of the petrogenesis of low-pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: a) the shape of the retrograde P-T-time path, and whether it is an artefact of repeated thermal events at different P-T conditions; b) the type of regional metamorphism; and c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration-melting reactions to form migmatites under peak M2 P-T conditions of c. 5.0-5.3 kbar and c. 750-800°C. Metapsammitic rocks that did not melt during M2 show spectacular garnet-orthopyroxene intergrowths that developed at c. 3.5-3.7 kbar and c. 700-750°C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike-parallel zones. Rehydration occurred within the sillimanite stability field at P-T conditions close to the granite solidus (c. 3.2-3.4 kbar and 650-700°C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: a) peak-M2 conditions were reached at c. 1594 Ma; b) the garnet-orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P-T-t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100°C) to the granite solidus over a period of c. 26 Ma. Lake kyanite-bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300-400 Ma Alice Springs Orogeny. The shape of the P-T-t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high-grade metamorphism, and that some other, longer lived heat sources, such as the burial of anomalously radiogenic, pre-tectonic granites is required.

Original languageEnglish
Pages (from-to)511-529
Number of pages19
JournalJournal of Metamorphic Geology
Issue number4
Publication statusPublished - 1 Jan 1998

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