How well established is isobaric cooling in Proterozoic orogenic belts? An example from the Arunta inlier, central Australia

Abstract

Orogenic pressure-temperature paths can only be deduced from metamorphic textures if they formed during a single metamorphic event, rather than recording the effects of two or more separate events. Erroneous tectonic models are likely to result if textures formed in several events are ascribed to one event. We illustrate this problem for the Arunta inlier in central Australia, which has been considered to be a good example of an isobarically cooled terrain. Rocks of the Anmatjira Range in the central Arunta inlier, which has been described in terms of isobaric cooling, actually record the effects of two low-pressure, granulite facies, metamorphic events of similar peak metamorphic grade, M 1 and M 2 , that are separated in time by a period of basin development and deposition of a succession of sedimentary rocks. Correlations of stratigraphic, structural fabric, and metamorphic texture data allow the effects of M 1 and M 2 to be distinguished. Without this fine control, the textures formed by superposition of M 1 upon M 2 , considered as a manifestation of one event, could easily be interpreted as indicating isobaric cooling, because the overprinting assemblages are of similar pressure but slightly lower temperature than the original assemblages. We call this apparent isobaric cooling; in fact, we now recognize textures reflecting decompression that formed immediately following the peaks of both M 1 and M 2 . It is now a matter for conjecture whether isobaric cooling is more apparent than real in other terrains for which it has been recorded.