TEMPERATURES OF GRANULITE-FACIES METAMORPHISM: CONSTRAINTS FROM EXPERIMENTAL PHASE EQUILIBRIA AND THERMOBAROMETRY CORRECTED FOR RETROGRADE EXCHANGE

Abstract

This study assesses temperatures of formation of common granulites by combining experimental constraints on the P–T stability of granulite-facies mineral associations with a garnet–orthopyroxene (Grt–Opx) thermobarometry scheme based on Al-solubility in Opx, corrected for late Fe–Mg exchange. We applied this scheme to 414 granulites of mafic, intermediate and aluminous bulk compositions. Our findings suggest that granulites are much hotter than traditionally assumed and that the P–T conditions of the amphibolite–granulite transition portrayed in current petrology textbooks are significant underestimates by over 100°C. For aluminous and intermediate granulites, mean corrected temperatures based on our method are 890 ± 17 and 841 ± 11°C, respectively (uncertainties reported as 95% confidence limits on the mean), consistent with minimum temperatures for orthopyroxene production by fluid-absent partial melting in these bulk compositions. In contrast, mean temperatures based on Grt–Opx Fe–Mg exchange equilibria, using the same thermodynamic data, are 732 ± 22 and 723 ± 11°C, respectively, well below the minimum temperatures for Opx stability. For mafic granulites, the mean corrected temperature using our method is 816 ± 12°C, similar to the mean temperature of 793 ± 13°C from Fe–Mg exchange. Reasons for the differences between the mafic granulites and aluminous–intermediate granulites are unclear but may be due to the lower Al concentrations in Opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations. We discuss a number of well-known granulite terrains in the context of our findings, including the Adirondacks, the Acadian granulites of New England, the incipient charnockites of southern India and Sri Lanka, and the Kerala Khondalite Belt. Our findings carry implications for thermotectonic models of granulite formation. A computer program to perform our thermobarometry calculations, RCLC, is available from the Journal of Petrology website at http://www.petrology.oupjournals.org or from the authors at http://www.geo.ucalgary.ca/~pattison/drm_pattison-rclc.htm .