An appraisal of peak temperatures and thermal histories in ultrahigh-temperature (UHT) crustal metamorphism: the significance of aluminous orthopyroxene

• Main Author: Simon L. Harley
• Format: Report
• Language: English
• Published: Department of Geology and Geophysics, University of Edinburgh 1998
• Subjects: metamorphism; granulite; ultrahigh temperature; geothermometry; orthopyroxene; mineral chemical zoning; Mather Peninsula; Napier; Rauer Islands; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=2338; http://id.nii.ac.jp/1291/00002338/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=2338&item_no=1&attribute_id=18&file_no=1

Ultrahigh-temperature (UHT) crustal metamorphism occurs where peak metamorphic temperatures of 900-1100℃ have been attained at pressures in the range 7-13 kbar. Mineral assemblages in metapelites that are constrained by experimentally-derived and calculated FMAS and KFMASH petrogenetic grids provide good evidence for UHT conditions. Sapphirine+quartz is stable only at >1040℃ in reduced rocks, and orthopyroxene+sillimanite+quartz is restricted to pressures greater than 8 kbar at T>900℃ in KFMASH. New Fe-Mg exchange thermometry and thermometry based on Al in orthopyroxene coexisting with garnet allows a re-evaluation of the peak or near-peak temperatures in UHT terrains. The J. GANGULY et al. (Contrib. Mineral. Petrol., 126,137,1996) garnet-orthopyroxene Fe-Mg thermometer yields temperature estimates in the range 900-1100℃ for several UHT case studies, but closes at lower temperatures in other cases. Thermometry based on the high Al_2O_3 contents (8-12wt%) of orthopyroxene coexisting with garnet, in sillimanite- and sapphirine-bearing assemblages that are constrained from the grids to be >1000℃, support the UHT nature' of the areas for which reliable data are available. For example, the FAS-system orthopyroxene Al-geothermometer (L. Y. ARANOVICH and R. G. BERMAN, Am. Mineral., 82,345,1997) yields average temperature estimates of 962±51℃ for granulites from the Napier Complex, Rauer Islands, Sri Lanka, Eastern Ghats and similar UHT areas. Elemental mapping of Al zoning in orthopyroxene coexisting with garnet or other aluminous phases can provide insight into pre- and post-peak thermal histories of UHT terrains. In the Napier Complex orthopyroxene coexisting with sapphirine+quartz zones from over 12 wt% Al_2O_3 in cores down to only ca. 9wt% Al_2O_3 in rims and recrystallised grains. This zoning reflects cooling through 80℃ within the stability field of sapphirine+quartz, and leads to a new estimate of 1120±20℃ for the peak temperature of regional metamorphism in this UHT terrain. At Mather Peninsula in the ...