Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe, Norway

• Published in: Journal of Metamorphic Geology
• Main Authors: JANÁK, M., RAVNA, E. J. K., KULLERUD, K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2012
• Subjects: Tromsdalstind; Tromsø
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.2011.00971.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.2011.00971.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.2011.00971.x

Abstract Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular ( X Grs = 0.25–0.29), but lower pyrope‐content ( X Prp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim ( X Ca Grt = 0.18–0.21) and symplectite clinopyroxene ( X Na Cpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine ( X Mg = 0.86–0.87) and spinel ( X Mg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data ...