Low-pressure and high-temperature metamorphism of basalts: insights from the Sudbury impact melt sheet aureole and thermodynamic modelling

Low-pressure and high-temperature (LP-HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high-grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least similar to 925 degrees C at similar to 1-3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase-orthopyroxene-clinopyroxene-ilmenite-magnetite +/- melt assemblage. Peak temperatures reached similar to 875 degrees C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts similar to 500 to 750 m from the SIC contact are characterized by a similar two-pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino- and orthopyroxene along an isobaric cooling path. Metabasalts similar to 750 to 1,000 m from the SIC contact are characterized by a hornblende-plagioclase-quartz-ilmenite assemblage indicating temperatures up to similar to 680 degrees C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10-20% melt generation in the inner similar to 500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O-2 (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high-Ca amphibole and silicate melt at relatively low pressures (similar to 1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP-HT conditions.