CONSONORM_HG: a new method of norm calculation for mid- to high-grade metamorphic rocks

The CONSONORM_HG norm provides a standardized solution to approximate metamorphic assemblages for mid- to high-grade metamorphic rocks. Other applications for this norm include: (i) the standardized classification of metamorphic rocks; (ii) the interpretation of hydrothermal alteration using indices calculated from normative minerals; (iii) the estimation of the carbonate content of rocks, even when CO2 has not been analysed. CONSONORM_HG is designed for rocks dominated by silicates, Fe-Ti oxides or carbonates, and it approximates the main mineral assemblages of amphibolite, granulite, greenschist and blueschist facies metamorphic rocks. The norm calculates silicate assemblages using one of the three Al-Ca-NaK-FeMg tetrahedra, a convenient way of representing a large number of silicate assemblages, for each of the 17 P-T conditions modelled and based on the Fe-Mg-Mn composition of the sample. In addition to silicate minerals, CONSONORM_HG calculates Fe-Ti oxides and other accessory minerals from minor elements, sulphides from analysed sulphur and/or from analysed metals and also carbonates from analysed CO2 or from normative CO2 estimated from the loss on ignition. CONSONORM_HG also integrates many reactions to address silicon deficits, as well as quartz-carbonate reactions to approximate natural assemblages better. The normative calculation is validated using published whole-rock analyses and petrographic descriptions and its various applications are discussed.