Abstract

Alkaline-silicate igneous complexes can host significant resources of critical metals and contain a diversity of co-genetic rock types from silica-oversaturated (quartz normative) to silica-undersaturated (feldspathoid normative) compositions. The origin of these coexisting rocks is an ongoing petrological problem, due to the separation of the two regimes by thermal divides. Here we apply new thermodynamic models to investigate the co-genetic formation of these rock types using two case studies: 1) the Blatchford Lake Igneous Complex (Canada), which also hosts a world-class REE deposit associated with the silica-undersaturated units; and 2) the ‘giant dyke’ complexes in south Greenland, which our group visited this summer. We quantify the effects of fractionation, crystallisation pressure and crustal assimilation to identify key conditions under which evolved alkaline melts, which are optimal for critical metal mineralisation, may be generated.