Petrogenesis of cogenetic silica-undersaturated and -oversaturated rocks: Quantifying the role of crustal assimilation

Abstract

Crustal assimilation during fractional crystallisation is commonly proposed as a mechanism to generate evolved silica-oversaturated (i.e. quartz normative) rocks alongside cogenetic silica-undersaturated (i.e. feldspathoid normative) rocks. However, the amount of required assimilation, and the impact of varying crystallisation parameters (e.g. pressure, oxygen fugacity), remain poorly understood, despite a petrogenetic framework for these cogenetic rocks being necessary for understanding their varied magmatic critical metal enrichment. Here we apply new thermodynamic models to a case study of the Younger Giant Dyke Complex in south Greenland, a layered intrusive complex with cogenetic evolved silica-undersaturated and -oversaturated rocks, to explore quantitatively the role of these processes during fractionation and assimilation. A range of assimilation scenarios is explored, alongside thermal and isotopic perspectives on these scenarios. Our calculations show that a phase equilibria approach can tightly constrain assimilant amounts (e.g. 0–≤ 15% assimilation by mass of country rock granite in this complex), because of the narrow compositional range that divides silica-undersaturated from silica-oversaturated melts, whereas thermal and isotopic approaches may permit wider ranges of assimilation estimates because their model input parameters tend to be more variable. In general, silica-undersaturated primitive melts crystallising at more reduced conditions and/or higher pressures require more crustal assimilation to become silica-oversaturated. Consequently, the formation of evolved silica-undersaturated rocks is favoured when fractionation occurs at lower oxygen fugacity and higher pressures, because magmas are more protected from the effects of assimilation. Understanding such behaviour may be an important part of decoding why some alkaline-silicate complexes develop high concentrations of critical metals.