Crustal melting vs. fractionation of basaltic magmas: Part 2, Attempting to quantify mantle and crustal contributions in granitoids

Abstract

Granitic melts may form either directly, by melting of pre-existing crustal rocks, or by fractionation of mafic to intermediate magmas, typically mantle-derived. Each model is applicable to distinct portions of the Earth at different times. Whenever there is an important flux of mafic magmas from the mantle, differentiation of basaltic magmas dominates. In contrast, locations with a lower mafic magma flux, elevated thermal gradients and/or a fertile, thick crust are dominated by crustal melting and reworking. This only partly overlaps with the dichotomy between magmatic arcs and collision zones, as places like e.g. inverted back-arcs also feature large-scale crustal melting, whereas post-collisional domains include a sizeable mantle-derived component. Petrogenesis of individual granitic suites probably cannot be accounted for by pure, end-member models, as these suites typically feature a fair proportion of hybrid or ambiguous granitoids. Thus, we explore various petrogenetic and geological scenarios leading to the formation of hybrid granitoids at various scales. Finally, we outline possibilities to quantify the respective contributions of crust and mantle involved in their formation – hopefully, paving the way for a better understanding and more rigorous discussion of the mechanisms of crustal growth and recycling.