Crustal melting vs. fractionation of basaltic magmas: Part 1, granites and paradigms

Granitoids are a major component of the continental crust. They play a pivotal role in its evolution, either by adding new material (continental growth), or by reworking older continental crust. These two roles correspond to two main ways of forming granitic magmas, either by partial melting of pre-existing crustal rocks yielding granitic melts directly, or by fractionation of mantle-derived mafic to intermediate magmas. Both models represent endmembers, or paradigms that have shaped the way the geological community envisions granitoids, their occurrence, features, formation and meaning for crustal evolution and differentiation of the whole planet. In this paper, we expose the two competing paradigms and their implications. We explore the evidence on which each model is based, and how each school of thought articulates a comprehensive view of granitic magmatism based on field geological, petrological, geochemical (including isotopes) and physical constraints; and how, in turn, each view shapes the thinking on crustal growth and evolution, and the interpretation of proxies such as age and Hf isotopic patterns in detrital zircon databases. We emphasize that both schools of thought build a different, but internally consistent view based on a large body of evidence, and we propose that each of them is, or has been, relevant to some portions of the Earth. Thus, the key question is not so much “which” model applies, but “where, when and to which extent”.