Tonalite–granodiorite suites as cotectic systems: A review of experimental studies with applications to granitoid petrogenesis

Granodiorites and tonalites (Granotonalites in this review paper) form large batholiths at active continental margins and collisional orogens. Although the relation with subduction is obvious, the mechanisms of magma generation remain among the most debated topics in Earth Sciences today. The most outstanding features of these calc-alkaline batholiths are summarized in this review. The problem has been addressed by laboratory experiments along decades. Dispersed data from experimental studies are compiled here in order to give phase equilibria constraints of general application. It is apparent from chemical data that major-element compositional trends follow cotectic patterns whose composition is largely dependent on intensive variables. The comparison between experiments and batholith chemical trends produces an empirical thermodynamic framework useful to understand in a first approach the origin and evolution of magmas forming granite batholiths. A new projection scheme of rock compositions onto the pseudoternary plane defined by the Fe+Mg, anorthite, orthoclase, is given. This can be taken as first approach to phase equilibria model of batholiths. It has been revealed useful to distinguish processes controlled by increasing (melting) or decreasing (crystallization) magma temperature from processes controlled by assimilation or magma mixing. The available experimental data, although scarce, can be used to tentatively depict phase equilibria in the projected pseudoternary F–An–Or diagram. Although important limitations are imposed by the projective procedure, phase equilibria relations in equilibrium with liquids can be drafted in the diagram. The stability fields of mineral in areas close to the liquid are defined according to reported phase assemblages in the experimental studies of reference. The diagram is robust to identify cotectic and non-cotectic relations. Phase equilibrium constraints and rock textures have revealed that one of the most important parameter controlling phase equilibria in granite magmas is the initial water content. Typically, calc-alkaline granitoids, from Qtz-diorite to granotonalite and granite, have crystallized from strongly undersaturated (water) magmas. Low initial water contents (Wⁱ) in the range 1–2wt.% H₂O may account for residual saturated melt fractions as low as 0.1–0.2 for plutons emplaced at 400MPa. The water content of these residual melt fractions is enough to stabilize hydrous minerals, namely Hbl and Bt as predicted by experiments. The initial water content is a primary feature of the magma and it is directly related to the melting reaction and, more precisely, to the temperature of magma generation. The study of compositional trends supports magma fractionation by crystal-liquid separation as a relevant role. As a first approach, the pressure–composition relations in the melts can be seen as a response to the composition of the solid saturating assemblage, and particularly related to the ratio Grt/Pl in the solid residue.