Role of mafic cargo in discriminating Andean from postcollisional I-type granites

The major-element geochemistry of I-type granitoid batholiths from Andean and postcollisional settings reveals that entrainment of a mafic mineral cargo is a key process controlling compositional trends. Using CaO-MgO systematics as a proxy for phase relations linking granitoid melts with their near-liquidus and cumulate mafic assemblages, I analyzed a compiled database of granitic and intermediate rocks from well-constrained tectonic environments. Although mafic cargo entrainment occurs in both settings, its geochemical effects differ according to the mineral assemblage involved. Andean granitoids are characterized by a clinopyroxene-dominated mafic cargo, producing liquid-solid mixing trends largely collinear with cotectic liquid evolution and resulting in coupled enrichments in CaO and MgO. In contrast, the mafic cargo of postcollisional batholiths is dominated by biotite and Mg-hornblende aggregates formed after liquid-orthopyroxene reaction, generating Mg-rich, Ca-poor magmas that deviate markedly from cotectic liquid trends. Thermodynamic modeling with Rhyolite-MELTS shows that these contrasting behaviors arise from differences in near-liquidus phase assemblages, with a limited influence from initial magma water content. The clinopyroxene-dominated cargo of Andean magmas is compatible with an oceanic crustal component in arc-related sources, whereas such a component is absent in postcollisional extensional settings. These results demonstrate that major-element relations grounded in phase equilibria may provide a robust framework for discriminating granitoid tectonic settings.