EPMA and LA-ICP-MS of amphiboles, quartz, apatite and plagioclase from the Valle Fértil batholith

Magma reservoirs in the middle to upper crust play a crucial role in the evolution of continents and volcanic activity, yet their architecture, compositional evolution, and melt extraction mechanisms remain incompletely understood. The Valle Fértil batholith (VF), an exhumed section of the Ordovician Famatinian arc, provides an exceptional opportunity to investigate these processes in situ. This study integrates bulk-rock and mineral geochemistry (plagioclase, amphibole, quartz, and apatite) with textural observations and geochemical modeling to reconstruct the magmatic evolution of the middle to upper part of the Famatinian arc section. Amphibole compositions reveal a systematic compositional evolution from Intermediate to Silicic lithological units, reflecting progressive differentiation. Trace element variations in amphibole indicate fractional crystallization as the dominant control on magma evolution. Apatite compositions record relative changes in oxygen fugacity of the system, and they capture the chemical evolution of the magmas as well as the amphibole. Chemometric reconstructions of amphibole equilibrium melts indicate derivation from dacitic to high-silica rhyolitic compositions, closely resembling silicic volcanic rocks from the Famatinian arc. Thermodynamic modeling of fractional crystallization, with and without assimilation, demonstrates that primitive arc magmas can generate these compositions under mid-crustal conditions (~0.4-0.6 GPa). These findings suggest that the middle crust of the Famatinian arc functioned as a vertically stratified magmatic distillation column, linking more mafic lower crustal units with high-silica magmas found in the upper crust, confirming the dominant role of trans-lithospheric differentiation columns in the evolution of arc crusts.