Melt extraction and magma storage in the middle crust: insights from amphibole and apatite geochemistry in the Famatinian arc

Abstract

Magma reservoirs in middle to upper crust play a crucial role in the evolution of continents and volcanic activity, yet theirarchitecture, compositional evolution, and melt extraction mechanisms remain incompletely understood. The Valle Fértilbatholith (VF), an exhumed section of the Ordovician Famatinian arc, provides an exceptional opportunity to investigatethese processes in situ. This study integrates bulk-rock and mineral geochemistry (plagioclase, amphibole, quartz andapatite) with textural observations to reconstruct the magmatic evolution of the middle to upper part of the Famatinianarc section. Amphibole compositions reveal a systematic compositional evolution from Intermediate to Silicic lithologicalunits, reflecting progressive differentiation. Trace element variations in amphibole indicate fractional crystallization as thedominant control on magma evolution. Apatite compositions record relative changes in oxygen fugacity of the system, andthey capture the chemical evolution of the magmas as well as the amphibole. Chemometric reconstructions of amphiboleequilibrium melts indicate derivation from dacitic to high-silica rhyolitic compositions, closely resembling silicic volcanicrocks from the Famatinian arc. These findings suggest that the middle crust of the Famatinian arc functioned as a verticallystratified magmatic distillation column, linking more mafic lower crustal units with high-silica magmas found inthe upper crust, confirming the dominant role of trans-lithospheric differentiation columns in the evolution of arc crusts.