Skip to content

Igneous distillation caught in the act: Insights from Compositional Stratification in the Middle Crust of the Famatinian Arc

Giuliano Cesar Camilletti , Olivier Bachmann

Abstract

The Valle Fértil mountain range exposes a ~14 km section of the middle crust of the Famatinian magmatic arc, showcasing a compositional stratification with an intermediate unit (57--70% SiO₂) underlying a silicic unit (65--77% SiO₂). This arrangement provides the opportunity to investigate the evolution of magmatic reservoirs within the middle crust. We analyzed whole-rock major and trace elements, alongside with amphibole and plagioclase chemistry, to identify key petrological processes governing magmatic differentiation. Textural analyses show amphiboles and plagioclase forming an interconnected network with quartz, biotite and occasional K-feldspar occupying interstitial spaces. Minerals are arranged in a steep regional magmatic fabric across unit boundaries, with no significant disruptions, suggesting a homogeneous and continuous magmatic system.  The variation in AlIV vs AlVI in amphiboles, indicative of pressure changes, reveals relative depth variations during magma emplacement. A fractional crystallization trend emerges in plutonic rocks, with decreasing MgO and Sr and increasing Rb concentrations with decreasing relative paleo-depths. Amphibole chemometry indicates melt compositions evolving from dacitic to rhyolitic, resembling volcanic rocks of the Famatinian arc. The lack of Fe-Mg equilibrium between amphiboles and their host rock, together with mineral orientation, supports some melt extraction from static mush zones, occurring dominantly via mechanical compaction and/or percolation flow. These findings suggest that middle-to-upper crustal plutonic rocks acted as a vertically-extensive mushy source reservoir for evolved melts that reach shallow depths (upper crust and surface). The chemical stratification of the magmatic column clearly reflects progressive differentiation during magma ascent.