The Central Segment of the Famatinian Arc: Insights into the Construction of Ancient Arc Crust

Abstract

The Famatinian arc provides an extensive cross section through an ancient magmatic arc, elucidating the evolution of subduction-related magmatism and the generation of the continental crust. Although the mantle-crust transition is not exposed, integrated field observations and laboratory analyses allow the reconstruction of a spatiotemporal model for the arc's thermal structure and magmatic evolution. The resulting cross section demonstrates that depth-dependent petrological processes govern igneous differentiation. At greater depths, crystallization-driven fractionation and melt-mush interactions produce variations within mafic-ultramafic complexes. From the middle crust to volcanic levels, crystal fractionation, partial melting, and the reactive assimilation of prearc host crust drive the evolution toward intermediate and silicic compositions. Comparative analysis with other ancient arcs indicates that subduction-related plutonic-volcanic columns are density filtered and organized as vertically interconnected systems. Within these trans-lithospheric magmatic columns, the influx of mantle-derived melts, multi-level crystal-melt-fluid fractionation, and assimilation of preexisting crust occur simultaneously in space and time from source to surface. ▪ The Famatinian arc represents a 30-km-thick segment of Ordovician arc crust that has undergone differential uplift along several hundred kilometers of its paleo-axis. ▪ Mantle melt input, polybaric igneous fractionation, and intracrustal melting controlled lithological and chemical diversification across the magmatic column. ▪ Extensive igneous columns transition from melt-dominated magmatic systems in the middle crust to crystal-rich mushy reservoirs in the upper crust. ▪ Comparable lithological architectures in ancient arc sections allow assessment of how the building blocks of continental crust develop over convergent plate margins.