Magmatic Differentiation Exposed on a Crustal Scale: A Field Example from Sierra Valle Fértil, Argentina

Two endmember models of magmatic arc assembly have been proposed: (1) a model where an upper-crust silicic magma body is fed by a lower-crust mafic source area with the mid-crust largely uninvolved, serving as a transport zone, and (2), a model of progressive and spatially continuous construction of arc basement that requires a few million years of thermal maturation before voluminous silicic magmas accumulate in the upper crust. One way to assess the generality of these two models is to look at crustal cross-sections. A particularly well-exposed example of such a cross-section is within the Ordovician Famatinian magmatic arc, a 1500 km long suite of magmatic units ranging from deep mafic to shallower intermediate plutonic bodies and silicic eruptive products. The deepest complete section of the arc is exposed in the Sierras Valle Fértil-La Huerta (SVF) with magmatic crystallization pressures ranging from greater than 8 kb to 4 kb. The SVF displays continuous outcrop that preserves the geological record of the magmatic role of igneous input, thermal prograde processes, and assimilation during arc crust assembly, thereby allowing us to construct a time-composition-volume evolution of the SVF arc section. Field observations—such as large, multiply intruded gabbro bodies interspersed with metapelitic septa, the absence of regional vertical structures, a sharp horizontal contact between mafic and felsic units, minor pelite assimilation, and no remelting of precursory mafic forerunners (amphibolites)—constrain possible processes of magmatic differentiation and preserve complex features indicative of emplacement during a common thermal prograde event. CA-ID-TIMS U-Pb geochronology from gabbroic, dioritic, and granodioritic rocks in the SVF yields crystallization ages from 471.47 Ma to 467.14 Ma. This 4.33 Myr time frame for the initial construction and evolution of arc crust is consistent with examples from continental and oceanic arcs that indicate an active ‘thermal maturation time’ of 4-6 Myr is required to build a lower-crustal mafic complex and yield volumetrically significant silicic magmas in the mid-to-upper crust. Our findings suggest that arc construction is more like model (2)—dominated by continuous inputs of mafic magmas which, at varying scales, can be kept hyper-solidus for millions of years, followed by the progressive extraction of silicic material in a common thermal envelope once the crustal section has matured thermally. Our preliminary results are consistent with geophysical data and composition and timing of intrusive magmatic evolution from other arc cross-sections.