A bimodal source for the generation of tonalitic to granitic magmas in a non-subduction-related magmatic belt: An example from the Sierra Chica of Córdoba, Argentina

A notable consensus prevails about the hybrid nature of intermediate calc-alkaline magmatism. Accordingly, petrogenetic models envisage the addition of young mantle material and cortical recycling in those geological settings where intermediate magmas are dominant. Most exhaustively studied examples are: 1) magmatic arcs associated with subduction zone settings where oceanic crust and sediments are introduced into the mantle wedge, and 2) deep crustal sections are pervaded by hot, H2O-rich mantle-derived liquids. Nevertheless, back-arc or foreland terranes of several hundred kilometers in width, frequently characterized by high heat flux, crustal melting, recycling and melt fractionation, are crucial areas to evaluate the generation of intermediate magmatism. The Sierra Chica of Córdoba (Argentina) provides an excellent example to study these processes. The high-grade crustal segment shows metamorphosed pelitic and basaltic protoliths that were part of an old sedimentary succession. We evaluate the evolution of this bimodal source during two consecutive early Paleozoic orogenic periods when it was established as the source area of intermediate to silicic magmatism far from the contemporary magmatic arcs. Field relations and new geochemical and geochronological data revealed the occurrence of two magmatic events during Cambrian and early Ordovician periods. Cambrian migmatization and partial melting (∼528 to 505 Ma) is represented by irregular-shaped monzogranites and tonalites, while early Ordovician magmatism (∼480 Ma) is characterized by dyke-shaped pegmatites and tonalite-trondhjemites. The preservation of two contrasting igneous lineages suggests a genetic linkage between metapelitic and amphibolitic migmatites and between granitic and tonalitic-trondhjemitic partial melts. Coeval with this magmatism, Famatinian tonalitic-trondhjemitic to monzogranitic magmas intruded upper crustal levels in Sierra Chica and other parts of Sierras of Córdoba. Geochemical signatures point to a bimodal (mafic/felsic) source for this intermediate to silica-rich magmatism. The evolution of partial melts studied in the Sierra Chica is depicted on the basis of geochemical projections with fractionation and hybridization models, in order to establish the primary liquids of this Famatinian magmatic belt. Trends defined by Famatinian tonalites, trondhjemites and granodiorites suggest that primary Ca-rich melts (tonalites and trondhjemites of this study) evolved through fraction