GEOCHEMICAL ARCHITECTURE AND EVOLUTION OF THE FAMATINIAN MAGMATIC ARC CRUSTAL SECTION IN THE NW ARGENTINA

A comparison of geochemistry of five pre-Andean, South American Cordillera flare-ups shows good correlations between major elements for mafic and intermediate igneous rocks and estimated crustal thickness. This suggests a relationship between the crustal thickness and the extent of mantle melting driven by changes in the thermal structure of the mantle wedge. We thus hypothesize that mantle processes are the main cause of magmatic flare-ups in arcs.

In one of these arcs, the Ordovician Famatinian arc in NW Argentina, we are exploring the temporal and geochemical evolution of the arc in three crustal sections. A lower crustal (LC) section is represented by the Sierra de Valle Fértil, a mid-crustal (MC) section is represented by the W Sierra de Famatina and SW Sierra de Velasco, and an upper crustal (UC) section is exposed in the Sierra de Los Llanos, E Sierra de Famatina and Sierra de Narváez. A compilation of published and new data from these sections of 35 U-Pb zircon ages and ca. 200 whole-rock geochemical samples reveals no differences in the range of crystallization ages from these different crustal levels. Crystallization occurred at 486 ̶ 469 Ma (peak at 474 Ma) for the LC, 483 ̶ 442 Ma (peak at 470 Ma) for the MC, and 484 ̶ 463 Ma (peak at 475 Ma) for the UC. The timing of migmatization inferred from intrusive relationships (ca. 480 Ma in the MC) and zircon rim ages (477 Ma) of a migmatite in the LC suggests that crustal melting was synchronous to slightly older than indicated by the magmatic peak. Harker diagrams show that the three crustal sections of the arc experienced similar magmatic differentiation. They show a wide range of SiO₂ values (from 48 to 75%) with decreasing contents of TiO₂, Al₂O₃, MgO, FeO^t, and CaO, but increasing alkalis. Additionally, the LC shows a greater range of SiO₂ and higher variations in alkaline content than the UC or MC. This observation is consistent with the Sr isotope composition of the LC which trends towards metasedimentary members signifying crustal contamination. We conclude that mantle melting was the main process in the generation of the metaluminous magmas of the Famatinian arc in a relatively thin crust (ca. 43 km) with some contamination of magmas from melting of continental crust in the lower crustal levels. At shallow levels, both I- and small granitic S-type (Crd-Bt) plutons were emplaced together.