Tectonic evolution of the western margin of Gondwana inferred from syntectonic emplacement of Paleozoic granitoid plutons in northwest Argentina

Abstract

The Cerro Durazno, Brealito, and Cobres granitoid plutons belong to the eastern Ordovician magmatic belt of northwestArgentina and are associated with a network of NW- and N-striking ductile shear zones. A structural analysisof these plutons and their metamorphic host rocks was conducted to elucidate mid- to upper-crustal magmatism andtectonism at the western margin of Gondwana. Pluton emplacement took place at 480?455 Ma, toward the end ofa ca. 75-Ma-long episode of regional deformation and high-temperature metamorphism. The Cerro Durazno andBrealito plutons were probably emplaced as NE-SW-striking, vertical magma sheets that coalesced into large magmabodies under overall E-W shortening that generated SW-dipping metamorphic foliations at a later stage of plutonformation. These fabrics were superimposed to the magmatic structures in the granite plutons and to the high-grademetamorphic structures of the host rocks. At the northern pluton margins, ductile high-strain zones, notably theAgua Rosada Shear Zone north of the Cerro Durazno pluton, further localized ascent of granitoid magma. Similarly,emplacement of the Brealito and Cobres plutons is genetically related to prominent shear zones. Geometry andmetamorphic character of the shear zone network in the eastern Ordovician magmatic belt point to orogen-parallelextension and transverse shortening of the belt under variable metamorphic conditions. The width as well as protracteddeformation, high-temperature metamorphism, anatexis, and granitoid magmatism of the magmatic belt are keycharacteristics of a hot, wide orogen that evolved during the lower Paleozoic at the western margin of Gondwana.Remarkable geotectonic similarities with respect to the modern Andes point to comparable plate-tectonic conditionsat the lower Paleozoic and the Cenozoic convergent plate margins.