Abstract:

The Sierra de Fiambala, located in the northwestern Sierras Pampeanas, northwestern Argentina, exposes deep crustal igneous and metasedimentary rocks from the Paleozoic Gondwana margin. These rocks were affected by multiple episodes of deformation, metamorphism, and intrusion. The first phase of deformation involved the formation of a foliation (S1) during greenschist-facies metamorphism. The second phase of deformation was contemporaneous with amphibolite- to granulite-facies metamorphism and involved isoclinal folding with axial planar (S2) foliation. The intrusion of a granitic pluton (La Puntilla Orthogneiss) pre-dated the S2 foliation and metamorphism. The intrusion of the Fiambala Gabbronorite was contemporaneous with S2 structures and high-grade metamorphism in the Early Ordovician. The final intrusions were post-trectonic granites. The trace element chemistry of the Fiambala Gabbronorite indicates a subductionrelated setting for magmatism and deformation during the Early Ordovician. Pressure estimates from metamorphic assemblages coeval with the gabbronorite are 7 to 8 kbar (∼25 km). We interpret the Sierra de Fiambala to be an exhumed exposure from the deep levels of a magmatic arc that developed along the early Paleozoic Gondwana margin. The described tectonothermal events in the Sierra de Fiambala arc exposure are generally consistent with events that occurred in coeval deep-crustal and supracrustal exposures from other areas along the Gondwana margin. These observations allow a preliminary reconstruction of the various components from the early Paleozoic Gondwana active margin.