Timing constraints on building an intermediate plutonic arc crustal section: U- Pb zircon geochronology of the Sierra Valle Fértil-La Huerta, Famatinian arc, Argentina

Abstract

[1] The Sierra Valle Fertil Range in northwestern Argentina exposes a tilted crustal section through the Ordovician Famatinian arc, from >25 km to shallow crustal paleodepths. Fourteen new U-Pb zircon crystallization ages of magmatic rocks from Sierra Valle Fertil area show that this section of the arc was built over a short time interval during the Ordovician, between 485 and 465 Ma. Zircon rim ages demonstrate that high-grade metamorphism and migmatization were synchronous with magmatic emplacement. Inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the Famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto-South American continent in the Ordovician, which represents a part of Gondwana. Docking of the Precordilleran terrane outboard of proto-South America led to the cessation of arc magmatism in the Valle Fertil area and preservation of the arc in its early stages after <60 Myr of subduction during the Pampean and Famatinian magmatic stages. Our age data from the Sierra Valle Fertil indicate that wet mafic magmas were emplaced into the section at a rate typical for modern island arcs, ∼30 km3 km−1 Myr−1. The production of intermediate to felsic magmas in the upper plate was a result of partial melting of the metasedimentary framework and hybridization with mantle-derived melts and resulted in the generation of tonalites, granodiorites, and granites in the upper crust at a rate of ∼100 km3 km−1 Myr−1, similar to major magmatic flare-ups in the mature arcs of North American Cordillera. The Sierra Valle Fertil arc section is a type example of an immature Cordilleran arc, before major crustal thickening and crustal overturn takes place in the upper plate. The Famatinian arc section may also be an equivalent to the crustal architecture of more mature modern island arcs, such as parts of the Aleutians, Caribbean, and Japan.