The Famatinian orogeny in southern South America: evidence of lithosphere reworking and continental subduction in the early proto-Andean margin of Gondwana

Abstract

To interpret the geological and tectonic evolution of the Famatinian orogen and the magma sources in different episodes we look at evidence at the 27º?33ºS type sector in the Sierras Pampeanas, the Precordillera, and northeastern Patagonia. Previous geological, geochemical and geochronological results are reviewed together with new 177Hf/176Hf and 18O/16O data for dated zircon, U?Pb SHRIMP crystallization ages and whole-rock Sr and Nd isotope compositions. Evolution of the Famatinian orogen in three main stages at 486?468 Ma is invoked to explain different characteristics in four recognized domains in the type sector. (1) ca. 482-486?: shallow subduction stage; compression thickens the foreland and squeezes out the asthenospheric mantle wedge; trondhjemite plutons with adakitic signature are emplaced in the Foreland Domain; ensialic basins started opening and early metaluminous and peraluminous granites intruded the Central Domain. (2) 480?474 Ma: slab rollback; a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle; full development of ensialic basins and early emplacement of both metaluminous granites and highly-peraluminous batholiths. (3) 472?468 Ma: continental collision and slab breakoff due to subduction of continental crust; a flare-up magmatic episode at the western edge of the Central Domain; K-bentonites in the Precordillera, leucogranites in the Western domain and scattered emplacement of metaluminous and peraluminous plutons in all Famatinian domains. Stages (2) and (3) developed during a high-T regime (hot orogen). Although asthenospheric mantle was a necessary heat source for lithospheric melting, its contribution to the growth of Early Paleozoic crust was apparently very minor. Recycling of Mesoproterozoic lithosphere, including the subcontinental mantle, coupled with crustal melting of Early Paleozoic metasedimentary sequences, accounts for most of the Famatinian magmatism. Consistent results from the Central Andes and East Antarctica confirm that the early stages of SW Gondwana were dominated by lithospheric reworking processes.