A review of the Famatinian Ordovician magmatism in southern South America: evidence of lithosphere reworking and continental subduction in the early proto-Andean margin of Gondwana

RAPELA, C.W; PANKHURST, R. J.; CASQUET, C.; DAHLQUIS, J.A.; FANNING, C.M.; BALDO, E.G.; GALINDO, C.; ALASINO, P.H.; RAMACCIOTTI, C.D.; VERDECCHIA, S.; MURRA, J.A.; BASEI, M.A.S.

EARTH-SCIENCE REVIEWS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2018 vol. 187 p. 259 - 285

0012-8252

Along the proto-Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, the Early-Middle Ordovician Famatinian orogeny was the first orogenic event following assembly of the supercontinent. Previous isotope studies ofthe igneous and (meta-)sedimentary rocks of southwestern Gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin. To interpret the geological andtectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern South America, including the 700 x 600 km type sector of the orogen at 27º?33ºS 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. Four geological domains are recognized in the type sector of the Sierras Pampeanas (Western, Central, Eastern and Foreland Famatinian domains). Magmatism is mostly restricted to the interval 463 ±4 to 486 ±7 Ma, with the most intense period of emplacement between 468 and 472 Ma constituting a magmatic flare-up. Granitoid emplacement in both northeastern Patagonia and the Cordon de Lila (Puna Altiplano, Chile) was effectively synchronous with that in the Sierras Pampeanas, defining a continuous belt. Combined geochemical and isotopic data (whole-rock Sr, Nd; Hf, O in zircon) indicate that the source of calcic metaluminous suites is the subcontinental lithosphere ? both mantle and mafic lower crust ? with variable contamination by the Early Paleozoic metasedimentary country rocks. The lithospheric mantle involved is assumed to underlie the outcropping 1330?1030 Ma Grenville-age basement of the Western Domain, which exhibits tectonic characteristics of active continental margin in the north and oceanic arc-back arc in the south. The latter sector is the potential source of some minor Famatinian igneous rocks with less evolved isotopic compositions, although a restricted asthenospheric addition cannot be discarded in this case. Minor peraluminous granites are spatially associated with the metaluminous sequence, but major highly-peraluminous batholiths occur on the eastern flank of the Central Domain. Field relations and geochemical/isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post-early Cambrian metasedimentary sequence comprising the host country rocks. Episodic evolution of the Famatinian orogen in three main stages at 486?468 Ma is invoked to explain different Characteristics in the four recognized domains in the type sector. (1) ca. 482-486?: shallow subduction stage; generalized compression thickens the foreland and squeezes out the asthenospheric mantle wedge; trondhjemite plutons with adakitic signature are emplaced in the Foreland Domain, far from the continental margin; ensialic basins started opening and early metaluminous and peraluminous granites intruded the Central Domain. (2) 480?474 Ma: slab rollback; a mainlyextensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle; full development of the marine 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; voluminous metaluminous magmatism at the western edge of the Central Domain (the flare-up episode); 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 the high-T regime of a hot orogen. Although asthenospheric mantle was a necessary heat source for lithospheric melting, its material 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 mostof the Famatinian orogenic magmatism. Consistent results from the Central Andes and East Antarctica confirm that the early stages of the Terra Australis orogen in SW Gondwana were dominated by lithospheric reworking processes.