?El orógeno acrecional Devónico de la Cordillera Norpatagónica: nuevas perspectivas en la evolución geológica paleozoica de la Patagonia?.

RAPELA, C.W.

Buenos Aires

Conferencia; Ciclo de Conferencias 2021, Asociación Geológica Argentina; 2021

Asociación Geológica Argentina

The Andean basement of NW Patagonia contains remnants of a Devonian accretionary orogen that lastedfor about 50 Ma. We present and review U-Pb geochronology, major and trace element geochemistry andO- and Hf-isotope analyses of zircon for Devonian igneous and metasedimentary rocks from the westernedge of the North Patagonian Massif to the Pacific coast. Two contrasting magmatic domains are characterised.The Foreland Devonian Domain (FDD) of the North Patagonian Massif is dominated bysubduction-related calcalkaline granite, granodiorite and tonalite emplaced in the interval c. 405?395 Ma and here named the San Martín arc. They are generally enriched in alkalis, light rare earth andLarge Ion Lithophile elements, and have igneous zircon with d18O mostly in the range +6 to +9? andƐHft typically +3 to 5, indicating magmas that incorporated continental crustal material, possiblyMesoproterozoic. A younger (374 ± 3 Ma) body in this group has lower d18O (down to +5?) and Hfisotopecharacteristics at the primitive end of this range. The Pacific Devonian Domain (PDD) lies onthe western flank of the Andes and has uniformly primitive character. Granitic rocks here represent amore extended magmatic history (c. 395?355 Ma) and are mostly subalkaline tonalites with d18O(+4 and closer to Devonian depleted mantle composition. The PDD also has metabasalts of bothN-MORB and E-MORB affinities, interbedded with Devonian turbidite metasandstones in which detritalzircon O- and Hf-isotope compositions and morphology indicate coetaneous derivation from the FDDarc. The results lead to a tectonic model involving continental arc, forearc rifting, Chaitenia oceanic arcand marginal basin formation, double arc subduction, and marginal basin closure in Mississippian times.This is recognisable as part of a more extensive Devonian accretion scenario along the SW Gondwanamargin.