The Devonian accretionary orogen of the North Patagonian cordillera

Abstract

The Andean basement of NW Patagonia contains remnants of a Devonian accretionary orogen that lasted for about 50 Ma. We present and review U-Pb geochronology, major and trace element geochemistry and O- and Hf-isotope analyses of zircon for Devonian igneous and metasedimentary rocks from the western edge 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 by subduction-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 and Large Ion Lithophile elements, and have igneous zircon with δ18O mostly in the range +6 to +9‰ and ƐHft typically +3 to −5, indicating magmas that incorporated continental crustal material, possibly Mesoproterozoic. A younger (374 ± 3 Ma) body in this group has lower δ18O (down to +5‰) and Hf-isotope characteristics at the primitive end of this range. The Pacific Devonian Domain (PDD) lies on the western flank of the Andes and has uniformly primitive character. Granitic rocks here represent a more extended magmatic history (c. 395–355 Ma) and are mostly subalkaline tonalites with δ18O (<+6‰) that fall within or below the normal mantle field: with one exception their ɛHft values are > +4 and closer to Devonian depleted mantle composition. The PDD also has metabasalts of both N-MORB and E-MORB affinities, interbedded with Devonian turbidite metasandstones in which detrital zircon O- and Hf-isotope compositions and morphology indicate coetaneous derivation from the FDD arc. The results lead to a tectonic model involving continental arc, forearc rifting, Chaitenia oceanic arc and 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 Gondwana margin.