The North Patagonian batholith at Paso Puyehue (Argentina-Chile). SHRIMP ages and compositional features

New U–Pb SHRIMP zircon ages together with a geochemical and isotopic (Sr and Nd) study have been carried out on a hornblende gabbro, a biotite granodiorite, and host andesitic rocks of the North Patagonian batholith at Paso Puyehue, close to the Argentinean-Chilean border. The results yield ages of 18 ± 1 Ma for the gabbros and 12.4 ± 0.3 Ma for the granodiorites. An age of 125 ± 2 Ma was obtained for the host volcanic rock, and an age of 22 ± 2 Ma for the basaltic trachy-andesite. The intrusive suite is composed of dominant hornblende-biotite granodiorites and subordinate hornblende gabbros and diorites. These form a typical metaluminous calc-alkaline series. The time-isotopic magmatic evolution of plutonic and volcanic rocks shows an early Eocene beginning of disparate isotopic ratios as a consequence of coeval primitive and more crustal compositions with no spatial constraints. This disparate isotopic assemblage is also coeval with the presence of the Aluk-Farallon-SAM triple junction at this latitude and the shift of the calc-alkaline magmatism from the batholith axis to the back-arc. Considerations are made for the possibility of a Paleogene slab window by slab detachment, and the development of a new subduction front since the beginning of Nazca Plate development at the 23 Ma major ocean plate rearrangement.