Abstract

U-Pb zircon dating of a finegrained metamorphosed porphyritic siliceous magmatic rock from Rio Puelo (Continental Chiloé; ca.72.0◦W/41.7◦S) yields a Lower Devonian (ca. 395 Ma) igneous crystallization age. This is broadly similar to ages previously obtained for igneous rocks from the two contrasting Devonian magmatic belts located east and west of the main Andes in this part of Patagonia, namely the Foreland Devonian Domain (FDD) and the Pacific Devonian Domain (PDD). The dated zircon grains have oxygen isotopic ratios, characteristic of the FDD (δ18Ozircon 6.6 to 9.5‰), and this indicates a westerly extension of the igneous rocks of this domain as far as the NNE trending Liqui˜ne - Ofqui Fault Zone (LOFZ). Detrital zircon U-Pb ages for a metasedimentary rock from the FDD (Piedra Santa area; ca. 70.4◦W/39.2◦S) indicate a maximum depositional age of ca. 365 Ma, but it shows Devonian detrital zircon age spectra and O-isotope compositions (δ18Ozircon 6.5 and 7.9‰) similar to those of metasedimentary units in the PDD south and west of Rio Puelo. It is suggested that the Devonian igneous rocks of the FDD were subject to erosion shortly after their emplacement with much of the eroded material being deposited in the PDD arc after the accretion of Chaitenia. The Liqui˜ne Ofqui Fault Zone, still active today, lies in the contact area between the FDD and the PDD, suggesting that its location might be inherited from the original suture contact between these two Devonian magmatic belts.