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Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia
Institution:1. GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany;2. Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005 Paris, France;3. Instituto de Geociências, Rua do Lago, 562, Universidade de São Paulo, 05505-080 São Paulo, Brazil;4. Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18071 Granada, Spain;5. Andalusian Institute of Earth Sciences, CSIC-Univ. Granada, Spain;6. Carrera de Geología, Universidad Andres Bello, Sazie 2119, Santiago, Chile;1. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;1. Laboratoire de Géologie, Ecole Normale Supérieure, UMR CNRS 8538, 24 rue Lhomond, 75005, Paris, France;2. Sorbonne Universités, UPMC Univ Paris 06, UMR 7193, ISTeP, F-75005, Paris, France;3. CNRS, UMR 7193, ISTeP, F-75005, Paris, France;4. IUF, F-75005, Paris, France;5. Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl–Liebknecht–Straße 24–25, Potsdam–Golm, 14476, Germany;6. Eurasia Institute of Earth Sciences and Department of Geology, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey
Abstract:Rare remnants of a Mesozoic subduction high pressure (HP) accretionary complex are exposed on Diego de Almagro Island in Chilean Patagonia. We herein focus on the Lazaro unit, a coherent slice of oceanic crust exposed on this island that has been first affected by high temperature (HT) metamorphism followed by a lower temperature deformation event (LT). Its Pressure-Temperature-time (P-T-t) evolution is reconstructed using field and petrographic observations, phase relations, thermobarometry and geochronology. Remnants of a primary amphibolite to HP granulite-facies event in mafic rocks comprising garnet (with ilmenite exsolutions), diopside, trondhjemitic melt, pargasite, plagioclase ± epidote are reported for the first time in neosomes, indicating peak P-T conditions of 1.1–1.3 GPa and c. 750 °C. This peak T paragenesis has been thoroughly overprinted by a phengite-chlorite-actinolite assemblage during isobaric cooling down to c. 450 °C. U-Pb dating of zircon metamorphic rims from a metasedimentary rock yielded a homogeneous age population of 162 ± 2 Ma for the HT event. Sm-Nd dating of two peritectic garnet-bearing samples yield ages of 163 ± 2 Ma and 163 ± 18 Ma for the HT event. Multi-mineral Rb-Sr dating of a metasedimentary rock overprinted by LT deformation suggests retrograde shearing between 120 and 80 Ma. Our results show that the HT event in the Lazaro unit took place at around 160–165 Ma, shortly before the onset of Patagonian Batholith emplacement. Partial melting of subducted oceanic crust reported in the Lazaro unit is related to the early stages of hot subduction along the Gondwana western margin. The Lazaro unit remained at c. 40 km depth along the subduction interface for > 80 Ma, recording the deformation and long-term cooling of the subduction channel environment until the upper Cretaceous.
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