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Viviana Avendaño, Universidad de Chile (Chile)
Mauricio Calderón, Universidad de Chile (Chile)
Francisco Hervé, Universidad de Chile (Chile)
Antonio Simonetti, University of Alberta (Canada)
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The Mesozoic Rocas Verdes ophiolites are tectonically juxtaposed in the Andes of the southern South America. They are interpreted as the remnants of a Late Jurassic to Early Cretaceous marginal basin developed along the southwestern margin of Gondwana that underwent seafloor-type hydrothermal metamorphism. The Tortuga ophiolite is located in the Fuegian Andes (ca. 56°S), form part of the Scotia Plate and preserves an incomplete ophiolite pseudostratigraphy, lacking ultramafic rocks. It consists of layered and massive and mostly fresh gabbro at the bottom, overlaying massive diabase bodies that grade upward to a sheeted dike complex. Plagiogranites have not been detected yet. These components underlie a more than 600 m thick succession of volcanic rocks, including massive and pillowed basalts, volcanic breccias and intercalated horizons of chert and siltstones. The mafic complex is apparently capped by the turbiditic and volcanoclastic successions of the (folded) Yahgán Fm. The age of the Tortuga ophiolite has been indirectly constrained by biofacies associations in the Tithonian to Valanginiana (150 -136 Ma) lower member of the Yahgán Fm. The minimum age of the ophiolite is bracketed by the early Late Cretaceous granitoids (98-82 Ma), from preceding studies, that intrude the submarine volcanic successions at Seno Grandi. In this area we collected samples of pillow basalts and their interstitial material. At the edge of pillow structures, spherical and concentric aggregates of titanite which occurs in companion with chlorite, epidote, actinolite, albite and quartz were found. They appear as granular individual polycrystalline bodies of spherical shape made up mainly by cryptocrystalline (0.5 um) titanite as determined by SAED (selected area electron diffraction) and TEM (transmission electron microscopy) images. They preserve a characteristic core and mantle texture. Tubular microstructures also occur. These textures, comparable in size and form to microorganisms (microbes) of modern oceanic crust and ophiolites, are thought to be the result of microbial action and subsequent mineral precipitation during seafloor metamorphism. Sixteen spherical aggregates of titanite in the same sample were dated by the U?Pb LA-MC-ICP-MS methods and yielded a latest Early Cretaceous (Aptian) metamorphic age (118.3 ± 3.5 Ma). This age represents a minimum age for the mafic rocks of the Tortuga ophiolite and is at least ca. 30 my younger than the Late Jurassic age determined through the dating of plagiogranite in ophiolite complexes located to the north (Sarmiento ophiolite) and east (Larsen Harbour ophiolite) in the discontinuous belt of the Rocas Verdes ophiolites.
Acknowledgements: Martin Reich and Christian Nievas are acknowledged for their guidance during the work with the TEM in the Geology Department of the University of Chile. This abstract is a contribution to Project ARTG - 04 of CONICYT and PBCYT
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