Okayama University, Japan

Late Miocene-Pliocene Taitao Ophiolite is located approximately 50 km SW of the Chile triple junction where the Chile Ridge subducts beneath the South American Plate. Field and laboratory studies, together with satellite image analyses, revealed lithological distribution and deformation in the Taitao Ophiolite. Gabbros with N-MORB compositions and ultramafic rocks are folded into a complex pattern, whereas only evidence for block rotation was found in the overriding sheeted dike complex and volcanic rocks which have enriched compositions. We applied SHRIMP U-Pb and fission track dating on zircon separated from gabbros and a dacite dike in the sheeted dike complex. The radiometric ages of gabbros range from 5.9 + or - 0.4 Ma to 5.6 + or - 0.1 Ma. These ages coincide within error range and imply rapid intrusion and cooling of gabbros. U-Pb age of the dacite dike was 5.2 + or - 0.2 Ma. Center of magmatic activities must have migrated northward from gabbros to the dike complex during the oblique subduction of the Chile ridge. U-Pb ages of clastic zircons from volcaniclastic sequence were similar to those of gabbros. A part of ophiolite must have been uplifted above the sea level to separate depositional basin from the hinterland composed of a Mesozoic accretionary complex just after the emplacement. Nevertheless, patterns of ocean-floor metamorphism were similar to those of ODP Site 504B and other ophiolite in the world. Field relations and radiometric ages of granite plutons that surround the ophiolite indicate that their origin was closely related to the ophiolite emplacement. Detailed petrological study of 4 Ma Cabo Raper Pluton, located nearest to the trench, suggests that the magmas were formed due to partial melting of subducting oceanic crust under amphibolite conditions. Other plutons have U-Pb ages ranging from 5.7 Ma to 4.9 Ma. Our data indicate that the emplaced of the Taitao Ophiolite and related granites took place in a shorter period than previously thought. A short segment of the Chile ridge system must be emplaced during the 6 Ma ridge collision to form the Taitao Ophiolite. Most granite magmas with various compositions were also developed during the subsequent subduction of the same ridge.