NEW GEOCHRONOLOGICAL CONTROL FOR THE TECTONOMAGMATIC EVOLUTION OF THE METAMORPHIC BASEMENT, CORDILLERA REAL AND EL-ORO PROVINCE OF ECUADOR

Some 150 new isotopic age determinations on metamorphic rocks from the Cordillera Real and parts of El Oro Province in Ecuador, using K-Ar, Rb-Sr, and Sm-Nd methods, help to clarify a complex succession of magmatic and tectonic events. The earliest regional metamorphic/plutonic event recognized, from the Tahuin Group in El Oro, is dated as between ca. 220 and 200 Ma (Late Triassic-Early Jurassic). Similar but less well constrained ages were also obtained from orthogneisses of the Sabanilla and Tres Lagunas subdivisions in the Cordillera Real. Major calc-alkaline granitoids were emplaced ca. 190-150 Ma (Middle-Late Jurassic) in the eastern part of the Cordillera Real, to the north of 2.S. and throughout the sub-Andean zone. Between ca. 140 and 120 Ma (Early Cretaceous), the Oriente region was uplifted and eroded and the Cordillera was affected by an important shearing (dynamothermal) event which resulted in the resetting of older plutonic ages. From ca. 120 to 85 Ma, conditions were relatively stable, but during ca. 85 to 65 Ma (Late Cretaceous), the Cordillera and Oriente were again uplifted. This uplift corresponds to a second widespread thermal overprinting, which produced a regional disturbance in the K-Ar isotopic systems. Throughout the Cordillera, a number of generally small, undeformed, dominantly lower Tertiary plutons are also present. A few older (i.e., pre-Mesozoic) dates have been obtained but their interpretation remains uncertain.