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K-Ar analyses of mineral separates from tonalitic intrusive rocks in the Dominican Republic yield ages of 80 to 82 Ma for the unfoliated Medina stock and Jautia batholith at the southern end of the Cordillera Central. Hornblende separates from the El Bao and Loma de Cabrera unfoliated tonalite batholiths in the northern part of the Cordillera yield ages of about 68 to 70 Ma, although coexisting biotites in two samples are significantly younger. These ages, in combination with published mineral separate and whole-rock ages from the area, suggest that most unfoliated tonalite magma was emplaced from about 87 to 80 Ma and that later, less abundant tonalite magmatism took place at 50 Ma, and possibly at a less well-defined interval between 70 and 50 Ma. Geologic relations, considered in the light of these and other published K-Ar data, indicate that the foliated tonalite magmas were emplaced shortly prior to the 87- to 80-Ma interval. Although all episodes of magmatism are widespread, the distribution of measured ages suggests that most of the tonalitic magma in the southern part of the Cordillera Central was emplaced in the 87- to 80-Ma period, whereas volumetrically important tonalitic magmatism persisted to later periods in the northern segment of the Cordillera Central.

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