A medium-grained, foliated amphibolite found in the Cordillera Central of the Dominican Republic near the Haitian border yields concordant K-Ar ages of 123 ± 2 m.y. for hornblende and plagioclase. The rock exhibits a low Rb/Sr ratio (0.0217) and present-day 87Sr/86Sr ratio (0.7024) but differs in chemical composition from metamorphosed mafic rocks of the Duarte Formation. A previously reported K-Ar age of 127 ± 6 m.y., obtained on a hornblendite body from the Duarte Formation, agrees closely with the amphibolite age. The hornblendite also differs in composition from the metamorphosed mafic rocks of the Duarte Formation and from the amphibolite. The 123- to 127- m.y. ages are interpreted to represent a minimum date for a metamorphic event that affected the basement complex of Hispaniola. An unpublished K-Ar date for the Bermeja complex in Puerto Rico falls within this range, but older ages have been obtained from rocks in Cuba. A date of about 120 m.y. indicated by one interpretation of a previously reported conformable lead isotopic composition for galena from the Pueblo Viejo gold deposit in the upper part of the Los Ranches Formation volcanic rocks in the Dominican Republic also agrees closely with this 123- to 127-m.y. range; together with other data, it suggests that the extrusion of Los Ranchos volcanic rocks and metamorphism of the Duarte Formation could be genetically related.

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