San Juan De La Maguana And Cerro Los Frailes

Limburgitic and alkali olivine basalt flows cover about 85 km2 in an area northwest of San Juan de la Maguana filling depressions in the underlying Tertiary Loro conglomerate. The lavas are variably scoriaceous or massive rocks associated with poorly consolidated tuffs and ashes and scattered vents marked by small cinder cones. The limburgite is relatively homogeneous throughout the area, comprising olivine, magnesian salite, titanomagnetite and glass; plagioclase is rare. A smaller area of lavas covering about 4 km2 at Cerro Los Frailes 30 km to the southeast is petrographically similar and probably part of the same volcanic province. Although two chemical analyses, out of seven, are hypersthene normative, five are nepheline normative with one rock having 17% ne and 5.6% lc in the norm. They are very similar chemically to the nearby Saut d'Eau and Thomazeau lavas of Haiti (q.v.). Sr isotope ratios are available (Michael et al., 1978).