Saut D'Eau And Thomazea

Two areas of alkaline lavas of about 18x2 km and 5x2 km are located some 30 km north-northeast and northeast of Port au Prince. They lie north of the Cul de Sac graben and overlie Eocene to Miocene sedimentary rocks. At Saut d'Eau the lavas flowed from a 200 m high scoria cone, La Vigie, while the Thomazeau lavas were erupted near to the northern boundary fault of the Cul de Sac graben; the associated finely bedded pyroclastic rocks are taken to indicate fragmentation of the lava on entering a lake in the graben, or else the sea (Wadge and Wooden, 1982). The rocks from the two localities are very similar petrographically and are nepheline basalts, with or without hauyne and melilite. The typical rock consists of olivine (15%) and augite (15%) phenocrysts in a groundmass of augite, fresh nepheline (15%), zeolite (8-10%), magnetite, ilmenite, apatite and melilite (10- 12%), and a little carbonate and analcime. Some rocks contain 3-5% of hauyne euhedra and a large bomb from one locality contained small yellowish crystals of melilite. The norm of an analysed specimen gave 15.7 ne and 8.7% lc (Woodring et al., 1924). Sr isotope ratios are available in Wadge and Wooden (1982).