Elemental and mineral composition of DSDP Leg 84 Holes

Ultramafic rocks were recovered from the lower parts of holes at four sites (567, 566, 569, 570) drilled on Leg 84. These sites are on the landward slope of the Middle America Trench off Guatemala. Harzburgites, cumulative texture peridotites, gabbros, dolerites, and amphibolite facies rocks were recognized by pétrologie and geochemical analysis (Xray fluorescence, microprobe). Some of these rocks belong to an ophiolitic sequence. They form the basement of the landward slope of the Guatemala Trench upon which lower Eocene sediments were deposited. As a whole, the ophiolitic rock has much in common with various ophiolitic massives known on land all over the world (e.g., Antalya, Oman), particularly the ophiolitic sequence of the Santa Elena Peninsula of Costa Rica. The age, the abundance of harzburgites, and the structural position of the mafic and ultramafic rocks of Leg 84 suggest that they could represent an offshore equivalent of the Santa Elena ultramafic rocks. In any case, the first ophiolites to have been recovered from the slope of an active margin are those of the drilled mafic and ultramafic rocks of Leg 84.