Easter Island Dyke

This dyke, orientated approximately east-west, extends over a distance of 29 km but there are discontinuities due to shearing and displacement. Deeper levels of the dyke are exposed to the west, and a higher heterogeneous roof zone to the east. In the west the dyke is some 80 m thick and ultramafic, the rock consisting of 40% olivine, 25% Ti-augite, together with very minor antiperthite, interstitial Ti-alkali amphibole, biotite and 10% opaques including ilmenite, magnetite and Fe-Ni-Cu sulphides. Further east such olivine-bearing rocks occur along the margins of the dyke, the central zone consisting of about 35% antiperthite, 25-30% Ti-augite rimmed by green sodic pyroxene and blue sodic amphibole, and about 12% Ti-alkali amphibole with blue rims, biotite and ilmenite. There is a thin central pegmatitic zone of alkali feldspar, blue amphibole and abundant apatite. Further eastwards the dyke becomes more feldspathic still, the mafic border zone disappears, and the pyroxene decreases but becomes more sodic. The most easterly parts of the dyke are essentially albitites and bostonites with chloritized mafic minerals and abundant hematite throughout the rock. There are three lines of diatreme pipes running parallel to the dyke, some of the pipes of the middle line being in contact with the Easter Island Dyke. They contain various angular or rounded blocks of country rock in a matrix of comminuted rock fragments or red bostonite. Major and trace element analyses for 20 Easter Island dyke rocks are available (Badham, 1979).