Olivine Nodules in Trachyte from the Jos Plateau, Nigeria

Abstract

THE Cenozoic volcanics on the Jos Plateau consist almost entirely of small flows of alkaline olivine basalt, erupted from widely scattered small cinder cones now in various stages of erosion. The trachytic Bokkos plug1 is a rare exception. The trachyte forming the plug is composed chiefly of tabular anorthoclase and pale green clinopyroxene, and it contains olivine nodules ranging in size from small angular fragments to ovoids up to 15 cm long. They are typically lherzolitic in their mineralogy (and practically identical with nodules from a New Zealand phonolite2). Magnesian olivine is optically neutral with γ = 1.681 ± 0.002; enstatite has γ = 1.676 ± 0.002; chrome diopside is bright green in hand specimen and has β = 1.685 ± 0.002; spinel is red-brown in section, with n = l.78 ± 0.01.