Mantle—crust transition zone and origin of wehrlitic magmas: Evidence from the Oman ophiolite

Abstract

Abundant exposure and well-preserved outcrops in the Oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment. The mantle-crust transition zone shows large lateral variations in thickness and composition, but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations, and gabbro lenses. Compositionally, the transition zone marks a gradational passage from mantle to crust. The Moho may be placed at the base of the continuous gabbroic crustal section. Structures both within the transition zone and near the base of the crust are nearly parallel to the Moho, however, the dominant flow mechanisms differ from the upper mantle to the lower crust. Plastic flow dominates in the upper mantle, while viscous flow, resulting in strong magmatic fabrics, dominates at crustal levels. Wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted. The parent magma of the wehrlitic instrusions is in fact a crystal-melt mixture composed of olivine and chromite, probably including a component of mantle xenocrysts and xenoliths, and of reequilibrated basaltic melt.