Rodriguez Island is offset from the center of an elliptical submarine platform on the south side of the Rodriguez Fracture Zone, about 400 km west of the crest of the Mid-Indian Ocean Ridge. It consists of rather uniform alkali basalts (nepheline-normative), with a local development of phonolitic segregation veins and occasional pyroclastic deposits. Gabbroic xenoliths and xenocrysts, which are widely distributed and locally abundant, are interpreted as fragments of a layered intrusion (cryptic variation at least An64→38) formed by gravitative differentiation in a relatively high-level rnagma chamber at an earlier period of the volcano's history, and brought to the surface during a rejuvenation phase of volcanic activity. This phase, which has produced the present island, seems to have occurred after marine planation of the main volcanic infrastructure. A fringe of strongly false-bedded calcarenites on the windward side of the island is interpreted as an aeolian deposit formed by accumulation of debris derived from the lagoonal reef-flat during a period of glacially lowered sea level in Pleistocene times.

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