The geochronology and evolution of the young volcanic island of Réunion, Indian Ocean

Abstract

A detailed K-Ar study of subaerial lavas from Réunion, using whole rock samples, shows that the measured ages are fully consistent with the excellent stratigraphic control available. The amount of extraneous radiogenic argon present in three of the groups of basalt flows is about 1 ± 1 × 10 −13 mol/g, and for many of the younger lavas the content must be less than 3 × 10 −13 mol/g. However, extraneous argon has been detected in alkali feldspar and amphibole in hypabyssal drusy syenites that are exposed in the eroded core of Piton des Neiges volcano. The ages enable part of the history of the two volcanoes comprising Réunion to be elucidated. The subaerial basaltic shield building lavas of Piton des Neiges were erupted during at least three distinct episodes at about 2 m.y., 1.2-1.0 m.y. and 0.55-0.43 m.y. ago. Following a hiatus of about 0.1 m.y. a sequence of alkali andesites and basalts was erupted between about 0.35 and 0.07 m.y. ago. The younger, and still active volcano, Piton de la Fournaise, which is built entirely of basaltic lavas, was active from at least 0.36 m.y. ago. The focus of basaltic eruption has migrated progressively toward the southeast with time, and may indicate that Réunion has moved toward the northwest, away from the southwest branch of the mid-Indian Ocean ridge, across the source region for the magmas in the upper mantle.