Isotopic Dating and Geomagnetic Polarity Studies on Volcanic Rocks from Mauritius, Indian Ocean
Results of a geochronological and paleomagnetic investigation of the volcanic island of Mauritius are reported. Potassium-argon dates indicate that the main shield volcano was built subaerially between about 7.8 and 6.8 m.y. ago in the early Pliocene. Some evidence is presented for caldera formation following the construction of the volcanic shield. Profound erosion subsequently destroyed much of the volcano, leaving only peripheral steep-sided massifs. Lavas of the Younger Volcanic Series were erupted from about 3.5 m.y. ago until less than 0.2 m.y. ago, flooding the denuded stumps of the shield volcano. This activity was intermittent and a hiatus is recognized from about 2 to 0.7 m.y. ago, which we have taken as the break between the eruption of the Early and Late Lavas of the Younger Volcanic Series. The mean directions of magnetization of the three groups of lavas are not significantly different from one another and are close to an axial dipole direction. The combined paleomagnetic and dating studies on the lavas also yield further information on the geomagnetic polarity time scale; 16 results on rocks with ages lying between 3.5 and 0.17 m.y. are consistent with the established scale for this period of geological time. Of particular significance are results that confirm the Mammoth reversed event at 3.0 m.y. ago, and the age of the boundary between the Gauss normal and Gilbert reversed polarity epochs at 3.35 m.y. Nineteen results were obtained on rocks whose ages are between 4.6 and 7.9 m.y., and these data generally are consistent with the polarity time scale based upon extrapolations from the sea-floor spreading hypothesis. The evolution of the Mascarene Islands, comprising Mauritius, Reunion and Rodriguez is briefly reviewed and it is concluded that each island had developed independently, and that no correlation of age with respect to distance from the Mid-Indian Ocean Ridge is observed. The volcanism that has built Mauritius and Reunion may be a continuation of the development of the Mascarene Plateau with migration of the activity generally southward with time.