History of the Rodrigues Ridge, Indian Ocean: implications of the Réunion hotspot and the Central Indian Ridge

The Rodrigues Ridge (Indian Ocean) is a N100°E oriented submarine volcanic structure stretching eastward of the Mascarene Plateau and toward the Central Indian Ridge (CIR). The geodynamic origin of Rodrigues’ volcanism is a matter of debate because the ridge neither follows the track of the Réunion hotspot nor the fabric of the oceanic lithosphere. To decipher the origin of this volcanism, we investigated the construction history of Rodrigues Island (i.e., the emerged portion of the ridge), by means of geomorphology, field observations, geochronology, and geochemistry. The morphology of Rodrigues Island’s slopes, the shape of the coral shelf, and unconformities observed in the field suggest that the island was constructed in two stages, including formation of a subcircular shield edifice, followed by formation of a N070°E ridge. This scenario is confirmed by K-Ar dating of groundmass and (U-Th)/He dating of zircon from volcanic rocks, suggesting that the circular edifice grew from 2.7 Ma to 2.5 Ma. Then, after a ca. 0.3 Myr hiatus and subsidence of the island, volcanic activity resumed from 2.2 Ma to 1.1 Ma, resulting in formation of the present-day ridge shape of Rodrigues Island. These ages are much younger than the unpublished ages ranging from 9.7 to 7.5 Ma reported for submarine volcanic rocks dredged on the flank of the Rodrigues Ridges.

Major/trace element and Sr-Nd-Pb isotopic analyses of the samples further show that the two stages of subaerial volcanism are chemically relatively homogenous, but much more enriched in incompatible elements than samples from the submarine ridge. Rodrigues Island was thus built by rejuvenescent volcanism of the submarine ridge. Available bathymetric and paleomagnetic data show that the Rodrigues Ridge propagates toward the east onto a less than 3 Ma old oceanic lithosphere (Demets et al., 2005) toward the CIR as en-échelon N070°E segments, called the Three Magi and the Gasitao ridges. The subaerial ridge shape of Rodrigues Island may thus belong to this array of en-échelon segments formed in the past 3 Ma.

Collectively, all the pieces of information suggest that the N100°E Rodrigues Ridge grew by protracted volcanism from ca. 10 Ma to ≤7 Ma, then from ca. 3.5 Ma to ca. 1 Ma by the propagation toward the CIR and coalescence of en-échelon N070°E segments of rejuvenated volcanism. Intriguingly, this temporality is coeval with the volcanic activity of Mauritius and Réunion Islands along the track of the Réunion hotspot, and particularly with the rejuvenescent volcanism of Mauritius since ca. 3.5 Ma. This coincidence favors a scenario of Rodrigues volcanism formed by capture of the Réunion plume ascending material by the CIR. We will discuss our results and their implications on the volcanic, structural and  geomorphology history of the Rodrigues Ridge.