U–Pb geochronology of Seychelles granitoids: a Neoproterozoic continental arc fragment

Abstract

New high-precision U–Pb zircon ages for 14 granitoid rocks of the Seychelles, including samples from Mahé, Praslin, La Digue, Ste. Anne, Marianne, Fregate and Recifs Islands, yield dates between 748.4±1.2 Ma and 808.8±1.9 Ma, interpreted as representing magmatic crystallization ages. U–Pb zircon ages as young as 703 Ma have been reported by other workers, and the time span of Seychelles magmatic activity, therefore, is ∼100 Myr. The vast majority of ages, however, fall in the period 748–755 Ma, suggesting a major period of granite plutonism at this time. At least some Seychelles dolerite dikes have equivalent ages, indicating the contemporaneity of granitic and basaltic magmas, and supporting field and chemical evidence for the production of minor quartz dioritic rocks by hybridization and magma mingling. Possible correlatives of the late Neoproterozoic (∼700–800 Ma) granitoids and dolerites of the Seychelles include volcanic and plutonic rocks in Madagascar and northwestern India (Malani Igneous Suite, Rajasthan), which may have formed in a continuous continental (Andean-type) arc located at the western margin of the Rodinia supercontinent. This idea is more consistent with the time span of magmatism, petrologic character of the igneous rocks, and paleomagnetically determined reconstructions, than the commonly held view of an intra-plate extensional setting for the Seychelles and Malani Igneous Suite.