AGES AND GEOLOGICAL SIGNIFICANCE OF THE IGNEOUS ROCKS FROM SEYCHELLES

Isotopic age dating of igneous rocks from the Seychelles and its relat ed Gondwana fragments reveals seven major igneous events related to fo ur phases of rifting during the fragmentation of Gondwana. Two phases of rift-related Late Precambrian granitic magmatism occurred on Mahe b etween similar to 710-680 Ma, followed by the Praslin/La Digue/Felicit e granites at similar to 665 Ma, then by basic dykes, which intruded t hese granites at similar to 620 Ma. The Early Jurassic (similar to 190 Ma) layers of devitrified ash can be correlated with the Karoo flood- basalt volcanism and its equivalents. Five important igneous events ar e recorded in the Cretaceous/Tertiary: i) similar to 135 Ma: basic dyk es temporally equivalent to the Marion hotspot-related Lebombo and Mov ene volcanics of South Africa and Mozambique. ii) similar to 124-113 M a: igneous interbeds possibly correlative of the postulated volcanic d evelopment of the Saya de Mahla from the Marion hotspot iii) similar t o 82-?65 Ma: tholeiitic basalts representing the formation of the Amir ante ridge complex iv) similar to 70-60 Ma: representing the Deccan ev ent and divisible into pre-Deccan tholeiitic (similar to 70-68 Ma), ma in Deccan basic (similar to 68-63 Ma) and post-Deccan acidic magmatism (similar to 63-60 Ma) v) similar to 47 Ma: basalts extruded when the d rift of Seychelles/Mascarene coincided with the Deccan-Reunion hotspot at Saya de Mahla. Acid volcanics from the northeast coast of Madagasc ar, and offshore the Indian west coast are related to the beginning (s imilar to 96 Ma) and end (similar to 84 Ma) of rifting between Seychel les/India and Madagascar.