|
Northwest Indian Ocean and Gulf of Aden |
1 Geologisch-Paläontologisches Institut, Universität Kiel, Olshausenstr. 40, D-2300 Kiel, Germany
2 Department of Geography and Geology, University of St Andrews, Fife KY16 9ST, UK
The rocks of the Seychelles can be divided into two age groups, namely Precambrian granites and younger (Cretaceous/Tertiary) intrusive rocks. The latter can be further subdivided into alkaline ring complexes (as found on the islands of Silhouette and North Island) and basic dykes (on Praslin, Felicité and Mahé islands). Evidence from offshore seismic work and drill holes suggests that Cretaceous/Tertiary magmatism occurred over the whole Seychelles Bank, producing both flood basalts and central volcanic complexes. The flood basalts extend at least as far south as 10°S/60°E.
The younger igneous rocks of the Seychelles show close similarities to the Deccan igneous rocks of India. Tholeiitic dykes from Praslin have previously been shown to resemble Bushe Formation tholeiites from the Deccan, and here we show that the Felicité Island dykes also resemble Bushe. We show also that the alkaline dykes of Mahé and North Island are chemically similar to the dykes at Murud on the west coast of India. Isotopically the Seychelles undersaturated rocks fall within the fields of the Deccan tholeiites.
In India, alkaline magmatism post-dates the tholeiitic magmatism; the age difference is of the order of 3 Ma. This is similar to the age difference between shield-building and rejuvenated-stage magmatism on Hawaiian volcanoes, which has been related to reactivation of the volcanoes by the passage of the Hawaiian Arch. We propose that the Deccan alkaline magmatism is a continental equivalent of oceanic rejuvenated-stage volcanism.
