Palaeomagnetic and petrological reconstruction of the Seychelles

Abstract

Abstract The geology of Mahe-Island and adjoining islets, Seychelles, consists of three types of late Precambrian granitic rock series: (1) a gneissose granodiorite-gneissose tonalite and amphibolite-leucogranodiorite series, (2) a porphyritic granite-pink granite series, and (3) a grey granite-gneissose grey granite series. All these granitic rocks are cut by dykes of doleritic compositions, mostly with a WNW-ESE trend. The gneissose granodiorite is a calcalkaline I-type granite with normative Ab ⪢ Or, the porphyritic granite is a sub-alkaline A-type granite with normative Ab > Or, and the grey granite is an alkaline A-type granite with normative Ab > Or. RbSr radiometric analyses yielded ages of 713±19 Ma for the gneissose granodiorite, 683±16 Ma for the porphyritic granite and 570±5 Ma for the grey granite. Petrological, geological, geochronological and geochemical studies suggest that the late Precambrian granitic rocks of the Seychelles are very similar to, and are well correlated with those of the Arabian-Nubian shield. 408 late Precambrian granite specimens were collected from the Mahe island group, Seychelles. Progressive alternating field demagnetization and thermal demagnetization have been successfully performed to isolate stable remanence with well-grouped direction from natural remanent magnetization of the granites. The stable remanences of the granite fall around a northerly mean direction, of which the declination is 348.5° and the inclination +48.7°. The stable remanences of the Mahe island group granite yield a palaeomagnetic pole at 54.0°N, 38.2°E. From the above data, in conjunction with reported polar wander path and plate rotation poles, it is concluded that the Seychelles were originally located near the eastern end of the Horn of Africa and have drifted to the present position since continental break-up.