1989 年 1989 巻 34 号 p. 9-22
Late Precambrian granitic rocks occurring in the Mahé and Praslin island groups of the Seychelles Islands are divided petrographically and chemically into two assemblages. One is older (ca. 710Ma) I-type assemblage of granodiorite and tonalite, and the other is younger (ca. 680Ma) A-type assemblage of sub-alkaline granite and typical alkaline granite.
Fe and Na contents of the ferromagnesian minerals increase systematically from I-type granodiorite through A-type sub-alkaline granite to A-type alkaline granite. These evolutionary trends of the ferromagnesian minerals suggest an intimate genetic relation between I-type magma and A-type magma in Seychelles.
The same association of I-type and A-type granites as in Seychelles occurs in the Hijaz region of the Arabian Shield, where the activity of I-type granite took place between 820 to 715Ma and that of A-type granite did between 686 to 517Ma. Such an association of granites is considered to occur extensively in the Arabian-Nubian Shield.
Undoubtedly, the Seychelles Islands are continental fragment left behind as a result of the disruption of Gondwanaland and, as stated above, petrological correlation suggests that the Seychelles Islands originally joined with the Arabian Shield.