The archipelago of
São Sebastião,
including its principal island
Ilhabela,
is emplaced into the central part of the
Ribeira Belt
(Northern
Mantiqueira Province).
Similar to
Itatiaia in the north,
the archipélago is composed of intrusive bodies which were formed during the upper Cretaceous (80 Ma)
by
alkaline magmatism,
probably caused by the deflected melts of the Trindade Mantle Plume.
In that time, molten alkaline rock (magma) intruded into the Earth´s crust,
where it slowly cooled and crystallized, forming ingeous rocks. As this happened
below the surface these rocks are called intrusive or plutonic rocks.
