Tonian Arc Magmatism in Central Madagascar: The Petrogenesis of the Imorona-Itsindro Suite

The East African Orogen is one of the largest orogens that formed during the Ediacaran to Cambrian amalgamation of Gondwana. In the Mozambique Belt, the East African Orogen represents the amalgamation of Neoproterozoic India with the Congo-Tanzania-Bangweulu Block. In the Arabian-Nubian Shield, the orogen consists of a pre-Neoproterozoic continental terrane surrounded by Neoproterozoic juvenile oceanic arc-like terranes. A large ocean named the Mozambique Ocean divided these Neoproterozoic landmasses. Many oceanic sutures tie together the various terranes in the northern orogen, but the location of potential sutures become less obvious moving south. The ca. 100 My duration of subduction of the Mozambique Ocean resulted in voluminous magmatism in Madagascar, named the Imorona-Itsindro Suite, but the location of possible suture zones in Madagascar is controversial. Understanding the petrogenesis of this suite is critical for developing Neoproterozoic paleogeographic plate reconstructions. The Tonian age of the ImoronaItsindro Suite is well documented, but the geochemistry of the suite provides ambiguous evidence for the tectonic setting. Geochemically, these rocks are predominantly calc-alkaline, with characteristics consistent with emplacement within a volcanic or continental arc. The suite has variable but mostly high Sr-87/Sr-86 and low Nd-143/Nd-144 signatures, indicating significant crustal involvement. Changes in subduction-zone dynamics, accompanied by variable crustal anatexis and assimilation, contributed to geochemical ambiguities that hav(e) prompted some authors to suggest an active rift as an alternative tectonic model. However, the geochemical signatures are more compatible with an Andean-like arc tectonic setting. We suggest that a prolonged history of subduction (>100 My) provided sufficient time for the continental arc to mature and for the development of shallow mantle source metasomatism.