From passive margin to volcano–sedimentary forearc: The Tonian to Cryogenian evolution of the Anosyen Domain of southeastern Madagascar

• Similar detrital zircon spectra are present in most post-Archaean sedimentary rocks.
• Regional cross-cutting relationships date these rocks to the Tonian and Cryogenian.
• Intercalated volcanic rocks argue for a similar depositional age.
• Described strata were deposited along the western margin of the Indo-Antarctic plate.
• The tectonic domains of Madagascar acquired their current configuration in the Tonian.

The Anosyen Domain of southeastern Madagascar represents a part of a vast sequence of predominantly siliciclastic and carbonaceous sedimentary and felsic volcanic rocks that can be traced from Madagascar through southern India, Sri Lanka, and into East Antarctica. We present new age and geochemical data from the Iakora and Horombe groups: units that represent the sedimentary and volcanogenic components of the Anosyen Domain. Our data demonstrate that the siliciclastic sedimentary rocks of the Iakora Group contain predominantly 2400–1600 Ma aged detrital zircon with a modal age peak at approximately 1850 Ma. This age spectrum is comparable to those reported from the majority of the other sedimentary rocks exposed in Madagascar. The felsic volcanic rocks of the Horombe Group are of both metaluminous and peraluminous composition and have a similar chemistry and the same age as the Imorona–Itsindro Suite (840–700 Ma), a voluminous suite of continental arc-related granitoids that intrude central and northern Madagascar.Taken together we interpret the strata of the Anosyen Domain to have been deposited in an expansive continent margin basin that existed on the western ocean-facing margin of the Indo-Antarctic Craton. We infer this basin records both passive margin and forearc phases of deposition and, on the basis of the regional similarities in detrital zircon spectra and cross-cutting relationships, suggest the older passive margin phase of deposition occurred after c. 980 Ma. The younger forearc phase of deposition occurred in the Cryogenian synchronous with the development of the Imorona–Itsindro magmatic arc (840–700 Ma). This suite reflects east-dipping subduction beneath the western margin of the Indo-Antarctic Craton and defines a linear belt of intrusions that, similar to the forearc sediments of the Anosyen Domain, can be traced from East Antarctica through Madagascar and potentially into East Africa, regions we take to have been continuous within the pre-Gondwana Indo-Antarctic plate. The volcanic and plutonic rocks of the Malani Igneous Suite (780–730 Ma) may represent the coeval backarc basin.