| Summary: | 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.
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