Towards unravelling the Mozambique Ocean conundrum using a triumvirate of zircon isotopic proxies on the Ambatolampy Group, central Madagascar

Abstract

Madagascar occupies an important location within the East African Orogen (EAO), which involves a collection of Neoproterozoic microcontinents and arc terranes lodged between older cratonic units during the final assembly of the supercontinent Gondwana. The detrital zircon record of Proterozoic metasedimentary rock packages within the Antananarivo Domain (the Ambatolampy, Manampotsy, Vondrozo, Itremo and Ikalamavony Groups) is used to identify pre-orogenic tectonic affiliations of the region—affiliations that control interpretations of the evolution of the Mozambique Ocean and the formation of this part of central Gondwana. Here we focus on the Ambatolampy Group, a previously poorly known group of high-grade siliciclastic metasedimentary rocks, and compare these data to similar data from the other sedimentary packages in central Madagascar (the Vondrozo, Sofia, Manampotsy, Itremo, Molo, Ikalamavony, Androyen, Ambodiriana, Iakora and Maha Groups). New U–Pb (SHRIMP) zircon data for the Ambatolampy Group yield detrital age maxima of ~3000Ma, ~2800–2700Ma, ~2500Ma, ~2200–2100Ma and ~1800Ma. The youngest near-concordant detrital zircon age is 1836±25Ma, which we suggest represents the maximum depositional age of the Ambatolampy Group, in contrast to younger ages reported elsewhere. The detrital spectra are very similar to those from the Itremo Group and we suggest that they correlate with each other. Metamorphic zircons and zircon rims constrain the minimum depositional age to be ~540Ma; the interpreted age of metamorphism. New δ18O data (SHRIMP-SI) and hafnium (MC-LA–ICP–MS) isotopic data complement the U–Pb data and provide new constraints on the age, geochemistry and provenance of the metasedimentary rocks. We suggest that central Madagascar contained a Mesoproterozoic (to possibly Tonian) siliciclastic sedimentary basin, within which the Ambatolampy Group, and the Itremo and Maha Groups (and possibly part of the Iakora Groups) were deposited. Later sedimentary systems are represented by the Ikalamavony, Ambodiriana, Manampotsy and Molo Groups.