Genesis of the Tonian Imorona–Itsindro magmatic Suite in central Madagascar: Insights from U–Pb, oxygen and hafnium isotopes in zircon

Abstract

Madagascar occupies an important location within the East African Orogen (EAO). The EAO comprises an assemblage of Neoproterozoic microcontinents and arc terranes lodged between older cratonic blocks during the final assembly of the supercontinent Gondwana. The Imorona–Itsindro Suite of central Madagascar represents voluminous Tonian-aged (850–750 Ma) magmatism with controversial petrogenesis. Early work proposed arc magma generation coinciding with oceanic plate subduction during closure of the Mozambique Ocean along the ‘Betsimisaraka Suture’ in eastern Madagascar. Recently, others have questioned the existence of such a suture in Madagascar and rather suggest extension related emplacement into the middle and upper crust through a system of pre-existing structures. New U–Pb (zircon) geochronological data coupled with in-situ oxygen and hafnium isotopic analyses demonstrate that the Imorona–Itsindro Suite had several source components. Most of the Tonian-aged magmatic rocks were derived by mixing between ancient crust and mantle derived melts. δ18O values show variation that indicates significant involvement of crustal material and hydrothermal fluids. Predominantly low negative εHf(t) values are also variable and indicate significant crustal involvement in the genesis of the Tonian magmas. A compilation of all available geochronological data shows magmatism was essentially continuous for ∼100 Myr but with periods of increased activity at ∼800 Ma, ∼791 Ma and ∼784 Ma. Temporal analysis shows magmatic cycles of enrichment and depletion on the scale of 15–40 Ma. Spatial variations in isotope compositions reflect the heterogeneity of probable crustal source rocks present in the Ikalamavony, Itremo, Antananarivo and Masora Domains. A tectonic model is proposed for the Imorona–Itsindro Suite as a long-lived Andean-like arc on the margin of the Mozambique Ocean. The longevity and temporal isotopic trends are interpreted as reflecting cycles of arc advance and retreat.