New evidence for a continental rift tectonic setting of the Neoproterozoic Imorona-Itsindro Suite (central Madagascar)

The Neoproterozoic Imorona-Itsindro Suite of central Madagascar represents one of the crucial petrological records of the supercontinent transformation from Rodinia into Gondwana, whereas its tectonic setting remains open to debate. Here newly reported and existing zircon U/Pb-Hf-O isotope data are used to better understand the tempo-spatial features and petrogenetic mechanisms of this enigmatic suite. The following results are at least more compatible with, if not suggestive of, a continental rift setting than the traditionally accepted continental arc setting. (1) This suite is present throughout central Madagascar, including the Manampotsy-Anaboriana Domain and probably also the Antongil Sub-domain. Particularly, the high-flux magmatic pulse at ∼790 Ma has a width of more than 300 km. (2) This suite is in essence an igneous expression of lithospheric reworking with no asthenosphere contribution being clearly detected. (3) The felsic and mafic end-members of this suite have independent sources and were derived from the crustal basement and lithospheric mantle, respectively. (4) Input of mantle materials (via magma mixing process) into the crust-derived felsic end-member increased at the ∼790 Ma high-flux period, as indicated by a discernible positive zircon εHf(t) excursion. (5) Felsic end-member magma systems may have interacted with meteoric water at a high temperature, as implied by a previously reported zircon low-δ18O signature. Thus, there is perhaps no Neoproterozoic Mozambique Ocean suture across the Archean basement of central Madagascar. The most important implication of this study is that an elongate Andean-type orogenic system assumed in many Neoproterozoic paleogeography maps may not have been developed along the western margin of the supercontinent Rodinia.