THE TRANSITION FROM WITHIN-PLATE TO CONVERGENT MARGIN MAGMATISM IN EAST GONDWANA AT THE DAWN OF THE CRYOGENIAN PERIOD: U-PB, HF AND O ISOTOPE RATIOS IN ZIRCON FROM THE BEMARIVO DOMAIN, NORTH MADAGASCAR

New U-Pb and Hf-O isotope zircon data from the Bemarivo domain, north Madagascar demonstrate a transition in the mantle geodynamics beneath East Gondwana at the start of the Cryogenian period. The Bemarivo is redefined as a unified domain of continental crust composed of: (1) an apparent basement consisting of two groups of Proterozoic metasedimentary rocks – the Betsiaka Group of Paleoproterozoic age and the Sahantaha Group of Mesoproterozoic age – each having a detrital zircon provenance of Mesoarchean, Neoarchean and Paleoproterozoic source rocks, (2) two principal suites of Neoproterozoic intrusive igneous rocks - the Antsirabe North (ANS, 760-745 Ma) and Manambato Suites (MS, 720-700 Ma) - each having very different chemical and Hf-O (zircon) isotopic characteristics, and (3) two groups of Neoproterozoic volcano-sedimentary strata - the older Serie Amphibolique Intermediaire (SAI, 771-755 Ma) and the younger Daraina-Milanoa Group (DMG, ~740-720 Ma) – that form a transitional assemblage of volcanic strata with the ANS and MS, respectively. Previous suggestions that the Betsimisaraka Suture divides the Bemarivo domain into two distinctive terranes is invalid because igneous rocks of the ANS, MS, and SAI are found on both sides of the putative boundary and the ANS and MS were emplaced into the same section of continental crust, albeit at different times (Tonian and Cryogenian). Intrusive granitoids of the ANS exhibit ferroan and A-type chemical affinities that mimic those of the Imorona-Itsindro Suite (I-IS, 860-740 Ma) of central Madagascar. Both the ANS and I-IS have evolved ε_Hf (t) values and a range of δ¹⁸O values (5.5-0.5) implying (1) their felsic and mafic magmas were derived from continental crust and lithospheric mantle, respectively, and (2) their magmas interacted with meteoric water at high temperature. In contrast, igneous rocks of the MS (720-700 Ma) exhibit calc-alkaline affinity, a limited range of juvenile ε_Hf (t) values, and mantle-like δ¹⁸O values, implying their generation involved partial melting of asthenospheric mantle and juvenile crust, and little interaction with meteoric water. Thus, Madagascar, as a key part of East Gondwana, witnessed a major shift in mantle geodynamics from intracratonic magmatism of Tonian age (840-745 Ma) to convergent margin magmatism of Cryogenian age (720-700 Ma) that roughly coincides with the break-up of Rodinia and the onset of world-wide, Sturtian glaciation.