U-PB ID-TIMS DATING OF GRANITOID INTRUSIONS FROM THE AMBATOLAONA UNIT, CENTRAL MADAGASCAR: SIGNIFICANCE AND TECTONIC IMPLICATIONS

Reconstruction models of Neoproterozoic Gondwanaland positioned Madagascar in the heart of East African Orogen, between East and West Gondwana. Accordingly, Madagascar occupies an important role in the tectonic assembly of Gondwana supercontinent. However, the tectonic evolution and crustal development of the Island remain controversial due to the scarcity of precise isotopic ages for the basement rocks and plutonic intrusions. The purpose of this study is to present high precision ID-TIMS U-Pb single zircon ages on granitoid intrusions cutting the basement rocks of central Madagascar. The study area, the Ambatolaona migmatitic granite gneiss unit occupies the northeast central of the Antananarivo Block, the largest tectonic unit identified within the Malagasy Precambrian basement. The Ambatolaona unit comprises dominantly of granitoid gneiss with a uniform texture of thick leucocratic bands (>5 cm) interlayered with very thin mafic layers (2 mm). Gneiss schlieren and K-feldspar porphyroclast are also present within the migmatite granitic gneiss. A migmatitic granite (Andreba granite) and a weakly foliated granite gneiss (Ambohidronono granitic gneiss) within the Ambatolaona unit have yielded U-Pb zircon crystallization ages of 820.7±5.6 Ma and 778.9 ±7.3 Ma respectively. These new data confirm early suggestions that Neoproterozoic magmatism was much more widespread within the Antananarivo Block, however it remains unknown whether these ages represent distinct magmatic pulses or reflect a continuous granitoid emplacement. These intrusions could represent arc magmatism related to subduction of Mozambique lithosphere during the Neoproterozoic amalgamation of East and West Gondwana. Additional geochemical studies are needed to further constraint the origin of these plutons.