ID-TIMS U-Pb Zircon Ages and Petrogenesis of Granitoid Magmas in Central Madagascar: Geodynamic Perspectives

We present ID-TIMS U-Pb single zircon ages and major and trace element data for granitoid plutons from the Imorona–Itsindro and Kiangara suites in central Madagascar, in order to constrain the timing of igneous emplacement and investigate the petrogenesis and tectonic settings of these Neoproterozoic plutons. A U-Pb crystallization age of 779 ± 7 Ma was determined from an Imorona–Itsindro intrusion, while a Kiangara intrusion yields 777 ± 4 Ma, older than previously reported. The identical U-Pb ages suggest contemporaneous emplacement of some Kiangara and Imorona–Itsindro intrusions. Elemental data indicate that the two suites display trace element patterns similar to those of arc-type magmas. Some distinct geochemical features are apparent between these two suites. The Imorona–Itsindro sample displays I-type affinity with low REE abundances, whereas the Kiangara samples exhibit A-type signatures with higher REE contents. We suggest that the elemental differences between A-type and I-type reflect the contamination of mantle-derived magma by lower and upper crustal rocks, respectively. We suggest that the coeval A-type and I-type granitoids in central Madagascar were generated in a subduction system associated with slab rollback and back-arc extension like extension. The compositional diversity in these Neoproterozoic plutons reflects the evolution of the tectonic regime within a single geodynamic environment, similar to that proposed for plutons in other Precambrian and younger terranes.