U-PB ZIRCON AGES AND PETROGENESIS OF GRANITOID MAGMAS, CASE STUDY CENTRAL MADAGASCAR

Neoproterozoic plutonic rocks of central Madagascar have been the subject of extensive studies; however their origins and the tectonic settings remain inconclusive. We present high precision U-Pb zircon age by ID-TIMS method, and major and trace element data for selected samples from the Kiangara and Imorona-Itsindro suites to further constrain the timing of igneous emplacement, and to investigate their petrogenesis and tectonic settings. Our new ID-TIMS U-Pb zircon age for a granitoid intrusion of the Imorona-Itsindro suite indicates that its crystallization and emplacement occurred at 779±7 Ma. In contrast, a granitoid intrusion mapped as part of the Kiangara suite yields a U-Pb zircon crystallization age of 777±4 Ma, distinctly older than those previously reported for the Kiangara suite. This confirms that the emplacement of the Kiangara suite occurred earlier than 633 Ma. Additionally, the U-Pb age from the Imorona-Itsindro intrusion in this study is identical to that of Kiangara intrusion, suggesting that the emplacement of some of the Kiangara and Imorona-Itsindro granitic intrusions were contemporaneous. Moreover, elemental data in this study indicate that the Kiangara and Imorona-Itsindro samples display similar trace element patterns very similar to those of arc-type magmas. Nonetheless, some subtle but distinct geochemical features are apparent among the two suites; most notably, the dated Imorona-Itsindro sample displays I-type affinity with low REE abundances, whereas the dated Kiangara sample exhibits A-type signatures with higher REE contents. Overall, the geochemical data can be explained by partial melting of mantle metasomatized by slab-derived hydrous fluid, followed by extensive crustal contamination and fractionation. Based on the existing data, the coeval gabbroic rocks, A-type and I-type granitoids in central Madagascar can be attributed to a subduction system associated with slab roll-back and back-arc-like extension. Thus, the compositional diversity in the Neoproterozoic plutons in central Madagascar reflects the evolution of the tectonic regime within a single geodynamic environment, very similar to that proposed for plutons in other Precambrian and younger terranes.