The geology of the Neoproterozoic and Cenozoic rocks of North Madagascar

The island of Madagascar, one of the world's most biologically diverse habitats, owes its unique biodiversity to the history of geological separation and isolation from the African, Antarctic, and Indian continents over the last 65 million years. This history is punctuated by intervals of continental rifting and sedimentation, together with periods of basalt eruption and alkaline volcanic activity. Equally interesting is Madagascar's role in the history of Gondwanaland's assembly. This history involves break-up of continental fragments and their reassembly, all linked in time to the extraordinary geologic events that mark Cambrian transition to skeletonized life.; This dissertation focuses on two aspects of this history. The first part of the dissertation examines the petrology, geochemistry, and age of the metamorphic rocks in the Lokoho Region of North Madagascar. These rocks comprise critical elements of a continental collision zone between two blocks of the former Gondwanan supercontinent. I was able to identify a complex polyphase orogenic history that can be interpreted either as rocks produced during an arc-continent collision or as reworked rocks, produced during and before Neoproterozoic arc-continent collision. They also form part of a distinct set of temporal, spatial, and compositional characteristics for the Madagascar shield that provide important constraints on Neoproterozoic supercontinent reconstructions.; The second part of the dissertation describes the petrology, geochemistry, and geochronology of a sequence of alkaline igneous rocks, not previously described. The petrological and geochemical compositions have been examined with a focus on a potential first order petrogenetic understanding. The new datasets suggest that the Cenozoic volcanism in Madagascar originated from an enriched sub-lithospheric HIMU reservoir similar to other basaltic magmas generated throughout the Comores "hotspot" trace. The isotopic variations in the volcanic rocks indicate contamination of the magmas by only a small amount of crustal materials. Compositional effects on the magmas derived beneath the lithosphere may be important because the lithosphere may have acted as barrier to continuing upwelling of asthenospheric material undergoing adiabatic decompression. Upwelling may be passive in response to lithospheric thinning or active response to an ascending plume and has influenced the chemical signature of the volcanic rocks.