Alkali basalts Basalts Cenozoic Chain silicates Chemical composition Continental crust Crust Igneous rocks Isotopes Lead Magma contamination Major elements Mantle Metals Nd-144/Nd-143 Neodymium Neogene Nephelinite Nesosilicates Northern Tanzania Olivine Olivine group Orthosilicates Oxides Pb-206/Pb-204 Peralkalic composition Perovskite Petrology Provenance Pyroclastics