REE and HFSE mineralization in peralkaline granites of the Ambohimirahavavy alkaline complex, Ampasindava peninsula, Madagascar

Abstract The Cenozoic Ambohimirahavavy alkaline complex in the northwest of Madagascar is characterized, in its outer flanks, by a network of discontinuous granitic dykes mineralized in rare-earth elements (REE) and high-field-strength elements (HFSE). These granitic dykes contain alkali amphibole and pyroxene, are strongly ferrian, have a peralkaline affinity [molar (Na 2 O + K 2 O)/Al 2 O 3  > 1], but differ in textures and mineral assemblages. Based on the latter parameters, three varieties of granite could be distinguished: coarse-grained, pegmatitic and eudialyte-bearing granite. Detailed study of the REE- and HFSE-bearing minerals revealed the presence of both magmatic and hydrothermal phases. In coarse-grained granite (GR-I) prevails a magmatic miaskitic assemblage, rich in zircon. In the pegmatitic granite (GR-II), the rare metals are contained in secondary minerals, essentially amoeboid zircon and rare-earth fluorocarbonates, forming pseudomorphs after eudialyte. The eudialyte-bearing granite (GR-III), the richest in REE and HFSE, is characterized by a magmatic agpaitic assemblage, dominated by eudialyte. A skarn develops at the contact of GR-I with limestone. Mineralization in this zone is in the form of pseudomorphs of rare-metal-bearing minerals after aegirine–augite. Presence of a magmatic miaskitic vs. an agpaitic assemblage in the coarse-grained and eudialyte-bearing granites, respectively, is interpreted to result from crystallization of differently evolved melts that probably originated from a common basaltic parental magma. Origin from the melting of a metasomatized lower crust cannot be excluded. In the eudialyte-bearing granite, very high contents of Na and volatiles maintained the rare metals in the melt to the very last stages of crystallization, whereas in the coarse-grained granite, Na and volatiles were probably lost to an exsolving fluid, causing the precipitation of a miaskitic assemblage. Subsolidus remobilization of REE and HFSE took place via an alkali-, silica- and F-rich orthomagmatic fluid exsolved from the crystallizing peralkaline granitic melt.