Developing an Integrated Petrogenetic Model for Understanding REE Deportment of the Ampasibitika Intrusion and Associated Ion Adsorption Deposits

Abstract Alkaline-peralkaline igneous systems are promising sources of Rare Earth Elements (REE). Preservation bias has resulted in a gap the geological record for alkaline-peralkaline magmatic systems, with hypabyssal plumbing system linking magma chambers to extrusive volcanic rocks poorly represented. Large plutonic varieties these often proposed have fed (now eroded) volcanoes, and current peralkaline obscure at depth. The alkaline Ampasibitika Intrusion Madagascar is rare example where magmatic-volcanic interface between deeper level reservoir its genetically related caldera volcano exposed. This c. 24 Ma sub-volcanic intrusive comprises silica-undersaturated -oversaturated units, metaluminous peraluminous characters, varying styles REE-mineralisation; including supergene Ion Adsorption style REE occurrences overlying weather profiles. There two main suites: 1) concentric Marginal Dyke Swarm (MDS) formed quartz-microsyenite granite dykes, 2) Ring (ARD) comprising alkali feldspar syenites subordinate nepheline syenites, trachytes phonolites. We present new field observations geochemical data indicate that MDS was emplaced as series low-viscosity, volatile-rich melt batches, which coalesced roof zone intruded prior collapse; whereas ARD into ring fault heterogeneous mix variably evolved syenitic crystal mushes phonolitic trachytic-melt batches. As such, we suggest represents residual fraction reservoir, contains portions fractionating, silica-neutral -undersaturated syenite, cumulate assemblage. In this revised framework, assess major trace element geochemistry amphibole- clinopyroxene-group minerals gain insight evolution partitioning early phases. Ultimately, most REE-enriched dykes MDS, identified product volatile-rich, highly melts from reservoir. However, although REE-enriched, mineralogy does always not enable efficient release Adsorption-style mineralisation; instead, lower REE-content protoliths REE-host phases more amenable decomposition greater proportion REE.