Lithospheric Structure Beneath the Cretaceous Chilwa Alkaline Province (CAP) in Southern Malawi and Northeastern Mozambique

Abstract

AbstractWe used aeromagnetic and satellite gravity data to investigate lithospheric structure beneath the Cretaceous Chilwa Alkaline Province (CAP) in southern Malawi and adjacent Mozambique. The CAP consists of granites, syenites, nepheline syenites, basanites, phonolite dikes, and minor carbonatite bodies. The intrusions were emplaced in the Precambrian (Mesoproterozoic‐Neoproterozoic) terranes of the Southern Irumide and Mozambique orogenic belts. Aeromagnetic data show the CAP as overlapping circular anomalies typical of nested igneous ring complexes formed through caldera collapse mechanism. We used gravity data to infer that: (1) the CAP was sourced from ~30‐km wide igneous bodies now preserved in the upper crust at ~5‐km depth. (2) the CAP is underlain by up to ~45‐km thick crust (due to mafic magmatic underplating) and a lithosphere as thin as ~90 km. These data suggest that mafic magmatic underplating and lithospheric thinning occurred during a Cretaceous rifting event. We propose, based on these results and taking into account previous petrographic, geochemical, geochronological, and isotopic studies, that the silica undersaturated magmatic phase of the CAP was due to flux melting of the asthenosphere. This was followed by silica‐rich magmatic phase due to decompression melting of the asthenosphere as a result of lithospheric thinning. Lithospheric thinning and ascendance of the asthenospheric melt might have been facilitated by the presence of late Neoproterozoic to early Cambrian suture zone. The heat provided by the mafic magmatic underplating resulted in partial melting of the lower crust to form the silica‐saturated CAP intrusions from mixed magma sources.