LITHOSPHERIC STRUCTURE BENEATH THE MESOZOIC CHILWA ALKALINE PROVINCE IN SOUTHERN MALAWI AND NORTHEASTERN MOZAMBIQUE

We investigated the lithospheric structure beneath the Mesozoic Chilwa Alkaline Province (CAP) in southern Malawi and northeastern Mozambique using aeromagnetic and satellite gravity data (World Gravity Model 2012 (WGM 2012). The CAP is a syeno-granite and nepheline syenite plutonic province with minor intrusions of carbonatite bodies. It is found intruding Precambrian terranes of the East African Orogen and it is located on the southeastern edge of the Cenozoic Malawi rift which is considered to be the southernmost segment of the Western Branch of the East African Rift System. Some of the CAP’s bodies are clearly offset by the normal border faults of the Malawi rift. Previous petrographic, geochemical and isotopic studies have suggested that the CAP is underlain by a thin subcontinental lithospheric mantle (SCLM) possibly due to the Paleozoic-Mesozoic Karoo rifting event. Hence, mantle magmatic source has been favored as an origin for the CAP. However, melting of a thickened continental crust cannot be ruled out as the origin of the CAP as has been suggested for several alkaline intrusions. Our study found that: (1). The CAP is characterized by circular and overlapping magnetic anomalies typical of hypabyssal nested igneous ring complexes. (2) Upward continuation of the satellite gravity data suggested that the CAP was sourced from deeper magma chambers now preserved as broad batholiths at ~8 km depth. (3) Two-dimensional (2D) radially averaged power spectral analysis of the satellite gravity data showed that the CAP is underlain by relatively thick crust where the Moho can be as deep as ~45 km. It also showed that the CAP is underlain by relatively thin SCLM where the asthenosphere-lithosphere boundary (LAB) can be as shallow as 120 km. We suggest that thinning of the SCLM might have allowed for the ascendance and melting of the asthenosphere to form the CAP, but also provided heat source to partially melting the lower crust. This suggestion can be further tested by additional geochemical and isotopic studies. Our work highlights the importance of using potential field geophysical data for imaging complex continental lithospheric structure. Understanding the lithospheric structure beneath the CAP is helpful in guiding future mineral exploration because igneous ring complexes are important sites for economic mineralization.