Plume-Lithosphere Interaction and the Origin of Continental Rift-related Alkaline Volcanism--the Chyulu Hills Volcanic Province, Southern Kenya

Abstract

Geochemical data are presented for primitive alkaline lavas from INTRODUCTION the Chyulu Hills Volcanic Province of southern Kenya, situated The geochemistry of mafic, alkaline volcanic rocks erupsome 100 km east of the Kenya Rift Valley. In addition to their ted in continental areas can potentially yield valuable primitive compositions, a striking and ubiquitous feature is a strong information about the nature of large parts of the Earth’s but variable depletion in K relative to other highly incompatible interior that are otherwise inaccessible. In combination elements when normalized to primitive mantle values. Semi-quanwith geophysical evidence and clues from mantle-derived titative models are developed that best explain the petrogenesis of xenoliths, such geochemical data hold the key to underthese lavas in terms of partial melting of a source that contained standing the composition and evolution of the Earth’s residual amphibole (but not phlogopite). The presence of amphibole mantle. The purpose of this contribution is to investigate implies a source in the subcontinental lithosphere rather than the aspects of the geochemistry and mineralogy of the mantle asthenosphere. It is suggested that the amphibole is of metasomatic source region of the relatively primitive, rift-related, origin and was precipitated in the lithospheric mantle by infiltrating alkaline lavas of the Quaternary Chyulu Hills Volcanic fluids and/or melts derived from rising mantle plume material. A Province (CHVP) of southern Kenya (Fig. 1). raised geotherm as a consequence of the continued ascent of the The voluminous alkaline magmatism commonly asplume material led to dehydration melting of the metasomatized sociated with continental rifting, of which the East African mantle and generation of the Chyulu Hills lavas. It is proposed Rift System (Fig. 1) is a type example, has been the that the Chyulu Hills Volcanic Province represents an analogue for subject of geochemical investigations for decades (e.g. the earliest stages of continental rift initiation, during which Williams, 1970; Baker et al., 1971; Baker, 1987). One interaction between a plume and initially refractory lithosphere may problem frequently addressed by recent studies is the lead to the generation of lithospheric melts. identification of the source regions of primitive, mafic, rift-related magmas. A primary concern is centred around whether they are derived from the subcontinental lithospheric mantle (SCLM) or from sublithospheric sources in the asthenosphere or in mantle plumes. Some workers have argued that the large volumes of lava