The Geochemistry and Geochronology of the Quaternary Volcanism of the Kassa Volcanic Field (KVF), Jos Plateau, North Central Nigeria

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Thirty lava flows of alkaline olivine volcanic rocks of Quaternary age occurrence in the Kassa Volcanic Field (KVF), north central, Nigeria. Basalts contain phenocrysts of olivine, clinopyroxene, Fe-Ti oxides (magnetite and/or ilmenite) and abundant microlites of plagioclase forming the groundmass. There is a large compositional gap between basaltic rocks and felsic lavas discovered in the study area. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic lavas in this area. The Kassa Volcanic Field (KVF) basalts are similar in their chemical features to the basalts from other parts of the Jos, Biu Plateaux and Cameroun Volcanic Line (CVL). It is believed that the KVF basaltic magmas originate from an intra-lithospheric homogeneity reservoir. The previous isotopic studies suggested that they are related to mantle-derived magmas and they are resulted from the mixing at shallow crustal levels of a large fraction of basaltic magma with a minor amount of trachytic magma. Major element modelling of the basalt- trachyte evolution supports an evolution through fractional crystallization. The fluids have played a significant role in the mafic lavas genesis, as attested by the occurrence of Fe-rich minerals. An origin of the Kassa volcanic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, accompanied by volatile, halogen-rich fluid phases, may probably be the preferred hypothesis for the genesis of these lavas. The striking feature of the analytical results from the radiometric dating is an age convergence around 1.3 Ma. One sample from cone three (CN3) give slightly different value of 2.5Ma despite this, it cannot be considered to be significantly different and the samples from this cone presents more olivine and clinopyroxene phenocrysts. The assumed instantaneous cooling of the rocks, the freshness of the samples and the aforementioned age convergence lead us to regard the 1.3Ma age as mirroring the emplacement of the volcanic rocks. Moreover, the age range suggests a rather partial progression that decreases from the northern extremity to the southern portion of KVF (1.97 Ma-1.66 Ma- 1.38 Ma and 1.34 Ma).