Petrology and geochemistry of lavas from Gawar, Minawao and Zamay volcanoes of the northern segment of the Cameroon volcanic line (Central Africa): Constraints on mantle source and geochemical evolution

Abstract

The most important part of geochemical studies of lavas along the Cameroon Volcanic Line (CVL) has so far focused on the southern and central segments of the line leaving behind the northern-most part of the CVL with few and incomplete data. This study presents the case of the Gawar, Zamay and Minawao volcanoes, for which, for the first time, petrological and geochemical data comprising major and trace elements, and Strontium-Neodymium-Lead isotopes have been used to investigate the mantle source and the genetic relation between the felsic and mafic lavas in these areas. Volcanics in the Gawar are mainly basanites while those from Minawao and Zamay are essentially trachytes. The Nb/Ta and Zr/Hf ratios are similar for both the mafic and felsic rocks suggesting a dominant evolution of the magma by fractional crystallization. Primitive-mantle normalized multi-element diagrams of the mafic lavas are comparable to those reported in other volcanoes along the CVL; characterized by the depletion of Heavy Rare Earth Elements (HREE) and enrichment of Light Rare Elements (LREE), indicative of an enrichment and the presence of garnet in the source. Modeled results for melting using primitive mantle normalized values of Sm/Yb vs La/Sm indicate a low degree (<2%) of partial melting of a source with less than 4% garnet. This is consistent with chondrite normalized REE patterns characterized by the fractionation of HREE relative LREE. Trace elements composition and isotope data preclude the significant role of the continental crust in the magmatism of the mafic rocks. However, during evolution to the felsic lavas, the role of the crust became more important.The ranges for 87Sr/86Sr (0.70356–0.70362), 143Nd/144Nd (0.512928–0.512931), 206Pb/204Pb (19.62–19.72), 207Pb/204Pb (15.65–15.68) and 208Pb/204Pb (39.31–39.45) isotopic composition for mafic samples (Gawar) are limited and fall within the range of mafic lavas studied along the CVL. The isotopic signatures and trace element characteristics are consistent with an asthenospheric origin, however with contribution from the sub-continental lithospheric Mantle (SCLM). The two sources have imparted characteristics of more than two mantle components involving the Depleted MORB-like mantle, HIMU and Enriched mantle (EM1).