Major/trace elements and Sr–Nd–Pb isotope systematics of lavas from lakes Barombi Mbo and Barombi Koto in the Kumba graben, Cameroon volcanic line: Constraints on petrogenesis

Abstract

This study presents major elements, trace elements and Sr–Nd–Pb isotope data for fifteen (15) representative volcanic rock samples from volcanic edifices of Lakes Barombi Mbo (n = 9) and Barombi Koto (n = 6), located in the Kumba graben along the Cameroon Volcanic Line (CVL). The samples are dominantly basalts and trachy-basalts with MgO ranging from 7 to 9 wt.%. The lavas from the edifice of Lake Barombi Mbo are slightly richer in TiO2 (mean 3.3 wt%) and lower in CaO (mean 9.2 wt%) relative to those from the edifice of Lake Barombi Koto (2.9 wt% and 10.3 wt%, respectively). Trace element characteristics of the lavas suggest an enrichment of the source by carbonatitic metasomatism and melting of a heterogeneous source at varying depth in the garnet-spinel transition zone. Trace elements and isotopic composition of the lavas suggest insignificant crustal contamination. Modelling of the degree of partial melting suggest that the magmas were formed at varying depth (60–80 km) and by ~2% partial melting of a source containing less than 4% garnet. Samples from Lake Barombi Mbo formed at a higher degree of partial melting (2%–0.5%) and at a shallower depth relative to those from the volcanic edifice of Lake Barombi Koto (mostly <0.5%) and Mt. Cameroon. Interestingly, the relationship of Zr/Hf and 206Pb/204Pb suggests that the magma was affected by carbonatitic metasomatism whose effect decreased from Mt. Cameroon to Lake Barombi Mbo. Our results suggest that Lakes Barombi Mbo and Barombi Koto lavas are typical of lavas with HIMU-like signature and involved contributions from a dominantly sublithospheric mantle (SLM) and minor contributions from the sub-continental lithospheric mantle (SCLM). The SLM material rose and was hosted in the SCLM prior to melting.