Petrology and geochemistry of monchiquites from Tchircotché (Garoua rift, north Cameroon, Central Africa)

Summary

Dykes of Cenozoic age (37.5 ± 2.3 Ma) crop out in the Tchircotché area (Garoua rift, north Cameroon). They consist of a lamprophyric (monchiquite) series with diopside, subsilicic kaersutite and apatite phenocrysts, Ba–Ti-rich biotite microphenocrysts and Cr-diopside xenocrysts scattered in a matrix of analcitic composition containing oligoclase, albite and sanidine microlites and carbonate ocelli.

Major and trace element distributions are interpreted in terms of crystal fractionation of olivine, clinopyroxene, amphibole and Fe–Ti oxides. The Tchircotché monchiquites show a relatively restricted range of initial ⁸⁷Sr/⁸⁶Sr ratio (0.70366–0.70387), of ɛ_Nd values (+2.5 to +2.7), and rare earth element patterns similar to those of the least differentiated basalts of the Cameroon Line. This supports a common magma source region. This mantle source is infra-lithospheric and is strongly enriched in incompatible elements (light REE, Zr, Sr, Ba) probably transported by volatile- and halogen-rich fluids. The monchiquites appear to be derived by low degrees of partial melting as attested by steep REE patterns and high contents of other incompatible elements, suggesting the presence of residual garnet in the source. Several lines of evidence support the occurrence of phlogopite in the source region of Tchircotché lamprophyre magma.