Preliminary data on mantle xenoliths from the Wum maar, Oku Volcanic Group, Cameroon Volcanic Line (West Africa)

The Wum maar is located in the Oku Volcanic group, part of continental sector of the Cameroon Volcanic Line (CVL) in west Africa, which consists of volcanoes active from Eocene to recent. The continental part of the CVL is located on the metamorphic-igneous basement of the Neoproterozoic Central African Orogenic Belt (CAOB), which originated during Gondwana assembly. Some of the CVL lavas contain spinel-facies peridotite and pyroxenite xenoliths giving insight into the mantle lithosphere underlying the CAOB.

We studied xenolith suite (19 xenoliths) from the Wum maar, comprising 14 lherzolites and 5 websterites. The half of lherzolites (7) consist of minerals with fertile composition (olivine Fo₈₉, orthopyroxene Al 0.16-0.19 atoms per formula unit, clinopyroxene Al 0.28-0.31 a pfu, spinel Cr# 0.08-0.13). Clinopyroxene is REE-depleted and has ⁸⁷Sr/⁸⁶Sr ratios of 0.7017-0.7021. A reconnaissance study of crystal preferred orientation (CPO) by EBSD shows that at least in part of the rocks the clinopyroxene fabric is very weak, suggesting that its crystallization post-dates the primary deformation event recorded by the olivine-orthopyroxene framework. A smaller part of lherzolites (5) contains clinopyroxene the CPO of which fits that of the olivine-orthopyroxene framework, is LREE-enriched and has ⁸⁷Sr/⁸⁶Sr ratios of 0.7027-0.7028. One of these lherzolites contains amphibole (pargasite), which forms aggregates and schlieren and texturally is later than olivine-pyroxene host. CPO of amphibole, ortho- and clinopyroxene is decoupled from that of olivine in that rock. Two lherzolites have slightly depleted mineral compositions (olivine Fo_90-91, orthopyroxene Al 0.15 apfu, clinopyroxene Al 0.25 a pfu, spinel Cr# 0.18).

Websterites are dominated by orthopyroxene (Al 0.20-0.21 a pfu) whereas clinopyroxene (Al 0.30-0.31) is subordinate, and is characterized by LREE-depletion and ⁸⁷Sr/⁸⁶Sr ratios of 0.7019-0.7020. Spinel occurring in websterites is aluminous (Cr# 0.04-0.06), in some samples subordinate olivine (Fo₉₀) occurs. One of the xenoliths consists of millimetric monomineral layers of pyroxenes and olivine chemically identical to those occurring in websterites.  

The mineral chemical data coupled with mineral fabrics suggest that lherzolites with LREE-depleted clinopyroxene could have originated by late crystallization caused by melt metasomatism. The metasomatic agent is probably best represented by websterites, which contain LREE-depleted clinopyroxene with similar, depleted ⁸⁷Sr/⁸⁶Sr of 0.7019-0.7020 (compare to DM value of 0.7026, Workman and Hart 2005), confirming earlier findings of refertilization of the regional lithospheric mantle by highly depleted melts (Tedonkenfack et al. 2021). The addition of amphibole was connected with recrystallization of ortho- and clinopyroxene and with significant change of its ⁸⁷Sr/⁸⁶Sr signature to more radiogenic values.

Funding. This study originated thanks to the project of Polish National Centre of Research NCN 2017/27/B/ST10/00365 to JP. The bilateral Austrian-Polish project WTZ PL 08/2018 enabled extensive microprobe work.

References:

Tedonkenfack SST, Puziewicz J, Aulbach S, Ntaflos T., Kaczmarek M-A, Matusiak-Małek M, Kukuła A, Ziobro M: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Contributions to Mineralogy and Petrology 176: 37.

Workman RK, Hart SR (2005) Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters 231: 53-72.