Xenoliths are among the most direct windows geologists have into the otherwise inaccessible sub-continental lithospheric mantle. When basaltic magma ascends rapidly from depth, it can rip fragments of the wall rock it passes through and transport them fast enough that the fragments survive the journey. At Gangassao, the xenoliths are impressive specimens, measuring 4 to 8 centimeters across. They are mostly ovoid to sub-ovoid in shape, a form the authors attribute to long transport within a turbulent magma, and they display sharp contacts with their basaltic host lava, with no reaction rims—another sign that the ascent was too quick for the fragments to be chemically digested by the melt around them.

Under the microscope, the Gangassao xenoliths reveal a classic four-phase mantle assemblage of olivine, orthopyroxene, clinopyroxene and spinel, with no hydrous minerals such as phlogopite, amphibole or apatite. Modal proportions classify them firmly as spinel lherzolites. Two textural types are present: protogranular, characterized by large olivine crystals 5 to 8 millimeters across, sometimes rimmed by aggregates of small, strain-free polygonal olivine; and porphyroclastic, marked by elongated, kinked olivine crystals, some with distinctive holly-leaf shapes. According to the classification scheme of Mercier and Nicolas, these textures record recrystallization during partial melting and syn- to post-tectonic deformation at elevated temperatures—a history consistent with the tectonically active setting of the Adamawa plateau.

The chemical fingerprints of the minerals are equally telling. Olivine crystals are highly magnesian, with forsterite contents between 87.6 and 89.1, and mostly low calcium oxide contents of 0.07 to 0.18 weight percent, a combination characteristic of mantle-derived olivine rather than crystals grown in a magma. Interestingly, some olivine grains carry elevated CaO values of 0.35 to 0.47 weight percent, approaching magmatic compositions. The researchers interpret this as evidence of reheating after the xenoliths were sampled by the ascending basalt, with calcium diffusing within olivine or from adjacent clinopyroxene. Orthopyroxene is enstatite and clinopyroxene is chromium-rich augite, while the spinel phase is an aluminum spinel with very low TiO2, below 0.02 weight percent, and low chromium numbers between 11.0 and 11.8. In the olivine–spinel mantle array diagram, the compositions plot squarely within the mantle field.

To pin down the conditions under which these rocks last equilibrated, the team applied two-pyroxene geothermometry and geobarometry, using the formulations of Putirka alongside independent thermometers from Taylor, Wells, and Brey and Köhler. The results are remarkably congruent. Temperatures cluster mostly between 1100 and 1150 degrees Celsius, with pressures of 1.13 to 2.00 gigapascals, implying depths of 37 to 66 kilometers and a geothermal gradient close to 30 degrees Celsius per kilometer. Notably, the range of values calculated for one sample may trace the physical journey of the xenolith itself: from an initial position at about 2.00 gigapascals and 1247 degrees Celsius, around 66 kilometers deep, to re-equilibration with the host magma at roughly 1.20 gigapascals and 1100 degrees Celsius, about 40 kilometers down, just below the Moho. The final dash through the crust, the authors conclude, was too rapid for any further thermal or pressure adjustment.

Plotted on a temperature–pressure diagram, the Gangassao lherzolites fall within the field of incipient melting, close to dehydration solidus boundaries, and span the transition from the spinel stability field toward the plagioclase field. This distribution points to decompression during mantle upwelling, or exhumation from deeper levels followed by incomplete re-equilibration during ascent. The team proposes that the mantle beneath Gangassao has undergone repeated melt extraction, a process supported by rounded outlines of some olivine, clinopyroxene and orthopyroxene crystals and by suspected droplets of silicate glass around them. They identify filter pressing—the squeezing of melt out of a crystal mush—as the most plausible mechanism for separating the silicate liquid generated by decompression of the sub-continental lithospheric mantle, with the Tertiary uplift of the entire Adamawa plateau providing the driving force.

The authors are careful to note the limits of their evidence. The filter-pressing interpretation would ideally be backed by geochemical data showing enrichment of incompatible elements in extracted melts relative to their residues, which the present study does not provide. They also raise alternative mechanisms that could accomplish melt extraction without filter pressing, including fracture-assisted melt escape driven by melt overpressure and buoyancy-driven segregation arising from density contrasts between melt and residue. Degrees of melting estimated from spinel chromium numbers range from about 2.0 to 2.65 percent, though the team cautions that this formula was calibrated for mid-ocean-ridge peridotites and its applicability to continental rift-related lherzolites is debated. An independent estimate based on aluminum and magnesium contents of coexisting pyroxenes suggests the xenoliths experienced less than 15 percent melting, consistent with the low-degree melting inferred for the source of the Adamawa basalts themselves.

The second process acting on the Gangassao mantle is silicate metasomatism—the infiltration of melt into the solid rock, subtly altering its chemistry. Noticeable variations in the major-element compositions of coexisting minerals within and between the xenoliths point to this heterogeneous modification. Together, melt extraction and melt infiltration emerge as the main processes sculpting the lithospheric mantle beneath the plateau. The broader regional context reinforces the picture: across the Adamawa plateau, ultramafic xenoliths include nine lherzolite occurrences, three harzburgites, two wehrlites and one olivine websterite, each recording different temperatures, pressures and depths. At Youkou, just 40 kilometers southwest of Gangassao, a refertilization process was invoked; at Dibi, 50 kilometers to the south, mantle upwelling is recorded by the transformation of spinel into plagioclase and a Moho at only about 20 kilometers depth.

The geological setting ties all of this together. The Adamawa plateau is a dome-shaped tectono-volcanic horst within the Pan-African mobile belt, cut by a network of ancient strike-slip faults trending in multiple directions that likely penetrate down to the mantle. Geophysical data show the crust thinning from about 35 kilometers in the south to as little as 14 to 20 kilometers north of Ngaoundéré, while the asthenosphere–lithosphere boundary has risen from around 120 kilometers to roughly 80 kilometers beneath the plateau. The authors argue that the history of the Adamawa lithospheric mantle reflects the Tertiary rejuvenation of this Pan-African fault system, which channeled magmas upward and allowed them to sample the mantle at a range of depths. The Gangassao lherzolites, equilibrated at 37 to 66 kilometers, fit neatly alongside lherzolites from Ngao Sey and complement the shallower harzburgites and deeper websterites found elsewhere on the plateau.

What makes the discovery compelling is how much information a handful of fist-sized rock fragments can carry. Each xenolith is a small time capsule, preserving textures and mineral chemistry that record partial melting, deformation, metasomatism and rapid volcanic transport. Together they demonstrate that the mantle beneath the Adamawa plateau is not a single homogeneous reservoir but a patchwork, stitched together by repeated melt extraction and infiltration over hundreds of millions of years. As volcanic activity continues to expose fresh fragments across the Cameroon Volcanic Line, each new occurrence refines the map of this hidden layer, bringing scientists closer to understanding how continental mantle evolves when ancient faults, rising plumes and uplifting plateaus collide in one of Africa’s most geologically dynamic regions.

Subject of Research: Petrology and geothermobarometry of spinel lherzolite xenoliths revealing lithospheric mantle heterogeneity beneath the Adamawa plateau, Cameroon

Article Title: Lithospheric mantle heterogeneity under Adamawa plateau (Cameroon, Central Africa) evidenced from Gangassao lherzolite xenoliths

Article References: Feukam Feuto, J. B., Bardintzeff, J. M., Okomo Atouba, L. C., Fagny Mefire, A., Adama, H., Njankouo Ndassa, Z. N., Nikara Voundou, D., Nkouandou, O. F., & Bonin, B. (2026). Lithospheric mantle heterogeneity under Adamawa plateau (Cameroon, Central Africa) evidenced from Gangassao lherzolite xenoliths. Discover Geoscience, 4(1), Article 364. https://doi.org/10.1007/s44288-026-00728-7

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00728-7

Keywords: lithospheric mantle, lherzolite, xenoliths, Adamawa plateau, Cameroon, spinel lherzolite, geothermobarometry, melt extraction, silicate metasomatism, Pan-African faults, Ngaoundéré, petrology