gems, garnet, pyrope, Algeria

Pyrope garnet grains found in Cretaceous and Quaternary alluvial sediments in the El Kseibat area (Algeria) suggest the presence of kimberlites along the northeastern margin of the West African Craton. These garnets have been analyzed for major and trace elements to infer the composition and thermal characteristics of the underlying lithospheric mantle. Most of the garnet grains are lherzolitic (group G9), with some exhibiting high chromium content (>10% Cr₂O₃), and two grains classified as harzburgitic (G9/G10). Three metasomatised groups were identified: (1) depleted grains with low Sr, Ti, and Zr; (2) grains with low Sr and Ti but high Zr; and (3) grains enriched in both Ti and Zr. These groups represent different stages and intensities of mantle metasomatism. Rare earth element patterns normalized to chondrite further reflect diverse metasomatic processes. Geochemical evidence suggests that most of the garnets originated in Archean to Proterozoic mantle. The estimated geotherm (~40 mW/m²) aligns with typical cratonic values but is slightly higher than the 33 ± 8 mW/m² determined for the craton interior from heat flow data. Pressure–temperature estimates range up to 950 °C and 40 kbar for depleted garnets, and up to 1400 °C and >50 kbar for those affected by melt-related metasomatism (high Ti, Zr). Most garnets originate from depths of 100–170 km within the graphite stability field, though some may derive from the diamond stability field. Overall, the pyrope garnets from El Kseibat reflect a lherzolitic lithospheric mantle modified by multiple metasomatic events.