Detrital pyrope garnets from the El Kseibat area, Algeria: A glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton

Abstract

Pyrope garnet grains found in Cretaceous and Quaternary alluvial sediments in the El Kseibat area (Algeria) suggest the presence of kimberlites along the NE margin of the West African Craton. The garnets have been studied using major- and trace-element chemistry to obtain information about the composition and thermal properties of the lithospheric mantle beneath this area. Most of the garnet grains are lherzolitic in composition (group G9), but range up to high Cr contents (>10% Cr 2O 3); two grains are harzburgitic (G9/G10). Three differently metasomatised groups of pyrope garnets were distinguished: (1) depleted grains that have low Sr, Ti, and Zr contents; (2) grains having low Sr and Ti and high Zr contents; and (3) grains with high Ti and Zr contents. Each group reflects a different stage and intensity of metasomatism. A range of chondrite-normalised rare earth element (REE n) patterns also reflects several styles of metasomatism. Based on geochemical features, most grains appear to be derived from Archean to Proterozoic mantle. The geotherm estimated from the garnet compositions is typically cratonic ( ca 40 mW/m 2), but somewhat higher than that estimated for the interior of the Craton using heat flow constraints (33 ± 8 mW/m 2). Depleted garnets give P– T estimates up to 950 °C, 40 kbar; those showing melt-related metasomatism (high Ti, Zr) go up to 1400 °C and >50 kbar. Nearly all garnet grains were derived from the graphite stability field at depths of 100–170 km, but some may be derived from the diamond stability field. The pyrope garnets of the El Kseibat area indicate that their lithospheric-mantle source has a lherzolitic composition as a result of several stages of metasomatism.