Sheared peridotites from the cratonic mantle beneath Lesotho (Kaapvaal craton): Investigating deformation and related metasomatic processes from mid-lithospheric depth to the LAB.

Sheared peridotites give direct evidence for deformation processes in the subcratonic mantle shortly before their transport to the surface by kimberlites. We studied 12 sheared peridotites (11 garnet-peridotites, one spinel-bearing) from three localities in Lesotho, which were entrained in the Late Cretaceous (90Ma). The peridotites exhibit different degrees of depletion from more fertile (XFo =91) to depleted (XFo =93). Typical for sheared peridotites, textures reveal a bimodal grain size with mm-size porphyroclasts, which are surrounded by 10s – 100s µm neoblasts. Our suite can be split into two groups: (i) low-T (<1250°C) and (ii) high-T (1400 ± 50 °C). The low-T sheared peridotites derive from various depths (3.5 – 5.5 GPa) in the lithospheric mantle whereas the high-T sheared peridotites equilibrated at high pressures (6 ± 0.5 GPa), corresponding to the lithosphere asthenosphere boundary (LAB) beneath the Kaapvaal craton.

Crystallographic preferred orientations of olivine verify the presence of metasomatic agents during deformation in both groups and reveal various types: B-, C-, E-, AG- and bimodal CPOs. Increasing Ti-contents in olivine and orthopyroxene neoblasts, accompanied with an increase in temperature in the low-T group, reveal a connection between metasomatism and deformation during interaction with (failed) kimberlites.     In contrast, the porphyroclasts and neoblasts of all phases in the high-T peridotites have homogeneous major and trace element contents, implying (re-)equilibration (during deformation?). Trace element signatures in garnet, clinopyroxene and orthopyroxene indicate that metasomatism at the LAB caused either enrichments or depletions in the high-T sheared peridotites.