Sheared Peridotites from Kimberley (Kaapvaal Craton, RSA): Record of Multiple Metasomatic Events Accompanied with Deformation

Abstract

Abstract Sheared peridotites from the Kaapvaal craton may be broadly divided into two types: (1) high T and refertilized and (2) low T and highly depleted, which equilibrated at conditions lying either above or along the Kaapvaal craton conductive geotherm, respectively. Here, we have studied 14 low-T sheared peridotites from Kimberley entrained by several Late Cretaceous (90 Ma) kimberlites in order to constrain the nature and timing of the deformation. The sample suite comprises nine garnet peridotites (GPs) with various amounts of clinopyroxene ± isolated spinel, three garnet-free phlogopite peridotites (PPs) with minor amounts of spinel, one garnet–spinel peridotite (GSP) and one dunite. The peridotites have intense deformation textures, ranging from porphyroclastic to fluidal mosaic. Olivine and orthopyroxene compositions (Mg# = 91–94) indicate varying degrees of depletion, similar to coarse-grained peridotites from the same localities. Pre-deformation conditions of the GPs are preserved in the cores of large (>100 μm–mm diameter) porphyroclasts and give a range in temperature of 930–1000°C at pressures of 4.0 ± 0.4 GPa. The GSP was equilibrated at 840°C and 3.1 GPa. Projected onto a 40-mW/m2 geothermal gradient, the PP samples yield temperatures of 850–870°C at 3.3–3.4 GPa. Trace element measurements by laser ablation inductively coupled plasma mass spectrometry and electron microprobe indicate that the ‘cold’ sheared peridotites were influenced by several metasomatic events, ranging from ‘old’ pre-deformation metasomatism to interactions shortly before or during deformation. The old pre-deformation metasomatism is recorded in garnet, clinopyroxene and orthopyroxene porphyroclasts and implies interactions with phlogopite–ilmenite–clinopyroxene- or muscovite–amphibole–rutile–ilmenite–diopside-related metasomatic agents, which also led to crystallization of phlogopite in the garnet-free peridotites. A ‘young’ metasomatic event caused an enrichment in Fe, Ti, Ca and Y (+heavy rare earth elements) and is evident in zoned orthopyroxene and clinopyroxene and phlogopite, the crystallization of new clinopyroxene porphyroclasts and compositional heterogeneities in garnet. This young event marks the beginning of extensive kimberlite-related metasomatism in the late Cretaceous beneath Kimberley. The metasomatism caused the deformation (triggered by a kimberlite pulse?), resulting in the recrystallization of fine-grained, mainly olivine, neoblasts (down to <10 μm). These record the metasomatic conditions at the time of deformation, revealing an increase in temperature up to 1200°C accompanied by an increase in Ti content up to 300 μg/g. Crystal preferred orientations of olivine neoblasts suggest the presence of elevated concentrations of water (B, C, E type) or the presence of a melt during the deformation (AG type). We suggest that these high water contents led to hydrolytic weakening of the cratonic lithosphere and prepared the pathways for subsequent kimberlite magmas to reach the surface. We propose that the deformation is a byproduct of extensive metasomatism, resulting in a metasomatism–deformation cycle. In times of extensive magmatism and metasomatism, fluids and melts flow along the pathways established by previous metasomatic agents, leading to further hydrolytic weakening of these mantle segments. Later, deformation was initiated by a new pulse of melt/fluid, with one of the later pulses eventually reaching the surface and transporting fragments of sheared and undeformed peridotites with it. The remaining peridotite anneals after the period of extensive metasomatism and recrystallizes to become coarse-grained peridotite again.