Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

Abstract

Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of the lithospheric mantle in this region. Garnet-bearing rocks are amphibolebearing websterite with *5-10 vol% orthopyroxene. Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (*970-1,100°C at *2.3-2.6 GPa to *700-800°C at *0.5-1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P-T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling. In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya rift.

The making of an underplate: Pyroxenites from the Ethiopian lithosphere

“…4). Other xenolith localities in the region have reported either the presence of garnet or evidence of garnet breakdown (Orlando et al, 2006;Ferrando et al, 2008;Kaeser et al, 2009), which could impact the composition of co-existing clinopyroxene by partitioning some HREE into co-existing garnet. However, we see no evidence in any of the xenoliths from Dedessa for either garnet or texture of garnet breakdown and thus we suggest that these HREE characteristics in the clinopyroxene were the result of fractionation from a HREE-depleted alkaline magma, similar to the host basanite (Conticelli et al, 1999).…”

“…On the basis of these controls on pyroxenite formation, previous studies in the region have reached radically different conclusions in relation to the origin of metasomatic agents impacting the East African lithospheric mantle (Kaeser et al, 2009;Beccaluva et al, 2011). Orthopyroxene-rich pyroxenite xenoliths from the Marsabit volcanic field in Northern Kenya, and the Western Ethiopian Plateau have been attributed to the percolation of an orthopyroxene-saturated sub-alkaline magma through the lithospheric mantle (Kaeser et al, 2009;Beccaluva et al, 2011), however the origin of this metasomatic agent is contentious with plume and subduction origins suggested.…”

“…Existing studies of xenoliths derived from the lithospheric mantle in this region suggest that an early subductionrelated metasomatic overprint may be possible (Kaeser et al, 2009;Frezzotti et al, 2010).…”

“…Specifically, Kaeser et al, (2009) report on orthopyroxene-saturated melts that are parental to garnet websterites from Marsabit (northern Kenya) and suggest that the Si-rich melts that would be required to form these xenoliths was derived from the Pan-African 'convergent tectonic setting'. The influence of subduction on the lithosphere in this region is also supported by studies on granulite xenoliths which are interpreted to be formed from arc-cumulates that were possibly derived from the Pan-African subduction event (Ulianov et al, 2006).…”

Physics and Chemistry of Deep Continental Crust Recycling

“…Garnet-pyroxenites are found in the deep roots of continental and some island arcs, but unlike true eclogites, these rocks are cumulates and restites associated with deep magmatic differentiation (Jagoutz, 2010;Jagoutz et al, 2009;Lee et al, 2006Lee et al, , 2007Rodriguez-Vargas et al, 2005;Weber et al, 2002; Figures 2 and 3). Garnet-pyroxenites are also found within the continental lithospheric mantle (Barth et al, 2001(Barth et al, , 2002Beard et al, 1996;Coleman et al, 1965;Esperança et al, 1997;Fung and Haggerty, 1995;Ionov, 2002;Jacob, 2004;Jacob et al, 1994;Kaeser et al, 2009;Liu et al, 2005;Porreca et al, 2006;Pyle and Haggerty, 1998;Schulze, 1989;Selverstone et al, 1999;Smith et al, 2004;Song et al, 2003;Taylor and Neal, 1989;Taylor et al, 2003). These continental pyroxenites could represent subducted oceanic crust, cumulates, restites, melt-rock reaction products, or frozen magmas.…”