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MPI-01 General contributions to igneous petrology
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Pyroxenite xenoliths from Marsabit (Northern Kenya): Evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite
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Benjamin Kaeser, Institut de Géologie et Hydrogéologie (Switzerland)
Anne-Lise Jourdan, Institut de Géologie et Hydrogéologie (Switzerland)
Bettina Olker, Mineralogisches Institut, Universität Heidelberg (Germany)
Angelika Kalt, Institut de Géologie et Hydrogéologie (Switzerland)
Rainer Altherr, Mineralogisches Institut, Universität Heidelberg (Germany)
Thomas Pettke, Institut für Geologie, Universität Bern (Switzerland)
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Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites (Marsabit, northern Kenya) were analysed for microstructures and mineral compositions (major and trace elements) in order to constrain the thermal and compositional evolution of the lithospheric mantle in this region.
Garnet-bearing rocks are amphibole-bearing 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 that deformation was accompanied by extensive decompression and cooling (∼970-1100 °C at ∼2.3-2.6 GPa to ∼700-800°C at ∼0.5- 1.0 GPa), best interpreted as pyroxenite intrusion into thick continental lithosphere followed by the onset of continental rifting in East Africa (i.e. formation of the Mesozoic Anza Graben). Pyroxenite intrusion in a convergent tectonic setting during Paleozoic multiple accretion could explain the presence of orthopyroxene-saturated melts. During subsequent continental rifting garnet websterites were decompressed (garnet-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 garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the xenolith-hosting basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya Rift.∼
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