Publication Date:
2018-05-04
Description:
Petrological and geothermobarometric data from (garnet)-spinel peridotite and pyroxenite xenoliths in Quaternary
alkali-basalts at Mega (Sidamo region, Ethiopia) provide evidence for refertilization and thinning of the
lower subcontinental lithospheric mantle (SCLM) during Tertiary rifting events in the southern Main Ethiopian
Rift (MER). Samples of lherzolites, harzburgites, and olivine websterites contain spinel-pyroxene symplectites
thatwere formed by garnet breakdown reactions. P-T equilibration conditions, in part calculated using the reconstructed
garnet composition (pyrope), indicate the evolution of peridotites from the P-T conditions fromthe garnet
stability field (2.9–2.2 GPa; 945–1025 °C), through garnet-spinel transition (b 2.1–1.7 GPa at 906–1017 °C), to
the final equilibration in the spinel stability field (1.3 GPa at 862–1056 °C), corresponding to a nearly adiabatic
decompression path of about 50 km. Peridotite chemical compositions plot in an off-craton trend, with variable
fertility degrees, only in part due to partial melting processes. Rocks showvariable CaO contents (0.96–4.12 wt%),
CaO/Al2O3 ratios (1.42–4.46), and clinopyroxene contents (6–31 vol.%), reaching compositions more enriched
than pyrolite. Clinopyroxene shows evident LREE, U, Th, and Sr enrichments, particularly in harzburgites. Geochemical
data suggest heterogeneous refertilization of a refractory lower SCLM, induced by infiltration and migration
through porous flow of silicate-carbonate melts. Metasomatic processes drove melt compositions
towards low-viscosity highly-mobile C-O-H-richmelts, inducing cryptic and hydrousmodalmetasomatismin refractory
peridotites. Heterogeneous refertilization of the SCLM beneath the southern MER seems to have been induced
by upwelling of asthenospheric melts and/or by reactivation of ancient lithospheric metasomes.
Rejuvenation processes could have rheologically weakened the base of SCLM through gravitational instabilities,
and increased the extensional stresses, causing a destabilizing effect at the onset of the lithospheric thinning and
Tertiary rifting.
Description:
Published
Description:
231-248
Description:
1T. Deformazione crostale attiva
Description:
7T. Struttura della Terra e geodinamica
Description:
JCR Journal
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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