Relics of subcontinental mantle in the Cape Verde oceanic lithosphere

Mantle xenoliths from two Late Tertiary necks of Sal
Island were investigated in order to define the petrological
characteristics together with the compositional evolution of
the Atlantic lithospheric mantle underneath Cape Verde
Archipelago. They consist of mainly protogranular sp-bearing
lherzolites and harzburgites. Whole rock and pyroxene trace
element distribution, particularly HREE, suggest that
compositional variation from lherzolites to harzburgites
cannot be explained by a common progressive depletion
process of a sp-bearing mantle peridotite. High Cr2O3, low
CaO contents and upward-convex REE patterns in large
protogranular clinopyroxenes (La_N=3.6–5.3; Sm_N= 8.3–12.8;
Yb_N=2.4–4.8), coupled with high Cr2O3 and low HREE
contents in orthopyroxene, suggest in fact that lherzolites still
preserve geochemical fingerprints consistent with a fertile gtbearing
stability field. By contrast the majority of Cape Verde
harzburgites, are quite comparable to highly refractory mantle
lithologies commonly observed in the Atlantic Ocean
generated after extensive MORB extraction. This is also in
agreement with the occurrence of Late Jurassic MORB in the
islands of Maio and Santiago. On the other hand the abundant
presence of lherzolites-still recording equilibration in the
garnet stability field-suggests that a significant portion of
Cape Verde lithospheric mantle was not formed at mid-ocean
ridge, but most probably represent a portion of sublithospheric
continental mantle incorporated in the newly-formed oceanic
lithosphere. A few lherzolites and harzburgites show textural
evidences of metasomatic enrichments. Metasomatic fluids
react with the original paragenesis causing the formation of
secondary minerals, such as clinopyroxenes after
orthopyroxene and primary clinopyroxene destabilization and,
in few cases, k-feldspar and K₂O-rich glass (K₂O up to
8.78wt%). These agents were reconstructed using major and
trace element mass balance calculations between primary and
secondary parageneses. The resulting compositions have
geochemical features comparable to those of kimberlite (on
dry basis, MgO:17-27 wt%; K₂O/Na₂O:1.6-3.2 molar;
(K₂O+Na₂O)/Al2O3:1.1-3.0 molar; Rb:91-165 ppm; Zr:194-
238ppm). These magmas may result from very low partial
melting degree of deeper SCLM portions, left behind by
drifting of the African Plate during the Central Atlantic
opening