Kimberlite-like metasomatism and "garnet signature" in spinel-peridotite xenoliths from Sal, Cape Verde Archipelago: relics of a subcontinental mantle domain within the Atlantic oceanic lithosphere?

A mantle xenolith suite from two Late Tertiary necks on Sal Island (Cape Verde Archipelago) consists of nearly equivalent amounts of anhydrous spinel-bearing lherzolites and harzburgites, in which secondary metasomatic textural domains are superimposed on the original protogranular textures. Detailed petrographic studies, coupled with in situ major and trace element analyses of the constituent minerals and interstitial glasses, reveal the complex evolutionary history of the Cape Verde lithospheric mantle, from depletion in the garnet facies to re-equilibration and re-enrichment in the spinel stability field. Low CaO (16.4-18.0 wt %) and heavy rare earth element (HREE; Yb-n = 2.4-4.8), and high Cr2O3 (1.06-1.84 wt %) contents in the clinopyroxenes of the lherzolites can be quantitatively accounted for by (1) low-degree (similar to 4%) partial melting of a Primitive Mantle-like garnet lherzolite followed by (2) partial re-equilibration of the melting residuum from the garnet to the spinel stability field. This model is further supported by thermobarometric estimates (T = 975-1210 degrees C; P = 1.3-2.1 GPa), which cluster around the spinel-garnet boundary in the peridotite system. Secondary parageneses, regardless of the primary lithologies, are characterized by (1) two clinopyroxenes, cpx2-O and cpx2-C, respectively related to orthopyroxene and clinopyroxene destabilization after reaction with metasomatic fluids, and (2) glasses with anomalously high, even for continental settings, K2O contents (up to 8.78 wt %), together with K-feldspar. Major and trace element mass balance calculations between the primary and secondary parageneses suggest infiltration of a kimberlite-like metasomatizing agent (on volatile-free 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 194-238 ppm). The kimberlite-like metasomatism in the Cape Verde lithospheric mantle, together with the presence of lherzolitic domains, partially re-equilibrated from the garnet to the spinel stability field, may suggest the presence of subcontinental mantle lithosphere relicts left behind by drifting of the African Plate during the opening of the Central Atlantic Ocean.