Geochemical evidence for subcontinental lithospheric mantle in oceanic domain (Sal, Cape Verde Archipelago)
|Location||International Geological Congress,oslo 2008|
|Author||Bonadiman, Costanza۱; Coltorti , Massimo۱; Beccaluva, Luigi۱; Griffin, L.William۲; O’Reilly, Suzanne Y۲; Pearson, Norman۲|
|Holding Date||08 October 2008|
Cape Verde xenoliths show protogranular to porphyroclastic textures and comprise lherzolites and harburgites. The fertile nature of lherzolites is expressed in whole rock major elements (e.g. 39.7-45.5 wt% MgO; 1.2-4.6 wt% CaO; 0.5-2.0 wt% Al2O3), and in mineral compositions (cr#sp: ca. 0.3, Fo-ol: 87-89, (Al2O3)opx: 4.1-6.0 wt%). Orthopyroxene have mildly LREE-depleted patterns (YbN=0.23-0.36, LaN=0.03-0.5), whereas those of clinopyroxene are upward convex (YbN= 0.9-1.5, LaN=1.7-2.0). The refractory harzburgites (،ـ3 % cpx) have low Al2O3 in orthopyroxene (<3 wt%), high Cr# in spinel (0.3-0.8) and high Fo contents in olivine (>91.5). The refractory spinel harzburgites and the mildly depleted spinel lherzolites represent two unrelated rock series: modal proportions of clinopyroxene plotted against olivine define for the lherzolites a trend of decreasing clinopyroxene with increasing olivine, whereas the harzburgites show roughly constant, low clinopyroxene contents. Sr, Nd and Hf isotopic analyses on separated cpxs from five un-metasomatized lherzolites and one wehrlite reveal 87Sr/86Sr between 0.7031 and 0.7045, comparable to the Sr isotopic compositions of the basalts in the southern islands of the Archipelago. 143Nd/144Nd ranges from 0.512837 to 0.512955 and 176Hf/177Hf 0.283038- 0.2831012. On the whole, their isotopic signature varies from Depleted Mantle (DM) towards Enriched Mantle (EM) components, as recorded in many subcontinental litospheric mantle domains.
Many xenoliths bear extensive evidence of metasomatic reactions, characterized by secondary parageneses (ol+sp+cpx+glass+K-feld) around primary clinopyroxenes, spinel and orthopyroxene. Glass is common, both interstitially and as small inclusions in olivine and orthopyroxene. Despite recent interpretation of these textures as produced by host magma infiltration during transport to the surface, the large data set presented unequivocally indicate that they are the results of the interaction between a fertile peridotite with a metasomatic melts originally formed from very low partial melting degree of subcontinental lithospheric mantle. We conclude that spinel lherzolites from Sal, Cape Verde represent a relict of continental mantle in an oceanic domain.