Petrology of spinel lherzolite xenoliths from Youkou volcano, Adamawa Massif, Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes

摘要

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo(89.4-90.5)), enstatite (En(89-91)Fs(8.7-9.8)Wo(0.82-1.13)), clinopyroxene, spinel (Cr#(Sp) = (9.4-13.8)), and in some cases amphibole (Mg-# = (88.5-89.1)). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three-two pyroxene thermometers and range between 835 and 937 degrees C at pressures of 10-18 kbar, consistent with shallow mantle depths of around 32-58 km. Trends displayed by bulk-rock MgO correlate with Al2O3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al2O3 = 6.07-6.56 wt% in clinopyroxene) and the low CaO/Al2O3 ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.