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The role of crustal contamination in the potassic suite of the Karisimbi Volcano (Virunga, African Rift Valley)

par De Mulder, M.;Hertogen, Jan;Deutsch, Marie Sarah ;André, Luc
Référence Chemical geology, 57, 1-2, page (117-136)
Publication Publié, 1986-12

Article révisé par les pairs

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Résumé :

Late Pleistocene to Recent potassic, silica-undersaturated volcanics from the Karisimbi volcano (Virunga region, Western African Rift Valley, Rwanda and Zaire), have been analyzed for major and trace elements and Sr and Nd isotopic composition. The most primitive mafic lavas show considerable isotopic heterogeneity. Two different parental magmas can be recognized: a first one represented by ankaramitic and picritic K-basanites, a second one represented by K-basanites from which the Karisimbi 'main suite', comprising a complete K-basanite to K-trachyte differentiation series is derived. The isotopic variability of the primitive magmas suggests that the mantle beneath Karisimbi was heterogeneous on a rather small scale. Evolution of the main Karisimbi suite was dominated by concurrent low-pressure fractionation and assimilation. Crustal contamination resulted in an increase in 87Sr 86Sr (0.70539-0.70799) and a decrease in 143Nd 144Nd (0.51270-0.51234) with increasing fractionation. Potential lower-crustal contaminants include charnockitic and granulitic rocks outcropping in Uganda (West Nile, Bunyoro) and Zaire (Kasai). Major-element mass-balance calculations and calculated assimilation-fractional crystallization (AFC) paths in the ε{lunate}Sr-ε{lunate}Nd diagram show that most of the data of the main suite are in good agreement with interaction of a basanitic liquid with the proposed crustal component. © 1986.