PETROLOGY OF MANTLE XENOLITHS FROM MEGADO AND DILO; KENYA RIFT; SOUTHERN ETHIOPIA

摘要: A study has been made of ultramafic xenoliths embedded in the Quaternary basanitic lavas Dilo and Megado areas; southern Ethiopia. This is first time that have found at Dilo. In both are spinel lherzolites pyroxenites; garnet- and/or spinel-bearing pyroxenites occasionally containing amphibole. Temperature/pressure estimates indicate most last equilibrated upper mantle similar P-T conditions (T ~ 950-1050°C P 1.3-1.7 GPa); whereas some higher temperatures (~ 1150-1250°C). Based on mineral assemblage; four groups were distinguished; two which exhibit spectacular garnet coronas around spinels. Whole-rock composition (major trace elements) indicates garnet-free segregated from OIB-type melt. Successively; melt evolution caused crystallisation (so bearing formed) and; eventually; pargasitic amphibole generation amphibole- with consequent LILE-enrichment residual melts. Melt resembles hypothesised by Bedini et al. (1997) to explain metasomatism based reactive porous flow simulations. Thus; may represent “counter-part” melts responsible this area. Furthermore; OIB nature parental permeating suggests (and pyroxenite genesis) was related magmatic activity for ascent surface.