Can Orthopyroxene be Present in the Source of Toro-Ankole, East African Rift, Kamafugites?
Résumé
We have studied mineral-hosted melt, crystal and fluid inclusions from two ugandite, one mafuriteand two katungite samples from the Toro-Ankole volcanic province in the East African Rift, whichis the archetypal location for kamafugitic rocks. A main finding of our study is the presence oforthopyroxene as inclusions in an early generation of olivine from all three types of kamafugites,suggesting interaction of a carbonate-rich metasomatic agent with lithospheric peridotite mantlethat may have caused almost complete dissolution of orthopyroxene. This process was preceded,accompanied or followed by the formation of phlogopite–clinopyroxene veins resulting from inter-action of F-rich and low H2O/CO2metasomatic fluids with the mantle rocks, which then became thesource of the Toro-Ankole kamafugites. Pressure–temperature (P–T) estimates suggest that the par-ental kamafugitic melts last equilibrated with their source rocks at1668 kbar and11606130C.This implies that they could have originated significantly below the solidus of dry, carbonated peri-dotite, but above the solidus of phlogopite-bearing clinopyroxenite. We conclude that the Toro-Ankole kamafugites originated by very low degrees of partial melting at moderately oxidized condi-tions (DFMQ¼þ22604 atm log units, where FMQ is fayalite–magnetite–quartz buffer) under ahigh geothermal gradient of 60–80 mW m2, in response to lithospheric extension and probable as-sociation with an adjacent mantle plume. We estimate that differentiation of parental ugandite,mafurite and katungite magmas could have occurred at depths<12 km in theTrange 1150–850C.Laboratory-heated, homogenized melt inclusions trapped by a second generation of olivine andclinopyroxene are characterized by remarkable silica-undersaturation, compared with mid-oceanridge basalt and ocean island basalt magmas, with high concentrations of alkalis, Ti, Ba, Sr and Zr,but varying to very low concentrations of Al and Ca. Such alkali-rich, strongly evolved melts mighthave resulted from extreme (>95%) fractional crystallization of the parental magmas, assumingtheir chemical compositions to be similar to those of the respective lavas. However, this estimate isabout three times higher than the modal amount of phenocrysts in the lavas that could be rein-forced by the presence of excess co ~900–1000°C has occurred during the final stage of magma evolution at very shallow crustal depths or possiblydirectly in the lava flow.