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Can Orthopyroxene be Present in the Source of Toro-Ankole, East African Rift, Kamafugites ?

Gurenko, A.A. ; Sobolev, A.V., J. of Petrology

Can Orthopyroxene be Present in the Source of Toro-Ankole, East African Rift, Kamafugites ?

Gurenko, A.A. ; Sobolev, A.V.

Journal of Petrology, 2018, 59, 8, 1517-1550

Abstract :

We have studied mineral-hosted melt, crystal and fluid inclusions from two ugandite, one mafurite and two katungite samples from the Toro-Ankole volcanic province in the East African Rift, which is the archetypal location for kamafugitic rocks. A main finding of our study is the presence of orthopyroxene 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 mantle that may have caused almost complete dissolution of orthopyroxene. This process was preceded, accompanied or followed by the formation of phlogopite–clinopyroxene veins resulting from interaction of F-rich and low H2O/CO2 metasomatic fluids with the mantle rocks, which then became the source of the Toro-Ankole kamafugites. Pressure–temperature (P–T) estimates suggest that the parental kamafugitic melts last equilibrated with their source rocks at 16+-8 kbar and 1160+-130°C. This implies that they could have originated significantly below the solidus of dry, carbonated peridotite, 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 conditions (∆FMQ = +2.2+-0.4 atm log units, where FMQ is fayalite–magnetite–quartz buffer) under a high geothermal gradient of 60–80mWm-2, in response to lithospheric extension and probable association with an adjacent mantle plume. We estimate that differentiation of parental ugandite, mafurite and katungite magmas could have occurred at depths <12km in the T range 1150–850°C. Laboratory-heated, homogenized melt inclusions trapped by a second generation of olivine and clinopyroxene are characterized by remarkable silica-undersaturation, compared with mid-ocean ridge 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 might have resulted from extreme (>95%) fractional crystallization of the parental magmas, assuming their chemical compositions to be similar to those of the respective lavas. However, this estimate is about three times higher than the modal amount of phenocrysts in the lavas that could be reinforced by the presence of excess cognate crystals and/or xenocrysts in the lavas. Strong oxidation from FMQ+2 to FMQ+4 to +5.2 atm log units of the evolved mafuritic magmas at 900–1000°C has occurred during the final stage of magma evolution at very shallow crustal depths or possibly directly in the lava flow.

Voir en ligne : http://dx.doi.org/doi:10.1093/petro...

publié lundi 5 novembre 2018