Mineral chemistry and petrogenesis of an ultrapotassic-ultramafic volcanic rock

Abstract

Compositions of the major phenocryst minerals (olivine, phlogopite) and groundmass minerals (olivine, phlogopite, kalsilite), and a glass phase have been determined from a biotite mafurite occurring as an ejected block in the highly K-rich ultramafic rocks of south west Uganda. Comparison of the phenocryst mineral compositions with those determined from recent high pressure experiments on biotite mafurite composition suggests this rock may have formed by partial melting of a K-enriched mantle source containing both H₂O and CO₂ at approximately 1,250 ° C and 30 kb. The absence of crystalline leucite but its presence as a major component of the glass phase and textural relations in the groundmass indicate that the final consolidation of the biotite mafurite took place at pressures greater than atmospheric. The presence of phlogopite, olivine, kalsilite, and glass mainly of leucite composition may suggest that consolidation took place under the conditions where these phases were in equilibrium. Based on the experimentally determined conditions for the reaction of phlogopite break down to olivine+kalsilite +liquid+vapor, a crude estimation of the consolidation conditions for ejected blocks of biotite mafurite are 1,150 °–1,180 ° C at a $P_{\operatorname{H} _2 O} ( = P_{\operatorname{T} \operatorname{otal} } )$ of 1–2 kb.