The Petrology of the Lavas of Grande Comore

The four volcanic islands comprising the Comores archipelagoform a linear rise across the northern end of the MozambiqueChannel. Volcanism migrated westward from the now deeply dissectedand partially submerged Mayotte to the active Kartala volcanoon Grande Comore. The volcanic rocks on all the islands consistof silica–undersaturated basalts and their derivatives. Grande Comore is formed of two coaleseing shield volcanoes,La Grille in the north and Kartala in the south. The lavas ofKartala are entirely alkali basalts, with abundant ankaramiticand oceanitic varieties, containing an average of 3.7 wt percent normative nepheline. They represent a series resultingfrom low–pressure (< 8kb) fractional crystallizationof alkali basalt parent magma. The formation of this parentmagma can be explained as resulting from more than 10 per centpartial melting of garnet lherzolite upper mantle at pressuresaround 30 kb, with some high–pressure (> 25 kb) fractionationof harzburgite, garnet harzburgite, and either garnet wehrliteor ecogite, followed by polybaric fractionation of olivine±clinopyroxene during ascent. They La Grille lavas are basanitie, with an average of 11–5wt per cent normative nepheline and some normative leucite,and contain only olivine as an important phenocryst phase. Theyare explicable as originating from less than 10 per cent partialmelting of garnet I herzolite upper mantle, with substantialhigh–pressure (> 25 kb) fractionation of garnet wehrliteor eclogite. Interuption of this high–pressure(> 25kb) fractionation at different stages resulted in one trendof chemical variation, on which was superimposed a second trendby polybaric fractionation of lherzolite, wehrlite, and dunite,now found as inclusions in the lavas.