Differentiation of Alkali Basaltic Magma Towards the Salic Ends

Alkali basaltic magma fractionating towards the salic ends, in plutonic environment,. shows characteristic crystallisation behaviour for which both magma chemistry and physical conditions seem to be responsible. Both pressure and iron enrichment of the magma appear to inhibit reaction relation of olivine with the rest liquid. The hydrous magma may experience prolonged precipitation of chain-silicates whose field of crystallisation is extended at the expense of plagioclase and olivine. Both pyroxene and amphibole show parallel trend in compositional variations by inviting calcium, titanium and aluminium along with a progressive Fe-Mg diadochic replacement with fractionation. Compositions of the precipitating mafic phases highly influence the nature of residual liquid which may thus show wide variation in silica saturation. High pressure fractionation of an alkalic basic magma under favourable condition may even lead to a peralkalic silica rich differentiate.