PHASE-EQUILIBRIA IN THE SILICA-UNDERSATURATED PART OF THE KALSIO4-MG2SIO4-CA2SIO4-SIO2-F SYSTEM AT 1 ATM AND THE LARNITE-NORMATIVE TREND OFMELT EVOLUTION

The evolution of nephelinitic melts in equilibrium with mica-bearing l iquidus assemblages and melting relations have been studied on two sil ica-undersaturated joins of the KAlSiO4 - Mg2SiO4 - Ca2SiO4 - SiO2 - F system at atmospheric pressure by quench runs in sealed platinum caps ules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F(-) = O2-). The first join is the p seudo-ternary Forsterite - Diopside - KAlSiO3F2 system. Forsterite, di opside, F-phlogopite and leucite crystallisation fields and a fluoride -silicate liquid immiscibility solvus are present on the liquidus surf ace of the join. Sub-liquidus and sub-solidus phases include akermanit e, cuspidine, spinel, fluorite and some other minor fluorine phases. T he second system is the pseudobinary Akermanite - F-phlogopite join th at intersects the Forsterite - Diopside - KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (Ic) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + Ic + l = phl, and the reaction proceeds until forsterite or leuc ite are completely consumed. The reaction temperature and resulting ph ase association depend on batch composition. Thus, leucite is not stab le in the sub-solidus of the Akermanite - F-phlogopite join, but is pr eserved in a part of the Forsterite - Diopside - KAl-SiO3F2 system whe re forsterite reacts out, or does not crystallise at all. The phlogopi te-in reaction has an important effect on the composition of the coexi sting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from ''normal'' melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites r equires fractionation of phlogopite-bearing assemblages under volatile pressure.