MELTING EXPERIMENTS ON A SANIDINE PHLOGOPITE LAMPROITE AT 4-7 GPA AND THEIR BEARING ON THE SOURCES OF LAMPROITIC MAGMAS

Suprasolidus phase relations at pressures from 4 to 7 GPa and temperatures from 1000 to 1700 degrees C have been determined experimentally for a sanidine phlogopite lamproite from North Table Mountain, Leucite Hills, Wyoming. The lamproite is silica rich and has been Postulated to be representative of the magmas which mere parental to the Leucite Hills volcanic field. Near-liquidus phases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene. Below 5 GPa, jadeite-poor pyroxene is the only near-liquidus phase. Near-solidus assemblages consist of clinopyroxene, titanian potassium richterite and titanian phlogopite with either potassium titanian silicate above 5 GPa or potassium feldspar below 5 GPa. The potassium titanian silicate is a newly recognized high-pressure phase ranging in composition from K4Ti2Si7O20 to K4TiSi8O20. It coexists With coesite at pressures above 6 GPa at 1100-1400 degrees C. A previously unrecognized K-Ba-phosphate is a common near-solidus phase. The phase relationships are interpreted to suggest that lamproites cannot be derived by the partial melting of simple lherzolitic sources. However, it is proposed that sanidine phlogopite lamproites are derived by high degrees of partial melting of ancient metasomatic veins within a harzburgitic/lherzolitic lithospheric substrate mantle. The veins are considered to consist of phlogopite, K-Ti-richterite, K-Ba-phosphate and K-Ti-silicates.