Liquidus phase relationships determined on the join CaAIzSi20 8 (anorthi te)-MgzSiO 4 (forsteri te)-SiO 2 (quartz) at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel, enstatite and silica fields to expand. The boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the SiO 2 apex as pressure increases. Therefore, simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases, as has been found in other studies. Also, increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization. Although anorthite is unstable in simplified peridotite above 9 kbar in the system C a O M g O A I 2 0 3-SIO2, it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts. a simplified analog of the tholeiitic portion of the basalt tetrahedron of Yoder and Tilley (1962). Viewed from either compositional or mineralogical relationships, the CMAS system is particularly valuable in modeling the generation and crystallization of basalts because (1) it contains about 85% of the chemical constituents of basalt and about 90% of the chemical constituents of their peridotite source rocks in the mantle, and (2) it contains representatives of all the major anhydrous phases of basalts and peridotites. As part of a systematic program in this laboratory aimed at clarifying liquidus phase relationships in the system forsterite-diopside-anorthite-quartz as a function of pressure, we present here liquidus relationships on the join anorthite-forsterite-quartz at 10 kbar. This join forms the base of the tholeiitic part of the simplified basalt tetrahedron in the CMAS system.