Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks

Experimental data in the range of I bal to 13 kbar enable us to may the liq uidus equilibria relevant to Proterozoic (massif) anorthosites and related mafic rocks. Massif anorthosites are widely believed to have formed by accu mulation of plagioclase into high Al basaltic liquids, Mantle-derived basal tic liquids, fractionating at pressures sufficiently high (10-13 kbar) to c rystallize the highly aluminous orthopyroxene megacrysts typically observed in anorthosite massifs, reach plagioclase saturation at low normative sili ca contents. Peritectic-like equilibria (e.g, liq + opx --> plag + cpx + sp ) and a thermal divide on the plagioclase + pyroxene liquidus surface ensur e that mantle-derived liquids become nepheline normative with further cryst allization and crustal assimilation at depth. Such liquids cannot Produce t he full range of troctolitic-noritic to troctolitic-gabbroic mineral assemb lages observed in anorthosite massifs without extensive low-pressure granit e assimilation. Conversely the array of plausible anorthosite Parental liqu ids not only lies alone the trace of the plagioclase + two-pyroxene cotecti c from 10 to 13 X-bar, but also straddles the thermal divide on the plagioc lase + pyroxene liquidus surface. This condition requires mafic sou,ce regi ons, such as lower continental crust or foundered mafic plutons, Sor liquid s Parental to massif anorthosites and associated mafic intrusions.