Phase-Equilibria of Dikes Associated with Proterozoic Anorthosite Complexes

We have investigated the petrogenesis of Proterozoic massif anorthosites through experimental phase equilibria studies of two compositions representative of intrusive bodies associated with anorthosite plutons, an anorthositic dike from the Nain Complex and an average high-Al gabbro composition from the Harp Lake Complex. Experiments on both compositions show that liquidus plagioclase becomes distinctly more albitic with increasing pressure. The anorthositic dike composition does not represent a liquid or a simple suspension of plagioclase in liquid because the phase assemblages and mineral compositions produced in the experiments do not match those in thin section. The discrepancy appears to be caused by large-scale heterogeneity in the dike sample, which is also evident in the whole set of anorthositic dike compositions and may be caused by open-system crystallization. The high-Al gabbro composition has plagioclase, orthopyroxene, and high-Ca pyroxene in its liquidus at 11.5 kbar. Orthopyroxene crystals formed at 10-11.5 kbar are similar in major and minor element composition to the most aluminous orthopyroxene megacrysts, and liquidus plagioclase compositions at 10-11.5 kbar overlap the bulk of plagioclase compositions reported for the Harp Lake Complex. These features are consistent with both the orthopyroxene megacrysts and much of the plagioclase crystallizing in lower-crustal or upper-mantle magma chambers from a parental magma similar to the average high-Al gabbro and then intruding upward as mushes or crystal-rich suspensions.