Lower Crustal Cumulate Nodules in Proterozoic Dikes of the Nain Complex: Evidence for the Origin of Proterozoic Anorthosites

Nodules and xenocrysts dominated by high-A1 orthopyroxene occur in Proterozoic basaltic dikes that cut the Nain anorthosite complex, Labrador. This pyroxene (En73–68, Al2O3 = 6.5–4.5) lacks exsolution and occurs both as anhedral xenocrysts up to 10 cm in diameter and with euhedral plagioclase (An55) in ophitic nodules. Rarely, olivine (Fo70) occurs with orthopyroxene and Al-spinel with plagioclase. Scarce, more Fe-rich nodules contain two pyroxenes (orthopyroxene + pigeonite and pigeonite+augite) and coarse intergrowths of ilmenite and Ti-rich magnetite. Pyroxene pairs yield temperatures of 1250° to 1170 °C; coexisting oxide lamellae yield temperatures between 1145° and 1120 °C. The high subsolidus temperatures of the nodules contrasts with the low temperature of the host anorthosite at the time of dike emplacement and indicates a deep source for the nodules. Coexisting olivine (Fo70) and plagioclase (An54) suggest a maximum pressure of about 11 kb. The dominant orthopyroxene in these nodules is nearly identical in composition to the high-Al orthopyroxene megacrysts with exsolved plagioclase (HAOM) found in most Proterozoic anorthosites, and the ophitic nodules have textures similar to ophitic occurrences of HAOM in anorthosite. Rafting of cotectic nodules from the lower crust can explain occurrences of HAOM in shallow level anorthosites. The nodules and xenocrysts have compositions consistent with crystallization from magmas that were parental to the anorthosites. They lend support to models which derive anorthosites by fractional crystallization of basaltic magma near the base of the crust.