Petrology and Chemistry of an Early Proterozoic Lherzolite-Gabbronorite-Anorthosite Pluton of the White Sea Complex, Northern Karelia, Russia

Two of the most well-preserved igneous bodies in the early Preeambrian White Sea complex— the Severnyy and Yuzhnyy massifs on Pezhostrov Island—have been studied in order to gain a better understanding of ultramafic-mafic magmatism in the Belomorian tectonic block. These massifs represent portions of a single, differentiated pluton, ranging in composition from lherzolite to gabbronorite to anorthosite. Mineral-chemical and trace-element compositions of chill margins from this pluton were used to model the differentiation in this ancient magma chamber. Major-element compositions of minerals suggest that plagioclase in these rocks is not in equilibrium with the mafic minerals. This possibly is the result of suspension of less dense, early-formed plagioclase in more dense, early residual liquids. Later, as the liquid density decreased because of precipitation of mafic phases, plagioclase began to precipitate. We speculate that the liquid density did not decrease to a point where plagioclase would settle, unt...