Ancient conglomerates of the Kursk Magnetic Anomaly

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, until after 15 to 30% crystallization of the parent magma. However, this early-formed plagioclase would not have been in chemical equilibrium with the later-forming mafic silicates. Toward the end of crystallization in the chamber, plagioclase precipitated in equilibrium with the mafic minerals. Trace-element modeling indicates that the rocks that form the pluton originally were precipitated as liquid-dominated cumulates.

The trace-element and mineral-chemical compositions of the parent liquid (s) of the Pezhostrov pluton were enriched in the LILE (Sr, Rb, Ba, La) and depleted in the HFSE (Zr, Ti, Y) relative to present-day MORB. These magmas are suggested to be roughly boninitic in composition, and are similar to those parental to other mafic plutons of similar age worldwide, including the Stillwater intrusion, Montana, USA. Thus, this character of magmatism may represent an important episode of mantle melting worldwide during the late Archean and early Proterozoic.