DISPERSED MAFIC-ULTRAMAFIC INTRUSIVE MAGMATISM IN EARLY PALEOPROTEROZOIC MOBILE ZONES OF THE BALTIC SHIELD: AN EXAMPLE OF THE BELOMORIAN DRUSITE (CORONITE) COMPLEX

Abstract

The Early Paleoproterozoic (2.46-2.36 Ga) Belomorian drusite (coronite) complex was addressed to for the first time in discussing the geology, petrology, and genesis of dispersed intrusive mafic-ultramafic magmatism that developed between cratons and granulite belts of Early Precambrian age. The complex com- prises numerous small rootless synkinematic intrusions that are scattered throughout the Belomorian Mobile Belt (BMB). The rocks of the complex are compositionally close to the rocks of large layered intrusions in the neighboring cratons and compose, together with them, the Baltic Large Igneous Province (BLIP) of the silicic high-Mg (boninite-like) series. It is demonstrated that the magma generation regions were similar beneath the cratons and BMB, but, in contrast to the situation at the cratons, melt portions ascending from below the BMB could be accommodated only in small chambers, whose position was controlled by local heterogeneities induced by the tectonic flowage of the host rocks. Moreover, these chambers continuously changed their posi- tion, thus precluding the origin of large bodies and eventually giving rise to dispersed magmatism. Upon their crystallization, the intrusions were affected by tectonometamorphic reworking under amphibolite-facies condi- tions, so that relatively little altered rocks are now preserved only in the cores of these bodies. The rocks are characterized by the development of drusite (coronite) textures along the grain boundaries of primary magmatic minerals.