An Early Proterozoic Large Igneous Province in the Eastern Baltic Shield: Evidence from the Mafic Drusite Complex, Belomorian Mobile Belt, Russia

Abstract

During the Early Proterozoic (2.5 to 2.3 Ga), three types of coeval structural provinces developed in the eastern Baltic Shield—(1) the Karelian and Kola granite-greenstone cratons, (2) the relatively high grade Lapland-Umba granulite belt (LUGB), and (3) the Belomorian (White Sea) mobile belt (BMB). The LUGB represents a compensated compressional zone where synkinematic crustal-derived magmatism of the enderbite-charnockite series predominates. The BMB is a transitional nappe-folded zone between these high- and low-grade terranes, which consists mainly of reworked granite-greenstone lithologies of the adjacent cratons. These cratons were vast extensional areas with mantle-derived, siliceous, high-Mg (boninite-like) series (SHMS) magmatism. This SHMS magmatism occurs in volcano-sedimentary sequences, large layered intrusions, and dike swarms within graben-like structures. One of the more interesting types of tectonomagmatic activity occurred within the BMB and is expressed as the unique Drusite Complex. It is represented by thousands of small intrusions of mafic and ultramafic rocks, dispersed among the higher-grade BMB host rocks. Geological features of these intrusions show that their formation was synkinematic with deformations within the belt, although they have undergone later, post-solidification deformation and metamorphism. As a result, intrusions often were transformed into lenticular, boudin-like bodies with primary igneous textures preserved only in their central portions. Compositions of the Drusite Complex intrusions, although forming small, individual bodies with associated chill zones, are similar to large layered intrusions in adjacent cratons (plagioclase harzburgites and lherzolites, pyroxenites, troctolites, olivine norites and norites, gabbronorites, anorthosites, and diorites). The areal distribution of the drusite intrusions and their correlation with large layered mafic intrusions in adjacent cratons suggests a vast magma-generation zone beneath western Russia during the Early Proterozoic. The character and extent of magmatism suggests that during the Early Proterozoic (in Sumian— Sariolian time) the Kola and Karelian cratons were vast extensional areas above spreading plume heads. Within this scenario, the LUGB was an area of intense crustal sagging between these two cratons. The BMB was a transitional zone of tectonic flowage between the LUGB and the cratons, where movements were not as intense; there a nappe-folded structure formed. As a result, the intrusion of new melts occurred under rapidly changing conditions and a specific type of disseminated, intrusive magmatism—The Drusite Complex—emerged instead of the formation of layered intrusions. The petrologic and mineralogic compositions of the Drusite Complex intrusions are indistinguishable from coeval layered mafic intrusions of the adjacent Karelia and Kola cratons, suggesting similar parental magmas and a large zone of magmatism (i.e., large igneous province, or LIP) beneath the eastern Baltic Shield. These magmas were derived either from depleted mantle melts that had assimilated a significant crustal component, or from enriched mantle.