An early Proterozoic large igneous province in the eastern Baltic Shield: Evidence from the mafic Drusite Complex, Belomorian mobile belt, Russia

During the Early Proterozoic (2.5 to 2.3 Ga), three types of coeval structu ral provinces developed in the eastern Baltic Shield-(1) the Karelian and K ola 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 c rustal-derived magmatism of the enderbite-charnockite series predominates. The BMB is a transitional nappe-folded zone between these high- and low-gra de terranes, which consists mainly of reworked granite-greenstone lithologi es 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 tectono-magmatic activity occurred wit hin the BMB and is expressed as the unique Drusite Complex. It is represent ed by thousands of small intrusions of mafic and ultramafic rocks, disperse d among the higher-grade BMB host rocks. Geological features of these intru sions show that their formation was synkinematic with deformations within t he belt, although they have undergone later, post-solidification deformatio n and metamorphism. As a result, intrusions often were transformed into len ticular, boudin-like bodies with primary igneous textures preserved only in their central portions. Compositions of the Drusite Complex intrusions, al though forming small, individual bodies with associated chill zones, are si milar to large layered intrusions in adjacent cratons (plagioclase harzburg ites 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 intrusio ns in adjacent cratons suggests a vast magmageneration zone beneath western Russia during the Early Proterozoic. The character and extent of magmatism suggests that during: the Early Prote rozoic (in Sumian-Sariolian time) the Kola and Karelian cratons were vast e xtensional areas above spreading plume heads. Within this scenario, the LUG B 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 crato ns, where movements were not as intense; there a nappe-folded structure for med. As a result, the intrusion of new melts occurred under rapidly changin g conditions and a specific type of disseminated, intrusive magmatism-The D rusite Complex-emerged instead of the formation of layered intrusions. The petrologic and mineralogic compositions of the Drusite Complex intrusions a re indistinguishable from coeval layered mafic intrusions of the adjacent K arelia and Kola cratons, suggesting similar parental magmas and a large zon e of magmatism (i.e., large igneous province, or LIP) beneath the eastern B altic Shield. These magmas were derived either from depleted mantle melts t hat had assimilated a significant crustal component, or from enriched mantl e.