ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT Mesoproterozoic crust is widely exposed in the Grenville Province portion of northeastern Laurentia, where it is interpreted as an assemblage of two continental-arc segments separated by a composite arc belt (Quebecia) with island-arc remnants. A synthesis of the geologic context, types, and geochemical patterns of 1.5–1.35 Ga granitoids reveals a regional distribution in each segment, with dioritic to granitic plutonism variably associated with arc-related volcano-sedimentary belts in the south and inboard monzonitic to granitic plutonism in the north. In addition, belts of dioritic to granitic orthogneisses occupy intermediate positions in Quebecia and in the west. The inboard granites are consistently old in all segments (1.5–1.45 Ga), but the preserved volcano-sedimentary belts are older in the east and in Quebecia (1.5–1.45 Ga) and younger in the west (1.39? and 1.36 Ga), while the belts of orthogneisses show a large spread of ages at 1.45–1.37 Ga. Granitoids in the volcano-sedimentary belts and the orthogneisses include magnesian, calcic to calc-alkalic components to ferroan, alkali-calcic components. In contrast, the inboard plutons are dominantly ferroan and alkali-calcic to alkalic in the continental-arc segments, where they are locally associated with anorthosite-mangerite-charnockite-granite (AMCG) suites. Collectively, the different types of granitoid magmatism can be linked to an active margin, with subduction under northeastern Laurentia, involving arc building, arc rifting, back-arc opening and inboard extension, and amalgamation processes variably operating at different parts of the margin and at different times. In addition, the data provide a basis for comparison with other parts of the eastern to southwestern Laurentian margin in the 1.5–1.35 Ga time frame.

Beginning ca. 2.76 Ga, evolution of the Kola-Karelia crust was related to the intracontinental high-temperature metamorphic (up to granulite facies) and magmatic events in combination with formation of the basins related to rifting and infilling with intracontinental volcanic and sedimentary sequences initiated by plume-type processes in the mantle. The geological events corresponding to intracontinental evolution were expressed not only in the formation of new rock associations, juvenile to a significant extent, but also in reworking of previously formed rocks. The age, content, and mode of geological activity are somewhat different in the Kola and the Karelian-Belomorian regions. The Karelian-Belomorian region is oval in plan view. The long axis of this oval extends for 600–700 km in the meridional direction; its maximum width is 400–450 km. The southern part of this oval structure is cut off along the NW-trending boundary with the Paleoproterozoic Svecofennian accretionary orogen. The main constituents of the Karelian-Belomorian region are: epicontinental sequences of greenstone belts (Kostomuksha, Khedozero-Bolsheozero, Gimoly-Sukkozero, Jalonvaara) and paragneiss belts (Hattu, Nurmes); granulite-gneiss complexes and intrusive enderbite-charnockite series; sanukitoid-type granitoid intrusions and lamprophyre dikes, along with migmatization and emplacement of within-plate young granites; and local manifestations of granulite-facies metamorphism superposed on older rocks. Concentric spatial distribution of related geological units is characteristic of the Karelian-Belomorian region. The geometric pattern of the region can be satisfactorily explained assuming initial activity of a mantle plume ca. 2.76 Ga in the central part of the region. A peak of activity was related to the events that occurred ca. 2.74–2.70 Ga. The geochronological data show that a region of high-temperature processes expanded from its center (2.76–2.73 Ga) to the periphery (2.74–2.70 Ga). The concentric character of the tectonic structure was eventually formed as a result of these processes. Widespread high-temperature magmatism and metamorphism in combination with formation of synformal and linear sedimentary basins indicate the setting of anorogenic extension and vigorous influx of extracrustal heat, i.e., a large event related to a mantle plume. In contrast to the Karelian-Belomorian hot region, the coeval Kola region of intracontinental manifestations of high-temperature metamorphism and magmatism is characterized by oval-block geometry. This area, confined to the central part of the Kola Peninsula, extends for 600 km in the northwestern direction, having a width of ~200 km. It is possible that this area extends further to the southeast beneath the platform cover. The main tectonic units are the intracontinental greenstone belts (Sør-Varanger, Titovka, Uraguba, Olenegorsk, Voche-Lambina, Kachalovka, Runijoki–Khikhnajarvi, and Strelna system) in the Inari-Kola microcontinent, the granulite-gneiss Central Kola complex, and the Keivy volcanotectonic paleodepression. Sanukitoid intrusions play a modest role. The Keivy volcanotectonic paleodepression is situated in the eastern Kola Peninsula. Rocks of this tectonic unit are peculiar, and many of them have no obvious analogs in the Fennoscandian Shield or elsewhere. The major Neoarchean amphibolite-gneiss association consists of calc-alkaline to subalkaline garnet-biotite and subalkaline-peralkaline aegirine-arfvedsonite gneisses, as well as biotite-amphibole and amphibole gneisses, amphibolites, and rheomorphic alkali granites. In the western part of the paleodepression, gneisses (metavolcanic rocks) are cut through by small Sakharjok and Kuljok nepheline syenite intrusions. Geochronological estimates characterize two outbursts of magmatic activity separated by a long gap. The early outburst corresponds to magmatic crystallization of calc-alkaline metavolcanic rocks at 2.90–2.87 Ga. The second vigorous outburst documented at 2.68–2.63 Ga corresponds to eruption of subalkaline and subalkaline-peralkaline volcanic rocks, emplacement of alkali and nepheline syenites, and crystallization of gabbro-anorthosite of the Tsaga-Acherjok complex. The duration of the main magmatic phase is ~50 m.y., whereas the preceding gap lasted for ~200 m.y. A model of a volcanotectonic depression largely filled with pyroclastic flows seems plausible to explain pre-metamorphic events. Such manifestations of volcanic activity are inherent to intracontinental domains and related to activity of mantle plumes; similar processes can also develop in the back extensional zone of active continental margins. The synchronism of felsic volcanism and emplacement of the typically intracontinental gabbro-anorthosites form a sound argument in favor of an intracontinental setting for the Keivy paleodepression. The geometry of the Kola region can be satisfactorily explained in terms of mantle-plume activity noted ca. 2.76 Ga in the marginal part of this region; a peak of activity in its central part is related to the events that happened ca. 2.68–2.63 Ga.