The paper prese nts results of petrochem ical, geochem ical, a nd isotope-geochem ical study of the Patchemvarek a nd Sever ny i gabbroa northos ite mass ifs of the Kola Pe n i nsula. It was show n th

New geological, isotopic-geochronological, petrochemical, and isotopic geochemical data were obtained on the Porosozero and Kolmozero sanukitoid intrusions in the Kola region. The Porosozero differentiated intrusion was formed in four phases successively emplaced during approximately 60 Ma. Phase 1 consists of a gabbrodiorite-quartz monzodiorite-granodiorite-granite series. The zircon ages of granodiorite and quartz monzodiorite from the Porosozero are 2733 ± 6 and 2734 ± 4 Ma, respectively. Phase 2 of the intrusion comprises biotite leucogranites and aplite and leucoplagiogranite veins. The zircon age of the leucogranite is 2712 ± 6 Ma. Phase 3 consists of lamprophyre dikes of odinite-spessartite-vogesite composition. The emplacement age of the lamprophyres is constrained by the age of magmatic zircon from an odinite dike: 2680 ± 10 Ma. The age of the metasomatic zircon is 2629 ± 8 Ma. Phase 4 is composed of the youngest pegmatite veins. The Rb-Sr isochron age of the phase-1 rocks is 2724 ± 74 Ma. The zircon age of granitoids from the Kolmozero is 2736 ± 4 Ma. The rocks of the sanukitoid intrusions affiliate with the calc-alkaline series, have Mg# = 0.45−0.60, are enriched in Ba, Sr, K, P, and LREE, and contain elevated concentrations of Cr and Ni. Sm-Nd isotopic data on sanukitoids from both intrusions suggest that they were derived from a mantle source enriched in LILE and LREE and having ɛ_Nd(2740) from +1.02 to +0.36. It was melted approximately 140 Ma after its origin [T(DM) = 2.9−2.8 Ga]. The rocks of the Porosozero and Kolmozero are proved to be similar to magmatic sanukitoid series of Archean and Phanerozoic age whose genesis was controlled by mantle-crustal interaction in suprasubduction environments at active continental margins. Elevated concentrations of Ag and Au in rocks from the Porosozero make it metallogenically promising in terms of precious metals.

In 1990, the Sopcheozero Cr deposit was discovered in the Monchegorsk Paleoproterozoic layered mafic-ultramafic layered intrusion (Monchepluton). This stratiform early-magmatic deposit occurs in the middle part of the Dunite Block, which is a member of the Monchepluton layered series. The Cr₂O₃ average-weighted content in ordinary and rich ores of the deposit is 16.65 and 38.76 wt.%, respectively, at gradually changing concentrations within the rich, ordinary and poor ore types and ore body in general. The ores of the Sopcheozero deposit, having a ratio of Cr₂O₃/FeO_total = 0.9–1.7, can serve as raw materials for the refractory and chemical industries. The ore Cr-spinel (magnochromite and magnoalumochromite) is associated with highly magnesian olivine (96–98 Fo) rich in Ni (0.4–1.1 wt.%). It confirms a low S content in the melt and complies with the low oxygen fugacity. The coexisting Cr-spinel-olivine pairs crystallized at temperatures from 1258 to 1163 °C, with accessory Cr-spinel crystallizing at relatively low, while ore Cr-spinel at higher temperatures. The host rock and ore distinguish with widespread plastic deformations of olivine at the postcrystallization phase under conditions of high temperature (above 400 °C) and pressure (5 kbar). At the post magmatic Svecofennian stage (1.84 Ga), the deposit, jointly with the Monchepluton, was subject to diverse tectonic deformations.

The paper concerns the geochemical analysis of rocks from the ore-bearing layered intrusions that belong to two age groups of the Monchepluton and the Imandra–Umbarechka Complex (2.50 and 2.44 Ga) and the largest gabbro-anorthosite of the Main Ridge Complex (2.51–2.45 Ga). The intrusion of these complexes happened at different depths when the endogenous and geodynamic settings changed at the beginning of the Paleoproterozoic Era. Five megacycles are distinguished in a generalized cross-section of the two-chamber Monchepluton. The megacycles differ in rock composition, rock geochemical features, and mineralization types, i.e., the chromite, sulfide Cu–Ni–PGE and low-sulfide PGE types. The abrupt changes in isotope indicators (εNd, ⁸⁷Sr/⁸⁶Sr) mark their boundaries. At a depth of 2037–2383 m, the M-1 borehole intersects a standalone intrusive body that is essentially a magma feeder channel. The intrusive body’s geochemical characteristics and U–Pb isotope age correlate to the Monchepluton rocks. The gabbro-anorthosite massifs united in the Main Ridge Complex were intruded in the following order: the Monchetundra, Chunatundra, Volchetundra, and Losevo–Medvezhye tundras. The largest Monchetundra massif was formed as a result of multiple intrusions of mafic magmatic melt from the deep reservoirs. The melts intruded in two stages, i.e., 2.51–2.49 Ga and 2.48–2.47 Ga, and their composition changed gradually. The gabbro-pegmatites and coeval harrisite dykes are more recent ones (2.46–2.45 Ga). The summarized results of the U–Pb, Sm–Nd, and Re–Os systems research allowed us to establish genetic relations between the studied geological objects. We proposed a model where there was an uplift of a mantle plume to the lower crust area at the age of 2.5 Ga, the deep mantle reservoirs were formed, and a large-scale interaction happened between the parental magma and granulite–eclogite complex rocks. Local contamination and assimilation processes took place during the uplifting of magmas in areas where the magmatic feeding system contacted the host amphibolite–gneiss Archean complexes.

A comprehensive study of a 340 m thick lenticular-sheet body of ultramafic composition penetrated by structural well M-1 at a depth of about 2.2 km was accomplished. Its main volume is composed of plagioharzburgite; fine-grained rocks of norite and orthopyroxenite chilling zones are preserved on endocontacts. The rocks of the body are similar in composition to the rocks near the underlying ore-bearing layered intrusion – the Monchepluton. The age of intrusion of the ultramafic body is 2510 ± 9 Ma (U-Pb, ID-TIMS, zircon) and, taking into account analytical errors, is comparable with the formation period of the Monchepluton (2507-2498 Ma). According to the study of the Sm-Nd system in rocks and minerals, a positive value of the eNd (+1.1) parameter was established, similar to that in dunites and chromitites of the Monchepluton. Based on these results, the ultramafic body penetrated at depth was assigned to the magma feeding paleochannel through which the ultramafic, weakly contaminated magma entered the overlying magma chamber. This body is a unique example of a magma-feeding system for the ore-bearing layered intrusion of Precambrian age.