Paleoproterozoic Layered Intrusions of the Monchegorsk Ore District: Geochemistry and U–Pb, Sm–Nd, Re–Os Isotope Analysis

Abstract

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.

Layered Intrusions of Paleoproterozoic Age in the Kola and Karelian Regions

“…These characteristics are due to the succession of their major mineral phases with a decline in their crystallization temperature: Ol-(Ol + Opx)-Opx-(Opx + Pl)-(Pl + Opx + Cpx) and the enrichment of rocks in feldspar components (Al2O3 and CaO, as well as Na2O and K2O). A sharp bend in the differentiation trend on an MgO -SiO2 curve, indicating mono-mineral rock (orthopyroxenite) crystallization, is also typical of completely differentiated Monchepluton [1]. The Marginal Series clearly shows an inverse differentiation pattern for Al2O3 and CaO, as well as Fe2O3t, and TiO2 accumulation, as indicated by Unlike the Kivakka intrusion, the Marginal Series displays a less distinct inverse differentiation trend.…”

“…The layered intrusions of the Monchegorsk Ore Dictrict, such as Monchepluton, the Imandra-Umbarechka Complex, and the gabbro-anorthosite massifs of the Main Ridge Complex, were previously analyzed [1]. In this paper, the results of whole-rock analysis of layered intrusions, including "Drusite Complex" intrusions, are reported.…”

“…Volcanic rock samples were taken from two rock sequences near the Polisarka River mouth and from the upper reaches of the Pana River. Analyses of volcanogenic rocks were published in [1].…”

“…Some time ago, we analyzed variations in the primary ε Nd (T) ratio of layered intrusion rocks (including those described above) in the massifs of gabbro-anorthosite complexes (The Main Ridge, Pyrshin), in the massifs of the "Drusite Complex", western White Sea region, and in Vetreny Belt gabbronorite dykes and komatiitic basalts [1]. The bulk of analytical data, including those on volcanics, range from −1 to −3 ε Nd (T).…”

“…2.50-2.45 Ga, the uplift of a superplume, the intrusion of abundant mantle magma into the Earth's crust of Baltica Paleocontinent, and the formation of abundant large-scale intrusions of mafic-ultramafic composition with a rhythmically layered internal structure took place [1][2][3][4][5]. The intrusion of abundant high-Mg magma into the lower portion of the earth crust has heated it, resulting in arch formation.…”