Conditions of Formation of Layered Intrusions of the Monchegorsk Magmatic Cluster

Abstract

The paper presents geochronological data and results of petrological modeling of the Monchegorsk and Monchetundra layered massifs. The U-Pb ID-TIMS zircon dating of norite from the marginal zone of Mt. Nittis yielded 2506 ± 7 Ma, which coincides with previous data on similar rocks from other areas. Based on the comparison of sections of the Monchegorsk and Kivakka massifs, the pre-erosion thickness of the Monchegorsk massif is estimated as 3700 m. Calculations using rocks of the marginal zone of the Monchegorsk intrusion at a pressure of 6.5–5.5 kbar showed that the melt in equilibrium with Ol contained SiO₂ ≤ 55 wt %, TiO₂ ≤ 0.50 wt %, and MgO ≤14 wt %. The temperature of the primary magma could be equal to 1390°C. At a lower pressure (below 6.5 kbar), the obtained melt becomes more siliceous. In the Monchegorsk intrusion, the composition of intratelluric olivine according to this model could vary within 88–92 mol % Fo. The content of intratelluric olivine in the melt, depending on pressure, could vary from 11 to 24 vol %. Magmas that formed the Monchegorsk intrusion and the layered series of the Monchetundra intrusion (hole 742) were derived from different sources.