SM-ND, RB-SR, AND K-AR ISOTOPIC SYSTEMS UNDER REGIONAL METAMORPHISM: EVIDENCE FROM THE BELOMORIAN BELT, KOLA PENINSULA

Abstract

The Sm-Nd, Rb-Sr, and K-Ar isotopic systems were examined in Archean rocks and minerals of the Belomorian Belt, Kola Peninsula, subjected to regional metamorphism of the epidote-amphibolite facies. The Svecofennian metamorphism did not change Archean Sm-Nd characteristics of the biotite plagiogneisses and amphibolites. According to the determined TDM = 2.88-2.94 Ma and εNd(t) = + 2.5, the protolith of the gneisses has an old age of 2.9 Ma and originated from the depleted mantle not affected by any contamination by crustal material. Although the Rb-Sr system of the rocks was partly opened in Svecofennian times, their initial strontium ratio (-0.700) calculated from the zircon age also points to the depleted mantle as the protolith of the gneisses. In terms of the U-Pb zircon method, the Archean metamorphism has a young age of 2.73 Ma. The subsequent Proterozoic metamorphism of the epidote-amphibolite facies was responsible for the discordant ages studied by the Sm-Nd, Rb-Sr, and K-Ar methods in rock-forming and accessory minerals and ranging over the interval from 1.97 to 1.55 Ma. Most of these dates are related to sequential closing of the isotopic systems with cooling of the rocks during the retrograde stage of the Proterozoic metamorphism. When studied by the Sm-Nd or Rb-Sr method, similar minerals from gneisses and schists have discordant age values, which shows that fluid plays an important role in the equilibration of the isotopic systems. According to Sm-Nd and Rb-Sr isotopic data on apatite (a fluid-sensitive mineral), the fluid circulation is dated at 1.77 and 1.67 Ma, respectively. The average cooling rate of Belomorian rocks was about -2°C/Ma, which is calculated from the difference between the Sm-Nd age of minerals in garnet amphibolite and the Rb-Sr age of biotite, and from the temperatures at which these isotopic systems were closed. However, the thermal-temporal pattern of Belomorian rocks shows two stages of the retrograde metamorphism: (1) slow cooling with a rate much lower than 2°C/Ma, which was related to the long existence of deep-seated rocks, and (2) subsequent rapid cooling with a rate over 10°C/Ma, which is explained by the rapid collision-related uplift of the region. This interpretation of isotopic data is consistent with the occurrence of this area within the Belomorian Belt near its junction with the Imandra-Varzuga structure.