Heterogeneous Metamorphism in Metabasites of the Kem-Ludy Islands, Belomorian Mobile Belt

Abstract

The Kem-Ludy Islands is an understudied fragment of the Early Precambrian Belomorian eclogite province. This study presents detailed petrological analyses on four mafic rocks in a sheared gneiss matrix: eclogite, quartz–clinopyroxene–plagioclase–amphibole–garnet granofels, garnet amphibolite, and quartz-bearing gabbronorite. These rocks exhibit varying degrees of metamorphic transformations, mineral assemblages, and mineral chemistry. Epidote inclusions corresponding to an early metamorphic episode were identified in the garnet cores from the eclogite, granofels, and garnet amphibolite. Peak eclogite facies conditions reconstructed using mineral geothermobarometry yielded T ~ 670°C and P_min ~ 1.3 GPa. The granofels retains indirect evidence of the eclogite stage, while the garnet amphibolite and quartz-bearing gabbronorite completely lack such signatures. Amphibolite-facies metamorphism significantly varies among the rock, primarily due to differential fluid influence. Reconstruction of P−T conditions for the amphibolite stage, done for each rock using mineral geothermobarometry, phase equilibrium modeling (PerpleX), and multiequilibrium thermobarometry (TWQ), yields values of T = 610–730°C and P = 0.4–0.8 GPa. For the eclogite, a P–T path of near-isothermal (~670°C) decompression from ~1.3 to 0.6 GPa was established from two metamorphic stages. This P−T path is similar to those previously determined for some eclogites and metaultramafic rocks from the tectonic mélange in the Gridino area differs from the evolution for eclogites from the Salma and Kuru-Vaara localities.