New occurrence of eclogite in the Belomorian mobile belt: Geology, metamorphic conditions, and isotope age

Abstract

Finds of eclogitelike associations within the Belomorian mobile belt (BMB) in the areas of Gridino [1],Shirokaya and Uzkaya Salma bays, KuruVaara mine[2], and Krasnaya Guba [3] have different geologicalinterpretations in relation to their age and geologicalsetting, as well as geological models of the formation.The eclogitic mineral associations in basic and ultrabasic rocks of the Belomorian area have been knownfor more than 70 years. In the first part of the 20th century, they were registered in the BMB by the authors of[4–6]. Eclogites were repeatedly mentioned byK.A. Shurkin and colleagues (Institute of Precambrian Geology and Geochronology) in the 1950s. Wediscovered a number of new eclogite bodies on theislands of the Keretskii archipelago in the central partof the BMB; previously described [5–7] analogousobjects on Sidorov and Ileika islands have been studiedby the authors in detail as well.Two nappes with different rock compositions maybe distinguished in the geological structure of SidorovIsland. The upper nappe represented by strongly granitized biotite (rarely epidote) gneiss practically doesnot contain basic bodies. The lower nappe with athickness of <30 m is represented by gray granite–gneiss with numerous boudined bodies of basic rocks.Eclogitized bodies of basic rocks were registered in themost outcropped northern and southern parts of theisland. All of them have the character of boudins (up to30–40 m in diameter) surrounded by a granite–gneissmatrix (Fig. 1). Eclogitization is reflected in the formation of linear zones and veins composed of garnet,monoclinic pyroxene with a high Na content, andamphibole (Sample 202) in metabasites. Basic bodiesare usually altered with the formation of rims of intenseamphibolization along the perimeter (Sample 216) witha thickness up to 0.5 m and higher, and intersected bylate pegmatoid and carbonate–quartz veins(Sample 205). Sample 223 was investigated from thethin linear zone of eclogitization of the basic bodyfrom SW Ileika Island, which differs from metabasitesof Sidorov Island in the composition and form ofeclogitization. Metabasites of Sidorov Island correspond to the complex of gabbroanorthosites, whereasrocks of Ileika Island correspond to the complex ofmetaporphyrites–garnet gabbro [7].Eclogites have porphyroblastic and granoblastic,sometimes symplectitic texture. Garnet porphyroblasts are distributed in rock matrix represented bymonoclinic pyroxene with a dependent portion ofamphibole and plagioclase quite regularly. In additionto these minerals, biotite, quartz, magnetite, ilmenite,titanite, rutile, apatite, and pyrite (a total of <2–5% ofrock volume) were registered in eclogites. Garnet ischaracterized by poor zoning reflected in a decrease inthe grossular and pyrope contents from the center tothe margin of porphyroblast and an increase of almandine and spessartine contents. Inclusions of quartz,monoclinic pyroxene, rutile, amphibole, and chloriteare irregularly distributed in garnet. Monoclinicpyroxene of the matrix is represented by prismaticgrains and rarely symplectitic aggregates. According tothe composition, it corresponds to sodic (