A. E. Mel’nik

Paleoproterozoic eclogites in the salma area, Northwestern Belomorian Mobile Belt: Composition and isotopic geochronologic characteristics of minerals and metamorphic age

The paper represents results of a comprehensive geochemical and isotopic-geochemical (SIMS) study of eclogites from the northwestern part of the Belomorian Belt (Salma eclogites). A detailed fieldwork was carried out at the quarry of the Kuru-Vaara deposit of ceramic pegmatite in the northwestern part of the study area, in which tonalite-trondhjemite gneisses include bodies and blocks of eclogite and Grt-Aug eclogite-like clinopyroxenite and are cut across by numerous pegmatite veins. The least altered types of the Grt-Cpx rocks selected for our further research included: (1) widespread massive homogeneous fine-grained Grt-Omp eclogite that replaced gabbro and contained symplectites of Pl + low-Na-Cpx around omphacite and Pl-Hbl kelyphite rims around Grt; and (2) coarse-grained eclogite-like Grt-Hbl-Aug clinopyroxenite beds up to 20 cm thick in the central parts of high-Mg metaultrabasites, which are mostly tremolite-actinolite schists. The REE patterns of garnet, clinopyroxene, and amphibole from the eclogites confirm that they crystallized simultaneously, under a high pressure, and in the absence of plagioclase. Local U-Pb dates of the zircons and their geochemistry are at variance with the earlier hypothesis that the eclogite metamorphism occurred in the Archean. The eclogites and Grt-Hbl-Aug clinopyroxenite were determined to contain zircons of Svecofennian age (approximately 1900 Ma), which show all geochemical characteristics of classic eclogitic zircons and occur either as individual crystals or as rims around Archean magmatic zircons from the primary gabbroids.

First Lu-Hf Garnet Ages of Eclogites from the Belomorian Mobile Belt (Baltic Shield, Russia)

377 Eclogites from the north eastern border of the Bel morian Mobile Belt (BMB) have not only been used for regional geodynamic reconstructions [1], but also as first direct evidence for plate tectonic processes in the Mesoarchean [2]. M.V. Mints and coworkers (2010) dated zircons from eclogites of the Salma area related to the so called Belomorian Meso–Neoar chean eclogite province and interpreted the age of 2.87 Ga obtained for some zircons as the age of meta morphism of the eclogite facies [2]. However, rocks of the Salma area undoubtedly underwent eclogite meta morphism with an age of ~1.9 Ga in the Paleoprotero zoic [2–4]; for this reason, there is a question of whether or not zircon datings of 2.87 Ga correspond to the age of Mesoarchean eclogite metamorphism [1, 2], or whether this is the age of the magmatic protolith (gabbro) of eclogites [3, 4], or if it represents a low P metamorphic event. In this study we applied the Lu– Hf garnet clinopyroxene geochronology, which are rock forming minerals of apo gabbro eclogites of the BMB. Consequently, the Lu–Hf datings of garnet provide unambiguous evidence for the age of meta morphism of the eclogite facies, provided that the blocking temperature was not exceeded (see discus sion below).

New data on the age of eclogites from the Belomorian mobile belt at Gridino settlement area

The repeated isotopic and geochemical study of zircons of the eclogite from Stolbikha Island (Gridino settlement area) allows one to interpret the U-Pb age value of about 2700 Ma by central parts of zircon grains as a magmatic event time, probably rejuvenated to a degree by intense manifestation of the eclogite metamorphism of about 1880 Ma age. The Svecofennian high-pressure metamorphism caused a partial recrystallization of zircons of magmatic origin and the appearance of their rims showing typical geochemical characteristics of eclogite zircons.

New data on the age (U-Pb, Sm-Nd) of garnetites from Salma eclogites of the Belomorian mobile belt

The origin of garnetites, which are quite abundant in highpressure metamorphic complexes, is still debatable. The idea about primary magmatic differen� tiation of basites to Fe-Ti (garnetite protolith) and Mg (protolith of metabasite complimentary to garnetite) parts is the most popular (1 and others). There are assumptions about the formation of garnetite as a result of metamorphic differentiation from active infiltration of fluid (2) and metasomatism with the for� mation of a metasomatic column (3). Extensive garnetization of eclogitic bodies as linear bands up to the appearance of garnetite containing up to 50% garnet and more was registered in Salma eclog� ites within the northwestern part of the Belomorian mobile belt (BMB). The authors studied in detail the body of massive eclogites (Sample 46) with a size of up to 10 m in diameter in the key area of Salma eclogites, in the KuruVaara deposit mine. This body occurs in migmatizes tonolite-trondhjemite gneiss intruded by numerous veins of ceramic pegmatites (4). Eclogites are strongly amphibolized at the contact with host gneiss with the formation of a garnet amphibolite rim (Sample 50) with a thickness of 1-2 m. The garnetite layer with a thickness of up to 60 cm (Sample 48) occurs between the amphibolite rim and the eclogite. Garnetite (Sample 48) contains garnet porphyro� blasts with a size of ~1 mm (up to 50%), intergranular brownishgreen amphibole (20%), andesine (14%), rutile, and ore mineral (5%). In contrast to eclogitic garnet (Sample 46), garnet from garnetite contains numerous poikilitic inclusions of prevailing quartz (10% of the whole rock volume), abundant horn� blende and rutile, and single grains of monoclinic pyroxene and biotite. Garnetite (Sample 48) and eclogite (Sample 46) located within the same body differ significantly in the chemical composition. Garnetite differs from eclogite by the high concentration of FeO* (18.0 and 12.1 wt %, respectively) and TiO2 (1.38 and 0.43 wt %) and the low concentrations of MgO (6.1 and 12.1 wt %) and CaO (11.1 and 13.4 wt %). Garnetite is significantly enriched in V (by a factor of 6) and depleted in Ni, Cr, and Ba by one order of magnitude in comparison with eclogite. The concentrations of Y, Zr, Hf, Th, and REE in garnetite are almost two times higher. A difference in major and minor elements is regu� larly observed in characteristic minerals of garnetite and eclogite as well. Garnet from garnetite differs from eclogitic garnet by the high concentrations of Fe, Ca, HREE, Y, and V and by low contents of Mg and Cr (4); amphibole and monoclinic pyroxene, by the high Fe#, Ti, and V contents; and rutile, by the high con� centrations of V, Zr, and Hf and the low contents of Cr and Nb. The contrasting chemical compositions of garnetite and eclogite do not result in qualitative change of the mineral association upon transforma� tion of eclogite to garnetite, but have an impact on the compositions of rockforming, as well as accessory, minerals.

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

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 (

In Situ LA-ICP-MS Investigation of the Geochemistry and U-Pb Age of Rutile from the Rocks of the Belomorian Mobile Belt

Rutile is a common accessory mineral in variousmetamorphic and igneous rocks. Although its formulais very simple, a number of elements may substitute forTi in rutile, including Al, V, Cr, Fe, Zr, Nb, Sn, Sb, Hf,Ta, W, and U [1]. The chemical characteristics of rutile(primarily, the character of HFSE distribution) arewidely used as geochemical indicators of mineralforming processes [1, 2]; Zr content in rutile was calibratedas a geothermometer [e.g., 3]; and rutile is a suitablemineral for the U–Pb dating of metamorphism andgeothermochronological reconstructions [4]. Theresults of the U–Pb dating of rutile and titanite from therocks of the Baltic shield were used for the reconstruction of the thermal history of the Karelian craton andBelomorian mobile belt (BMB) [5, 6]. The authorsestablished a significant difference between the U–Pbages of rutiles and titanites from the rocks of the Karelian craton (Archean) and BMB (Paleoproterozoic).However, the character of trace element distribution inrutile, which may potentially provide insight into theparameters controlling crystallization processes, wasnot previously explored in the rocks of the Baltic shield.This paper reports the results of an in situ LA–ICP–MS investigation of the geochemistry and U–Pb age ofrutile from metamorphic rocks of two areas within theBMB, the Shueretskoe deposit of garnet in its centralpart and the Salma eclogites from the northwesternBMB. Although the rocks of these complexes wereimprinted penecontemporaneously by the youngestSvecofennian metamorphism, they are significantlydifferent in the mineral and chemical composition ofrutilebearing rocks and the geologic history of theirtransformation.In the Shueretskoe deposit, rutile inclusions wereseparated from a garnet megacryst (sample 6). The ageof the metasomatic crystallization of this garnet wasdetermined as 1837 ± 14 Ma by the in situ SHRIMP IIdating of coexisting zircon and monazite inclusions atthe Center of Isotopic Investigations, Karpinskii AllRussia Geological Institute [7]. The deposit is locatedin the central part of the BMB, at the mouth of theShuya River and hosted by the metamorphic rocks ofthe Belomorian complex. Economically important aremetasomatic garnet gedritite and glimmerite in garnetbearing gneisses and amphibolites. Garnet occurs inthe metasomatic rocks as irregular nodules or, morerarely, faced crystals up to 25 cm across. The metasomatism that resulted in the formation of the garnetdeposit occurred under peak metamorphic conditions at a temperature of 650–680°C and a pressureof 7.8–8.5 kbar [7].The Salma eclogites were investigated in the pit ofthe KuruVaara deposit, where tonalite–trondhjemitegneisses with bodies and blocks of eclogites and eclogitelike

Multistage Svecofennian metamorphism: Evidence from the composition and U-Pb age of titanite from eclogites of the Belomorian Mobile Belt

The first geochemical study of titanite from eclogites and associated rocks of the Belomorian Mobile Belt (BMB) by secondary ion mass spectrometry made it possible to establish the compositional features of this mineral in the garnet-bearing and garnet-free assemblages. Titanite from garnet-bearing assemblages is characterized by upward convex REE pattern and lowered HREE content relative to LREE, as well as the average GdN/YbN ratio around 16.5. Titanite from metaultrabasic rock inherits the specific features of the host rock, which should be taken into account when comparing with titanite from metagabbro. Results of U-Pb (TIMS) dating of titanite confirms the identification of the early and late stages of the Svecofennian metamorphism in the studied areas of BMB: early metamorphism with the peak eclogite facies conditions at around 1900 Ma, retrograde amphibolite facies metamorphism at 1870–1880 Ma, and late allochemical metamorphism accompanied by the pegmatite formation with an age of 1840 Ma.

Comments on the paper “The nature of “Svecofennian” zircon in the Belomorian Mobile Belt and some geological implications” by A.A. Shchipanskii and A.I. Slabunov

Eclogites were relatively recently found in the Belomorian Mobile Belt (BMB) (Volodichev et al., 2004; Shchipanskii et al., 2005; Konilov et al., 2004). The very first isotopic dates (Volodichev et al., 2004; Mints et al., 2010) were obtained for these rocks in the northwestern (in the Salma and Kuru-Vaara areas) and central (Gridino area) portions of BMB and corresponded to the Archean: approximately 2.72–2.87 Ga. Because no crustal eclogites older that 2.0 Ga (Möller et al., 1995) had been known before these dates were obtained, these eclogites were regarded as unique. It is commonly believed that no crustal eclogites could be formed in the Archean because the crust was then relatively thin (Kröner, 2010), and hence, the find of crustal eclogites of Archean age in BMB called for a fundamental revision of geodynamic reconstructions of the crustal evolution and was one of the main arguments invoked to support the hypothesis that currently operating geodynamic mechanisms of plate tectonic can be extrapolated to the Early Precambrian (Rozen et al., 2008). However, these finds were practically immediately followed by serious doubts that the primary estimates of the timing of the eclogite metamorphism in the Belomorian Belt may be incorrect (Mitrofanov et al., 2009; and others).

Reply to a comment in G. Meinhold “Geochemical discrimination of rutile from the Belomorian Mobil Belt”

335 1 The comment of G. Meinhold is concerned with the precise position of the dividing line between the eclogite and metapelite fields on the Cr–Nb discrimi nation diagram for rutile. To date it is a common knowl edge that rutiles from metabasic rocks show higher Cr and lower Nb concentrations compared with rutiles from metapelitic rocks. It is hardly possible to deter mine the absolutely precise position of the dividing line between the eclogite and metapelite fields on the dia gram for rutiles. The position is to be refined with the extension of the empirical database on rutile geochem istry. In fact the dividing line on the diagram for rutile in [1] is more consistent with the diagram from Mein hold’s article [2]. However, the use of the diagram from the later publication of Zack et al. [3] does not change the position of points in the respective fields. Therefore, a more correct variant of the capture of Fig. 1 in [3] is the following. Fig. 1. Covariations of (a) Nb and Cr and (b) Nb and Ta in rutile. (a) The fields of rutile compositions from eclogites and metapelites are after Meinhold [2]; (b) the lines of constant Nb/Ta ratios are shown, and analysis 200 of sample 6 is omitted.