A. V. Berezin

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.

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8–2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ∼1.9 Ga) episode of eclogite metamorphism of Archean rocks.

New U-Pb and Sm-Nd ages and P-T estimates for eclogitization in the Fe-rich gabbro dyke in Gridino area (Belomorian Mobile Belt)

New data from isotope geochronology (U-Pb, Sm-Nd) petrological study provide evidence of the Svecofennian age (∼1.9 Ga) for eclogitization in the Fe-gabbro dyke inductile shear zones of Gridinrea. P-T estimates of eclogitization were computed using the THERIAK/DOMINO software. A close timing relationship between dyke magmatism and eclogitization is inferred.

Complex Isotopic-Geochemical (Sm-Nd, U-Pb) Study of Salma Eclogites

Salma eclogites distinguished in the northwestern part of the Belomorian mobile belt (BMB) is one of the key objects supporting the application of the plate tectonics mechanism to the Early Precambrian [1]. In this connection, age dating of eclogite metamorphism acquires principal importance and requires a complex approach. As a result of local U–Pb dating and study of the geochemistry of zircons from Salma eclogites of two types (after basic and ultrabasic rocks), grains and rims of zircon crystals of Svekofennian age (~1.9 Ga) with a complete set of geochemical characteristics of standard eclogitic zircons were found [2].

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 (

Eclogite-like apogabbro rocks in Sidorov and Bolshaya Ileika islands, Keret Archipelago, White Sea: Compositional characteristics, metamorphic age and conditions

The paper reports the first data obtained by state-of-the-art analytical techniques on the composition of minerals and the metamorphic age and metamorphic parameters of eclogite-like rocks from Sidorov and Ileiki islands, Keret Archipelago, White Sea. The U-Pb zircon magmatic age of the metabasites lies within the range of 2400–2480 Ma. The rocks were subjected to eclogite metamorphism at 1870–1890 Ma, with this age estimate consistent with analogous estimates for eclogite metamorphism elsewhere within BMB. Simultaneously garnetite zones were produced. The metabasites were eclogitized at 12 kbar and 700°C, i.e., near the boundary between the eclogite and amphibolite facies of relatively high pressure, because of a local pressure increase during rock cooling in the presence of fluid. The retrograde metamorphic episode proceeded under pressures from 12 to 6.5 kbar at temperatures from 700 to 600°C. The contact amphibolization of the metabasites at a temperature close to 620°C and pressures of 2–2.5 kbar (low-pressure amphibolite facies) occurred at 1870 Ma and is pronounced in the form of an amphibolite rim around a boudin of eclogitized basite and in significant changes in the trace-element and REE composition of the eclogite zircon.

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.