Yu. B. Marin

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 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 (

Wolframoixiolite and niobian ferberite from zinnwaldite granitic rocks of the Chukchi Peninsula

The finding of wolframoixiolite with inclusions of niobian ferberite is described from zinnwaldite granite and ongonite of the Severny pluton of the Chukchi Peninsula. The optical, morphological, and chemical properties of minerals are characterized and compared with their analogues from other regions. The petrologic and mineragenic index implications of the minerals are discussed with allowance for our contemporary mineralogical knowledge on W-bearing Ta-Nb minerals.

The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

Zircons with an anomalously high content of Y and REEs in rims were first found in rocks of the Baltic shield (in a pegmatite vein in the Gridino eclogite-bearing melange and in quartz syenites of the North-Karelian greenstone belt); the total content reaching 17 wt %. The presence of such an amount of nonformula elements is caused by the isomorphic occurrence of them in zircon and is not associated with microinclusions of accessory minerals. The age of formation of these rims was determined as the Svecofennian, although the direct dating yields underestimated values of the age, which are void of geological sense.

Hadean-archean detrital zircons from Jatulian quartzites and conglomerates of the Karelian craton

Using local procedures, the new results on the isotope ages and composition of mineral inclusions were obtained for detrital zircons from Paleoproterozoic Jatulian terrigenous quartzites and polymictic conglomerates in Central and Western Karelia. For Eastern Laurasia, signs of the existence of Hadean and ancient Eoarchean matter were found for the first time (zircon grains of 3871 ± 38.6 and 3837 ± 42.1 Ma concordant ages). The multimodal distribution of ages within 3.45−2.61 Ga was revealed. The discovery of the oldest zircon grains provides the conditions for valid global correlations of geological events that determined the deposition and formation of the continental crust of the North Atlantic supercraton.

Accessory mineralization of rocks from Late Cretaceous intrusive series with Li–F granites in the Far East

Accessory mineralization of the Late Cretaceous intrusive series in the Far East was investigated on the basis of published data and the author’s original evidence. The composition of accessory minerals from leucogranite, monzonitoid rocks, and Li–F granites has been established. The trend in the evolution of Late Cretaceous granitoids is characterized by an increase in the mineral-forming role of iron and rare elements. Diverse accessory minerals and their typomorphic assemblages have been identified for Li–F granites and ongonites. The regional specificity of accessory mineralization in rare-metal granites consists in the leading role of the minerals W, Ta, Nb, Bi, Y, REE, and As. The uniformity of mineral species and mineral assemblages and the typomorphism and evolution of accessory minerals are inherent to the Far East belt of Li–F granites.

Volkhovite: A new type of tektite-like glass

Volkhovites are tektite-like glasses of mafic and ultramafic composition discovered in the fluvioglacial drift of the Valdai glaciation (10–60 ka ago) along the right bank of the Volkhov River (59°27′ N, 32°01′ E). Volkhovite particles are small in size (0.1–3.0 mm) and irregular in shape, with various microtektite aerodynamics (globular, droplike, and dumbbell-like forms). They are perfectly preserved, and, thus, postglacial in age. Sporadic volkhovite grains (up to 20 items per 1 kg of loam) were detected over an area 1.0 × 1.5 km2. Some samples from local spots 20 × 30 m2 in size are anomalously enriched, up to 1700 volkhovite grains per 1 kg of loam. Tiny Ti-Fe, Fe, and Cu-Au spherules, particles of quenched glass and cinder, and fragments of the Precambrian and Paleozoic rocks are spatially associated with volkhovites. The suggested cryptomagmatic model assumes that the glass and cinder are pre-Holocene in age, whereas volkhovites were formed in the post-glacial time as a result of the outburst of the slag-stone-melt-mud-gas mixture that ascended from the asthenosphere to the surface. Drops of the melt solidified in the air instantaneously to form tectite-like glasses, and the tuffisite-like agglomerate mixed with the fluvioglacial drift.