V. A. Glebovitskii

Isochron Re-Os age of gold from mayskoe gold-quartz vein deposit (Northern Karelia, Baltic Shield)

54 In contrast to most of the isotope systems of litho phile elements widely abundant in geochronological investigations (Rb, Sr, K, Ca, Sm, Nd, Hf, Lu, U, and Pb), the behavior of the rhenium–osmium system is mainly controlled by the chalcophile and siderophile properties of these elements [1]. This makes it possible to apply the Re–Os system for direct dating of ore mineralization of various genetic types and origins [2].

Thermodynamic conditions of metasomatism in high-temperature and high-pressure shear zones (Kandalaksha-Umba Zone, Kola Peninsula)

874 The Kandalaksha–Umba zone on the Kola Penin sula is the southeastern fragment of the Lapland gran ulite belt (Fig. 1), which is known for high pressure and high grade metamorphism. It was the study of the Lapland belt that allowed P. Eskola to distinguish the granulite facies in the general system of the mineral facies [1]. P. Eskola was the first to discover orthopy roxene–sillimanite rocks in Lapland granulites, which provide the main evidence for high pressure. M.D. Kry lova and L.A. Priyatkina [2] originally described such rocks in the Por’ya Guba area, and O.A. Belyaev [3] and S.A. Bushmin et al. [4] identified them as the products of acid leaching and related iron–magne sium metasomatism. N.E. Kozlova et al. [5] per formed microstructural investigations of the associa tions of orthopyroxene–sillimanite rocks and demon strated the paragenetic character and age links to the stage of shear deformations. Investigations of the con ditions of granulite metamorphism [6–9] demon strated wide variations of pressure from 7 to 12 kbar in the zone considered.

New data on the U-Pb (SHRIMP II) age of zircons from aluminous gneisses of the Archean Kola Group of the Baltic shield and the problem of their interpretation

New data on the U-Pb (SHRIMP II) age are obtained for aluminous gneisses of the Kola Group of the Baltic shield. Gneisses are typically ascribed to metasedimentary rocks with detrital zircons. Our work interprets the isotopic data based on the magmatic (tonalite) origin of gneisses and zircon that was established from study of the morphology and geochemistry of zircons. The age of crystallization of the protolith is 2.9 Ga. The existence of two stages of Archean granulite metamorphism is confirmed: an early stage (2.9 Ga) and later granulite metamorphism (2707–2656 Ma).

Age (U-Pb, SHRIMP-II), Geochemistry of zircon, and conditions of the formation of sapphirine-bearing rocks of the Central Kola granulite-gneiss area

Sapphirinebearing rocks are observed in the cen� tral part of the Central Kola granulite-gneiss area (CKGGA), ~10 km to the west of the Kitsa railway station (1), where two types of rocks are abundant, namely orthopyroxene-biotite orthogneiss separated by aluminous garnet-biotite and garnet-cordierite- sillimanite-biotite plagiogneisses related to the Kola Series. The contacts between the two rock types are tectonic. Several outcrops of sapphirinebearing rocks occur in the shear zone of the contact between the Vezhetundra orthogneiss body and aluminous gneiss. Sapphirinebearing rocks often compose elongated veinlike bodies with a length of >100 m and a thickness of < n m. In each area sapphirinebearing rocks are quite variable by the structure and mineral composi� tion: from acid rocks enriched in quartz to quartzfree melanocratic rocks with a remarkable content of sap� phirine, spinel, and corundum. A typical feature of sapphirinebearing rocks is their zoning, which may be described by infiltration metasomatic columns with a decrease in the number of phases towards their axial zones, with various reaction relationships between minerals; some of them provide evidence for the allochemical character of alterations.

The Oldest Granites of Russia: Paleoarchean (3343 Ma) Subalkali Granites of the Okhotsk Massif

The Paleoarchean age (3.34 Ga) of subalkali granite magmatism first established for the Kukhtui uplift of the Okhotsk Massif suggests a formation time of the mature continental K-rich crust in this region as early as the Paleoarchean. According to the geological structural, mineralogical–geochemical, geochronological, and isotopic–geochemical data, the Kukhtui uplift can be considered as the most ancient Paleoarchean province in Russia: the ancient consolidation core of the sialic protocrust of the Okhotsk–Omolon Craton.

Caledonian formation of gold-bearing sulfide depositions in Early Proterozoic gabbroids in the northern Ladoga region

We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ∼450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = 238U/204Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores.

Isotopic timing of the magmatic and metamorphic events at the turn of the Archean and Proterozoic within the Belomorian belt, Fenno-Scandinavian shield

It is proved that dating high-grade metamorphism events through dating of migmatites is quite efficient. Our investigation has made it possible to identify two events of 2500 and 2700 Ma and to estimate the age of an igneous protolith for both tonalite gneiss, the most ancient in the Belomorian belt, and related metagabbroid. Based on the upper crossing of the concordia and the discordia, the zircon core age is estimated at 2796 ± 63 Ma; this age is slightly different from that of a growth rim of rhythmically zoned prismatic zircon (2816 ± 110 Ma). A linear approximation of all measured points yields an upper crossing of 2803 ± 55 Ma. The error of these estimates is high for quite understandable reasons, and yet it should be taken into account when analyzing the geodynamic development regimes of Neo-Archaean endogenic processes.

From migmatites to crustal granites: Pb–Pb isotopic system of feldspars of partial melting products, Northern Ladoga region, Fennoscandinavian Shield

The isotopic–geochemical features of late and postorogenic granites of the S type and ambient migmatites are studied within the Russian part of the Svecofennian orogen of the Fennoscandinavian Shield. The spatial association of leucosomes of migmatites and granites of the S type and their similar petro- and geochemistry and distribution of Pb isotopes are evidence of the genetic similarity of their parental melts. The Borodinskoe pluton has a more primitive 206Pb/207Pb ratio, which indicates the presence of upper and U-poor lower crustal material in the source of granitic magmas. This conclusion is supported by the ɛNt(t) lower value of granites of this pluton relative to those of other plutons of the region.

Paleoproterozoic gabbro–diorite–granite magmatism of the Batomga Rise (NE Aldan Shield): Sm–Nd isotope geochemical evidence

The geochemical similarity and almost simultaneous (2055–2060 Ma) formation of Utakachan gabbro-amphibolite, Jagdakin granodiorite-diorite, Khoyunda granitoid, and Tygymyt leucogranite complexes, which inruded metamorphic formations of the Batomga Group are evidence of their formaton from unified magmatic source. All this makes it possibble to combine aforementioned complexes into the unified Early Proterozoic diferentiated gabbro-diorite-granite complex.

Geochemistry of biotite from charnockitoids in the granite-forming series (Nimnyr Block of the Aldan Shield)

The geochemical evolution of biotite (Bt) from garnet-bearing associations in the granite-forming series of the Nimnyr Block (Aldan Shield) is considered. The series includes the primary rock (gneiss or basic rock), autochthonous enderbite- or charnockite-gneiss (Lc1), migmatitic leucosome (Lc2, Lc4), and paraautochthonous charnockite (Lc3-ch1) and (Lc5-ch2 → grn). Bt in each series is characterized by an individual, significant difference in the concentrations of Ti, V, Cr; rarely Al, Fe, Mg, and Rb; and very rarely La, Ce, Nd, and Gd. The uniform trend of evolution of Bt compositions in the series was established for Al, KAl, (K + Na)/Al, rarely for Mg, Ti, and К. In contrast to the series with ch2, Bt becomes enriched in LREEs from autochthonous Lc1 to allochthonous Lc2 and Lc3. The different compositional variations of Bt in relation some minor elements is explained by the different composition of the material, which underwent melting, temperature variations (from 820 to 750°C), and changes in pH of the mineral-forming medium.