Alexander V. Samsonov

Relatively large crystals of baddeleyite (up to 100 μm) and zircon (up to 400 μm) were found in Cenozoic subalkaline andesites in the northern Sikhote-Alin. The reasons for such a rare association of Zr minerals in volcanics are discussed based on the petrological characteristics of the andesites, their U−Pb isotope dating, and contents of trace elements in the baddeleyite and zircon. The andesites contain rare clinopyroxene phenocrysts, which crystallized in the temperature range of 1090–1150°C. The fine-grained matrix is composed of pyroxene, plagioclase, hornblende, ilmenite, and felsic domains, which consist of K−Na feldspar, quartz and silicic glass and were formed at 926°C. The accessory baddeleyite and zircon are confined to the felsic domains. The obtained U−Pb age of the baddeleyite 52.7 ± 1.1 Ma (MSWD = 2.6) can be used as an estimation of the crystallization age of the andesite melts. The zircon ²⁰⁶Pb/²³⁸U age broadly varies within the range of 46 to 56 Ma. The youngest values (46–49 Ma) probably reflect the partial disturbance of the isotope system caused by the high contents of U (up to 1.3 wt %) and Th (up to 3.8 wt %). The baddeleyite shows a simultaneous decrease in concentrations of Hf (from 7742 to 2869 ppm), Y, and heavy REE, which may be explained by its competitive crystallization with amphibole. Well-pronounced negative Eu anomalies in the baddeleyite and zircon suggest their growth simultaneously with feldspars. High concentrations of HREE, U, and Th in the zircon indicate its crystallization from enriched residual melts. Zircon crystallization temperatures estimated using the Ti-in-zircon geothermometer (from 800 to 990°C) are comparable with temperature estimations for the felsic domains. The baddeleyite and zircon compositions imply that the minerals crystallized during the late stages of the melt evolution, perhaps in an intermediate magma chamber. In the magmatic history of the Sikhote-Alin, the Early Eocene andesites formed between the older Paleocene−Early Eocene A-type rhyolites (61–53 Ma) and the younger Eocene–Miocene basalts (40–20 Ma). This time span is thought to have been associated with lithospheric extension due to the break-up of the downgoing oceanic slab and the opening of a mantle window, with the Early Eocene andesites likely marking this tectonic event.

Petrological and geochronological (zircon, U-Th-Pb (LA-ICP-MS)) studies of rocks from the Poriya Guba tectonic mélange exposed on the Ozerchanka and Palenyi islands were carried out to decipher the composition and tectonic history of the Paleoproterozoic Lapland-Kola orogen (LKO). Tonalite–trondhjemite–granodiorite (TTG) (Grt)–Cpx–Opx gneisses dominate on Ozerchanka Island. They contain numerous bodies of mafic granulites and are intruded by syn- and post-tectonic granitoids. The TTG gneisses are Archean in age (>2.6 Ga, Т_Nd(DM) = 2.9–3.0 Ga). The HREE depletion in the gneisses indicates that the parental melts of their igneous protoliths were formed in equilibrium with a garnet-bearing residue. Mafic granulite bodies vary widely in geochemistry and likely represent fragments of several Paleoproterozoic mafic intrusions and dikes. Mineral assemblages in the gneisses recorded the early granulite-facies (Т = 780–820°С and Р = 8.6–9.4 kbar) and later amphibolite-facies (Т = 640–650°С and Р = 6.7–7.3 kbar) metamorphic events that occurred at 1.9 Ga according to zircon ages. On Palenyi Island, the banded Grt–Cpx–Opx gneisses are predominant and vary in composition from basaltic andesites to rhyolites. The volcanic protoliths of these rocks have island-arc geochemical signatures, a Paleoproterozoic age of 1958 ± 6 Ma, and juvenile origin (ε_Nd(1960) = +1.7 ÷ +3.1; Т_Nd(DM) = 2.2–2.3 Ga). These rocks were metamorphosed under the granulite-facies conditions at about 1.9 Ga. Two models can explain the presence of the Archean block in the Poriya Guba tectonic mélange, which is composed of the Paleoproterozoic juvenile island-arc complexes in the core of the LKO. First, this Archean block could represent a single fragment of Archean lithosphere that was separated during Paleoproterozoic continental rifting and opening of the Lapland–Kola ocean and then was tectonically juxtaposed with Paleoproterozoic subduction complexes during the Lapland-Kola collisional orogeny. Second, the Archean block may represent the margin of an adjacent Archean continent exposed in an erosional window within the Paleoproterozoic Poriya Guba tectonic nappe.

The study of the volcanogenic-sedimentary sequence in the lower part of the Zaonega Formation in the Paleoproterozoic Onega structure (Karelian craton, Fennoscandian Shield) has shown that tuffs and high-silica rocks predominate in its composition. High-silica rocks (SiO2 up to 94 wt. %) are depleted of all elements and probably representing chemogenic siliceous silt. Tuff rocks are close to N-MORB basalts in terms of major element content and rare element distribution character. This association is common to the early stages of continental rifting in the Phanerozoic and may indicate the formation of volcanogenic-sedimentary complexes of the Zaonega Formation in the environment of continental rifting. The mafic rocks in the lower part of the Zaonega Formation are geochemically identical to dolerite dikes and N-MORB-type basalts of 2.10–2.14 Ga age. Their formation was probably related to the same episode of large-scale stretching and thinning of the continental lithosphere of the Karelian craton in the mid–Paleoproterozoic. In this case, the age limit of the Zaonega and underlying Tulomozero Formations should be somewhat older than the 2.06–2.10 Ga interval accepted in modern regional stratigraphic schemes of the Paleoproterozoic of Fennoscandian shield.

The metamorphosed dacitic porphyry dykes were established for the first time in the western part of the Vorontsovska terrane, which is located in the Paleoproterozoic Volga-Don orogen at the margin of Archean Sarmatia and Volga-Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. They are ferrous, metaluminous rocks of calc-alkali series and belong to I-type granitoids. Sodium specialization, low concentrations of Mg, Cr, Ni, incoherent elements with sufficient REE fractioning and absence of Eu*-anomalies, high Sr/Y ratio and especially (Gd/Yb)n values (10), and also radiogenic Nd isotopic composition suppose the juvenile mafic source for the dacitic melts. According to petrogenetic estimations, such conditions could be caused by partial melting of depleted N-MORB type basites at the equilibrium with the eclogitic restite. Supposed mechanism for the dacitic magmas forming is the partial melting of the basites from the lower horizons of crust with highly enlarged thicknes (60 km) due to preceding collision processes.

In the western part of the Aldan terrane, in the middle reaches of the Tokko river, dolerite dikes have been studied. These dolerite dikes form a swarm of submeridional trend about 1 km wide. In the thickest dike, dolerites have well-preserved primary textural and structural features and mineral composition: plagioclase + pigeonite + augite + titanomagnetite. Dolerites from the chilled margins and inner parts of the dike are homogeneous in composition, correspond to low-Mg tholeiites, have low contents of Ti and other HFSE, with weak enrichments in light REE spectra and small negative Nb anomalies. Sm-Nd isotopic studies of magmatic dolerite minerals from the central part of the dike in isochron coordinates yielded a good linear correlation corresponding to an age of 2510 ± 64 Ma, which probably records the time of crystallization of the basaltic melt. The metadolerites in the shallow dike retain plagioclase-porphyritic structures, but the pyroxenes in them are completely replaced by amphibole and chlorite. Metadolerites are contrasted by low contents of MgO, Cr and Ni and higher contents of TiO2, Fe2O3, P2O5, Nb and all REEs. The differences in the composition of the dikes may be related to the long-term (about 65%) crystallization differentiation of the initial melt and the flow of residual melt from the shallow intermediate magmatic chamber along the opening cracks. Such conditions probably existed in tectonically stable intraplate settings. The age of the studied dolerites of the dike swarm is comparable to that of the anorogenic granites of the Nelyuki Complex (~2.4–2.5 Ga), which are widespread in the western part of Aldan granulite-gneiss Terrane. The data obtained complement the characterization of the intraplate anorogenic magmatism that occurred in the western part of the Aldan Shield in the Late Archean and marked the final consolidation of a large block of Archean crust in the Chara-Olekma granite-greenstone area.

The study of the volcano–sedimentary sequence in the lower part of the Zaonega formation in the Paleoproterozoic Onega structure (Karelian craton, Fennoscandian Shield) has shown that tuffaceous and high-silica rocks predominate in its composition. High-silica rocks (SiO₂ up to 94 wt %) are depleted in all elements and probably represent chemogenic siliceous silts. Tuffaceous rocks vary broadly in composition and were formed during mixing of tuffogenous basalt material and high-silica chemogenic sediments. In terms of levels of contents and the distribution character of trace elements, tuffogenous rocks are close to N-MORB volcanic rocks. This rock association is typical of the early stages of continental rifting in the Phanerozoic and may indicate the formation of volcano–sedimentary complexes of the Zaonega formation in the environment of continental rifting. The tuffaceous rocks in the lower part of the Zaonega formation are geochemically identical to dolerite dikes and MORB-type volcanic rocks 2.10–2.14 Ga in age. Their formation was probably related to this episode of large-scale extension and thinning of the continental lithosphere of the Karelian craton in the Middle Paleoproterozoic. In this case, the age limit of the Zaonega and underlying Tulomozero formations should be somewhat older than the 2.06–2.10 Ga interval accepted in the modern regional stratigraphic schemes of the Paleoproterozoic Fennoscandian shield.

Гигантские дайки основных пород маркируют ключевые этапы эволюции Земли в докембрии и являются предметом всесторонних исследований. В статье представлены результаты палеомагнитных и петромагнитных исследований и определения палеонапряженности B_др, полученные по коллекции образцов Великой дайки Кольского полуострова возрастом 2.68 млрд лет. Вычисленные по 5 сайтам (41 образец) среднее палеомагнитное направление характеристической компоненты намагниченности и палеомагнитный полюс Мурманского кратона составляют D = 117.6°, I = 77.1°, K = 40.9, α₉₅ = 12.1°, slat = 69.265°, slong = 34.35447°, plat = 51.5°, plong = 70.7°, dp/dm = 21.1°/22.6°, paleolat = 65°. Подробно изучены термомагнитные свойства пород и показано, что носителями остаточной намагниченности являются однодоменные и малые псевдооднодоменные (группа А) или многодоменные (группа Б) частицы магнетита. Оценка палеонапряженности по 12 образцам из группы А методом Телье-Коэ составляет B_др = (6.16 ± 0.92) мкТл, а соответствующее среднее значение виртуального дипольного момента VDM_2.68Ga = (0.85 ± 0.13) × 10²² Ам². Новые данные согласуются с результатами исследований других объектов архейского и протерозойского возраста и указывают на крайне слабое по величине магнитное поле Земли в позднем архее.

Mafic intraplate magmatism is the main source of information about the geodynamics of processes that lead to the breakup of continental blocks. The article discusses geodynamics of the breakup of the Archean supercraton Superia in the Middle Paleoproterozoic. The discussion is based on data on 2.1 Ga magmatism in the Karelian Craton, where mafic igneous rocks of this age are represented by tholeiites of two geochemical types: depleted and enriched. Geochemically close to N-MORB, depleted tholeiites were studied in the Northern Ladoga Region where they form dike swarms at ca. 2111 ± 6 Ma (U-Pb, SIMS, zircon) in the Hatunoiya locality, and pillow lavas and sills in the Lake Maloe Jänisjärvi locality. Enriched tholeiites were studied in the Lake Tulos locality where they form a large swarm of doleritic dikes of age 2118 ± 5 Ma (U-Pb, ID-TIMS, baddeleyite). The results of these studies provide deeper insight into 2.1 Ga mafic magmatism. Depleted tholeiites with N-MORB geochemistry have a wide spatial distribution in the Karelian Craton and could be formed via decompression melting of a depleted asthenospheric mantle, raising melts along the extension zones, and minimal contamination by the Archean crust. According to modelling results, enriched tholeiitic melts probably occurred due to differentiation and crustal contamination of rising depleted tholeiitic melts through more rigid Archean crustal blocks. Data on ca. 2.1 Ga mafic magmatism in the Karelian craton are difficult to explain within the mantle plume rise model, but are consistent with the model of lithosphere extension due to a retreat of a subduction zone in the northeastern margin of the craton, in the Lapland-Kola Ocean at 2.0–2.2 Ga. The intensive thinning and rupture of the Archean continental lithosphere and opening of an oceanic basin at the western margin of the Karelian craton were probably controlled by the suture zone of the junction of Neoarchean and Paleoarchean crustal blocks, traced in the western part of the Karelian craton. An additional factor that led to the ca. 2.1 Ga lithospheric breakup could be a rise of a deep-seated mantle plume in the Hearne craton, neighboring to the Karelian craton in the Archean Superia supercraton.

Mafic within-plate magmatism is the main source of information about the geodynamics of processes that lead to the breakup of continental blocks. The article discusses the problem of geodynamics of the split of the Archean supercraton Superia in the Middle Paleoproterozoic. The discussion is based on data on 2.1 Ga magmatism in the Karelian Craton, where basites of this age are represented by tholeiites of two geochemical types: depleted and enriched. Geochemically close to N-MORB, depleted tholeiites were studied in Northern Priladozhye where they form methadoleritic dike swarms at c. 2111 ± 6 Ma (U-Pb, SIMS, zircon) in the Khatunoiya locality, and pillow lavas and sills near Lake Maloye Janisjarvi. Enriched tholeiites were studied in the Lake Tulos locality where they form a large swarm of doleritic dikes at 2118 ± 5 Ma (U-Pb, ID-TIMS, baddeleyite). The results of these studies provide deeper insight into 2.1 Ga mafic magmatism. Depleted tholeiites with N-MORB geochemistry have a spatial distribution in the Karelian Craton and could be formed as a result of decompression melting of a depleted asthenospheric mantle, raising melts along the extension zones, and minimal contamination by the Archean crust. The simultaneous formation of enriched tholeiitic melts probably occurred at differentiation and crustal contamination of depleted tholeiites during melt migration through more rigid Archean crustal blocks. Data on basic magmatism with an age of 2.1 Ga in the Karelian craton, which are difficult to explain within the framework of the mantle plume rise model, are consistent with the model of lithosphere extension due to the retreat of the subduction zone in the northeastern framing of the craton, in the Lapland-Kola Ocean in the interval of 2.0–2.2 Ga years. The maximum thinning, discontinuity of the Archean continental lithosphere, and the opening of an oceanic basin at the western edge of the Karelian craton were probably controlled by the suture zone of the junction of the Neoarchean crust with the Paleoarchean blocks, a chain of which was traced in the west of the Karelian craton. An additional factor that led to breakup of the lithosphere 2.1 Ga ago could be the rise of a deep mantle plume in the Khern craton, which occupied a spatial position close to the Karelian craton in the Archean supercraton Superia.