T. B. Bayanova

Isotope data on the age and genesis of layered basic-ultrabasic intrusions in the Kola Peninsula and northern Karelia, northeastern Baltic Shield

Abstract New UPb zircon ages and SmNd data are presented for layered basic-ultrabasic intrusions of the Kola Peninsula and northern Karelia in the northeastern part of the Baltic Shield. All ages are in the range between 2.5 and 2.4 Ga, but there appear to exist three phases of intrusion. Thus the Monchegorsk Intrusion and the Fedorovy-Pansky Massif have ages between 2493 and 2470 Ma, the Kivakka and Lukkulaisvaara massifs of the Olanga group of intrusions between 2445 and 2435 Ma, while the Imandra Lopolith is 2396±7 Ma old. A narrow range of negative ϵNd(T) values between −2.3 and −1.2 suggests formation by partial melting of a semi-homogeneous enriched mantle source.

Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield: metallogenic, petrological and geodynamic implications

Abstract There are two 300–500 km long belts of Palaeoproterozoic layered intrusions in the Baltic (Fennoscandian) Shield; the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt. New U–Pb (TIMS) ages and radiogenic isotopic (Nd–Sr–He) data have been determined for mafic-ultramafic Cu–Ni–Ti–Cr and PGE-bearing layered intrusions of the Kola Belt. U–Pb zircon and baddeleyite data from gabbronorite and anorthosite bodies of the Fedorovo-Pansky, Monchepluton, Main Ridge (Monchetundra and Chunatundra) and Mt Generalskaya intrusions, and from gabbronorite intrusions and dykes associated with the Imandra lopolith, yield ages from c. 2.52–2.39 Ga. The age range of 130 Ma recorded in the Kola Belt samples, associated with at least four intrusive phases (three PGE-bearing and one barren), is significantly greater than that for intrusions of the Southern (Fenno-Karelian) Belt which clusters at 2.44 Ga. Nd isotopic values for the Kola Belt range from −1.1 to −2.4 and indicate an enriched mantle ‘EM-1 type’ reservoir for these layered intrusions. Initial Sr isotopic data for the Kola intrusions are radiogenic relative to bulk mantle, with ISr values from 0.703 to 0.704, but geochemical data and 4He/3He isotopic ratios of various minerals record a significant contribution from a mantle source rather than simply crustal melting. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic‐ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

Palaeoproterozoic to Eoarchaean crustal growth in southern Siberia: a Nd-isotope synthesis

Abstract Nd-isotope analyses from 114 rock samples are reported from the southern part of the Siberian craton to establish a first-order crustal formation scheme for the region. The Nd-isotope data show considerable variability within and among different cratonic units. In many cases this variability reflects differing degrees of mixing between juvenile and older (up to Eoarchaean) crustal components. The fragments of Palaeoproterozoic juvenile crust within the studied segment of the Siberian craton margin have Nd-model ages of c. 2.0–2.3 Ga. Voluminous Palaeoproterozoic granites (c. 1.85 Ga) were intruded into cratonic fragments and suture zones. These granites mark the stabilization of the southern Siberian craton. The complexity in the Nd data indicate a long history of crustal development, extending from the Eoarchaean to the Palaeoproterozoic eras, which is interpreted to reflect the amalgamation of distinct Archaean crustal fragments, with differing histories, during Palaeoproterozoic accretion at 1.9–2.0 Ga and subsequent cratonic stabilization at 1.85 Ga. Such a model temporally coincides with important orogenic events on nearly every continent and suggests that the Siberian craton participated in the formation of a Palaeoproterozoic supercontinent at around 1.9 Ga.

Paleoproterozoic Collisional and Intraplate Granitoids of the Southwest Margin of the Siberian Craton: Petrogeochemical Features and U-Pb Geochronological and Sm-Nd Isotopic Data

Accretion of the Archean blocks of the continental crust 1.95-1.90 Ga ago that caused the formation of the Paleoproterozoic supercontinent Pangea I (1) was accompanied by metamorphism and intensive granite formation in collisional orogenes. Paleoproterozoic collisional granitoids are widely represented in the uplifts of the Eurasian platform basement: on the Ukrainian, Anabar, and AldanoStanovoi shields as well as along the southwestern margin of the Siberian craton. They form extended belts consisting of domi� nating posttectonic microcline granites with minor tonalities, granodiorites and quartz syenites predomi� nantly of sodium-potassium calcalkaline magmatic series. Their characteristic feature is enrichment by largeionic and high field strength elements, light lan� thanoids, uranium, and especially thorium (2). Fur� ther sialization of the crust and increasing of the degree of its geochemical differentiation (maturity) were important consequences of this powerful process of granite formation. The extended belt of the Paleoproterozoic grani� toids marks uplifts of the Siberian craton basement along the modern southwestern and southern margins tracing in the structures of the Yenisey ridge, Prisaya� nie, Pribaikalie, and AldanoStanovoi shield. Forma� tion of the belt was accompanied by the first occur� rence of sodiumpotassium granitoid magmatism, which was the most powerful for this segment of the Earth and occurred in the interval 1.90-1.75 Ga ago. Granite formation was caused by accretioncollisional and, as was established recently (3), subsequent intra� plate processes. In the present work by the example of the investiga� tion of the Taraka massif of the south Yenisey Ridge, the multistage character of the rock associations com� posing the massif varying in age as well as in composi� tion was proved. For the first time, the results of the petrogeochemical and isotopegeochronological investigations of the intraplate (anorogenic) granites were presented. These granites were distinguished by the authors within this massif due to their enrichment in thorium and rare earth elements. The variety of geo� dynamic conditions of granitoid formation and pro� longed multistage evolution of the Paleoproterozoic granite formation were noted. Wider development of anorogenic granites within the mentioned belt was shown.

New geochronological data on metamorphic and igneous rocks from the Gridino Village area (Belomorian eclogitic province)

In the Belomorian eclogitic province, eclogites aretraceable in the meridional direction from SalmaStrait (Imandra Lake) via the islands and coast of theKrasnaya Estuary of the White Sea [1] and the GridinoVillage area [2] to the Onezhskaya Estuary coast. Thestudy area is located in the wellexposed coastal zoneand adjacent islands of the White Sea near the villageof Gridino. An eclogitebearing complex approximately 50 km long (from the northwest southeastward) and 10 km wide was described in [2] as anArchean eclogitebearing melange and thoroughlyillustrated in [3] using rocks from Stolbikha Island asan example. The complex includes eclogites andeclogitized mafic dikes [2, 4]. The mafic rocks demonstrate distinct successive stages of metamorphic transformations under eclogite (maximal pressure 15–16 kbar),elevatedpressure granulite (

Age and isotopic geochemical characteristics of Archean carbonatites and alkaline rocks of the Baltic shield

Occurrences of Archean alkaline rocks are insignificant. Only a few complexes in the Canadian Shield (Superior Province), Greenland, Australia (Yilgarn Block), and South Africa are known to date [1]. They are composed of alkali and nepheline syenites, foidolites, carbonatites, peralkaline granites, lamprophyres, and potassic volcanic rocks. The age of the oldest alkaline rocks is estimated at ~2.7 Ga. The Neoproterozoic and Phanerozoic alkaline rocks occur in three main geodynamic settings: (1) continental rifts, (2) oceanic islands, and (3) subduction zones (peralkaline granites in back-arc zones). The Early Precambrian alkaline rocks formed at hotspots of the oceanic crust and are unknown in continental rifts. Therefore, the geodynamic setting of the Archean alkaline rocks is interpreted as a subduction-related environment and the depleted mantle is thought to be their source. The Late Archean subduction-related alkaline complexes presumably formed at the final stages of the evolution of greenstone belts, while the depleted source is accounted for by the absence of metasomatic processes in the Archean mantle [1]. High concentrations of Sr and REE, which are typical of alkaline rocks, make it possible to neglect contamination in the course of later superimposed processes and consider the primary isotope ratios as true characteristics of the mantle. This is very important for understanding the oldest (Archean) processes of lithosphere formation, because the initial geodynamic setting and role of mantle‐crust interaction are often masked by numerous superimposed processes.

The Salma Eclogites of the Belomorian Province, Russia: HP/UHP Metamorphism Through the Subduction of Mesoarchean Oceanic Crust

Publisher Summary Eclogite-facies mafic rocks occur within gray gneisses of TTG affinity in the northeastern part of the Belomorian Province, Kola Peninsula. These are characterized by widespread omphacite-breakdown textures and locally preserved relics of omphacite. Thermobarometry indicates a clockwise PT path. Garnet inclusions suggest a prograde path passing from surface-weathering conditions through the low-grade green schist facies (pumpellyiteactinolite facies) before reaching the eclogite facies. Peak metamorphic conditions are estimated to be about 700‑750°C and > 14‑15 kbar. Needle-shaped inclusions (rods) of quartz in omphacite suggest that the peak P-T conditions of studied eclogites could reach significantly higher pressure than estimated in the present study. The retrograde path passed through granulite facies to upper amphibolite facies by near-isothermal decompression. The results of UPb dating and Hf-isotope analysis of zircons from the eclogites and cross-cutting felsic vein can be used to infer an approximate 2.89 Ga age for the oceanic crust, which was subducted and underwent eclogite-facies metamorphism between 2.87 and 2.82 Ga. The granulite-facies overprint is likely to have occurred at 2.72 Ga. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9‑1.8 Ga) orogeny. These new data imply that plate tectonic processes (“hot subduction”) operated at least locally in the late Mesoarchean. The petrology and geochemistry of the Salma eclogites and related TTG rocks can be best explained by subduction of Archean oceanic crust. The adakitic nature of the felsic vein inside the Salma eclogites suggests that partial melting of hydrated eclogites could produce Archean TTG-type magmas.

U-Pb ID-TIMS age of the Tiksheozero carbonatite: expression of 2.0 Ga alkaline magmatism in Karelia, Russia

The Tiksheozero carbonatite in northern Russian Karelia is a transitional type between alkaline ultramafic — carbonatitic and alkaline gabbroic suites. The complex is dominated by pyroxenite with a variety of subordinate mafic and ultramafic phases and nepheline syenite. Carbonatite occurs in a main central body and in veins. In this study we have obtained a reliable age for the complex by single grain ID-TIMS U-Pb analyses of zircon and baddeleyite. The age of 1999 ± 5 Ma is important because it places the emplacement of the alkaline complexes in the context of craton-wide extension and break-up events which preceded the initiation of a major Paleoproterozoic orogenic cycle. The Paleoproterozoic age also emphasizes the fact that not all members of the Kola alkaline province are of Paleozoic age.

New data on the Early Paleozoic gabbroid and granitoid magmatism age within the Dzhida zone of Caledonides (Southwestern Transbaikalia, North Mongolia)

Dealing with the problem of the formation and evolution of oceanic and continental crust is one of the fundamental issues in the study of folded areas. The Central Asian fold belt (CAFB), which includes the Dzhida zone of Caledonides, refers to an area where according to some researchers opinions [1–3, etc.], crust forming processes occurred from the Late Riph� ean to the beginning of the Mezozoic. Upon that, as

Geochemical and U–Pb zircon age characterization of granites of the Bathani Volcano Sedimentary sequence, Chotanagpur Granite Gneiss Complex, eastern India: vestiges of the Nuna supercontinent in the Central Indian Tectonic Zone

Abstract The Central Indian Tectonic Zone (CITZ) marks the suture zone where the North and South Indian cratonic blocks amalgamated to form the Greater Indian Landmass (GIL). It consists of three broad domains from west to east: the central CITZ occupying the central region of mainland India juxtaposed between two mobile belts, namely the Sausar Mobile Belt (SMB) in the south and the Mahakoshal Mobile Belt (MMB) in the north; the Chotanagpur Granite Gneiss Complex (CGGC) lying east of the main CITZ; and the easternmost Shillong Plateau Gneissic Complex (SPGC). The studied granites are from the Bathani Volcano Sedimentary sequence (BVSs) from the northern margin of the CGGC. These are high-K, calc-alkaline, I-type granites related to arc magmatism and are interpreted to have formed by partial melting of an igneous source at upper-crustal depths. The granitic magma underwent extensive fractional crystallization of plagioclase, biotite, K-feldspar and ilmenite during emplacement. The U–Pb (ID-TIMS) zircon emplacement age is c. 1.7–1.6 Ga for these granites. This episode of magmatism can be correlated to the global event of the Nuna supercontinent assembly also reported from the MMB of the central CITZ. We infer that the BVSs is the eastern continuation of the MMB of the central CITZ.