Isotope Geochemical Characteristics Research Articles

Isotopic-Geochemical Features of Zircon and Its Significance for Reconstructing the Geological History of Paleoarchean Granulites in the Ukrainian Shield

This paper presents the results of a complex study (morphology of grains, internal texture in cathodoluminescence and backscattered electrons, microprobe analysis, Lu–Hf data) of five groups (generations) of zircon crystals differing in age and separated from the same granulite sample pertaining to the Bug River Complex of the Ukrainian Shield. The data show that the oldest zircon crystals of the first group (3.74 Ga in age) are xenogenic and initially crystallized from a granitic melt; zircon of the second group (3.66 Ga) formed from a mafic melt contaminated by felsic country rocks. The third group (3.59 Ga) is represented by zircons that formed about 100 Ma later than the second group under conditions of granulite-facies metamorphism and with the participation of fluid-saturated anatectic melt. Two Paleoproterozoic zircon groups (~2.5 and 2.1 Ga) also formed under granulite-facies conditions; to a certain extent, their structure and composition were controlled by fluid. The geochemistry of all zircon generations provides evidence for their crystallization in the continental crust, but from the sources differing in the contribution of mantle-derived material and in oxygen fugacity.

Pb–Pb isotopic systematics of orogenic gold deposits of the Baikal–Patom fold belt (Northern Transbaikalia, Russia) and estimation of the role of neoproterozoic crust in their formation

The paper reports the results of Pb isotope study of several gold deposits of the Russia’s largest metallogenic province of Northern Transbaikalia. Potential sources of the ore material are considered by the example of new and previously published Pb–Pb data on nine deposits and occurrences of different scales. The comparison of Pb–Pb isotope-geochemical characteristics of ores, Paleozoic granitoids, as well as metamorphic pyrite from barren metasedimentary sequences shows that the Neoproterozoic terrigenous–carbonate rocks of the Baikal–Patom fold belt (BPB) served as the main source of lead and other components in the mineral-forming systems of the deposits. Significant variations of Pb isotope ratios typical in general of the considered deposits of the BPB reflect the initial isotopic heterogeneity of Pb source. This heterogeneity is caused by mixing of two geochemical types of continental crust during sedimentation: old (Early Precambrian) crust of the Siberian craton with long-term geochemical evolution and newly formed Late Precambrian crust. Pb–Pb data serve in support of the hydrothermal–metamorphogenic hypothesis of the formation of gold deposits of the BPB.

Geochemistry of Precambrian volcanosedimentary rocks of the Karsakpai Group in southern Ulutau (Central Kazakhstan)

We have analyzed the isotope-geochemical features of volcanosedimentary rocks of the Karsakpai Group in southern Ulutau (Central Kazakhstan): mafic volcanics, siliceous and siliceous-ferruginous sediments, and quartz-sericite-chlorite schists. The close association of ferruginous quartzites with intraplate volcanics indicates that they formed in a tectonically active basin. The Nd isotope composition of ferruginous quartzites was governed by synchronous underwater volcanism, whereas the 143Nd/144Nd value of schists was additionally controlled by the Nd isotope composition of older sources. The Mesoproterozoic Nd model ages and positive εNd(t) values of the metaterrigenous rocks of the Karsakpai Group indicate the presence of Mesoproterozoic juvenile material in the provenance. The minimum Nd model ages suggest the lower boundary of sedimentation of 1.3 Ga.

The Adamawa-Yadé domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Bafia area, Cameroon)

New field observations, ages and geochemical data are presented for the Adamawa-Yadé domain, forming part of Central African Orogenic Belt in Cameroon. This belt delineates the northern margin of the Congo craton and is part of the Braziliano/Pan-African orogenic system, which was formed during amalgamation of the West Gondwana continent at the end of the Neoproterozoic.U–Pb dating indicates that the protoliths of some orthogneisses from the Adamawa-Yadé domain were emplaced during three distinct periods. Tonalite-Trondhjemite-Granodiorite (TTG) suites in the Makénéné area intruded at ca. 3.0–2.5Ga, and were affected by partial melting at ca. 2.08–2.07Ga, during the Eburnean orogeny, and by a magmatic event with a metamorphic overprint at 0.64–0.61Ga, during the Pan-African orogeny. A dioritic orthogneiss from the Maham area, north of Makénéné, was emplaced at 2.07Ga and also reworked at 0.62–0.61Ga. The youngest granitic orthogneisses, sampled in the vicinity of Bafia, Bep and Ngaa-Bapé, yield intrusion ages of 0.64–0.63Ga. Some of these Neoproterozoic plutonic rocks show shoshonitic affinity and reveal εNd(630Ma) values ranging from −1.2 to −6.9 (1.7>TDM>1.1Ga), and 87Sr/86Sr initial ratios of 0.706 and 0.704, providing evidence for crustal reworking during the Pan-African collision. The isotopic geochemical characteristics provide evidence that potassic magmas result from the interaction between the Archaean TTG continental crust and magmas derived from a mantle enriched in silica and incompatible elements during subduction underneath a continental margin. The Bafia group appears as a complex geological entity comprised of an Archaean/Palaeoproterozoic basement and its Neoproterozoic sedimentary cover, both deformed and recrystallized at ca. 0.61Ga.The crustal evolution of the Adamawa-Yadé domain is very similar to that inferred for the northern margin of the Congo craton (the Ntem complex and Nyong series). This and the sedimentary record suggest that the Adamawa-Yadé domain represents an Archaean/Palaeoproterozoic microcontinent, which was detached from the northern margin of the Congo craton in the early Neoproterozoic, but became re-accreted together with the Mayo Kebbi (magmatic) arc during the Pan-African orogeny.

Age and sources of matter for the Kedrovskoe gold deposit, Northern Transbaikal tegion, Republic of Buryatia: Geochronological and isotopic geochemical constraints

The paper presents new geochronological and isotopic geochemical data on gold mineralization of the Kedrovskoe deposit. The deposit is located in the northeastern part of the Transbaikal metallogenic province, Russia’s largest. The Early Permian age (273 ± 4 Ma) of mineralization based on the results of Rb–Sr study of metasomatic rocks is correlated with the age of the final phases of Hercynian magmatism in the Baikal–Muya Foldbelt. The Sr, Nd, and Pb isotopic geochemical characteristics of mineralization show that the host rocks are involved in the formation of the latter. It has been established that ore lead was supplied to the hydrothermal system of the deposit mainly from a geochemical reservoir represented by the Neoproterozoic juvenile continental crust of the Baikal–Muya Foldbelt.

Results of U–Pb dating of zircons from wehrlite of the platinum-bearing Feklistov massif (Shantar Archipelago, Russia)

The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.

Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey

Chromite, ultramafic and mafic rocks from Eldivan, Yaprakli, Ayli Dag, Kure, Elekdag and Kizilirmak in northern Turkey have been studied to determine their mineral and whole-rock geochemical, and Re-Os isotope geochemical characteristics. Most of the studied peridotites display depleted but commonly V-shaped chondrite-normalized rare-earth element (REE) patterns while some peridotites as well as pyroxenites from all areas exhibit light REE depleted patterns. Olivine (forsterite 82 to 92 mol%) and spinel (chromium number 13 to 63) in the studied peridotites exhibit a wide range of compositions. Compositions of spinels suggest that peridotites from Eldivan, Ayli Dag and Kure experienced relatively large degrees of partial melting (~ 15 and 19 wt%), whereas those of the Kizilirmak area most likely reflect lower melting degrees (~ 4–6 wt%). Whole-rock and mineral chemical data indicate that the ultramafic rocks are similar to abyssal and supra-subduction zone peridotites. The ultramafic rocks of the investigated areas exhibit a wide range of 187Re/188Os (0.12 to 6.6) and measured 187Os/188Os (0.122–1.14), while the basaltic rocks from Kure, Eldivan and Kizilirmak areas have high 187Re/188Os (128–562) and measured 187Os/188Os (0.724–1.943). On the other hand, chromite from Eldivan, Elekdag and Kizilirmak show high Os contents (21.81–44.04 ppb) and low 187Re/188Os (0.015–0.818) and 187Os/188Os (0.122–0.133). Re-Os model ages (TChur) for all analyzed samples yielded scattered ages ranging from Jurassic to Proterozoic. Overall, geochemical data are interpreted to reflect different degrees of partial melting, melt – rock interactions and metasomatic effects that produced a heterogeneous mantle in a supra-subduction setting.

Results of chemical dating of monazite from the Talnakh economic intrusion (Russia)

Economic copper, nickel, and platinum-group element (PGE) deposits are associated with the Talnakh ultramafic–mafic intrusion of the Noril’sk Province in the northwestern part of the Siberian Platform. The results of the chemical Th–U–Pb dating of monazite from the ore-bearing melanotroctolites of the Talnakh intrusion are presented. The isotope-geochemical features of monazite are compared with zircon, sulfides, and baddeleyite.

Discovery of rare hydrothermal alterations of oligocene Dolomite reservoirs in the Yingxi area, Qaidam, West China

Comprehensive research into core and micro-structure characteristics, petrology, trace elements, and carbon and oxygen isotope geochemical characteristics led to the initial discovery of a rare dolomite reservoirs reformed by hydrothermal fluid in the Yingxi area of Qaidam Basin; however, the degree of hydrothermal alteration is weak. The evidence mainly includes the following aspects: (1) the main rock reservoirs are dense, mainly developed ankerite and calcite with a micritic structure. Albite, pyrite, barite, celestite, and other hydrothermal minerals are occasionally found in fractures. (2) Trace amounts of FeO and MnO enriched with carbonate minerals with an average amount of 4.112 and 0.186%, respectively, are present. They were significantly higher than penecontemporary metasomatism dolomites. The amount of Ca2+ and Mg2+ also saw increases, indicating the hydrothermal fluid is an alkaline brine rich in Ca2+, Fe2+, Sr2+, Mn2+, and some Mg2+. (3) REE has a large range of changes and some of them have higher ΣREE values, which suffered from hydrothermal alteration. Clear Eu negative anomalies appeared on REE distribution model curves. It reflected the REE composition characteristics of the lake environment’s specific hydrothermal fluid. It formed in a deep lake with an alkaline brine environment. (4)The change to the range of δ18OPDB values is −6.8 to −3.6‰, with an average of −5.2‰. The values are between hydrothermal dolomite and penecontemporaneous metasomatism dolomite. This shows that the different types of dolomite have characteristics consistent with a penecontemporaneous metasomatic origin before hydrothermal fluid in a medium–low temperature condition reformed them. The degree of hydrothermal alteration is dependent upon the degree fractures developed as well as the distance between the sample and the hydrothermal channel. In short, the initial discovery of dolomite reservoirs reformed by hydrothermal fluid opens up new horizons for deep tight oil and gas exploration in Qaidam Basin’s Yingxi area in China. These results are substantial in the search for quality “desert” reservoirs present in deep stratum in the Yingxi area.

Characteristics, origin, and role of salt minerals in the process of hydrocarbon accumulation in the saline lacustrine basin of the Yingxi Area, Qaidam, China

In recent years, an important discovery in exploration has been made in $$ E_{3}^{2} $$ of Oligocene in the Yingxi Area of the Western Qaidam Basin. Based on core observation, systemic microscopic characteristics analysis, mineralogy analysis and geochemical analysis, the mineral characteristics, origin, and role in the process of hydrocarbon accumulation in Oligocene in the saline lacustrine basin are systemically investigated in the Yingxi Area of the Qaidam Basin: (1) In this area, there are four important salt minerals: halite, gypsum, glauberite and celestite, which form saline layers with a certain thickness or develop in matrix carbonate rock in the form of mineral particles. In the upper reservoir group in the middle of saline layers, the saline layers are thicker, mainly rock salt, where thin oil layers are developed. In lower reservoir groups, the saline layers are thinner, mainly gypsum. (2) Salt minerals possess geochemical characteristics of “low content of Mn”. The matrix carbonate rock associated with salt minerals has “low temperature” carbon and oxygen isotope geochemical characteristics, and petrological characteristics without the hydrothermal minerals and distribution characteristics with a very wide range. All the above characteristics demonstrate that the salt minerals in this area are a typical deposition type in the saline lacustrine basin, and not a hydrothermal brine precipitation type. (3) “Salt” plays a critical role in the process of hydrocarbon accumulation. Salt minerals play the roles of crack-filling and sealing to prevent dissipation of oil and gas. In addition, salt layers play the role of sealing and covering due to high capillary breakthrough pressure and plastic characteristics. Salt minerals promote secondary pore development in dolomite, and exhibit the negative effect of occupying certain reservoir space. The results of this research can provide important guidance for oil and gas exploration in subsalt layers of saline lacustrine carbonate rocks.

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling, China: insights from LA-ICP-MS zircon U–Pb ages, Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

ABSTRACTWest Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO2 (63.59–76.22%) and low P2O5 (0–0.2%) concentrations, a high K2O/Na2O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous εHf(t) (−4.425 to 1.067 for Xujiaba and −4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.

Geochemical and Lu/Hf isotopic (LA–ICP–MS) systematics of detrital zircons from the Upper Ordovician sandstones of the Bashkir Uplift (Southern Urals)

Analyses of trace elements and Lu/Hf isotopes have been carried out in already U–Pb dated detrital zircons from the Upper Ordovician sandstones of the southern part of the Bashkir Uplift. The concentrations of trace elements in the zircons suggest that they were derived from rocks of intermediate (62%), basic (24%), and felsic (9%) compositions as well as alkaline rocks (2%). The Lu/Hf systematics of the zircons demonstrated a wide variation of both eHf (from +9.5 to–8.7) and model ages of the parental rocks (1.60 Ga < T DM C < 3.28 Ga). Comparison of the isotopic–geochemical characteristics of the detrital zircons from different levels of the Riphean–Paleozoic sequence of the Bashkir Uplift against those from the Early Cambrian Brusov Formation of the Mezen Basin and Early Neoproterozoic Dzhezhim sandstones of Timan Ridge suggests that at the end of the Late Precambrian near the Uralian margin of Baltica a large block of Late Mesoproterozoic–Early Neoproterozoic crust existed, comprising a significant proportion of melanocratic rocks.

Polymetamorphism of the Chupa Sequence of the Belomorian mobile belt (Fennoscandia): Evidence from the isotope-geochemical (U-Pb, REE, O) study of zircon

U-Pb age and isotope-geochemical features were determined for zircon from kyanite gneisses and amphibolites of the Chupa Sequence of the Belomorian mobile belt (BMB) of the Fennoscandian shield. The cores of the zircon from the gneisses marks the Neoarchean events within 2700–2800 Ma known in the BMB, while those of the amphibolites correspond to the age of magmatic crystallization (2775 ± 12 Ma). The inner rims of zircon from the amphibolites and gneisses likely record two different Neoarchean metamorphic events (2650 ± 8 and 2599 ± 10 Ma, respectively). The outer rims record Paleoproterozoic metamorphism with an age of 1890 Ma, which formed the modern appearance and mineral assemblages of the rock association. The value of δ18O in the zircon from the gneiss is 8.6‰ in cores, slightly decreases to 8.0‰ in inner rims, and sharply decreases to 3.9‰ in outer rims. The value of δ18O in the zircon from the amphibolite is around 6.2‰ in cores, increases up to 8.6 in inner rims, and decreases to 5.2‰ in outer rims. A significant decrease of δ18O is likely related to the anomalous composition of Svecofennian metamorphic fluid restricted to local shear zones. The geochemical features of the zircons in combination with their morphology and anatomy make it possible to distinguish zircon generations of different age and change in metamorphic environments.

Paleozoic tholeiitic magmatism of the Kola province: Spatial distribution, age, and relation to alkaline magmatism

This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380–390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume–lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.

Hydrogeological, Hydrogeochemical and Isotope Geochemical Features of Geothermal Waters in Simav and Environs, Western Anatolia, Turkey

Simav geothermal field is located in Simav graben within the Menderes Massif in Western Anatolia, Turkey. The geothermal waters in marbles, limestones and basalts can be classified as Na-HCO3-SO4 type exchange waters with the dominant cations in increasing order of Na+K>Ca>Mg while the anions are HCO3>SO4>Cl. The Cl-SO4-HCO3 ternary diagram shows that the waters are peripheral/immature and more likely related to groundwaters heated by steam from deeper reservoirs. Quartz, aragonite, calcite and chalcedony are oversaturated at discharge temperatures while at a recomputed temperature according to the reservoir composition, only quartz, dolomite and chalcedony are oversaturated. The silica thermometer shows reservoir temperatures between 83 and 1820C. The Na-K-Ca-Mg correction cation thermometers give reservoir temperatures close to the measured reservoir temperature on site in Eynal which is between 148 and 163 0C. The waters plot along the meteoric water line suggesting the source from local meteoric water and are rich in δ18O. This situation points to the existence of water-rock interaction in the system and/or boiling due to the high temperatures in the reservoir.

Hydrogeological, Hydrogeochemical and Isotope Geochemical Features of Geothermal Waters in Tekkehamam and Environs, Western Anatolia, Turkey

From Early to Middle Miocene, the continental rift zones of the Büyük Menderes, the Küçük Menderes and the Gediz were formed by extensional tectonic features, which generally strike E-W and are represented by a great number of geothermal waters, epithermal Hg, Sb and Au mineralizations, and volcanic rocks of Middle Miocene to recent age. The geothermal waters and epithermal mineralizations are related to faults, which strike preferentially NW-SE and NE-SW and are located transversely to the general strike of the rift zones. These faults are probably generated by compressional tectonic stress, which leads to the deformation of uplift between two extensional rift zones. One of these continental rift zones is the rıft zone of the Büyük Menderes which is ascribed to a great number of geothermal waters such as those issuing in very important locations of Kızıldere, Tekkehamam, Salavatlı, Germencik and others with a geothermal capacity of 860 MWe in the next future. The geothermal waters of Tekkehamam and surroundings are identified to belong to the Na+K>Ca>Na and HCO3>SO4>CI facies. According to the Cl-SO4-HCO3 diagram the geothermal waters might be heated by a magmatic source due to the high content of sulfate and boron in geothermal waters. Geochemical thermometers were applied to the collected samples in the region. According to the Na-K-Mg diagram (1), part of the geothermal waters can be considered as equilibrated geothermal waters. According to the results of geochemical thermometers, the reservoir temperatures of geothermal waters range from 160 to 250°C. The δ2H values of geothermal waters are between -61.9 to -51.8, while δ18O values range from -9.23 to -5.84. The tritium contents of geothermal waters are between 0.7 to 3.3 TU. These results show that there is no mixing with cold groundwaters.

Hydrogeological, Hydrogeochemical and Isotope Geochemical Features of the Geothermal Waters in Kurşunlu, Western Anatolia, Turkey

The geothermal waters in Kurşunlu and environs within the continental rift zone of the Gediz in the Menderes Massif can be considered as Na-HCO3 type waters. In these waters, we describe Na+K>Ca>Mg as dominant cations and HCO3>Cl>SO4 as dominant cations. The results of geochemical thermometers (quartz, Na-K and Na-K-Ca) show reservoir temperatures 120°C in Çamurlu and 148-205°C which correspond with calculations based on SiO2 versus enthalpy mixing model and enthalpy versus Cl mixing model of the geothermal waters in Kurşunlu and Çamurlu. The mixing ratio of geothermal waters with ground waters can be calculated due to indicators such as Cl-, B, Li, and temperatures; there is a mixing ratio of 42,5 percent geothermal waters and 57,5 percent groundwaters in Çamurlu. In Kurşunlu, we have calculated a mixing ratio of 83 percent geothermal waters and 17 percent groundwaters.

Age and sources of the Paleoproterozoic premetamorphic granitoids of the Goloustnaya block of the Siberian craton: Geodynamic applications

This paper reports the results of geological, geochronological, and isotope geochemical investigations of two premetamorphic granite massifs of the Goloustnaya block of the Baikal salient of the basement of the Siberian craton and granite gneisses from the migmatite–gneiss sequence of this block. The U–Pb zircon age of the granites of the Khomut massif is 2153 ± 11 Ma. The age of the Elovka massif was previously determined by us as 2018 ± 28 Ma. The Khomut and Elovka granites underwent structural and metamorphic transformations accompanied by migmatization. An age of 1.98–1.97 Ga was obtained for the structural and metamorphic processes in the Goloustnaya block from the analysis of margins of zircon grains from the Khomut granites and zircon from the granite gneisses. The biotite granites of the Khomut massif show transitional I–S-type geochemical characteristics, which allowed us to suggest that they were derived by melting of a crustal source of intermediate–acid composition. The Khomut granites show positive eNd(T) values from +2.0 to +2.2 and a Nd model age of 2.4 Ga, which may indicate their formation owing to the reworking of the Paleoproterozoic juvenile continental crust. The combined isotope geochemical data are consistent with collision of island arcs as a possible environment for the formation of the Khomut granites. The formation of these granites was not related to the development of the structure of the Siberian craton, similar to a few other anorogenic magmatic complexes of the margin of the Chara–Olekma terrane of the Aldan shield with ages of ~2.2–2.1 Ga, including the granites of the Katugin complex. The biotite–amphibole granites of the Elovka massif with an age of ~2.02 Ga are geochemically similar to I-type granites. The geochemical characteristics of these granites, including elevated Sr and Ba and low Nb and Ta contents, were inherited from a subduction-related source. Negative eNd(T) values from–0.9 to–1.8 and rather high contents of K2O and Th allow us to suppose a metamagmatic crustal source for the granites of the Elovka massif. The combined isotope geochemical characteristics of the Elovka granites suggest that a mature island arc or an active continental margin is the most probable environment of their formation. The estimates of the age of structural and metamorphic processes affecting the Goloustnaya block (1.98–1.97 Ga) coinciding with the time of similar transformations in the central part of the Aldan shield and eastern Anabar shield (1.99–1.96 Ga) indicate wide occurrence of collisional events of similar age in the Siberian craton and allow us to consider this age interval as an early large-scale stage of the formaiton of the structure of the Siberian craton.

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.