Petrogeochemical Characteristics Research Articles

Ospe–Luvtuaivench massif of metabasic rocks, Kola Peninsula, Russia: geologic structure and petrogeochemical and isotope geochemical evidence for its relation to the Imandra complex of layered intrusions

This paper reports the results of a combined study of the geology, petrochemistry, geochemistry, and isotope geochronology of the Ospe–Luvtuaivench massif of metabasic rocks in the Kola region consisting of three blocks (from northwest to southeast): Luvtuaivench, Ospe, and Ekostrov. The massif is approximately 17 km long and 6 km wide and is dominated by meso-and melanocratic orthoamphibolites (metagab-broids) with 5–15-m-thick layers of leucocratic epidote orthoamphibolite (metagabbro-anorthosite), which are changed upsection by metaferrogabbro with a horizon of ilmenite–magnetite ore at least 1 km in lateral extent and approximately 50 m thick. The petrogeochemical characteristics of metabasic rocks from the three blocks of the massif are identical and similar to those of the rocks of the Imandra complex of layered intrusions. In addition, metaultrabasic bodies (chlorite–actinolite, actinolite–chlorite–serpentine, and actinolite–serpentine rocks) of varying thickness were found in the Luvtuaivench and Ekostrov blocks. Based on their structural features differing from those of similar rocks of Imandra layered intrusions and petrogeochemical characteristics, these rocks were interpreted as xenolithic bodies. New U–Pb zircon ages were obtained for the metagabbroids of all blocks of the Ospe–Luvtuaivench massif: 2445 ± 5 Ma for the Luv-tuaivench block, 2455.8 ± 3.5 Ma for the Ospe block, and 2447 ± 4 Ma for the Ekostrov block; these estimates characterize the time of massif crystallization and are similar to the age of formation of typical intrusions of the Imandra layered complex. A U–Pb zircon age of 2444 ± 3 Ma was obtained for the crystallization of the Mount Yagel’naya gabbronorites of the Imandra complex. The following eNd(2450) values were obtained for the rocks of the massif: +0.61 for the Luvtuaivench block,–1.58 for the Ospe block, and –1.27 for the Eko-strov block; the gabbronorites of the Mount Yagel’naya massif yielded eNd(2450) =–0.99. These values are similar to those of the Paleoproterozoic layered intrusions. The similarity of the Ospe–Luvtuaivench massif to typical massifs of the Imandra complex in terms of internal structure, petrochemical and isotope geochemical characteristics, and age of rock formation provides compelling evidence for the interpretation of the massif as part of the Imandra complex.

The Klyuchevskoe gold ore deposit (Eastern Transbaikalia): Formation conditions and petrogeochemical features of rocks and ores

It was found that the magma chambers in the Amudzhikan complex (J3) were characterized by close degrees of their differentiation and occurred at depths corresponding to the lower continental crust. The formation of explosive breccias proceeded during each period of the ore-forming process. The magma chambers of early breccias occurred at great depths. The late breccias contain carbonate cement and are characterized by an increased REE content.

Geochemistry of the eclogites of the Maksyutov Complex, South Urals, and genetic nature of their protoliths

Detailed mineralogical-petrographic and petrogeochemical characteristics are reported for eclogites of the Maksyutov Metamorphic Complex (MMC). It was established that protoliths of different varieties of eclogites (high-Ti, moderate- and low-Ti eclogites, graphite eclogites, and eclogites of the layered body) were mafic magmatic rocks of different affinity. The petrogeochemical comparison of MMC eclogites with the Early Paleozoic lithotectonic complexes of the South Urals shows that the eclogites are close to basalts formed in different geodynamic settings that existed in the region in the Cambrian (?)-Ordovician-Silurian(?) time. Subduction of oceanic crust and subsequent rise to the surface resulted in the spatial juxtaposition of bodies, the protoliths of which were formed at the initial and island arc stages of the evolution of the Paleo-Ural Ocean. Thermodynamic calculations of mineral assemblages of eclogites showed that low-Ti eclogites (680–700°C, 24 kbar), graphite eclogites (660–710°C, 17–18.8 kbar), and eclogites of the layered body (610–730°C, 16–18 kbar; 410–430°C, 12.5–13 kbar) were formed at similar temperatures but at a large scatter in pressure. It was concluded that the pressure variations were caused by the tectonic juxtaposition of bodies during exhumation of eclogites formed at different depths of subducted plate.

Miocene magmatism and tectonics within the Peri-Alboran orogen (western Mediterranean)

The aim of this paper concerns Miocene igneous activity in the Alboran Sea and Peri-Alboran area (northern Morocco, western Algeria and Betic Cordilleras in Spain), considering its age and its location with regard to major tectonics structures.We have compiled previous K-Ar isotopic ages of lavas and plutonic boulders and intrusives with an error of ±1σ and completed this set by a new K-Ar isotopic age for andesitic tuffites from Alboran Island. Geochemistry of most of these samples has been considered after previous analyses completed with new data for Spain magmatism. These two sets of data allow us to place the magmatic activity within the regional stratigraphy and tectonics and their chronological framework of the three major tectonic phases of the Maghrebian orogen, at 17Ma (Burdigalian), 15Ma (Langhian) and 9Ma (Tortonian). Petro-geochemical characteristics are compared through time and geographical locations. A major goal of this coupled approach is to help the elaboration of possible geodynamical processes.As an application, we present the case study of the Dellys, Djinet and Thenia region (east of Algiers) where the successive magmatic events between 19.4±1 and 11.6±0.5Ma are closely related to the local tectonics and sedimentation.The Peri-Alboran igneous activity is placed in a multidisciplinary framework. Timing of activity is defined according to the ages of the neighbouring sedimentary units and the K-Ar ages of igneous rocks.In Spain, the Cabo de Gata-Carboneras magmatic province displays late Oligocene and early Miocene leucogranitic dikes, dated from 24.8±1.3 to 18.1±1.2Ma; three following andesitic to rhyolitic events took place around 15.1±0.8 to 14.0±0.7Ma, 11.8±0.6 to 9.4±0.4Ma, 8.8±0.4 to 7.9±0.4Ma; this last event displays also granitic rocks. Lamproitic magmas dated between 8.4±0.4 and 6.76±0.04Ma were emplaced after the Tortonian phase.In Morocco, after the complex building of the Ras Tarf volcanic edifice, major calc-alkaline to shoshonitic volcanoes were built between 9.0±0.5 and 4.8±0.5Ma, in particular the large Gourougou volcanic complex. Near Oujda, volcanic activity of alkaline affinity leads to multiple emissions of basalts throughout Pliocene times until the beginning of Pleistocene, between 6.2±0.3 and 1.5±0.1Ma.In the Alboran domain, an age of 19.7±0.8Ma is reported (this study) for the andesitic tuffites that form the emergent part of the Alboran Island. This age is comparable to that of the Algerian tuffites and cherts “silexites” and the Malaga ones in Spain. Younger activity, completely separated from the previous one, forms the low-K basaltic andesitic dikes from Alboran Island, dated between 9.1±0.5 and 7.5±0.3Ma. Along the Alboran Ridge both low-K and high-K andesites to dacites were emitted in the estimated range of 10.7–8.7Ma. Low-K and high-K andesites to dacites sampled at ODP sites 977 and 978 into the East Alboran Basin, are dated between 12.1±0.2 and 9.3±0.1Ma.We propose to relate with the Trans-Alboran lineament only the post-Tortonian igneous activity younger than 9Ma.

The Shakhtama porphyry Mo ore-magmatic system (eastern Transbaikalia): age, sources, and genetic features

Two intrusive complexes are recognized at the Shakhtama deposit: Shakhtama and ore-bearing porphyry. The U–Pb zircon dates (SHRIMP II) are 161.7 ± 1.4 and 161.0 ± 1.7 Ma for the monzonites and granites of the Shakhtama complex and 159.3 ± 0.9 and 155.0 ± 1.7 Ma for the monzonite- and granite-porphyry of the ore-bearing complex. The igneous complexes formed in a complex geodynamic setting in the late Middle Jurassic and early Late Jurassic, respectively. The setting combined the collision of continents during the closure of the Mongol-Okhotsk ocean and the influence of mantle plume on the lithosphere of the Central Asian orogenic belt. The intrusion of the Shakhtama granitoids took place at the end of the collision, and the intrusion of porphyry of the ore-bearing complex, during the change of the geodynamic setting by a postcollisional (rifting) one. The complexes are composed of monzonite–granite series with similar geochemical characteristics of rocks. The performed geological, geochemical, and isotope-geochemical studies suggest that the sources of magmas were juvenile crust and Precambrian metaintrusive bodies. The juvenile mafic crust is considered to be the predominant source of fluid components and metals of the Shakhtama ore-magmatic system. The granitoids of both complexes include calc-alkalic high-K rocks with typical geochemical characteristics and with characteristics of K-adakites. These geochemical features indicate that the parental melts of the former rocks were generated at depths shallower than 55 km, and the melts of the latter, at depths of 55–66 km. K-adakite melts resulted from the melting of crust submerged into the mantle during the lithosphere delamination, which was caused by the crust thickening as a result of the repeated inflow of basic magma into the basement of the crust and tectonic deformations in its upper horizons. The high-Mg monzonitic magma produced under these conditions ascended and was mixed with melts generated in the upper horizons, which accounts for the high Mg contents of the Shakhtama granitoids. The similar compositions and petrogeochemical characteristics of the granitoids of the Shakhtama and porphyry complexes point to the same sources, transport paths, and evolution trend of their parental melts. This indicates that the igneous rocks of both complexes are products of the same long-living magmatic system, which produced Mo mineralization at the final stage. The favorable conditions for the ore production in the magmatic system during the formation of the porphyry complex appeared as early as the preceding stage—during the formation of the Shakhtama complex, which we regard as a preparatory stage in the evolution of the ore-magmatic system.

The Talazhin plagiodunite–troctolite–anorthosite–gabbro massif (East Sayan): petrogeochemistry and ore potential

The petrology and ore potential of the Talazhin massif located in northwestern East Sayan are studied. The internal structure of the intrusion, the petrographic composition of its rocks, and their metallogenic, petrostructural, and petrogeochemical features are considered. The probable temperature and chemical composition of the parental magma of the pluton were computed using the KOMAGMAT-3.52 program on the modeling of equilibrium crystallization. The obtained data indicate that the Talazhin massif is a rhythmically layered plagiodunite–troctolite– anorthosite–gabbro intrusion formed from low-Ti high-alumina olivine–basalt melt. It is promising for Cu–Ni–PGE mineralization.

Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value THf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).

Late Jurassic-Neocomian volcanism of the northern Okhotsk region: Geology, tectonic settings, and mineralization

The paper deals with new data on the geology, geochronometry, and petrogeochemistry of the Upper Jurassic-Neocomian basaltic andesites constituting the base of the Cretaceous volcanogenic section in the northern Okhotsk region. The geochronometric data confirm the Late Jurassic-Neocomian age of these rocks (147–138 Ma, K-Ar method). Their characteristic features provide grounds for considering these rocks as fragments of the Uda volcanic belt. The comparison of the petrogeochemical characteristics with similar parameters of other different-age tectonic structures of the region under consideration that were formed in different geodynamic environments (intracontiental and continental marginal volcanogenic belts, island arcs) revealed similarity between the conditions of the formation of the Late Jurassic-Neocomian (Uda belt) and Albian-Cenomanian (Okhotsk-Chukotka belt) volcanics: both characterize continental marginal environments. It is assumed that the Late Jurassic-Neocomian volcanism associates with Au-Ag mineralization, which is also expected in other similar although poorly known volcanic structures.

Alteration Geochemistry of Volcanic Rocks around Sarcheshmeh Porphyry Copper Deposit, Rafsanjan, Kerman, Iran: Implications for Regional Exploration

AbstractTo discriminate the mineral potentiality of the trachybasalt around the Miocene Sarcheshmeh porphyry copper deposit, petrogeochemical characteristics of more than 45 samples of the volcanic rocks were studied. Sarcheshmeh is one of the world's largest Miocene porphyry copper deposits in a continental arc setting and contains about 1200 million tonnes of ores with an average grade of 1.2 percent copper, 0.03 percent molybdenum, 3.9 g/t Ag and 0.11 g/t Au. The biotized and sericitized trachybasalts around the Sarcheshmeh deposit are associated with chalcopyrite, pyrite and molybdenite and and are enriched in Cu (>3108 ppm), K2O (>4.2%), Rb (>155 ppm) and MgO (>2.9%), but depleted in yttrium (<11 ppm), MnO (<0.06%), CaO (<0.6%), Na2O (<0.33%), Sr (<107 ppm), and Ba (<181 ppm). The propylitized trachybasalts are enriched in CaO (>9.1%), Na2O (>3.2%), MnO (>0.24%), Y (>18.2 ppm), and Ba (>323 ppm). The results demonstrate that the diagrams of loss on ignition − Cu, Cu − Y, K2O/K2O + Na2O + CaO − Cu and Y − MnO may be used as an exploration guide for undiscovered porphyry copper mineralization in the Central Iranian volcano—plutonic copper belt.

The modern fluid-magmatic system of the Pyatogorsk volcanic center

In the area of Caucasus Mineral Waters (CMW), referred to the “volcanic center”-type structures, an interrelated annular zonality of structural, petro-geochemical, geothermal, and hydrochemical features is found. The probability of peripheral magmatic source existence at 9–15 km depth is shown.

Study on the geochemical characteristics of ocean-ridge and oceanic-island volcanic rocks in the Nan-Uttaradit zone, northern Thailand

Field investigations and laboratory integrated research as indicated that ophiolite melange in the Nan-Uttaradit zone, northern Thailand, consists of fragments of tectonites such as metamorphic peridotite (extremely silicified serpentinite), cumulates (pyroxenolite, gabbro, and gabbro-diorite), ocean-ridge basalt, oceanic-island basalt and radiolarian silicalite, and it was formed during D3-P. The rock series, rock types and petrogeochemical characteristics of metamorphic tholeiites in the Nan area of the Nan-Uttaradit zone are similar to those of ocean-ridge basalts (C1) in China’s Ailaoshan zone. As for the Hawaiites in the Nan area of the Nan-Uttaradit zone, their major elements, REEs and trace elements are similar to those of oceanic-island basalts in China’s Jinshanjiang zone (P1 1). In the Uttaradit area of this zone the metamorphic alkaline basalts show transitional petrogeochemical characteristics between ocean-ridge basalts and oceanic-island basalts, which were still formed in oceanic-island environments. The above-described basalts are all oceanic volcanic rocks and they are the most important part of the Paleo-Tethys oceanic crust in the Nan-Uttaradit zone.

Granitoids of the Osnitsky complex of Ukrainian Shield and the Brno Massif of Moravia: Comparison of structural-geological, petrophysical and petrogeochemical features of origin and petrological significance

In this work we summarise basic results of our studies for the areas of tectonomagmatic activities in Moravia and at the western slope of Ukrainian Shield. The processes occurred there have resulted in development of volcuno-plutonic belts related to different blocks of the Earth’s crust. There is a likeness in the origin and characteristics of granitoids in different areas of tectonomagmatic activity of the Earth’s crust. This often forms a very close set of their composition and structural features reflecting the regularities of their origin. In spite of evident difference in the geological and structural positions of those granitoids, similarities and approaching trends in the changes of their structural, petrochemical and petrophysical characteristics should be marked. These characteristics can be taken as indicators of magmatism and mineralisations and can also serve as important distinguishing features of the ore-monitoring structures.

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.

Basalts of the Pantalassa ocean in the Samarka terrane, Central Sikhote Alin

Basalts developed on the right bank of the Matai River belong to the Samarka terrane (Central Sikhote Alin), which is a fragment of the Jurassic accretionary prism. They associate with Carboniferous-Permian reef limestones, Permian pelagic cherts, Jurassic hemipelagic cherty-clayey deposits, and terrigenous rocks of the near-continental sedimentation area. The petrogeochemical features of the basalts provide insight into the character of the volcanism in different settings of the ancient Pantalassa ocean. In terms of chemistry, the Carboniferous-Permian basalts are similar to the within-plate ocean-island basalts related to plume mantle sources. They were presumably formed in an oceanic area with numerous islands and seamounts. The Permian basalts associated with cherts are tholeiitic in composition and were formed from depleted mantle in a spreading center located in the pelagic area. The Jurassic basalts are of plume origin and, in terms of geochemistry, occupy an intermediate position between OIB and E-MORB. They were presumably formed in a convergent zone in a geodynamic setting of rapid oblique subduction.

Petrogeochemical features of Oligocene-Early Miocene volcaniclastic rocks of the Sea of Japan

Petrographic and geochemical studies showed that the Oligocene-Early Miocene volcaniclastic rocks from the southern part of the Sea of Japan are ascribed to the high-potassium aluminous rocks of the subalkaline volcanic series of active continental margins. A comparative analysis revealed the spatiotemporal relation of Oligocene-Early Miocene subaerial volcanism of the Sea of Japan with Late Cretaceous and Eocene-Early Miocene ignimbrite volcanism of the East Eurasian margin. This allows us to refer the volcaniclastic rocks of the Sea of Japan to a stage of ignimbrite volcanism that occurred during relative quiescence against a general extension in the continental margin setting.

Petrogeochemical features of metaterrigenous rocks of the Kan Block of the Eastern Sayan: Reconstructions of provenances and sedimentation conditions

Chemical and isotopic compositions of Proterozoic metaterrigenous rocks of the Kan Block (Central and Idar terranes) of the Eastern Sayan are studied. The results of the reconstruction of their provenances and sedimentation conditions are presented. The rocks under investigation correspond by their petrogeochemical composition to graywackes of island arcs. The combination of geochemical and isotope data shows that sediments of the Central terrane had a local provenance represented by Early Proterozoic subduction magmatic complexes, whereas sediments of the Idar terrane formed probably as the result of mixing of terrigenous material related to the destruction of rocks of Meso-Neoproterozoic oceanic and more ancient continental crust.

Petrogeochemical characteristics of the kimberlites from the Middle Markha region with application to the problem of the geochemical heterogeneity of kimberlites

A comparative geochemical investigation of kimberlites from the Middle Markha region and traditional diamond-bearing areas of Yakutia supported the division of kimberlites into two geochemical types. One of them includes kimberlites from the traditional diamond-bearing areas of Yakutia, and the other is represented by the kimberlites of the Middle Markha region and the rocks of the Zolotitsa field and V. Grib pipe of the Arkhangelsk province. The obtained representative parameters of the two kimberlite types indicate a sharp geochemical contrast between them, and the individual features of correlation relationships between the elements in the rocks of the two types suggest that these differences are related to fundamental genetic factors, which concern primarily the group of highly charged incompatible trace and radioactive elements. The presence of geochemically contrasting rocks within a rather uniform petrochemical association and the geochemically specialized occurrences of kimberlites and related rocks are consequences of repeated metasomatic transformations of mantle rocks under the influence of deep-derived fluids or volatiles released during the recycling of subneous source and subsequent derivation of geochemically specialized types of deep magmas showing signatures of individual mantle sources.

Major Characteristics of the Lajishan Orogenic Belt of the South Qilian Mountains and Its Geotectonic Attribute

Abstract The Lajishan orogenic belt is one of the E‐W‐trending Caledonian orogenic belts within the Qinling‐Qilian orogenic system. It was formed upon the Jiningian basement by intensive taphrogenesis. Its major characteristics comprise the prominent faulting along the north and south boundaries, the highly complicated petrological and petro‐geochemical features of the volcanic rock series, and the development of a new type of ophiolite suite. In terms of tectonic analysis and the sequential analysis of tectonic settings of magmatic rocks, it is suggested that the Lajishan orogenic belt has undergone a complete “opening‐closing” cycle, which can be further divided into 3 second‐order “opening‐closing” cycles. The composite characteristics of the “opening‐closing” movement show that Laji Mountain is a typical fault orogenic belt. The fault orogenic belt is one of the most important types of intracontinental orogens. It is of critical theoretical and practical significance to summarize the characteristics and the diagnostic criteria of this kind of orogenic belts, and study the mechanism of their formation and build models of their evolution.

New evidences for the formation of granitic rocks by remelting of the sial crust in Southern Hunan

Comparatively analyzing 47 REE distribution data of the Southern Hunan geosynclinal arkose-silicarenite and various period granitic rocks and the results of their partially melting experiment series in company with the occurence and petrogeochemical features of various period granitic rocks, the authors suggest that all of the above-mentioned salic rocks can be partially melt to produce granitic magma.

Petrology, Geochemistry, and Tectonic Setting of Plagiogranites of the Chelyuskin Ophiolite Belt

Results of research on the geological, petrochemical, and isotopic-geochronological charac- teristics of plagiogranites from the Chelyuskin ophiolitic belt, on the northern part of East Siberia's Taymyr Peninsula, are presented. Petro-geochemical features and REE distributions for this tonalite-trondhjemite series resemble those of plagiogranites from different ophiolitic complexes. The plagiogranites considered here belong to the low-potassium series of ophiolitic mafics—gabbro, gabbro-dolerite dikes, and basalts. Their spatial relationships; low K2O, Rb, Nb, Ta, U., and Th contents; similar REE patterns; and tonalite and trondhjemite Nd- and Sm-Nd- isotopic ratios typical of mafic rocks confirm the cogenetic nature of these rocks. Zircon U-Pb dating and an Sm-Nd isotopic study suggest a Late Riphean age for the plagiogranites. We regard the 740 ± 38 Ma age as the upper age boundary for the formation of the Chelyuskin ophiolitic belt, and Sm-Nd model ages (850–785 Ma) as its lower boundary. Tonalite-tron...