O. A. Levchenkov

Early Paleozoic batholiths in the northern part of the Kuznetsk Alatau: Composition, age, and sources

The paper reports geological, chemical, and geochronological data on the Early Paleozoic granitoid and gabbro-granite associations, which compose the Kozhukhovskii and Dudetskii batholiths in the northern part of the Kuznetsk Alatau. The Kozhukhovskii batholith located in the Alatau volcanoplutonic belt is made up of tholeiitic, calc-alkaline, and subalkaline rocks that were formed in two stages. The first stage corresponded to the formation of granitoids of the Tylinskii quartz diorite-tonalite-plagiogranite complex (∼530 Ma, Tylinskii Massif, tholeiitic type) in an island arc setting. The second stage (∼500 Ma) produced the Martaiga quartz diorite-tonalite-plagiogranite complex (Kozhukhovskii Massif, calc-alkaline type) and the Krasnokamenskii monzodiorite-syenite-granosyenite complex (Krasnokamenskii Massif, subalkaline type) in an accretionary-collisional setting. The Dudetskii batholith is situated in the Altai-Kuznetsk volcanoplutonic belt and contains widespread subalkaline intrusive rocks (Malodudetskii monzogabbro-monzodiorite-syenite and Karnayul’skii granosyenite-leucogranite complexes) and less abundant alkaline rocks (Verkhnepetropavlovskii carbonatite-bearing alkaline-gabbroid complex), which were formed within the age range of 500–485 Ma. Our Nd isotopic studies suggest mainly a subduction source of the rocks of the Kozhukhovskii batholith (εNd from + 4.8 to + 4.2). Subalkaline rocks of the Dudetskii batholith exhibit wide isotopic variations. The Nd isotopic composition of monzodiorites and monzogabbro of the Malodudetskii Complex (εNd = + 6.6), in association with the elevated alkalinity and high Nb and Ta contents of these rocks, testifies to the predominant contribution of an enriched mantle source at the participation of a depleted mantle source. The lower εNd (from + 3.2 to + 1.9) in its syenites possibly indicates their generation through melting of metabasic rocks derived from enriched mantle protolith. The rocks of the Karnayul’skii Complex have lower Nb and Ta contents at similar εNd (+3.6), which suggests some crustal contribution to their formation.

Two metamorphic stages in the Svecofennian domain: Evidence from the isotopic geochronological study of the Ladoga and Sulkava metamorphic complexes

The Ladoga, Russia, and adjacent Sulkava, southeastern Finland, metamorphic complexes are the two largest “granulite” provinces of the Svecofennian domain. In this area, the domain is composed of outer and inner zones. Sulkava is situated in the inner zone, which principally can be compared to the accretionary arc complex of Southern Finland. Ladoga is situated in the outer zone, which is correlated with the accretionary arc complexes of central and Western Finland. The complexes contain different metamorphic assemblages, which are caused by the different composition of the sedimentary protoliths: the rocks of the Sulkava metamorphic complex are higher in Al and K than those of the Ladoga Complex. Pb-Pb step leaching dating was used to determine the age of prograde sillimanite from both complexes. The dates thus obtained constrain metamorphic peaks for the Sulkava and Ladoga complexes at 1799 ± 19 Ma and 1878 ± 7 Ma, respectively, which is consistent with the U-Pb monazite ages of gneisses from both of the complexes. The differences in the ages of the metamorphic minerals from these complexes reflect the Early Svecofennian (1.89–1.86 Ga) and Late Svecofennian (1.83–1.79 Ga) metamorphic stages in the Fennoscandian Svecofennides.

The Paleozoic age of high-pressure metamorphic rocks in the Dakhov Salient, northwestern Caucasus: Results of U-Pb geochronological investigations

The Dakhov Salient is a small exposure of the preAlpine crystalline basement within Jurassic rocks at the northwestern termination of the Front Range of the Greater Caucasus (Fig. 1). Together with the southeastern Beskes and Sakhrai salients, the Dakhov Salient makes up a partly exposed northern framing of the Middle Paleozoic island-arc complex of the Front Range. Like crystalline rocks of the Blyb Salient at the southern framing of the island-arc complex, rocks of the Dakhov Salient are traditionally considered the pre-Paleozoic basement of the Front Range. The Dakhov Salient is mainly composed of pre-Alpine granitoids (primarily, granodiorites), which intrude metamorphic rocks exposed as a narrow band in the northern area. The Dakhov Salient is easily accessible, and ravines of the Belaya River and Syuk Creek are excellently exposed. Nevertheless, several essential aspects of the geology of the Dakhov Salient remain controversial. The age of host rocks is the most important issue. In the 1970s, the K–Ar dating of granitoids and metamorphic rocks of this salient yielded a Neoproterozoic value of 985–612 Ma [4, 5]. Since then, the Dakhov Salient has become a rare reference (in terms of geochronology) object in the Greater Caucasus. This salient is always mentioned for the substantiation of the pre-Paleozoic age of its metamorphic basement [1, 2]. However, there are grounds to doubt the reliability of the datings mentioned above [6]. We performed U–Pb geochronological investigation of one of the oldest components of the Dakhov Salient— metaaplite veins assigned to the symplectitic garnet amphibolites of the Belaya River canyon. We also carried out K–Ar dating of granodiorites intruding the metamorphic rocks. The Paleozoic Age of High-Pressure Metamorphic Rocks in the Dakhov Salient, Northwestern Caucasus: Results of U–Pb Geochronological Investigations

Isotopic geochronology of the archean posttectonic association of sanukitoids, syenites, and granitoids in central Karelia

The U-Pb geochronological study (by the classic technique and on an ion microprobe) of syenites from central Karelia has established their Archean age. The age values obtained for individual massifs are 2735 ± 15 Ma for syenites from the Sjargozero Massif and 2745 ± 10 Ma for syenite from the Khizhjarvi Massif. The syenites are demonstrated to have been emplaced nearly synchronously with sanukitoid massifs in central Karelia, whose average age is 2743 ± 3 Ma (Bibikova et al., 2005). The syenites of the Sjargozero Massif and granodiorites of the Ust-Volomsky Massif were determined to have practically identical ages of 2735 and 2738 Ma, respectively, a fact also corroborating the coeval character of the syenites and granodiorites. Some zircon grains from the Sjargozero syenites contain cores with an age of about 2755 Ma, which suggests that the syenites could have been contaminated with the material of the host volcanic rocks of basaltic and andesitic composition that were metamorphosed at 2750–2760 Ma. The results of the isotopic geochronologic research indicate that the different rock groups composing the Archean postorogenic association of sanukitoids, syenites, and granitoids in central Karelia have been generated in a single stage at approximately 2740 Ma, i.e., 60–70 m.y. after the origin of the syntectonic tonalites. The zircons have elevated Th/U ratios, which is consistent with the mantle genesis of the rocks. Significant crustal contamination was identified in the most acid members of the sanukitoid series: syenites and granitoids. Our data obtained for zircons from the sanukitoids and syenites of the Karelian craton in the Baltic Shield are in good agreement with the results obtained on the sanukitoids of the Canadian Shield.

Geochemistry of metasomatic zircons from the Terskii greenstone belt

Local isotopic and geochemical studies of the zircons from metasomatites of the Terskii greenstone belt allowed us to determine two stages of metamorphism (2680 and 2025 My) and two stages of metasomatosis (2600 and 1800 My). Almost all the zircons were either metasomatic or affected to some degree by metasomatic processes caused by the enrichment of zircons in light rare earth elements, Th, U, Sr, and Ba, and flattening of the Ce anomaly.

First find of cerianite in zircons from metasomatites of the Terskii greenstone belt, Baltic shield

) is one of the most scarce rareearth minerals. Because of difficulties in discovery andidentification of this mineral, E.I. Semenov proposedconsidering cerianite as an object of nano (not micro)mineralogy [1]. Since its discovery in the 1950s, nomore than 20 reliable finds of this mineral (mainly inthe form of microscopic grains and microinclusions inother minerals) have been documented. Cerianite wasdiscovered for the first time in Sudbury (Canada) inthe carbonate dikes cutting across nepheline syenites[2]. In addition to Ce, cerianite contains isomorphicadmixtures of other LREE (La, Pr, Nd). Significantamounts of Th and other elements (Xe, Cs, Pb, Rh)were found in cerianite from pegmatites of East Antarctica [3]. Supergene cerianite occurs as a weatheringproduct of bastnaesite [1] and fluocerite [4], as well asin the weathering crust of different compositions [1,5–7]. In the oceanic Fe–Mn ores, cerianite isobserved as submicron films on apatite [8]. The formation of cerianite in granites is considered to berelated to hydrothermal alteration, accompanied byfluid leaching of Ce from monazite [9]. Cerianite associates with minerals and ores of noble metals: Au, Pt(Sukhoi Log deposit [10]); Au, Pd, and REE (oremanifestations of the Polar Urals [11] and others]).Overgrowths of cerianite of a few microns thick ondiamond phase were found in the carbonado fromBrasilia [12].Cerianite is a sensitive indicator of conditions ofmineral formation, because Ce is the only compatibleREE, which in natural conditions can occur not onlyin trivalent but also in tetravalent form. Cerianite isformed only in strongly oxidizing conditions, mainlyin alkaline solutions (fluids) [13].Submicron inclusions of cerianite were found by usin the zircon from twomica garnet–chlorite–quartz–plagioclase metasomatite (sample 2508) in the Terskiigreenstone belt in the southern framing of the Imandra–Varzuga structure (southeastern part of the KolaPeninsula). Complex isotopic–geochemical study ofzircons from metasomatites of the Terskii greenstonebelt allowed us to distinguish two stages of metamorphism (2680 and 2025 Ma) and two stages of metasomatism (2600 and 1800 Ma) [14]. Practically all zircons of different ages were completely or partiallymetasomatized, which caused their enrichment inREE (especially LREE), Th, U, Sr, and Ba, and formation of flat REEs with a reduced Ce anomaly.The structural features, majorelement composition of the zircons, and the composition of foreignmineral inclusions, if present, were studied in theregime of compositional contrast on a JEOL JSM6460LV scanning electron microscope equipped withan Oxford INCA detector at the St. Petersburg MiningInstitute. U–Pb dating of zircons from metasomatiteswas conducted on a SHRIMPII ion microprobe atthe Center for Isotopic Research, Karpinskii AllRussia Research Institute of Geology. The REE and traceelement composition of zircons was analyzed at thesame grains and points as U–Pb dating, on a CamecaIMS4f ion microprobe at the Yaroslavl Branch of thePhysicoTechnical Institute of the Russian Academyof Sciences.Zircon containing cerianite differs in the heterogeneous structure. Its weakly altered right upper part(point 2.1 in Fig. 1) retains traces of faceting and primary growth zoning. At the final stage of the Svecofennian metasomatism, most zircon was transformed with the formation of “patches” of irregularreniform shape, which form a band up to 100 µm insize with a characteristic dark tint in backscatteredelectrons (BSE) (point 2.2 and inset

Early migmatites in the prograde metamorphism zone of gneisses in the northern domain of the Ladoga Region: U-Pb evidence based on monazite

The North Ladoga region is the southeastern fragment of the Svecofennian Belt. The metamorphic rocks of this region were traditionally regarded as constituents of a zonal metamorphosed complex composed largely of the Kalevian (late Paleoproterozoic) turbidite formation [1]. Later data [2] provided evidence for tectonic juxtaposition of the high-temperature core of the complex with less metamorphosed rocks. These relationships made it possible to distinguish the northern (ND) and southern (SD) domains [3]. The relatively low-grade metamorphic rocks of the ND are related to the Sveco-Karelian pericratonic zone; the high-grade metamorphic rocks of the SD, to the Svecofennides that are devoid of the Archean granitic basement (Fig. 1). The geochronological data obtained over recent years furnished convincing evidence that ultrametamorphism of rocks in the SD occurred within a rather narrow time interval (1880‐1870 Ma ago) [4]. The results of geochronological study of migmatites in rocks of the Sveco-Karelian pericratonic zone (ND) are reported in the present communication for the first time. The ND structure is controlled by mantled gneiss domes with the Archean granite‐gneiss basement in their cores. The ND is a domain of zonal metamorphism with southward increasing grade from greenschist to amphibolite facies and poorly developed intrusive magmatism (Fig. 1). The thermobarometric estimates of metamorphic grade in the ND and metamorphic mineral assemblages confirm a decrease in metamorphic grade from south to north. The parameters of peak metamorphism in the amphibolite-facies zone are T = 650‐730 ° C and P = 3.8‐5.0 kbar [5]. The ND rocks also bear indications of retrograde transformations of the andalusite‐muscovite subfacies, which are most intense along the narrow submeridional tectonic zones. The SD is characterized by the predominance of granulite-facies metamorphic rocks. Hence, the SD is a high-temperature granulitic block with numerous gabbro, enderbite, tonalite, and granite intrusions. This block is considered a high-temperature core of the zonal metamorphic complex. The SD is thrust over the ND along the low-angle Meier fault zone [3].

U-Pb Geochronology of Migmatite Leucosomes Based on Zircon SIMS Measurements and Correlation with TIMS-ID Data on Monazite

The use of an advanced analytical method for the local study of isotope systems requires control of results by traditional and tested methods and approaches. This is particularly true of local U‐Pb analysis of metamorphic zircons with poorly developed or thin metamorphogenic rims. Repeated measurements for statistics, which is usually necessary during mass spectrometric analysis with application of secondary ions (SIMS), are objectively difficult for such zircon, while the correct TIMS-ID (isotopic dilution) dating is problematic. At the same time, even a limited number of local measurements in different generations (and rims) of zircons may yield useful information and reveal real stages in substance evolution. This is evident from local SIMS dating of zircons from migmatite leucosomes substantiated by U‐Pb geochronological data (TIMS-ID) on monazite. Zircons from migmatite veins developed in garnetbearing gneisses of the North Ladoga region served as the object of these studies. This region is characterized by wide development of Archean and Proterozoic rocks constituting a zoned metamorphosed complex located south of the epi-Archean Karelian Craton. This region is defined as the Northern Domain [1] with zoned metamorphism ranging from the green-schist facies in the north (near the boundary of the Karelian Craton) to the amphibolite facies in the south. Migmatite melting products are observable in the so-called zone of the second sillimanite [2]. The formation of this sillimanite is accompanied by the disappearance of andalusite and staurolite; simultaneously, muscovite becomes unstable. The morphological features of veins as well as their composition allow the conclusion that these products were formed due to partial melting of host gneisses. The T and P metamorphism conditions of rocks from this zone were 650‐700 ° C and 4.5‐5.0 kbar, respectively [1]. The rocks of the Northern Domain are locally subjected to regressive transformation into the andalusite‐muscovite subfacies. Such a transformation is most intense along narrow meridional tectonic zones. Two leucosome generations are definable in the examined outcrop. Both of them are deformed and almost conformable relative to gneissose patterns of the host rocks. The leucosomes are usually a few centimeters across. Zircons and monazites for isotopic mea