Stages in the formation of uranium mineralization in the Salla-Koulajarvinskaya zone (Northern Karelia): Geological and isotope geochronological data

On the basis of U–Pb, Rb–Sr and Sm–Nd isotopic data, it is shown that formation of uranium mineralization in the Paleoproterozoic Salla-Koulajarvinsky belt (Northern Karelia) was a long-lasting mult-stage process that developed over more than 1 Ga: from the Paleoproterozoic to the Paleozoic. The first stage, 1.75 Ga ago, corresponds to the Svekofennian metamorphic event—regional albitization. The process was dated by the Rb–Sr (isochronic age of albitites is 1754 ± 39 Ma) and U–Pb methods (the age of rutile is 1756 ± 8 Ma). At this stage, with a lower temperature limit of 400–450°C, conditions were favorable for the mobilization and migration of uranium, but not for its deposition in minerals. The second stage, 1.62 Ga ago, was a time of alteration of rocks at the regressive stage of the Svekofennian metamorphic event, when carbonate and chlorite rocks formed after albitites. The age of this stage was estimated as 1627 ± 42 Ma according to ThO₂, UO₂, and PbO contents in uraninite. Probably, the deposition of uraninite took place at this stage at temperature not higher than 300–350°C. The final, third stage, 385 Ma ago, corresponds to the Paleozoic tectonic activation and formation of Caledonian alkaline intrusions. Uranium minerals were probably redeposited at this stage; the U–Pb age of brannerite is 385 ± 2 Ma.     

Interaction between mafic and felsic magmas in subvolcanic environment (Tastau igneous complex,eastern Kazakhstan)

The paper reports the results of integrated geological, petrological, geochemical, and geochronological studies of the Tastau igneous ring complex in the Zaisan orogen of eastern Kazakhstan. Interaction between felsic and mafic magmas has been studied. Hybrid rocks are represented by gabbros and diorites injected into a granitic magma chamber. They occur as dikes and pillow-like and globular mafic bodies variously disintegrated and mixed with host granitoids. The age of synplutonic rocks is 242 ± 20 Ma (U/Pb zircon dating), which is, with regard to analytical error, substantially younger than it was presumed.Mechanisms of interaction between felsic and mafic magmas have been studied. They include mechanical (mingling) and chemical (mixing) interaction, which produce composite mixtures and hybrid rocks. The ratios of mafic to felsic components involved in the formation of intermediate rocks were calculated from major elements by regression analysis and tested with regard to rare and trace elements. The model for mingling includes rapid quenching of the mafic melt when it is injected into the granitic magma chamber, decomposition of crystalline fragments, dispersion of fragments and crystals in the magma chamber under conditions of rapid turbulent flow, and enrichment of felsic magma with femic components to produce monzonitic magmas.     

Sequence of geologic events in the central and northeastern parts of the Lapland Granulite Belt: Isotopic geochemical zircon data and results of geological-petrological studies

Geological and petrological studies of rocks were conducted to reproduce the succession of endogenic processes in the central and northeastern parts of the Lapland Granulite Belt along the course of the Lotta River. Mineralogical-geochemical data on zircon from these rocks (the anatomy of zircon crystals seen in BSE and CL imagery; U, Th, Hf, and Y concentrations; and REE patterns) and their crystallization temperatures (calculated using the Ti concentration) make it possible to correlate the newly obtained geochronologic data with the sequence of metamorphic events inferred from petrological data. Three major episodes of enderbite-forming processes are distinguished: (i) early episode at 1.99 Ga, (ii) main episode at 1.940–1.925 Ga; and (iii) late episode at 1.89 Ga. The early medium-pressure metamorphic episode is dated at 1.97–1.96 Ga. The diatexis of the quartz-feldspar granulites with the derivation of garnet plagioclase leucogranites took place at 1.917–1.909 Ga.     

Paleoproterozoic Age of Carbonates and Trondhjemites of the Central Azov Group: Sr Isotope Chemostratigraphy and U–Pb Geochronology

Doklady Earth Sciences - The Sr isotope characteristic of carbonate rocks in the Central Azov Group has been reported (0.70322–0.70352), the Nd model age of silicate sediments has been...     

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.     

REE distribution in zircon from reference rocks of the Arctic region: Evidence from study by the LA-ICP-MS method

The results of the LA-ICP-MS analysis of the concentrations of REEs, U, Th, and Hf in zircon from Paleo- and Neoarchean reference rocks of the Kola region (garnet–amphibole gneiss, basic and acid granulites, and granite pegmatite) are reported. A new methodology of the study of accessory zircons has been validated and modified. The accuracy of the results is confirmed by analysis of standard zircons Temora 1 and 91 500 and by comparison with the data obtained in other laboratories.     

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

Doklady Earth Sciences -     

Mesoarchean mafic dykes of the Belomorian eclogite province (Gridino Village Area,Russia)

Archean processes of eclogitization in the Gridino metamorphic association (the Belomorian eclogite province) developed in mafic dykes, boudins, and acidic rocks of the Archean continental crusts. To determine the U-Pb age of the intrusion of the latest dykes, the geochronological samples were taken from the dyke of ferriferious metagabbro that cross-cuts the dyke of eclogitzed and granulitized olivine gabbronorite. The igneous zircons were dated by the SHRIMP II technique. The zircons showed a concordia age of 2846 ± 7 Ma, which is considered as the time of intrusion of a mafic melt. The younger low-thorium zircon rims of 2.78–2.81 Ga age around the igneous cores are typical formations that appeared under metamorphic conditions in equilibrium with a migmatite melt, and may characterize the time of formation of the granite leucosome under metamorphism, probably of eclogite facies.     

Metamorphic evolution of the archean eclogite-like rocks of the Shirokaya and Uzkaya Salma area (Kola Peninsula): Geochemical features of zircon,composition of inclusions,and age

Zircon from the eclogite-like rocks of the Shirokaya and Uzkaya Salma area (Kola Peninsula) was studied using a complex of mineralogical and geochemical methods (CL, BSE, microprobe, and REE distribution). Different zones distinguished within zircon crystals were dated on a SHRIMP-II mass spectrometer. Mineral and chemical compositions of inclusions in the zircons were analyzed. Based on these studies, the following stages of the formation and transformation of the rocks were determined: (1) formation of basic protolith of the eclogite-like rocks of the Shirokaya and Uzkaya Salma area 2.94–2.93 Ga ago; (2) the granulite-facies metamorphism of the eclogite-like rocks of the Shirokaya Salma 2.72 Ga ago; (3) the onset of decompressional cooling with formation of Cpx-Pl symplectites at 2.70 Ga ago; and (4) final metamorphic reworking together with surrounding TTG under the amphibolite-facies conditions at 1.89 Ga ago. The studied rocks and minerals revealed no isotope-geochemical or geochronological signs of eclogite metamorphism. Geochemistry of the primary magmatic zircons showed that the protolith of eclogite-like rocks was gabbro rather than MOR basalts. The formation of garnet in the rocks of the Uzkaya and Shirokaya Salma area is dated at 2.70 and 1.89 Ga ago, which is consistent with petrological observations of later formation of garnet relative to omphacite. Obtained data led us to prefer a magmatogenic model, which suggests that omphacite in the rocks of the Shirokaya and Uzkaya Salma was presumably formed during crystallization from basic melt, rather than during eclogite-facies metamorphism.     

Paleoarchean U–Pb (SIMS SHRIMP-II) Age of Mafic Granulites from the Bug Complex,Ukrainian Shield

Doklady Earth Sciences - Data on the U–Pb (SIMS SHRIMP-II) age (3628 ± 38 and 2845 ± 65 Ma) of zircon from the inclusion of mafic granulite in gneiss–enderbites of the Bug...