Geochronological limits of subalkaline magmatism in the Ket-Kap–Yuna igneous province,Aldan Shield

Comparative U-Pb (zircon- and sphene-based, for the first time) and K-Ar (biotite-, amphibole, and whole rock-based) dating of monzonitoids and subalkaline granitoids of the Ket-Kap and Uchur volcanic-plutonic complexes (Ket-Kap-Yuna Igneous Province is one of the areas of tectonic-magmatic activation of the Aldan Shield), respectively, has been made.     

Capabilities and constraints of the Pb-Pb dating of metamorphogenic minerals using the step-leaching method

Based on particular examples, this paper considers the capabilities and constraints of the step-leaching Pb-Pb dating (PbSL) of metamorphogenic minerals. It was shown that stepwise leaching allows the separation of fractions enriched in uranogenic and thorogenic Pb isotopes, which can be used for the determination of mineral ages and the time of crystallization of coexisting equilibrium and, occasionally, disequilibrium monazites. The presence of monazite in a mineral is indicated by a high Th/U ratio similar to that of monazite. The main limitation of the method is related to the presence in minerals of both disequilibrium domains of the mineral matrix and disequilibrium monazite microinclusions. By the example of minerals studied, we discussed three scenarios for the development of the U-Th-Pb isotopic systems of metamorphogenic minerals.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

U-Pb age of granitoids from the Elenovsk Complex (Kokchetav block, Northern Kazakhstan)

The U-Pb geochronological study of zircons from alkali granitoids of the Elenovsk Complex constituting the Zhilanda and Lesnoi massifs (northern Kazakhstan) located several kilometers away from each other revealed their close ages: 433 ± 2, 440 ± 10, and 426 ± 12 Ma.     

Interaction of metamict zircon with fluids of various composition

Interaction of metamict zircon with solutions of various composition (H₂O, 1 m NaCl + 0.5 m HCl, 1 m NaCl, 1 m NaHCO₃, 0.1 m Na₂CO₃, 1 m Na₂CO₃, 2 m Na₂CO₃, and 1 m NaOH) was examined at temperatures of 25–800°C, pressures of 1 and 5 kbar, and exposure times of 3, 7, and 14 days. The choice of fluid compositions was determined by the fact that aqueous, chloride, and carbonate solutions are the principal components of postmagmatic, metamorphic, and metasomatic fluids that can affect zircon in nature. Occurring in contact with these solutions, metamict zircon congruently and incongruently dissolves competing with the sorption of elements, crystallization of newly formed phases, and the restoration of the crystal structure. The intensities of these processes are controlled by the temperature, pressure, exposure time, and the pH and composition of the solution.     

Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study

 The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na₂CO₃ solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeleyite accommodating U from 2M Na₂CO₃ solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism. Received: 4 October 1997 / Accepted: 6 December 1999     

Age of the Irkut block of the Prisayan uplift of the Siberian platform basement: Dating minerals from metamorphic rocks

New geochronological (U-Pb, Pb-LS, Sm-Nd) studies were carried out for minerals from metamorphic rocks (aluminous plagiogneisses with sillimanite (kinzigites) and potassium shadow migmatites) to establish the sequence of metamorphic events in the Irkut block of the Prisayan marginal uplift of the Siberian platform basement. Obtained data permit the distinguishing of two main stages of regional metamorphism under the granulite and amphibolite facies conditions: 2480–2560 and 1860–1900 Ma. New age data in general are consistent with previously published zircon estimates of the Neoarchean and Paleoproterozoic ages of the granulite-facies metamorphism of the Irkut block. This gives grounds to consider the geochronological studies of garnet and monazite as promising tool for distinguishing age boundaries of metamorphic transformations in the areas of polycyclic evolution.     

U-Pb age of autochthonous paleoproterozoic charnockite in the Aldan Shield

Autochthonous and parautochthonous charnockites in granulite facies of the Aldan Shield (the Aldan River upper flow) were dated. According to the geological observation data, the autochthonous and parautochthonous granite formation included successive development of nebulite (Lc1), its melting product such as early diatectite (Lc3), later “layer-by-layer” migmatite (Lc4), and diatectite (Lc5). The concordant ages of Lc1 and Lc3 were estimated at 2436 ± 10 and 2453 ± 14 Ma. The age of Lc5 was estimated by the upper concordia crossing at 1960 ± 8 Ma likely corresponding to the diatectic melt crystallization period. The process is accompanied by repeated high-temperature alterations of nebulite, diatectite, and their zircons yielding a concordant age of 1945 ± 13 Ma. This zircon making up the overgrowth rims is characterized by remarkable enrichment in uranium and thorium. The granulite facies metamorphism is confirmed by dating of monazite from migmatite after metapelite (1947.7 ± 8.7 Ma). The two main stages of the autochthonous and parautochthonous charnockite formation initiated the development of the crust magmatic chambers. The first stage (2430–2450 Ma) was synchronous to allochthonous high-K alkali granite in the Olekma granite-greenstone region. The second stage (1900–1960) implied the formation of autochthonous and parautochthonous charnockites under the granulite facies conditions and development of allochthonous charnockite and granite in the central part of the granulite areal.     

The Borodinskii Massif of potassium porphyric granite: Results of U–Pb dating and its tectonic position (Fennoscandian Shield)

It has been demonstrated for the first time that the Svecofennian crustal porphyric granites in the southeastern part of the Fennoscandian Shield are clearly subdivided into two age groups of 1.87 and 1.80 Ga. The representative of the first group is the Kuznechenskii Massif of porphyric granites with a U–Pb age of 1874 ± 4 Ma belongs to the group of plutons formed during the orogenic stage of Svecofennide evolution. The Borodinskii Massif with an age of 1797 ± 2 Ma is a member of the second group: these plutons were formed under tectonically stable (epiplatform) conditions. Therefore, the formation of porphyric granites occurred as the result of repeated generation of crustal magmas during multistage accretional growth of the Svecofennian crust.