V. N. Smirnov

Zircon geochronology of the Klyuchevskoi gabbro-ultramafic massif and the problem of the age of the Mohorovicic paleoboundary in the Central Urals

The Klyuveskoi gabbro-ultramafic massif is the most representative ophiolite complex on the eastern portion of the Uralian paleoisland arc part. The massif is composed of dunite-harzburgite (tectonized mantle peridotites) and dunite-wehrlite-clinopyroxenite-gabbro (layered part of the ophiolite section) rock associations. The U-Pb age was obtained for the accessory zircons from the latter association using a SHRIMP-II ion microprobe at the Center for Isotopic Research at the Karpinskii Russian Geological Research Institute. The euhedral zircon crystals with thin rhythmic zoning from dunites are 441.4 ± 5.0 Ma in age. Zircons from olivine clinopyroxenite show three age clusters with sharply prevalent grains 449.0 ± 6.8 Ma in age. Two points give 1.7 Ga, which is probably related to the age of the mantle generating the layered complex. One value corresponds to 280 Ma, which possibly reflects exhumation of ultramafic rocks in the upper crust during the collision of the Uralian foldbelt. Thus, dunites and olivine pyroxenites from the Klyuchevskoi massif are similar in age at 441–449 Ma. The bottom of the layered part of the ophiolite section corresponds to the M paleoboundary and, consequently, the age of the Mohorovicic discontinuity conforms with the Ordovician-Silurian boundary in this part of the Urals.

The U-Pb SIMS zircon age and geodynamic conditions of formation of granitoides of the Verkhisetsk batholith, the eastern slope of the Middle Urals

The U-Pb SIMS age dating of zircons from different-age granitoid assemblages varying in composition of the Verkhisetsk batholith shows that it comprises rocks of three age groups, formed at different stages of the Ural Mobile Belt. The first age group is represented by quartz diorites (396 ± 5 Ma) with insignificant distribution in area. Their formation was synchronous to island arc volcanism, manifested in the area of study from the second half of the Emsian to late Givetian-early Frasnian. According to this, we could consider these granitoides as comagmatic to island arc volcanites. The second age group includes tonalites and trondhjemites (367 ± 4 Ma), comprising the western part of the batholith. On the basis of similarity between these rocks and granitoides of modern active continental margins in material composition, it is assumed that they formed throughout the island arc-continental stage of development of the Ural Mobile Belt. Granitoides of the third age group, dominating in the Verkhisetsk batholith, formed as a result of several homodrome rhythms of granodiorite-granite intrusions of moderate-potassium composition during a short period of time (315–300 Ma). Their formation is related to the initial stage of the collision stage of development of the region, lasting from the early Bashkirian to Late Permian in the Middle Urals, which is fixed by deposition of flysch and molasse sediments in the Ural Foredeep. The data obtained change our understanding significantly of the character of evolution of granitoid magmatism and the place of rock assemblages studied in the geological history of the Urals.

Geodynamic conditions of formation of granitoids of the Verkhisetsk batholith (Eastern Slope of the Middle Urals)

684 The Verkhisetsk batholith (the Verkhisetsk massif), which occupies a vast area (80 to 30 km) in the vicinity of the city of Yekaterinburg (the upper reaches of the Neiva and Iset rivers), is one of the largest and most structurally complicated granitoid massifs of the Urals. Within this massif a number of rock associations varying in the chemical composition, relative age, and level of deformation were distinguished. Due to this, the Verkhisetsk batholith is of great importance for studying the evolution of orogenic granitoid magma tism in the Urals and the geodynamic conditions of their formation.

The First Zircon U-Pb Dating (SHRIMP II) for the Silurian Ophiolites in the Urals

The rocks of the ophiolite association are wide� spread within the Ural mobile belt. For this reason dating of their formation is of great interest due to its tectonic significance, allowing interpretation of the history of geological development of the region. For a long time, the geochronological dating of ophiolite rocks has been limited to the search for faunal remains in interlayers of sedimentary rocks among the volca� nites of the upper part of the ophiolite section. Over the last 20 years, in connection with enhancement of the analytical possibilities, a set of isotope data for rocks of the ophiolite association has been obtained (mainly from the Polar segment of the Urals) [1–5 etc.]. The age data available indicate that the forma� tion of the Uralian ophiolites is connected mainly with two separate time intervals on the geochronological age scale. Older rocks (604–490 Ma) formed during the Late Vendian–Cambrian; the younger rocks (410–370 Ma) formed during the Devonian. Single intermediate dating (Ordovician and Silurian), dated by the Ar–Ar method [6, 7], is not regarded as conclu� sive evidence, so it lies in a very wide time interval (70–90 Ma for the same geological object, dated by the Ar–Ar method). At the same time, according to the present geological evidence, the predominant part of ophiolites on the eastern slope of the Urals formed between the Late Ordovician and the Silurian. Based on the zircon U–Pb age for ophiolite gabbro from the eastern zone of the Middle Urals using the SHRIMP II ion microprobe (Center for Isotope Research, VSEGEI), good evidence of the occurrence of Sil� urian ophiolites in the Urals was obtained for the first time. The Eastern zone of the Urals is known as a zone made of essentially Middle Paleozoic volcanogenic formations and comagmatic intrusive bodies, which extends along the eastern border of the Trans Ural region, eastward of the zone of granite batholiths (the main granite axis of the Urals) [8 etc.]. The ophiolite rocks, which are widespread within the zone, are rep� resented by dunite–harzburgite massifs, layered ultra� basite–gabbro complexes, and parallel dolerite dykes

Zircon geochronology of mantle ultramafics from the Klyuchevskoi massif of the Urals

dunite–wehrlite–clinopyroxenite–gabbro com� plex constitutes several tectonic blocks in the southern and western parts of the massif. The rocks are dis� tinctly layered. The lower part of the section repre� sented by variably serpentinized dunites with bodies of syngenetic disseminated and subordinate massive chromite ores is exposed in the southeastern part of the massif. They are overlain by a rhythmically banded sequence represented by alternating dunites, wehr� lites, and olivine pyroxenites with the gradual disap� pearance of the first two varieties in the upper part, which is entirely composed of clinopyroxenites. Higher, the last rocks give way to amphibolized gab� bro. The most compete section of this complex is observed along the Sysert and Iset rivers upstream of their confluence. The U–Pb zircon dating revealed that both dunites and olivine clinopyroxenites of the layered complex are 441–449 Ma old, i.e., coeval within the error limits [5].

Sr, Nd, and Hf Isotope Composition of Rocks of the Reft Gabbro–Diorite–Tonalite Complex (Eastern Slope of the Middle Urals): Petrological and Geological Implications

This paper reports Rb–Sr and Sm–Nd isotope data on the gabbro–diorite–tonalite rock association of the Reft massif (eastern margin of the Middle Urals) and Lu–Hf isotope data on zircon populations from these rocks. In terms of Nd and Hf isotope composition, the rocks of the studied association are subdivided into two distinctly different groups. The first group consists of gabbros and diorites, as well as plagioclase granites from thin dikes and veins cutting across the gabbros. In terms of 43Nd/144Ndi = 0.512518–0.512573 (εNd(T) = +8.6...+9.7) and 176Hf/177Hfi = 0.282961–0.283019 (εHf(T) = +15.9...+17.9), these rocks are practically identical to depleted mantle. Their Nd and Hf model ages show wide variations, but in general are close to their crystallization time. The second group is represented by tonalites and quartz diorites, which compose a large body occupying over half of the massif area. These rocks are characterized by the lower values of 143Nd/144Ndi = 0.512265–0.512388 (εNd(T) = +3.7...+6.0) and 176Hf/177Hfi = 0.282826–0.282870 (εHf(T) = +11.1...+12.7). The TDM values of the second group are much (two–three times) higher than their geological age (crystallization time), which indicates sufficiently long crustal residence time of their source. The initial 87Sr/86Sr in the rocks of both the groups varies from 0.70348 to 0.70495. This is likely explained by the different saturation of melts with fluid enriched in radiogenic Sr. The source of this fluid could be seawater that was buried in a subduction zone with oceanic sediments and released during slab dehydration. Obtained data make it possible to conclude that the formation of the studied gabbro–diorite–tonalite association is a result of spatially and temporally close magma formation processes in the crust and mantle, with insignificant contribution of differentiation of mantle basite magma.

A Large Paleoseismodislocation in the Southeastern Part of the Cherskii Seismic Belt, Northern Priokhotye

The structure of a large paleoseismodislocation located on the southeastern flank of the Cherskii seismic belt, in the upper reaches of the Ola River, 125 km NNE of Magadan, is studied. Detailed morphologic, morphometric, and structural descriptions of this paleoseismodislocation are made. The main parameters of the landslide body and the dammed lake formed by it are determined. The correlating deposits formed in the dammed lake and upon the surface of rupture are studied, and their radiocarbon age is determined.

First data on Early Carboniferous intrusive magmatism of the eastern margin of the Middle Urals: Geodynamic conditions and U–Pb isotope constraints

U–Pb LA ICP–MS dating of zircon from rocks of the Nekrasov gabbro–granitoid complex within the eastern margin of the Middle Urals was performed. The average U–Pb age calculated from three concordant measurements (326 ± 8 Ma) shows that their intrusion occurred at the Serpukhov Stage of the Early Carboniferous. According to the ideas on periodization of magmatic processes within the eastern sector of the Middle Urals, the formation of this complex corresponds to the final episodes of the continental marginal (supersubduction) magmatism.

The first Lu–Hf zircon isotope data for gabbro–diorite–tonalite associations of the Urals

The Lu–Hf isotope systematics of zircon from the gabbro–plagiogranite association (gabbro, diorite, tonalite, and plagiogranite), which is one of the most typical associations of igneous rocks in the Urals, was studied for the first time. The isotope study yielded a unified age limit of 433 Ma, which corresponds to the time of formation of this rock association. The younger “rejuvenated” ages characterize superimposed thermal impact events, induced by the volcanic arc activity, as well as collisional and postcollisional processes. Here, the initial 176Hf/177Hf(t) ratio in the studied zircon from gabbro and plagiogranite corresponds in fact to a highly LILE-depleted (DM) mantle.

The Sr, Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in εNd(T) = 8.6–9.7 and εHf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: εNd(T) = 3.7–6.0, εHf(T) = 11.1–12.7, and TDM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary 87Sr/86Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.