A. I. Rusin

Zirconology of ultrabasic rocks of the Buldym massif (Il’meno-Vishnevogorskii Complex, Southern Urals)

235 The Buldym massif (BM) of ultrabasic rocks occurs among metamorphic rocks of the Precambrian Vishnevogorskaya Suite (Fig. 1) in the northern part of the Il’meno–Vishnevogorskii Complex (IVC). The wide popularity of the massive is associated with the presence of pyrochlore carbonatite and phlogopite– vermiculite mineralization located in the sublatitudi nal zones of tectonic dislocations. Prefold, most likely Archean–Proterozoic ultrabasic rocks include regen erated olivine, enstatite–olivine rocks, and secondary lizardite serpentinite. Although the structural–mate rial peculiarities and geological setting of the massif are well known [1, 2], geochronological data on the ultrabasic rocks are almost absent. The solution of age problems is favored by SHRIMP dating [3] of zircons from ultrabasic rocks (Tables 1, 2) collected in differ ent parts of the massif (Fig. 1). The chemical compo sition of the studied ultthat reported previously [1].

Zircons, zircon geochronology, and petrogenetic problems of the lherzolite massifs of the South Urals

The paper reports the results of mineralogical and isotope-geochronological study of zircons from the Uzyansky Kraka (UK) Massif, which represents the part of the large (more than 900 km2) lherzolite allochthon thrusted onto the Paleozoic sequences of the East European margin. The massif shows a distinct stratification (from the top downward): spinel lherzolites, garnet pyroxenites, and dunites. The formation of stratified section is considered to be related to the decompression uplift of mantle lherzolite block. Zircons from the massif rocks were dated using SHRIMP-II ion microprobe. The oldest relict datings characterizing endogenous transformations of protolith were established in the zircons from the lherzolites (2037 ± 20 and 1132 ± 6 Ma), garnet pyroxenites (953 ± 11 Ma), and dunites (632 ± 11 Ma). All rock associations contain zircons with ages within 590–550 and 445–390 Ma, which mark the stages of mantle stratification of lherzolite block into complementary series and their emplacement at the upper crustal level. Age values within 299–196 Ma were found only in the dunites and date the influence of the Paleozoic strike-slipping. Our studies led us to conclude that the modern structure of the Ural collision orogen contains the fragments of subcontinental lithosphere, which were previously described only for the massifs of the root zones of the Western and Central Europe. Some general petrogenetic questions of lherzolite massifs from orogenic regions are discussed.

UHPM eclogite of the Maksyutov complex (Southern Urals)

This study reports the first find of a jadeite-bearing eclogite spatially associated with UHP ultramafic rocks of unit 1 of the Maksyutov complex (near the village of Karayanovo). Two phases of elcogitization were recognized based on mineralogical data, petrographic observations, and isotope geochronology. The first UHP metamorphic stage (533 ± 4.6 Ma, P > 4.4 GPa, T > 700°C) was defined by the assemblage jadeite + grossular-almandine + rutile ± phengite. This assemblage was later transformed into omphacite + grossular-almandine + phengite + albite + clinozoisite + titanite at a retrograde phase of stage I (392–485 ± 2–4 Ma, P > 3.1–3.4 GPa, T > 633–740°C) with decreasing pressure and temperature. The second prograde phase (360 ± 3 Ma, P > 1.1–2.2 GPa, T > 450–550°C) of HP metamorphism was marked by the development of a chlorite rim (almandine-grossular—pyrope-almandine-grossular, diopside, clinozoisite) around the eclogite body. A comparison shows that the studied eclogite and ultramafic rocks followed a common P-T-t path. Jadiet-bearing eclogites and meta-ultramafic rocks represent UHPM fragments of the mantle and crust, which were tectonically emplaced in quartzite schists of unit 1.

Zirconology of dunite from the sarany chromite-bearing ultramafic complex (Middle Urals)

737 The Sarany ultramafic complex including the Northern (NM) and Southern (SM) massifs is located on the western slope of the Middle Urals among poorly metamorphosed Vendian rocks (Fig. 1). Despite the small sizes, these massifs contain unique deposits of chromite ores related to the layered series of ultramafic rocks [1–3]. The massifs are character ized by a lens like shape, meridional orientation, steep dipping, and strong tectonic violation [4]. Widely abundant hydrothermal formations are represented by thin veins of carbonate, uvarovite, and chlorite com positions. Ultramafic rocks of massifs, especially those of the Northern massif are intruded by numerous dykes of metadiabase, metapicrite, alkaline, and sub alkaline diabases. The eastern surrounding of the mas sifs is characterized by the sill like intrusion of gabbro anorthosite, which is considered an individual intru sive phase [1, 2] or an element of macrolayering [3].

Isotope geochemical characteristics of zircon from dunite, clinopyroxenite, and gabbro of the Uralian Platinum Belt

513 Dunite, clinopyroxenite, and olivine gabbro of the Uralian Platinum Belt (UPB) are united in a single complex, the origin of which and geological relation ships between rocks of which have been a widely debated topic until now. This also regards the age data obtained by different isotope methods, which range from the Archean to the Carboniferous [1, 4–7]. To confirm the geochronological interpretation of the results of U–Pb dating of zircon (SHRIMP) from dunite and clinopyroxenite of the Nizhnii Tagil and gabbro of the Volkovskii massifs in the UPB (Fig. 1), we performed comparative study of its isotope geochemical (REE and Lu–Hf) peculiarities.

Zirconology of lherzolites in the Nurali Massif

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445–448 and 439–440 Ma) wass the time of lherzolite magmatism of 10–15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro–diorite intrusions crowning in the Nurali Massif.

Nature of zircon in lherzolites (Uzyansky Kraka massif, Southern Urals)

Uzyansky Kraka massif (UK) is located 40 km to the southwest of the town of Beloretsk near the village of Uzyan and is represented by a separated fragment (50 km 2 ) of the large Krakinsky lherzolite allochthone overthrusted on the Paleozoic volcanogenic‐sedimentary strata of the Eastern European continental passive margin [1]. As was established by detailed investiga

Zirconology of ultramafic rocks from the Vostochnotagilskii massif (Middle Urals)

441 The Vostochnotagilskii massif (VTM; also known as Krasnoural’skii) is one of the largest in the Middle and North Urals [1]. This massif occurs in the merid ian direction for >300 km (Fig. 1, the southern part of massif) and controls the Serovsko–Maukskii ophiolite complex separating the Tagil and Magnitogorsk paleo island arc zones. The massif is composed of dunite and harzburgite serpentinized to various degrees. Chrysotile serpentinite prevails in the central part intruded by gabbro diabase and granitoid dykes, whereas antigorite serpentinite predominates in the marginal zones. The exocontact zones of dykes are characterized by bimetasomatic zoning with the for mation of vermiculite, chlorite–actinolite, talc, and talc–carbonate rocks. The freshest rock varieties are preserved in the central part of massif. Poor exposure does not provide unambiguous evidence for relation ships between dunite and harzburgite; according to the composition of chrome spinellids, ultramafic rocks form a continuous series [2]. At the same time, transi tional varieties between dunite and harzburgite were not found in the largest Kutuzovskii mine located at the latitude of Krasnoural’sk.

Zirconology of ultrabasic rocks of the Karabash massif (Southern Urals)

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530–560 Ma) and Early–Late Ordovician (440–480 Ma).