A. A. Krasnobaev

Geodynamic settings and history of the Paleozoic intrusive magmatism of the central and southern Urals: Results of zircon dating

The main stages of the Paleozoic intrusive magmatism in the Urals, 460–420, 415–395, 365–355, 345–330, 320–315, and 290–250 Ma, as well as two virtually amagmatic periods, 375–365 Ma (Frasnian-early Famennian) and 315–300 Ma (Late Carboniferous), are recognized. The Cambrian-Early Ordovician pause predated the onset of igneous activity in the Ural Orogen, while the Early Triassic pause followed by an outburst of trap magmatism postdated this activity. The interval from 460 to 420 Ma is characterized by mantle magma sources that produced ultramafic and mafic primary melts. The dunite-clinopyroxenite-gabbro association of the Platinum Belt and miaskite-carbonatite association are specific derivatives of these melts. The rift-related (?) Tagil Synform functioned at that time. The volcanic-plutonic magmatism in this oldest magmatic zone of the Uralides comprises gabbro, gabbro-granitoid, and gabbro-syenite series and comagmatic volcanic rocks. After a break almost 20 Ma long, this magmatism ended in the Early Devonian (405–400 Ma) with the formation of small K-Na gabbro-granitoid plutons. The magmatic intervals of 415–395, 365–355, and 320–315 Ma are characterized by the mantle-crustal nature. The first interval accompanied obduction of the oceanic lithosphere on the continental crust. The subsequent magmatic episodes presumably were related to the subduction of the island-arc (?) lithosphere beneath the continent and to the collision. The intense granitoid magmatism started 365–355 Ma ago. As in the following interval 320–315 Ma, the tonalite-granodiorite complexes, accompanied by hydrous basic magmatism, were formed. Amphibole gabbro and diorite served as a source of heat and material for the predominant tonalite and granodiorite. The Permian granitic magmatism had crustal sources. Thus, the mantle-derived Ordovician-Middle Devonian magmatism gave way to the mantle-crustal Late Devonian-Early Carboniferous plutonic complexes, while the latter were followed by the crustal Permian granites. This sequence was disturbed by rifting and formation of continental arcs accompanied by specific Early Carboniferous Magnitogorsk gabbro-granitoid series and Early Permian Stepnoe monzodiorite-granite series, which deviate from the general evolutional trend.

Zirconology of Navysh volcanic rocks of the Ai Suite and the problem of the age of the Lower Riphean boundary in the Southern Urals

185 Volcanic rocks of the Navysh Complex occur in the base of the Lower Riphean Ai Formation of the Burzyan Series within the Bashkirian meganticlinorium (South ern Urals). They represent practically the only object suitable by the material–geological and geochronolog ical criteria for estimation of the age of the lower bound ary of not only the Ai Formation, but the whole strato typical Riphean section of the Southern Urals. Trachybasalts prevail in the composition of Navysh volcanic rocks; vein and subvolcanic bodies are observed as well. The peculiarities of structure, com position, metasomatic and dynamic alterations of vol canogenic and sedimentary rocks of the Burzyan Series are considered in detail in [1–3]. Complex application of K–Ar, Rb–Sr, and U–Pb methods demonstrated that the age of volcanic rocks of the Navysh Complex is 1615 ± 45 Ma; the age of their transformations resulting from bostonization is 630 ± 60 Ma. Furthermore, the mineralogical and geochemical properties of zircons, the presence of dif ferent inclusions and melt inclusions among them, the existence of successive generations, the participation of corrosion and crushing, the redistribution of impu rities, and the influence of metamict nature were described in detail in [2]. In addition to significant variations of the age isotope ratios for most zircons (from 670 to 1490 Ma), fractions with concordant ages of 1350 ± 15 Ma were distinguished. For many years this dating of Navysh volcanic rocks was used for substantiation of the lower age boundary of the Riphean in different stratigraphic schemes including the stratigraphic scheme of the Russian Precambrian [4] and the scale of geological time suggested by W.B. Harland et al. [5] at the level of 1650 Ma. Development of modern methods of isotope investigations provided the possibility to specify the age “content” of the dating obtained and to estimate its agreement with the geological information on the age boundaries of Riphean sections of the Southern Urals more precisely. Zircons from the new samples of volcanic rocks of the Navysh Complex were studied by the SHRIMP methodology [6]. The results were dis couraging. In two samples from the Ushat Brook (Fig. 1), zircons from basaltoids, which were included in the Navysh volcanogenic complex by all previous researchers, were dated as 441.8 ± 8.2 and 437 ± 11 Ma; this level was obtained in other areas of the Bashkirian meganticlinorium [7]. Ages of zircons from 11 samples ranged within 1350–1400 Ma provid ing evidence for the Mashak (Middle Riphean) level of their formation. This result not only complicated the task, but contradicted the entire amount of accumu lated knowledge as well. The studies were continued due to the newly obtained data of the structure of crys tals reflecting their polygenetic nature. This conclu sion was supported by both the structural and age peculiarities of crystals (Fig. 2) extracted from fine grained porphyric andesite occurring on the left bank of the Malyi Navysh River at the base of the NW slope of the Efremov Mountains (Sample 1392, Fig. 1).

Isotopic-geochemical features and age of zircons in dunites of the platinum-bearing type Uralian massifs: Petrogenetic implications

This paper presents results of isotopic (Cameca IMS1270 NORDSIM and SHRIMP-II ion microprobes) and geochemical (LA-ICP MS) study of zircons in three dunite samples of the Uralian-Alaskan-type massifs of the Urals: Kosva, Sakharin, and Eastern Khabarny. The zircons in the dunites share common features. Each sample contains the following genetic and age groups of zircons: (1) xenogenic zircons of the Archean and Proterozoic age; (2) zircons of magmatic appearance, which in age and geochemistry are close to the zircons from associated gabbroids; (3) postmagmatic zircons that presumably crystallized from hydrothermal solutions. The xenogenic zircons of the Archean age in each of three samples comprise transparent fragments, which are depleted in U and other trace elements and presumably have mantle origin. Xenogenic zircons of the Proterozoic age (1500–2000 Ma) occur as oval grains with surface abrasion, the traces of their redeposition. The geochemical features of the xenogenic zircons unequivocally demonstrate their affiliation to the continental crust—the basement of the Uralian orogen.The zircons of magmatic habit in all the dunite samples are close in age to the associated gabbroids: 435–432 Ma in the Kosva Massif, 378–374 in the Sakharin Massif, and 407–402 Ma in the Eastern Khabarny Massif, and mark the age of dunite formation. In addition, the magmatic zircons from dunites and associated gabbroids share similar geochemical features. These data could serve as additional argument in support of cumulate origin of dunites in the Uralian-Alaskan-type complexes. The postmagmatic zircons are most enriched in trace elements and were presumably formed from a fluid phase, which was responsible for the recrystallization of dunites and redistribution of Cr-spinel and PGE mineralization.

The akhmerovo granite massif: A proxy of mesoproterozoic intrusive magmatism in the Southern Urals

U‐Pb (SIMS SHRIMP II) zircon dating of granites of the Akhmerovo granite massif, which is located in the central part of the Beloretsk metamorphosed complex (southern Ural), defines reliably its Lower Riphean (Mesoproterozoic) age. Based on the zircon geochronometer, the duration of the crystallization of granite massifs, such as the Akhmerovo granite massif, can reach 30 Ma.

Zircon geochronology of the Mashak volcanic rocks and the problem of the age of the lower-middle Riphean boundary (Southern Urals)

In the type sections of the Riphean within the Bashkirian mega-anticlinorium (Southern Urals), the Mashak Formation represents a basal unit of the Middle Riphean erathem. The formation comprises throughout its area of distribution the alternation of volcanic, volcano-sedimentary, and sedimentary sequences and is divided into the lower, middle, and upper subformations. The volcanic rocks containing zircons (four samples, rhyodacite and rhyolite collected at Mashak, Berezyak, and Bolshoi Shatak ranges) are largely confined to the lower subformation. Analyses were performed using a SHRIMP II methodology, with special attention to the mineralogical characteristics of zircons, including their habit, morphology, preservation, and inclusions. All zircons show similarities in their mineral chemistry and geochemistry, which are indicative of the geochemical affinity of the volcanic rocks. At the same time, all zircon grains are characterized by specific typological parameters, which may equally reflect the parameters involved in the development of such volcanic rocks under different conditions. The integrated U-Pb age of zircons (SHRIMP II, VSEGEI, St. Petersbrug) from the four samples is 1383 ± 3 Ma. On the basis of the age of the Berdyaush gabbro-granitoid intrusion (up to 1410 Ma), the most likely age of this boundary is 1400 Ma, which is equated to the Calymmian and Ectasian of the International Stratigraphic Scale.

The polygenous-polychronous nature of zircons and the problem of the age of the Barangulov gabbro-granite complex

The first precision U‐Pb (SIMS SHRIMP II) zircon datings of gabbros ( 728 ± 8 Ma) and granites ( 723 ± 10 Ma) have been reported for the Barangulov gabbro‐ granite complex located in the northern Uraltau zone of the southern Urals. The temporal evolution of the magma chamber has been traced based on zircons. The available data suggest that gabbros and granites of the study region are dated at 725 ± 5 Ma. Based on these data, metasedimentary rocks of the Mazarin and Arvyak formations, which host the Barangulov gabbro‐granite complex, are older than 725 Ma. This estimate supports the correlation of metamorphic rocks of the Mazarin and Arvyak formations of the northern Uraltau zone with rocks of the Karatau Group in Riphean type sections of the Bashkirian anticlinorium, southern Urals. The Barangulov Complex (BC) of the closely inter

Stability of zircon in dunite at 1400–1550°C

The results of experimental studies of interaction between zircon crystals and dunite at 1400–1550°C are given. At 1400°C no interaction of zircon with dunite takes place. At higher temperatures zircon interacts with olivine forming eutectoid mixture of baddeleyite and pyroxene grains. Change of free energy at interaction is close to zero and process is reversible. At 1550°C partial melting of dunite occurs, but character of interaction with dunite remains the same. Experiments results verify phase relationships in liquidus area at diagram. The obtained results allow explaining the ancient age of zircons in ultrabasite of folded areas.

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].

Isotope geochemistry and geochronology of the gabbro of the Volkovsky Massif, Urals

Abstract40Ar-39Ar, Sm-Nd, U-Pb, and Lu-Hf isotope data are reported on the gabbro of the Volkovsky Massif, the only massif of the Uralian Platinum Belt wherein economic copper-iron-vanadium and high-grade gold-palladium mineralization is present. The massif is made up of gabbro blocks with concentrically zoned structure and diorite intrusions in its core. In the northeast and southwest, the gabbro is cut by syenite of the Kushva Massif. Gabbro blocks mainly consist of the olivine-anorthite gabbro, while labradorite two- pyroxene gabbro intersects both olivine-anorthite gabbro and Ti-magnetite and copper-PGE mineralization developed in them. The study of both gabbro types by Sm-Nd isochron and U-Pb (SHRIMP II) zircon methods with subsequent REE and Lu-Hf isotope analysis of zircon made it possible to date reliably (428 ± 7 Ma (SHRIMP) and 436 ± 21 Ma (Sm-Nd)) postore labradorite gabbro and, correspondingly, the upper age limit of the mineralization of the Volkovsky Massif. Ore-bearing olivine-anorthite gabbro contain four different-age zircon populations: 2682 ± 37–972 ± 18 Ma, 655 ± 15 to 565 ± 9 Ma; 450 ± 12 Ma, and 343 ± 8 Ma. Hf-Nd isotope systematics showed that zircon with an age of 450 ± 12 Ma presumably marks the formation age of the rocks, the older zircon was trapped, while zircon with an age of 343 ± 8 Ma was formed during low-temperature transformation of the rock and sometimes contains excess radiogenic Hf. Proterozoic xenogenic zircon was inherited from diverse rocks of ancient crust, while the oldest grain with an age of 2065 Ma was possibly formed in a deep mantle source. Vendian zircon was presumably also entrapped, and its morphology and geochemistry point to the crystallization from a basaltic melt. The abundance of pre-Paleozoic zircon in the olivine-anorthite gabbro suggests significant contribution of ancient material in their petrogenesis. This material could serve as source of ore components (metals and sulfur) for unique copper-sulfide gold-PGE mineralization of the Volkovsky Massif.

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.