O. V. Udoratina

Baltica in the Cordillera

U-Pb ages of detrital zircon suites from Paleozoic strata in the Arctic Alaska–Chukotka terrane (AAC), Alexander terrane, northern Sierra terrane, and eastern Klamath terrane of the North American Cordillera suggest an exotic Gondwana or Baltic origin. We evaluate these hypotheses with U-Pb ages of detrital zircon suites from Cambrian–Devonian strata of northern Baltica. Precambrian zircon populations (ca. 0.8–3.0 Ga) from Baltica compare well with similar age detritus in the AAC and Cordilleran terranes, but the amount and age of younger Neoproterozoic and Ordovician–Silurian components are variable. The AAC shares its stratigraphy with Baltica and has the most similar detrital zircon suites. Closing the Arctic places the AAC against the Lomonosov Ridge and the edge of Baltica in pre-Cretaceous time. After the Caledonian orogeny and before the Ural Mountains formed, the Baltica, AAC, and Cordilleran margins shared a Devonian–Carboniferous rift history and became along-strike portions of a Carboniferous–Permian continental margin. This rifting event might have been responsible for the initial separation of Baltica and Caledonian affinity terranes from this margin.

The Zr/Hf ratio as a fractionation indicator of rare-metal granites

The Zr-Hf geochemical indicator, i.e., the Zr/Hf ratio (in wt %) in granitic rocks is proposed to be used as the most reliable indicator of the fractionation and ore potential of rare-metal granites. It was empirically determined that the fractional crystallization of granitic magma according to the scheme granodiorite → biotite granite → leucogranite → Li-F granite is associated with a decrease in the Zr/Hf ratio of the granites. The reason for this is the stronger affinity of Hf than Zr to granitic melt. This was confirmed by experiments on Zr and Hf distribution between granitic melt and crystals of Hf-bearing zircon (T = 800°C, P= 1 kbar). The application of the Zr/Hf indicator was tested at three classic territories of rare-metal granites: eastern Transbaikalia, central Kazakhstan, and the Erzgebirge in the Czech Republic and Germany. The reference Kukul’bei complex of rare-metal granites in eastern Transbaikalia (J3) is characterized by a uniquely high degree of fractionation of the parental granitic melt, with the granites and their vein derivatives forming three intrusive phases. The biotite granites of phase 1 are barren, the leucogranites of phase 2 are accompanied by greisen Sn-W mineral deposits (Spokoininskoe and others), and the final dome-shaped stocks of amazonite Li-F granites of phase 3 host (in their upper parts) Ta deposits of the “apogranite” type: Orlovka, Etyka, and Achikan. The Kukul’bei Complex includes also dikes of ongonites, elvanes, amazonite granites, and miarolitic pegmatites. All granitic rocks of the complex are roughly coeval and have an age of 142±0.6 Ma. The Zr/Hf ratio of the rocks systematically decreases from intrusive phase 1 (40–25) to phases 2 (20–30) and 3 (10–2). Compared to other granite series, the granites of the Kukul’bei Complex are enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but are depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From earlier to later intrusive phases, the rocks become progressively more strongly enriched or depleted in these elements, and their Zr/Hf ratio systematically decreases from 40 to 2. This ratio serves as a reliable indicator of genetic links, degree of fractionation, and rare-metal potential of granites. Greisen Sn, W, Mo, and Be deposits are expected to accompany granites with Zr/Hf < 25, whereas granites related to Ta deposits should have Zr/Hf < 5.

Age constraints for the Pre-Uralide-Timanide orogenic event inferred from the study of detrital zircons

The PreUralide-Timanide orogeny is the oldest collision event, which is more or less reliably recorded in the presentday Arctic region (9, 10). This collision was determined by the convergence at the Vendian- Cambrian transition (or in the initial Cambrian) between the Timan-Waranger margin of the Baltica continent, an ancient framework of the East European Platform and the Bolshezemlskaya margin of Arc� tida. The PreUralide-Timanide orogeny was inferred from the study of PreUralide inliers among the Uralide complexes (2, 10, and references therein), the distribution of detrital zircons of corresponding ages in different Arctic areas (1, 2, 13, and references therein), and mostly the analysis of granitoid associa� tions in the northern areas of the Urals and basement of the Pechora basin (6, 8, 9, and references therein). It is known that northern segments of the western Urals and basement of the Pechora basin are intruded by Late Precambrian and Cambrian granites with ages from ~730 to 510 Ma (Fig. 1) (2, 6, 8, 9, and others). By their composition, these granites may be attributed to both subductionrelated and collisional types (6, 9). The ambiguous nature of these rocks hampers limiting the period when subductionrelated tectonic environ� ments gave way to continental collision settings. This means that granites that reflect convergence environ� ments cannot serve as a tool for an accurate age deter� mination for the onset of collision between Baltica and Arctida. This problem requires other independent tools. 1 We suggest to use erosional products of the Pre� Uralide-Timanide orogen that are represented by Upper Precambrian-Lower Paleozoic detrital rocks developed in the northern part of the East European Platform as one such tool. This approach is based on the concept that the composition of allothigenic min� erals constituting detrital rocks as well as their isotopic compositions and ages reflect the corresponding parameters of provenances, i.e., domains that yielded eroded material for detrital rocks. By analogy with recent collision orogens (Alps, Himalayas, and others), we assume that the Pre� Uralide-Timanide orogen represented a mountainous folded structure, which was subjected to intense ero�

The first results of U/Pb dating and isotope geochemical studies of detrital zircons from the neoproterozoic sandstones of the Southern Timan (Djejim-Parma Hill)

This report presents the first results of U/Pb dating, isotope-geochemical, and geochemical studies of detrital zircons from the Neoproterozoic clastic rocks of the Southern Timan. Sixty-one zircon grains were treated, including 51 from red-colored sandstones and 10 grains from aleurosandstones of the Djejim Formation of the southern Chetlas-Djejim zone (Djejim-Parma Hill). It was found that the U/Pb-ages of zircons from the rocks of the Djejim Formation, varied from ∼2.97 to ∼1.20 Ga. The studies of microelement composition in 47 grains (of 61 U/Pb isotope ages obtained), on the basis of several empirical regularities found formerly, show that the detrital zircons had originated from “granites” (22 grains), “diorites” (12 grains), or their volcanic analogues, or more rarely, from “syenites” and “basites” (5 and 8 grains, respectively). The Lu/Hf isotope system of zircons allows one to estimate the model ages (TDMC) of the substrate magmatic rocks being parental to the zircons considered. In particular, Archean zircons are characterized by ∼2.84–3.36 Ga model ages of magmaforming rocks. For some of the grains, their model ages (∼2.84 Ga) are close to those of zircons as such (∼2.7–2.8 Ga), which points to the juvenile character of the substrate from which the parent magma of the zircons treated was fused. For Proterozoic (to Middle Riphean) zircons, the Lu/Hf isotope system allows one to estimate the model age of the substrate of their parental rocks within ∼2.00–3.36 Ga, which shows that these rocks were formed under the recycling of the Archean and Early-Proterozoic crust. The ages obtained for detrital zircons, as well as model ages of the substrate of the corresponding parental magmatic rocks, are quite comparable to the age of crystalline complexes of the ancient framework of the East European Platform (EEP), formed in the course of the Archean, Early-Proterozoic, and Early-Middle Riphean tectonomagmatic events. This permits us to conclude that the Neoproterozoic detrital complexes of the Timan were formed owing to the erosion of earlier Neoproterozoic and Early Precambrian complexes constituting the Neoproterozoic Baltica continent, presenting complexes of the passive margin of this continent. A variety of ages of detrital zircons from sandstones and aleurosandstones from the Djejim Formation of Djejim-Parma Hill, and of the estimates of magmatic rocks parental to these zircons, may be characterized as a Baltic Provenance signal.

First results of U/Pb dating of detrital zircons from Early Paleozoic and Devonian sandstones of the Baltic-Ladoga region (south Ladoga Area)

The first results of U/Pb isotopic dating (LA ICP MS) of detrital zircons from sands from the Middle Cambrian Sablinka Formation, Upper Cambrian Ladoga Formation, Low Ordovician Tosna Formation, and calcareous sands from Syas’ Formation (Sargaevskii horizon of the Upper Frasnian) from Baltica-Ladoga Glint (BLG) of the Southern Ladoga area are presented. The obtained ages of detrital zircons span the intervals 492.7 ± 5.1-3196.4 ± 5.1 Ma (Sablino Formation); 577.9 ± 7–2972.6 ± 13.4 Ma (Ladoga Formation); 509.4 ± 8.5–3247.6 ± 10.1 Ma (Tosna Formation); 451.1 ± 14.7–2442.2 ± 6.9 Ma (Syas’ Formation). A comparison of the obtained isotopic ages of detrital zircons to ages of crystalline complexes composing the Kola-Karelian, Svecofennian, and Sveconorwegian domains of Baltic Shield and Pre-Uralian-Timanian structures of Subpolar and Polar Urals and basement of Pechora Basin was carried out. It is proposed that the Middle Paleozoic sedimentary basin accumulated Upper Frasnian rocks of Syas’ Formation. The basin ranged northward from the present-day BLG and occupied the eastern part of the Baltic Shield.

First results of U-Pb dating of detrital zircons from basal horizons of Uralides (Polar Urals)

The most modern and the most selfconsistentideas about the structure of the Uralian thrustfold beltare reported in [1]. This book develops insights byN.P. Kheraskov [2] who believed that there are two agegroups whose units are widespread in the Urals; theyare the Uralides and the ProtoUralides (below, PreUralides).

First Results of Isotopic Dating of Detrital Zircons from the Clastic Rocks of the Pre-Uralides-Timanides Complexes: Contribution in the Late Precambrian Stratigraphy of the Enganepe Uplift, Western Polar Urals

The upper horizons of the Earth’s crust in the eastern and northeastern framing of the East European platform consist of two units. The upper unit is made up of Late Precambrian and younger (mainly sedimentary) complexes; the lower unit consists of Late Precambrian complexes termed in total as Pre-Uralides‐Timanides [1, 2]. The Pre-Uralides‐Timanides form uplifts and anticlinoriums extending as an uninterrupted chain along the western slope of the Urals. They are also exposed at Timan and the Kanin Nos peninsula. Age analogs of the Pre-Uralides‐Timanides complexes occur as fragments on Kildin Island, in the Rybachii, Srednii, and Varanger peninsulas, on Paikhoi (Amderma region) and Vaigach Island, and in the southern part of the Novaya Zemlya archipelago. These complexes were recovered by several boreholes in the Pechora plate and, according to geophysical data, are traceable in the Barents Sea shelf [2, 3].

Zr/Hf ratio as an indicator of fractionation of rare-metal granites by the example of the Kukulbei complex, eastern Transbaikalia

The concept of granitic melt fractionation as the main process in the concentration of rare elements in granites calls for the development of a reliable method to determine the evolutionary sequences of granite series. We propose to use for this purpose a zirconium-hafnium indicator, the Zr/Hf weight ratio in granitic rocks (Zaraisky et al., 1999, 2000). By the example of three classic regions of rare-metal deposits, eastern Transbaikalia, central Kazakhstan, and Erzgebirge (Czech Republic and Germany), it was empirically shown that the Zr/Hf ratio of granites decreases during the fractional crystallization of granite magmas in the sequence granodiorite → biotite granite → leucogranite → lithium-fluorine granite. The reason is the higher affinity of Hf compared with Zr to a granite melt. This implies that the crystallization and settling of accessory zircon will cause the progressive enrichment of Hf relative to Zr in the residual melt. As a result, the Zr/Hf ratio decreases regularly in the series of sequential phases of granite intrusion related to a single magma chamber from granodiorite to biotite granite, leucogranite, and Li-F granite (from 45-30 to 10-2). Our experimental investigations supported the preferential enrichment of haplogranite melt in Hf and zircon crystals in equilibrium with melt in Zr (T= 800°C and P = 1 kbar). The Zr/Hf indicator was tested by the example of the wellknown Kukulbei rare-metal granite complex of eastern Transbaikalia (J3), which is unique in the degree of fractionation of initial granite melt with the formation of three phases of granite emplacement and vein derivatives. An important feature of the complex is its “short” differentiation trend. It was supposed that the granite magma of the first phase is parental, and the later phases forming small intrusive bodies in large massifs of biotite granites of the first phase are sequential products of its crystallization differentiation in a magma chamber. The biotite granites of the first phase are barren. The leucocratic granites of the second phase are accompanied by tin-tungsten greisen deposits (e.g., Spokoininskoe), and the upper part of cupola-like stocks of Li-F amazonite granites of the third phase host apogranite-type tantalum deposits (Orlovka, Etyka, and Achikan). In addition to three granite phases, the Kukulbei complex includes dikes of ongonites, elvans, amazonite granites, and chamber miarolitic pegmatites. All of the granitic rocks of the complex have similar isotopic ages of 142± 0.6 Ma. The Zr/Hf ratio decreases systematically from phase 1 (40–25), to phase 2 (20–10), and phase 3 (10–2). The ongonites, elvans, and pegmatites have similar Zr/Hf ratios (15-5), falling between the ranges of leucocratic muscovite granites and Li-F granites. Compared with other granite series, the granitic rocks of the Kukulbei complex show specific petrographic and geochemical features: they are strongly enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From the early to late intrusion phases, the degree of enrichment and depletion in these element groups increases regularly. This is accompanied by a significant decrease (from 40 to 2) in Zr/Hf, which can be used as a reliable indicator of genetic relations, degree of fractionation, and rare-metal potential of granites. Granites with Zr/Hf values lower than 25 are promising for prospecting for Sn, W, Mo, and Be greisen deposits, whereas the formation of Ta deposits requires Zr/Hf values lower than 10.

Structural Patterns of Pre-Uralides in the Enganepe Uplift (Polar Urals) As a Reflection of the Cambrian Collision of Baltica and Arctida

The East European Platform (EEP) is bordered by the Ural foldbelt of Hercynides on the east and the Pechora Plate (PP) with the adjacent Barents Sea shelf in the northeast. The upper horizons of the Earth’s crust in the Urals, PP, and Barents shelf are characterized by a two-level structure. The lower stratigraphic‐structural level consists of Late Precambrian complexes (Pre-Uralides‐Timanides), whereas the upper level is composed of Postcambrian complexes. In terms of the composition and structure, PreUralide‐Timanide complexes are subdivided into two groups [4]. The first group (southwestern Pre-Uralides‐ Timanides) is mainly represented by sedimentary rocks. These rock complexes outcrop in Timan and compose a part of the adjacent PP basement. In the southeast, the complexes are traced in the Urals (Kvarkush anticlinorium and Bashkir Uplift). In the northwest, they outcrop in the northeastern Baltic Shield (Kildin Island and Rybachii, Srednii, and Varanger peninsulas) and take part in the structure of adjacent areas of the Barents Shelf. The second group (northeastern Pre-Uralides‐ Timanides) includes not only sedimentary rocks, but also volcanic and volcanosedimentary complexes, granitoids, gabbroids, and rare ophiolites. These complexes make up the northeastern part of the PP basement and outcrop within several positive structures in the northern part of the western slope of the Urals (e.g., the Enganepe Uplift). The available arguments allow correlation of Late Precambrian complexes of the Wedel Jarlsberg Land (WJL) on southwestern Spitsbergen with the Pre-Uralide‐Timanide complexes. The

Age of granitoids in the Man’khambo and Il’yaiz plutons, the northern Urals: U-Pb data

The Man’khambo and Il’yaiz plutons are situated in the northern Urals at the main divide of the Urals (basins of Shchugor, Ilych, and Severnaya Sos’va rivers). These plutons cut the Upper Riphean‐Vendian country rocks [1, 2] and, along with the latter ones, belong to Preuralides of the Central Ural Uplift. The Man’khambo and Il’yaiz plutons cover about 700 and 200 km 2 , respectively, and are the largest granitoid bodies of this tectonic unit. The plutons are localized in the core of the Man’khambo Anticlinorium, a part of the Lyapin‐Kutim Meganticlinorium of the Central Ural Uplift. The Man’khambo Pluton is a petrotype of the Sal’ner‐Man’khambo plutonic complex recognized in the regional scheme of correlation of igneous complexes and in legends of the state geological maps. The closely located plutons are similar in mineral and chemical compositions (Table 1) and consist of two intrusive phases. The first phase is composed of the major granite and leucogranite (main facies) and the subordinate quartz diorite and granodiorite (hybrid facies). Granitic rocks of the first phase occupy up to 90% of the exposed areas of the plutons. The second phase consists of leucogranite and alaskite. The dike suite is represented by aplite-like granites and aplites; pegmatites and rhyolites are less abundant. According to the petrochemical classification, the rocks of both phases belong to granite, leucogranite, subalkali granite and leucogranite, since the (Na 2 O + K 2 O) content