A. V. Travin

The Vendian accretionary event in the southwestern margin of the Siberian Craton

The present-day southwestern margin of the Siberian Craton is represented by its Early Precambrian inliers (Angara‐Kan and Sayan) and Mesoproterozoic‐ Neoproterozoic marginal continental region that includes the Yenisei Range and Sayan areas (Fig. 1). The Early Precambrian evolution of structures of the Siberian Craton culminated with formation of the Early Proterozoic Angara continental-margin foldbelt, intense intrusion of granitoids (1.87‐1.84 Ga ago), and its general consolidation [1]. Mesoproterozoic‐Neoproterozoic fold‐thrust complexes in the trans-Angara part of the Yenisei Range and largely riftogenic complexes of the Sayan region demonstrate all features typical of the ensialic marginal continental and intracontinenal structures. Precambrian blocks and terranes in folded structures of the Central Asian foldbelt were united into the Sayan‐Yenisei accretionary belt. The belt includes Early and Late Proterozoic island-arc and oceanic terranes (Central, Idar, and Shumikha‐Kirel terranes in the Kan block; Arzybei, Isakovka, and Predivinsk) or terranes with substantially metasedimentary sections (Derba) and fragments of oceanic and island-arc structures (Kuvai Group) overlain by Vendian‐Cambrian strata in the Man Trough. The age of accretionary‐collision processes, which promoted amalgamation of Precambrian terranes and their amalgamation with the Siberian Craton margin, remains an open problem. Nearly synchronous metamorphism and granite intrusions in two or more terranes, as well as formation of synchronous molasses in foreland basins, serve as indicators of the accretionary‐ collision processes. The purpose of this work is to determine, based on the above-mentioned criteria, the age of the main event that produced the Sayan‐Yenisei accretionary belt and terminated the Neoproterozoic history of the Siberian Craton. We carried out complex Ar‐Ar dating of amphiboles and biotites from metamorphic rocks in Precambrian terranes and from the marginal zone of the Siberian Craton, U‐Pb dating of zircons from collision-related granitoids, and sedimentological studies in Vendian foredeeps. Given below is a brief characteristic of Precambrian terranes, which were the main objects of study. Three terranes (Central, Idar, and Shumikha‐Kirel) are recognized in the structure of the Kan block (Fig. 1). They differ from each other in lithology and age of the constituting rock associations and are characterized by tectonic boundaries [2]. The Central Terrane is represented by the Paleoproterozoic lithotectonic complex and constitutes an old “core” of the block under consideration. The lower part of the section is largely composed of metavolcanics of the tholeiitic‐basaltic and calc-alkaline associations, while metasediments prevail in its upper part. Orthogneisses (metadacites) aged 2.3 Ga are characterized by positive e Nd (from 1.8 to 3.0) and T (DM) = 2.4‐ 2.5 Ga values. Similar values of the model age (2.3‐ 2.6 Ga) are established also for graywackes. Based on characteristic geochemical parameters, the metamorphosed volcanogenic‐terrigenous complex is comparable with igneous associations and graywackes formed in subduction settings. The stratified complex is intruded by Late Vendian (555 ± 5 Ma) trondhjemites of the Verkhnii Kan Massif [2]. The Idar Terrane is separated from the Central Terrane by the thrust zone. The lower part of the stratified complex is composed of metamorphosed rocks of the komatiitic‐tholeiitic series, which are comparable with oceanic volcanics in geochemical properties. They enclose intrusive ultramafic and ultramafic‐mafic massifs crosscut by plagiogranite veins. The section is supplemented in the southeastern direction with a sequence of garnet-bearing biotite and amphibole paragneisses (metagraywackes), which are similar (in the composition of trace elements) to terrigenous sediments of island arcs. Paragneisses are characterized by a wide range of model Nd age varying from 1.3 to 2.5 Ma.

Late Mesozoic volcanism of the eastern part of the Argun superterrane (Far East): Geochemistry and 40Ar/39Ar geochronology

This study presents geochronological data (40Ar/39Ar) obtained for Mesozoic volcanic rocks superimposed upon different-aged rocks from the eastern portion of the Argun superterrane. The new data on their age corresponds to the two episodes of magmatic activity distinctly manifested within the East Asia during the Mesozoic and proves the asynchronous development of the volcanic complexes, which make up the eastern and western flanks of the Umlekan-Ogodzha volcanoplutonic belt. It is assumed that the Umlekan-Ogodzha belt is a near EW section of a series of different-aged near NS-trending volcanoplutonic belts subparallel to the Pacific margin.

U-Pb Dating and Sm-Nd Systematics of Igneous Rocks in the Ol'khon Region (Western Baikal Coast)

The Ol’khon region represents a fragment of the West Baikal collision belt that resulted from Early Caledonian accretionary‐collisional processes related to the closure of the Paleo-Asian ocean in the southern margin (in present-day coordinates) of the Siberian Craton [1‐3]. This communication is dedicated to analysis of the internal structure and age of protoliths from this region based on U‐Pb dating and the Sm‐Md isotopic composition of igneous complexes. The U‐Pb (SHRIMP-II) age of rocks was measured at the Center of Isotopic Studies (VSEGEI, St. Petersburg) using the technique described in [4]. The Sm‐Nd isotope composition of bulk rock samples was determined by the standard method [5] at the Laboratory of Isotope Geochronology and Geochemistry (Geological Institute, Kola Scientific Center, Russian Academy of Sciences, Apatity). Figures 1 and 2 demonstrate experimental results obtained for two periods (500‐485 and 475‐465 Ma ago) (Figs. 1, 2), which reflect the scale and spatial distribution of synmetamorphic and intrusive igneous rocks in the southwestern part of the Ol’khon region. 1

Geochronology of Mesozoic Granitoids and Associated Molybdenum Mineralization in the Western Part of the Dzhugdzhur-Stanovoi Superterrane

The Dzhugdzhur superterrane [1] located at the south-eastern margin of the Siberian Craton is one of the keystructures of the eastern margin of Asia. It is made up of thetraditionally distinguished Early and Late Precambriancomplexes and numerous Early–Late Mesozoic intrusiveand volcanoplutonic associations [2 and others]. The func-tioning of magmatic and ore systems of different ages andtypes in various geodynamic settings in the course of com-plex and multistage evolution of tectonic structures pro-duced no less intricate metallogenic specifics of the region.

The Uspensk intrusion in south Primorye as a reference petrotype for granitoids of the transform continental margins

Identification of the granitoid complexes as petrotypes of definite tectonic settings remains one of the most important geological problems taking into account the large volume of fundamental tectonic and applied regional works. To the present, the granitoids of within-plate environments, island arcs, subductionrelated continental margins, and collisional settings have been substantiated and studied in detail [1‐3 and others]. At the same time, the granitoid associations of transform plate boundaries have been studied significantly less well. Transform continental-margin settings (Californiantype complex settings) related to subsidence of the oceanic ridge beneath a continent with the formation of slab windows and wide development of strike-slip faulting were distinguished for the first time at the western margin of North America [4 and others]. A similar regime was later substantiated for the Early Cretaceous and Paleogene stages of the Sikhote Alin evolution [5, 6]. Unlike the Californian coast, the transform margin setting in the Russian Far East was not complicated by mantle plumes that were unrelated to subduction. Therefore, this region is a unique object for distinguishing magmatic complexes typical of the transform‐continental setting.

Age of the Berezitovoe Gold-Base Metal Deposit in the Western Selenga-Stanovoi Superterrane and Its Relation to Magmatism

The Selenga‐Stanovoi Superterrane [1] in the southeastern margin of the North Asian Craton is one of the key structures of eastern Asia. Its geological structure is mainly composed of conditionally defined Early and Later Precambrian rock complexes, as well as numerous Paleozoic‐Mesozoic intrusive and volcanoplutonic associations. The intricate and multistage evolution of tectonic structures was characterized by the functioning of magmatic and ore-forming systems of various ages and types related to different geodynamic settings, resulting in the formation of no less complicated metallogenic specialization of the region. Numerous data obtained recently make it possible to specify the ages of igneous and metamorphic rock complexes and, correspondingly, to revise the existing concepts of the regional geological structure. However, isotopic‐geochronological information on most ore objects remains insufficient. Therefore, it is rather difficult to correlate tectonic, magmatic, and ore-forming processes. According to metallogenic models, the majority of ore objects are Mesozoic structures [2 and others]. However, these models are based largely on the macroscopic association of mineralization with different magmatic processes. Systematic geochronological studies of ore deposits with application of advanced methods were introduced only during the past few years [3‐5 and others]. In this communication, we

Aptian basaltic andesites in the Amur-Zeya depression: First geochemical and 40Ar/39Ar geochronological data

The eastern margin of Asia hosts numerous Mesozoic‐Cenozoic depressions or sedimentary basins: Amur‐Zeya, Bureya, Torom, Uda, Middle Amur, Alchan, and others (Fig. 1). Their formation is closely related to general geological development of the continental margin. Therefore, concepts on their origin were transformed in accordance with the development of concepts on the regional geodynamic evolution. The great body of geological, geochemical, geochronological, paleontological, and paleomagnetic data accumulated in recent years has provided insight into the Mesozoic and Cenozoic tectonic events that produced the present-day structure of East Asia. At the same time, geochronological and geochemical data on volcanic rocks in continental sedimentary basins are scarce, although precisely such information is crucial for understanding the origin of these structures. In the present communication, we discuss geochemical and 40 Ar/ 39 Ar geochronological data obtained for volcanics of the Amur‐Zeya Depression (AZD), which is among the largest structures of this kind in the Russian Far East.

First geochronological data on felsic lavas from the Ezop-Yamalin volcanoplutonic zone, Khingan-Okhotsk volcanogenic belt

The Khingan–Okhotsk volcanogenic belt [3, 7, 10,and others], which includes a series of NE-trending vol-canic zones, has been studied by several researchers [4–7, 9–11, and others] because of the presence of veryhigh-grade tin ore deposits. However, the age and com-position of the host rocks and their geodynamic settingstill remain debatable.According to literature data [3–8, 10, 11, and oth-ers], the belt is subdivided into the Khingan–Olonoi,Ezop–Yamalin, Komsomol’sk, Badzhal, and otherzones (Fig. 1), including several volcanic fields on thecoast of the Tugur and Ul’ban bays, which are under-lain by complexes of three regional structures of differ-ent ages: the Mongol–Okhotsk orogenic belt, Badzhalterrane, and Bureya–Jiamusi superterrane [8]. Applica-tion of precise methods yielded new geochemical andage data on the magmatic complexes of the Khingan–Olonoi, Komsomol’sk, and Badzhal zones [4–6, 10, 11,and others], whereas only scanty data are available onthe composition and age of the Ezop–Yamalin zone.Previous works were devoted to study of the composi-tion of felsic rocks of the zone [6] and the dating of thecomagmatic granitoids [1].In the present paper, we attempted to determine theage of lavas in the Ezop–Yamalin zone that includes theEzop and Yamalin volcanic structures with a total areaof 5000 km

New data on the age of lamproite-lamprophyre magmatism in the Urals

Several occurrences of lamproites on the eastern slopeof the Urals are objects of our investigation (Fig. 1). Thelamproite occurrence situated in the central and easternpart of the Paleozoic Magnitogorsk island-arc system ata few tens of kilometers northeast from Magnitogorskwas described in [4, 9]. These lamproites make up apetrogenetic series with lamprophyres (monchiquite,camptonite, minette, and kersantite) identified as theKalymbai Complex [9]. The second lamproite occur-rence (Pervomaiskii area) is located in the eastern partof the East Ural Uplift 40 km south of Chelyabinsk nearPervomaiskii Settlement [3, 7]. This area includes anumber of pipe-shaped magnetic anomalies, as well asdiatremes and dikes that crosscut the Devonian rocks. Inaddition, we previously studied the lamproite occurrencenear Skalistyi Settlement 25 km west of the town ofTroitsk in the Transural region [6], where lamproite dikescrosscut adamellites of the Lower Sanarka pluton.Based on Rb–Sr and K–Ar datings, the ages of thelamproites mentioned above vary from 198 to 295 Ma[1–4]. Lamproites of this age are related to the postcol-lision extension of the Earth’s crust [9], tectonomag-matic reactivation of the young platform along latitudi-nal transverse faults [4], or manifestation of an epicon-tinental hot spot [11].Lamprophyres have been investigated in the Urals toa lesser extent than lamproites, although they occur inmany places and vary in age. The best studied and mosteasily accessible lamprophyre related to the alkaline–ultrabasic potassic magmatism is represented by a dikein the Shartash granitic pluton on the outskirts of Yeka-terinburg [5]. This occurrence is situated at the conti-nental margin west of the Murzinka–Adui Block of theEast Ural Uplift. The K–Ar age of the Shartash dike,which is appreciably altered at the contact with granite,ranges from 261 Ma (reaction amphibole at the contact)to 274 Ma (phlogopite) [5].In this communication, we present the first results of