V. A. Ponomarchuk

Geology, Geochronology, and Geodynamics of the Khan Bogd Alkali Granite Pluton in Southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

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.

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.

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