I.Yu. Safonova

Late Paleozoic faults of the Altai region, Central Asia: tectonic pattern and model of formation

Abstract The present kinematic and dynamic analysis of large-scale strike-slip faults, which enabled the formation of a collage of Altai terranes as a result of two collisional events. The Late Devonian–Early Carboniferous collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent resulted in the formation of the Charysh–Terekta system of dextral strike-slip faults and later the Kurai and Kuznetsk–Teletsk–Bashkauss sinistral strike-slip faults. The Late Carboniferous–Permian collision of the Siberian and Kazakhstan continents resulted in the formation of the Chara, Irtysh and North-East sinistral strike-slip zones. The age of deformation of both collisional events becomes younger toward the inner areas of the Siberian continent. In the same direction the amount of displacement of strike-slip faulting decreases from several thousand to several hundred kilometers. The width of the Late Paleozoic zone of deformation reaches 1500 km. These events deformed the accretion-collision continental margins and their primary paleogeographic pattern.

Fragments of Vendian-Early Carboniferous Oceanic Crust of the Paleo-Asian Ocean in Foldbelts of the Altai-Sayan Region of Central Asia: Geochemistry, Biostratigraphy and Structural Setting

Abstract Detailed geological, geochemical and biostratigraphic studies of rocks from basaltic-sedimentary terranes in the Kurai and Katun accretionary wedges (Vendian-Middle Cambrian units), the Charysh-Terekta strike-slip zone (Late Cambrian-Early Ordovician units), and the Chara ophiolite-bearing strike-slip zone (Late Devonian-Early Carboniferous units) have been undertaken. The Early Cambrian accretionary wedges record a stage of the Kuznetsk-Altai island arc evolution. The Charysh-Terekta strike-slip zone records evidence of the Late Devonian collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent. The Chara ophiolitic zone was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents. Our study of these fragments of oceanic crust led us to conclude that intra-plate volcanism was active at the early stages of the Paleo-Asian oceanic evolution, in a period from the Vendian to the Early Carboniferous. Fragments of weakly to strongly differentiated oceanic and island-arc basalts have been preserved in accretion-collision zones and give information about chemical composition, petrology and tectonic setting of the oceanic crust at these times. The geochemical data indicate that the Altai and East Kazakhstan metabasalts could have been formed at mid-oceanic ridges, oceanic islands or oceanic plateau of the Paleo-Asian Ocean. Our interpretation of structural, lithological, geochemical and biostratigraphic data shows that the structure and composition of the oceanic lithosphere of the Paleo-Asian Ocean were similar to those of the present Pacific Ocean.

Geochemical Evolution of Intraplate Magmatism in the Paleo-Asian Ocean from the Late Neoproterozoic to the Early Cambrian

A group of oceanic islands and/or seamounts (hereafter, paleoseamounts) was produced by oceanic hot-spot magmatism in the Late Proterozoic-Early Cambrian in the southwestern margin of the Paleo-Asian Ocean. They were accreted to the Kuznetsk-Altai island arc in the Late Cambrian and were subsequently incorporated during the closing of the paleocean into the accretionary complexes of the western part of the Altai-Sayan area (southwestern Siberia, Russia). The major-and trace-element compositions and Sr and Nd isotopic systematics of pillow lavas and basalt flows from the Kurai (600 Ma) and Katun’ (550–530 Ma) paleoseamounts of Gorny Altai characterize the evolution of Hawaiian-type magmatism in the Paleo-Asian Ocean during that period. The obtained data show a significant change in lava composition between 600 and 550–530 Ma. The tholeiitic basalts of the Kurai Paleoseamount (600 Ma) from the southern part of Gorny Altai have lower incompatible element contents and higher 147Sm/144Nd values compared with the younger tholeiitic and alkali basalts of the Katun’ Paleoseamount (550–530 Ma), whose rocks are exposed in northern Gorny Altai. The trace-element compositions of the Katun’ lavas are similar to those of the Hawaiian tholeiites, and their 147Sm/144Nd ratios are lower than those of the Kurai basalts. It was suggested that the older Kurai Paleoseamount was formed above a thinner oceanic lithosphere, i.e., closer to a paleospreading axis compared with the younger Katun’ Paleoseamount. The observed temporal variations in the chemical and isotopic characteristics of lavas are probably related to differences in the degree of melting of the heterogeneous mantle owing to the different thickness of the oceanic lithosphere above which the Kurai and Katun’ paleoseamounts were formed. During the Ediacaran, a plume developed beneath the younger and, consequently, thinner lithosphere of the Paleo-Asian Ocean. The higher degree of melting in the mantle column resulted in a more considerable contribution from the refractory depleted material of the upper mantle. After 50–70 Ma, i.e., in the Early Cambrian, the plume affected a thicker lithosphere, its mantle column became shorter, and the degree of melting was lower. Owing to this, the basaltic melt was more contributed by incompatible element enriched less refractory material of the lower mantle.

Petrogenesis of the island-arc complexes of the Chara zone, East Kazakhstan

The study of clinopyroxenes and melt inclusions provided direct (independent on secondary alteration) information on the petrogenesis of the island arc complexes of the Chara zone, East Kazakhstan. It was shown that magmatism of this zone evolved from primitive island-arc systems with boninites to mature island arc with calc-alkaline melts. In terms of trace and rare-earth element distribution, the melt inclusions in the clinopyroxenes of the Chara zone differ from mid-ocean ridge basalts, being closer to the island-arc calcalkaline series. Based on inclusion composition, the parental melts of the considered complexes crystallized within 1150–1190°C with decreasing iron, magnesium, calcium, and sodium contents. Simulation based on melt inclusion data in clinopyroxenes indicates that the melts contained up to 1 wt % water, which was confirmed by direct ion-microprobe determination of 0.84 wt % H2O in the inclusions. Calculated liquidus temperatures are consistent with homogenization temperatures of the inclusions. Our calculations on the basis of inclusion data testify that the primary melts of the studied basaltic series of the Chara zone were generated from the mantle protolith within temperatures of 1350–1530°C at depths of 50–95 km. Similar parameters are typical of the generation of the tholeiitic and boninitic island-arc magmas in the modern ocean-continent transition zones of the Pacific type. In general, the study of clinopyroxenes and melt inclusions suggests that the considered complexes of the Chara zone were formed with the participation of tholeiitic and calcalkaline volcanogenic systems of basaltic, basaltic andesite, and, possibly, boninitic composition in the paleogeodynamic setting of evolving ancient island arc.

U-Pb ages of detrital zircons from modern sediments of the Yangtze River and stages of orogeny in Southeast Asia

280 The problem of periodicity in continental growth is important for defining global cycles of the Earth evo lution. One of the methods appropriate for solving it is mass U–Pb dating of detrital zircons and monazites from sandy sediments of large world rivers that drain vast territories [1]. The distribution spectra of U–Pb ages obtained for zircons or, less commonly, monazite from sediments of modern river systems yield infor mation on the main stages of orogeny, continent for mation and composition of terranes constituting the basement of river basins and serving as sources of sed imentary material [2]. The distribution of the obtained ages is compared with that of the ages of bedrock com plexes in the basin. The coincidence of peaks in age spectra obtained for zircons and rocks in drainage areas provides grounds for the conclusion on the largely juvenile character of the continental crust in the particular region [1]. On the other hand, the dis crepancy between them implies crustal material recy cling (i.e., reworking of previously subducted zircon bearing crustal material) and/or substantial changes in the provenance usually due to tectonic processes. For tectonic reconstructions, spectra of ages available for zircons from modern sediments are compared with ages obtained for zircons from older sequences [2].

Paleoproterozoic granitoids from the basement of the central Siberian Platform (Borehole Mogdinskaya-6): U-Pb age and composition

334 The Siberian Platform is the largest structure of Northeast Asia, which records practically its entire geological history since the Early Archean. The base ment of the platform crops out in the Aldan Shield in the southeast, Anabar Shield, Olenek Uplift in the north, and as small inliers along its southwestern periphery (Kan, Sharyzhalgai, Irkutnyi, and Angara– Kan) (Fig. 1). The remaining part of the platform is overlain by a thick cover of Phanerozoic sedimentary and igneous rocks. The age and composition of the buried basement of the Siberian Platform is deter mined by analogy with the well known metamorphic and igneous rocks cropping out in the above men tioned shields and inliers. The data on the occurrence, depth, thickness, and lateral distribution of regional rock complexes of the basement under the cover are usually derived from geophysical data whole rock composition of blocks (terranes) and specific features of boundaries between these blocks (Fig. 1).