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Detrital zircons of Ordovician terrigenous sequences are studied in various Southern Uralian tectonic units. The age of detrital zircons of the West Uralian and Transuralian megazones, ...Taganai–Beloretsk Zone, and Kraka allochthons spans from the Late Archean to the end of the Vendian– beginning of the Cambrian; Early Precambrian and Early–Middle Riphean zircons are the most abundant. Vendian–Cambrian detrital zircons are strongly dominant in the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone; the zircons of other ages are absent or extremely rare. The Vendian–Cambrian detrital zircons of all Southern Urals zones probably derive from volcanic and granitic rocks of the marginal continental belt, which are part of the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone. The Lu–Hf isotopic characteristics of Vendian–Cambrian detrital zircons indicate that their parental rocks formed on a heterogeneous basement that includes blocks of juvenile and ancient continental crust. According to a model of the pre-Ordovician tectonic evolution of the Southern Urals, at the end of the Late Riphean, the passive margin of the East European Platform collided with a block on a heterogeneous basement. The formation of the block terminated with the Grenville Orogeny. After collision, a volcano-plutonic belt originated in the Vendian–Cambrian at the actively evolved margin of the East European Platform.
Metagabbroid garnet amphibolite formed after high-pressure granulite with an estimated P-T peak of 12-16 kbar at 700-790°C occurs at the sole of the Kempirsai ophiolite allochthon (Southern Urals). ...Garnet amphibolite includes high-Fe varieties with the assemblages of garnet and relics of pyroxene, as well as high-alumina rocks composed of garnet, pyroxene, corundum, and sapphirine. The Ediacaran and Early-Middle Palaeozoic sequences underlying the allochthon were metamorphosed under the conditions of amphibolite facies. Our studies were aimed at estimation of the peak metamorphic age of garnet amphibolite. The mean .sup.206Pb/.sup.238U (SIMS, SHRIMP II) age obtained for the zircons from garnet amphibolite is 392 ± 4 Ma. The estimated age characterizes the timing of metamorphism related to mantle magmatism accompanying obduction.
► New SHRIMP ages for garnet pyroxenite from Chu-Yili Mountains of South Kazakhstan reveal so far unknown early Paleozoic HP belt with a peak metamorphism at ∼490
Ma. ► Chemical composition of the ...garnet pyroxenite points at an ultramafic mantle protolith. Block-in-matrix structure of the belt implies exhumation in subductional channel. ► Upper Cambrian arc related granodiorite dated as 509
Ma delineates magmatic arc in the NE of the HP belt. ► The HP belt was formed in subductional and collisional setting due to convergence of Anrakhai arc and North Tianshan microcontinent with subduction towards the NE.
The mechanism and age of Palaeozoic accretion in the Central Asian Orogenic Belt remain poorly constrained. One of the most complex belts extends from the Kokchetav area in northern Kazakhstan to the Kyrgyz northern Tianshan. It represents an assemblage of small blocks with Palaeoproterozoic continental crust, intervening slivers containing early Palaeozoic ophiolites and/or deep-marine sediments, and a number of HP and UHP metamorphic complexes. The HP–LT metamorphic rocks provide important clues for reconstructions of the overall structure and evolution of the accretionary collage.
This study is aimed to constrain the metamorphic age and tectonic implications of HP garnet pyroxenites and enclosing mica schists in the Anrakhai area of the Chu-Yili Mountains of southern Kazakhstan. The HP belt is located in the central part of the Anrakhai metamorphic block and extends NW–SE between Neoproterozoic gneisses in the SW and undated ophiolites and granodiorites in the NE. Garnet pyroxenites and their retrograded equivalents form tectonic boudins and blocks in garnet–muscovite and muscovite schist of sedimentary origin. Metamorphic zircons from two garnet pyroxenite samples yielded a SHRIMP mean
206Pb/
238U age of 489.9
±
3.1
Ma. This age is interpreted to reflect the time of Early Ordovician peak metamorphism and ongoing subduction in the area as constrained by geological data and suggests that HP metamorphism was related to subduction. Exhumation of the HP rocks occurred between 490 and ∼475
Ma, based on the Early Ordovician age of overlap assemblages. Detrital zircons from a garnet–muscovite schist enclosing the pyroxenites were dated by LA-ICP-MS and range in age from 694 to 2557
Ma. They suggest a maximum late Neoproterozoic age for deposition of the sedimentary protolith and derivation from a continental source including Neoproterozoic to Archaean crustal components.
Granodiorites with chemical characteristics of island arc rocks intruded into Proterozoic gneisses in the NE of the HP belt, and magmatic zircons from one sample yielded a SHRIMP
206Pb/
238U mean age of 508.3
±
3
Ma. This signifies the existence of a Cambrian magmatic arc. The early Palaeozoic age of the HP garnet pyroxenites indicates that the Anrakhai block is not part of a extensive Precambrian microcontinent, as previously thought, but represents a package of tectonically interlayered slivers, made up of Precambrian basement and early Palaeozoic rocks. Stacking of these heterogeneous domains may be due to subduction of continental crust, mutual underthrusting of continental and ophiolitic rocks, wedge extrusion of HP rocks, and strike-slip deformation in a subduction and/or collision setting. The Anrakhai collision zone is part of larger accretionary belt which formed by the end of the Early Ordovician and may extend from the Kyrgyz northern Tianshan to the Kokchetav area in northern Kazakhstan.
U–Th–Pb (SHRIMP II) isotopic dating of accessory zircons from Vendian (Ediacaran) ash tuffs of the Basa formation section (Asha Group, Southern Ural) was performed for the first time. The concordant ...age obtained (573.0 ± 2.3 Ma) can be interpreted as the upper age limit of the onset of the Timan Orogeny in the structure of the Southern Urals. Fossil remains of
Kuckaraukia multituberculata
and several paleopascichnids, belonging to the genera
Palaeopascichnus
and
Orbisiana
, were previously found in the rocks of the Upper Basa Subformation, and the first occurrence of
Arumberia banksi
was established. The age obtained indicates a more ancient age of this Vendian assemblage compared to the previously accepted one.
The Sakmara Allochthon (Southern Urals) comprises a widely distributed folded stratum of rhyolites and ignimbrites, which are characterized by Nb–Zr–REE geochemical specification. This stratum ...overlies unconformably folded Paleozoic complexes, including Devonian ones. Rhyolites contain potassium feldspar and quartz phenocrysts, as well as granite xenoliths. According to geochemical features, volcaniс rocks were formed under the intraplate setting; in terms of the chemical composition, they belong to the A-type granites. The
40
Ar/
39
Ar age of the felsic rock groundmass of rhyolites is 303 ± 2 Ma. The feldspar phenocrysts yielded ages of 306 ± 3 and 337 ± 3 Ma. The first estimate, as well as the matrix age, is associated with the formation of the volcanic complex, while the second value is probably associated with the age of xenogenic material. The complex of rare-metal enriched rhyolites characterizes the stage of the terminal Late Carboniferous rift magmatism in the rear part of the active continental margin. This complex formed prior to collision and thrusting of ophiolitic allochthons.
The new data obtained on conodonts from the Lower Ordovician of the Kidryasovo, Akbulaksai, and Kuagach formations (the central part of the Sakmara Zone of the Southern Urals) allow to evaluate the ...biogeography of conodonts of this region. The comparison of conodont assemblages of the Southern Urals with well-studied conodont assemblages of the Baltoscandian Basin situated in the northwestern part of the East European Platform show that these assemblages were biogeographically isolated. This shows that basins of the central and eastern parts of the East European Platform were separated by land in the Early Ordovician.
The basement of the Zheltav sialic massif (Southern Kazakhstan) is composed of different metamorphic rocks united into the Anrakhai Complex. In the southeastern part of the massif, these rocks form a ...large antiform with the core represented by amphibole and clinopyroxene gneissic granite varieties. By their chemical composition, dominant amphibole (hastingsite) gneissic granites correspond to subalkaline granites, while their petroand geochemical properties make them close to A-type granites. The U–Pb geochronological study of accessory zircons yielded an age of 1841 ± 6 Ma, which corresponds to the crystallization age of melts parental for protoliths of amphibole gneissic granites of the Zheltav Massif. Thus, the structural–geological and geochronological data make it possible to define the Paleoproterozoic (Staterian) stage of anorogenic magmatism in the Precambrian history of the Zheltav Massif. The combined Sm–Nd isotopic—geochronological data and age estimates obtained for detrital zircons indicate the significant role of the Paleoproterozoic tectono-magmatic stage in the formation of the Precambrian continental crust of sialic massifs in Kazakhstan and northern Tien Shan.
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