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The combined petrographic, petrological, geochemical and geochronological study of the Neoproterozoic gneisses of the Sarychabyn and Baskan complexes of the Junggar Alataw of South Kazakhstan elucidate the Precambrian tectonic evolution of the Aktau–Yili terrane. It is one of the largest Precambrian crustal blocks in the western Central Asian orogenic belt. The U‐Pb single‐grain zircon ages indicate that granite‐gneisses formed from the same source and crystallised in the early Neoproterozoic ca. 930–920 Ma. The chemical composition of gneisses corresponds to A2‐type granites. The whole‐rock Nd isotopic characteristics (εNd(t) = –4.9 to –1.0 and TNd(DM‐2st) = 1.9 to 1.7 Ga) indicate the involvement of Paleoproterozoic crustal rocks in magma generation. Early Neoproterozoic ca. 930–920 Ma A‐type granitoids in the Aktau–Yili terrane of South and Central Kazakhstan might reflect within‐plate magmatism adjacent to the collisional belt or a local extension setting in back‐arc areas of the continental arc. [ABSTRACT FROM AUTHOR]

In this study, we investigated the high-pressure (HP) metamorphism of the Precambrian continental crust exposed in the Zheltau terrane in South Kazakhstan (Koyandy complex) and the Chu-Kendyktas terrane in the North Tien Shan of Kyrgyzstan (Aktyuz, Kemin and Kokdzhon complexes) within the SW part of the Central Asian Orogenic Belt. HP quartz–feldspar lithologies of the Koyandy complex consist of migmatized kyanite-bearing garnet–mica paragneisses, garnet–kyanite paragneisses and their derivatives associated with eclogites. Paragneisses demonstrate prograde evolution involving mica dehydration melting and producing magnesium-rich garnet, kyanite and K-feldspar at the near-peak to retrograde stages at pressures of 15–18.5 kbar and temperatures of 800–870°C. The widespread growth of micas in these rocks reflects lower stages of retrogression at P = 10–12 kbar and T = 720–770°C. The age distributions of the cores of detrital zircon grains from the paragneisses indicate a predominance of Neoproterozoic and minor occurrence of Mesoproterozoic and Palaeoproterozoic sources of their protoliths. The ages of ∼487–485 Ma obtained from the zircon rims of the paragneisses reflect the timing of their HP metamorphic re-equilibration. These age clusters are consistent with the age estimates obtained from the rims of zircons in the eclogite-bearing garnet gneisses of the adjacent Aktyuz complex in the North Tien Shan. The P–T paths and zircon ages obtained from the high-grade quartz–feldspar gneisses of the Zheltau and Chu-Kendyktas terranes are thus interpreted to indicate involvement of the crustal material derived from the Precambrian basement (magmatic zircons aged ca. 844 Ma) and its Ediacaran–Cambrian sedimentary cover (detrital zircons with maxima at 1 Ga and 800–600 Ma) in the latest Cambrian subduction processes induced by the closure of the oceanic basins assigned to the Palaeo-Asian Ocean. [ABSTRACT FROM AUTHOR]

The Yili Block in the Western Tianshan orogen is a key area for understanding the early crustal formation and evolution of the Central Asian orogenic belt, due to the widely‐distributed Precambrian rocks. Also, it hosts a lot of medium– to large‐scale sedimentary exhalative (SEDEX) Pb‐Zn deposits that mainly occur in Proterozoic metamorphosed clastic‐carbonate rocks. In this study, LA‐ICP‐MS U‐Pb analyses were carried out on detrital zircons in siltstones of the Precambrian Haerdaban Group in the Haerdaban Pb‐Zn deposit and magmatic zircons in the diorite dyke that cuts through the strata and orebodies. The maximum depositional age of the siltstones was determined to be about 604 Ma, the diorite having formed at approximately 500 Ma. As such, the Haerdaban Group was most likely formed in the Neoproterozoic Sinian, rather than the previously considered Mesoproterozoic Changchengian. Detrital materials of the Haerdaban Group were mostly derived from the Nanhua–Sinian mafic dykes and granitic rocks around Lake Sayram at the northern margin of the Yili Block. It is proposed that the Yili Block, together with the Kazakhstan and Central Tianshan blocks and the Tarim Craton, might all pertain to the same Rodinia supercontinent, which has great potential for targeting large to super‐large SEDEX Pb‐Zn deposits. [ABSTRACT FROM AUTHOR]

The results of a study of Late Precambrian effusive and plutonic rocks of the Dyusembai and Aktass volcanic–plutonic associations of the western part of the Southern Ulutau are reported. A U‒Th‒Pb isotope–geochronological study of accessory zircons (SHRIMP II) showed that the formation of rhyolite–granite associations occurred in the second half of the Tonian in the Neoproterozoic (~830 Ma, Dyusembai association, and ~800–790 Ma, Aktass association). The formation of parental melts for effusive and plutonic rocks of both associations occurred via dehydration melting of metatonalite (metagreywacke) formations of the Early Precambrian continental crust in an intraplate environment. The Neoproterozoic evolution of the Southern Ulutau took place in an environment of an active continental margin. The formations of the eastern part of the Southern Ulutau were formed within the ensialic island arc, while the western part, in the area of riftogenic magmatism, upon extension in the rear area. The Tonian magmatism in the Southern Ulutau, as well as in the other terranes of the Ulutau‒Moyunkum Group, indicates their incorporation into the basement of a large volcanic–plutonic belt marking subduction of the oceanic lithosphere beneath the northwestern margin of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

The metamorphosed iron ore volcanogenic–sedimentary strata of the Karsakpai series in the eastern Ulutau Precambrian terrane of Central Kazakhstan are studied. The crystallization age of 745 ± 3 Ma is first dated for the andesidacitic tuffs by the U–Th–Pb (SIMS) method, which indicates their formation at the end of the Tonian period of the Neoproterozoic Era. [ABSTRACT FROM AUTHOR]

Several ribbon‐like continental fragments with Precambrian basement were wrapped within Palaeozoic accretionary complexes in the southwest Central Asian Orogenic Belt (CAOB). Their origins and tectonic evolution are essential for understanding the accretionary orogeny of the CAOB and crustal growth versus continental reworking in Phanerozoic. The southern Yili Block in NW of China is one of the major continental fragments in SW CAOB. In order to better constrain the tectonic settings of the Precambrian basement of the southern Yili Block, we present new data of field structures, whole‐rock geochemical compositions, zircon U–Pb ages and Lu‐Hf isotopic ratios of the Neoproterozoic gneissic granites. Ductile deformation characterized by mylonitic foliations and stretching/mineral lineation is commonly recognized in the gneissic granites, which were cross‐cut by undeformed granitoids and mafic dykes. The gneissic granites yielded zircon U–Pb ages of 968 ± 5 Ma, 948 ± 8 Ma, and 873 ± 9 Ma. These gneissic granites show geochemical features of high potassium, peraluminous, calc‐alkaline series, enrichment of Th, K, Rb and depletion of Ba, Nd, Sr. They show zircon εHf(t) values from −5.5 to 2.0 and two‐stage zircon Hf model ages between 1.7 and 2.1 Ga. These new results indicate that the Neoproterozoic gneissic granites in the southern Yili Block derived from partial melting of meta‐greywackes that most likely had formed in a continental arc setting. A comparison of stratigraphy, magmatism and detrital zircon ages suggests that the southern Yili Block is similar with the northern Yili Block and Chinese Central Tianshan Block in term of Precambrian basement, and these continental fragments might have originated from Neoproterozoic orogenic belts. [ABSTRACT FROM AUTHOR]

The western part of the Central Asian Orogenic Belt, which comprises folded areas in Kazakhstan, Kyrgyzstan, and northwestern China, includes a number of large Precambrian sialic massifs that are framed by deformed and dismembered Paleozoic ophiolites and by island arc and flysch formations. The basements of the massifs are commonly made up of diverse metamorphic complexes, some of which were metamorphosed under high and ultrahigh pressures in the Early Paleozoic at ~480–530 Ma. These metamorphic formations are the Zerendy Group of the Kokchetav massif in northern Kazakhstan; Akdzhon Group of the Issyk-Kul massif in the northern Tien Shan); Aktyuz, Kemin, and Koyandy complexes of the Chu-Kendyktas and Zheltau massifs in southern Kazakhstan and the northern Tien Shan; and the Kassan Group of the Ishim–Naryn massif in the central Tien Shan. The paper reviews data on the structures, compositions, and metamorphic evolutions of the high- and ultrahigh-, and medium-pressure metamorphic rocks of these massifs. Numerous P–T assessments have been made for the near-peak and/or post-peak retrograde metamorphism, and some prograde P–T paths have been calculated for the key rock types over the past three decades of the studies. Near-peak and/or post-peak metamorphic ages and some ages of retrograde metamorphism are estimated for most of the high- and ultrahigh-pressure rocks. The paper discusses problems faced by the researcher when building geodynamic models for the high- and ultrahigh-pressure complexes in various massifs of the western part of the Central Asian Orogenic Belt. It is shown that any reliable model shall be underlain by detailed information on the compositions, ages, and formation environments of the protoliths for the ultrahigh-, high-, and medium-pressure rocks and complexes. Moreover, the structures and compositions of Paleozoic complexes surrounding the Precambrian massifs shall also be taken into consideration. [ABSTRACT FROM AUTHOR]

Based on the results of U–Th–Pb (SIMS) dating, an Early Neoproterozoic (924 ± 4 Ma) age was obtained for granite–gneiss of the Sarychabyn Complex, which is one of the oldest complexes in the Junggar Alataw Range. This value constrains the minimum age of the metasedimentary rocks of the Sarychabyn Group hosting granite–gneiss as well. The most likely age of metasediments is Late Mesoproterozoic–Early Neoproterozoic, which is consistent with the age of lithologically similar complexes in the adjacent regions of Kazakhstan and China. The age of the gneiss of the Sarychabyn Complex coincides with the ages of anorogenic granitoids and felsic volcanic rocks from the Aktau-Mointy massif of Central Kazakhstan and the ages of granitoids in the Wenquan Complex in China. An episode of granitoid magmatism at ~920 Ma, widespread in the Aktau-Junggar microcontinent, is a characteristic feature that distinguishes it from other Precambrian crustal blocks of Kazakhstan and the Northern Tianshan. [ABSTRACT FROM AUTHOR]

Detrital zircons from quartzite–schist sequences of the Aktau–Mointy Massif (Central Kazakhstan), which occur in various structural positions relative to the Early Neoproterozoic felsic volcanics, were studied. The obtained U–Pb zircon dating results suggest similarity of detrital zircon age populations with a lower limiting depositional age of 1220 Ma in all quartzite–schist sequences. The different positions of quartzite–schist sequences relative to the felsic volcanics are due to subsequent deformations. The ages of detrital zircons studied fall mostly within the intervals of 1149–2105 and 2354–3159 Ma with major peaks at 1218, 1338, 1468, 1581, 1628, 1743, 1851, and 2520 Ma and minor peaks at 2016, 2619, 2696, 2791, 2823, and 3114 Ma. The structural features of quartzite–schist sequences and the presence of heavy minerals that are unstable during long-term transport indicate the predominance of local source areas, which either form the buried part of the basement of the Aktau–Mointy Massif or have been eroded completely. Within the Aktau–Mointy Massif, it enables us to use the U–Pb ages and Lu–Hf isotope compositions of detrital zircons from quartzite–schist sequences to reconstruct the history of Precambrian continental crust formation. Results suggest that the production of juvenile continental crust prevailed only during Middle Mesoproterozoic, while formation during the Mesoarchean–Neoproterozoic time accompanied magmatic reworking of the older crustal complexes. [ABSTRACT FROM AUTHOR]

U–Pb geochronological studies of detrital zircons from quartzite–schist sequences of the Akbastau Formation of the Chu Block (northwestern part of the Chu–Kendyktas terrane, Southern Kazakhstan) have been provided. The concordant ages of detrital zircons are predominantly within the intervals of 1672–2115 Ma with peaks at 1697, 1780, 1857, and 2066 Ma. Individual zircon grains display ages of 2291–2332 Ma with peaks at 2303 and 2322 Ma. Neoarchean ages of 2608–2747 Ma with a peak at 2681 Ma characterize another significant zircon population. The lower limit of deposition for the Akbastau Formation of the Chu Block, corresponding to the youngest statistically significant zircon population, is estimated at 1.7 Ga. [ABSTRACT FROM AUTHOR]

For the first time, the U-Pb age is determined for detrital zircons of quartzite-schist sequences, which are part of the Precambrian basement of the Aktau-Mointy Block (Central Kazakhstan) along with Neoproterozoic felsic volcanic (925-920 Ma) and granitic (945-917 Ma) rocks [6]. We analyzed 219 zircon grains from small-grained quartzites of the northern part of the block (Mt. Bol’shoi Alabas) including 206 grains with concordant age (1149-1273, 1276-1975, 2354-2592 Ma). These ages indicate the Mesoproterozoic, Paleoproterozoic, and Neoarchean rocks as provenances. The youngest statistically significant age peak of 1209 Ma indicates that the quartzite-schist sequences accumulated 1200-900 Ma ago (at the end of the Mesoproterozoic and beginning of the Neoproterozoic) prior to the formation of the Early Neoproterozoic felsic rocks and granites. [ABSTRACT FROM AUTHOR]

The Ediacaran–Cambrian transition records distinct evolutionary changes of metazoans. The Ediacaran fossils ( i.e. , Ediacara-type biota and tubular fossils) are contrasting with the diverse small skeletal fossils (SSFs) from the early Cambrian. The apparent dissimilarities hindered studies deciphering their evolutionary relationships. This also led to a popular assumption that there exists a mass extinction of the Ediacara biota and cloudinids at the Precambrian–Cambrian (Pc–C) boundary. Here we report for the first time a transitional fauna which consists of typical elements of Ediacaran, i.e. cloudinids and related tubicolous organisms, together with Cambrian SSFs including protoconodonts, anabaritids and siphogonuchitids from South China and Maly Karatau (Kazakhstan). The sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous. Their chronostratigraphic assignment in South China has been debated for years. Based on the new fossil assemblage, the siliceous strata of the Daibu Member (Northeast Yunnan, South China) and the basal Kuanchuanpu Formation (South Shaanxi, South China) can be assigned to the earliest Cambrian SSF biozone ( Anabarites trisulcatus – Protohertzina anabarica Assemblage Zone) and thus can be considered of early Cambrian in age. A new subzone of the earliest SSF zone in eastern Yunnan (South China) is proposed herein defined as Ganloudina symmetrica – Rugatotheca typica Interval Subzone. The new fauna demonstrates that the cloudinids, originally confined to the late Ediacaran, persisted into the Cambrian Fortunian, and no major extinction event occurred at the Pc–C transition. [ABSTRACT FROM AUTHOR]

A typical feature of the Precambrian complexes of the Kokshetau, Ishkeolmess, Erementau-Niyaz, and Aktau-Dzhungaria massifs of Northern and Central Kazakhstan is the presence of the end Mesoproterozoic-beginning of the Neoproterozoic quartzite-schist sequences in these sections. The lower and upper parts of these sequences are mostly composed of schists with interlayers of quartzites and marbles and of quartzitic sandstones, respectively. It is suggested that the quartzite-schist sequences represent the sub-platform cover of a large continental block and were formed in the regressive basin with widely abundant facies of submarine deltas and a littoral shoal. The presence of horizons and the lenses enriched in zircon-rutile heavy concentrate with the amount of accessory minerals of 10-70% characterizes the quartzite-schist sections of the Kokshetau and Erementau-Niyaz massifs. The U-Pb age of zircons from one such locality in the central part of the Erementau-Niyaz massif was analyzed by LA-ICP-MS. The Concordia ages of zircons are in the intervals 1041 ± 13-1519 ± 14, 1623 ± 14-1931 ± 14, and 2691 ± 14-2746 ± 14 Ma. One age was 2850 ± 14 Ma. The age distribution is characterized by clear peaks of 1.08, 1.20. 1.34, 1.46, 1.65, 1.89, and 2.70 Ga and weak peaks of 1.13 and 1.68 Ga. The age of the majority of zircons ranges from 1309 ± 14 to 1519 ± 14 Ma. Our data indicate that mostly Neoproterozoic rocks with a subordinate role of Paleoproterozoic and Neoarchean complexes served the feeding sources for the quartzite-schist sequence of the Erementau-Niyaz massif. The Mesoproterozoic and Paleoproterozoic events identified for the detrital zircons of the Erementau-Niyaz massif are completely manifested only in Laurentia. In the first approximation, these events coincide with the assembly and breakup of the Columbia/Nuna supercontinent (∼1650-1580 and 1450-1380 Ma) and assembly of the Rodinia supercontinent (1300-900 Ma). [ABSTRACT FROM AUTHOR]

Paleomagnetic studies of thick lava series are one of the most reliable sources of data on the ancient geomagnetic field. However, most of such data are younger than 5 Ma, with much fewer results on the rest of the Cenozoic and the Mesozoic. Two wholesome results are available for the Precambrian but none for the Paleozoic. Late Permian basalts and rhyolites from northeastern Kazakhstan were studied to obtain first estimates of the geomagnetic-field characteristics during that period. We present preliminary results on part of the collection (66 flows (sites)) from a section ~1600 m thick. The characteristic component of reversed polarity was isolated by stepwise demagnetization at all the sites with a slight error. This component is of prefolding age and, most likely, primary. No abnormal magnetization direction is observed in the data, and the average directions of the characteristic component at the sites are tightly clustered (D = 243.3°; I = −57.0°; k = 79.1; α95 = 2.0°; 65 sites). As compared with the published data on Cenozoic and Mesozoic thick lava series, secular variation was much weaker in the Late Permian than in the Mesozoic or Cenozoic, and the geomagnetic field was less disturbed. Secular-variation models based on the Late Cenozoic data show even more dramatic differences. [ABSTRACT FROM AUTHOR]

• The Precambrian quartzite-schist sequences of the Aktau-Mointy terrane were deposited at ca. 1150–920 Ma. • Age peaks ca. 1.2–2.1 and 2.3– 2.9 Ga obtained for detrital zircons. • There are three main events in the evolution of the Pre-Proterozoic crust of Aktau-Mointy terrane.: Neoproterozoic (ca. ~2.5–3.4 Ga), Paleoproterozoic (ca. ~1.5–2.0 Ga) and Mezoproterozoic (~1.20–1.50 Ga) • The Aktau-Mointy terrane has a close tectonic affinity to the Yili Block, Chinese Central Tien Shan, Kokchetav, Ishkeol'mes, Erementau-Niyaz and Issyk-Kul terranes in the Precambrian. • The Aktau-Mointy terrane was probably located in southwest of the Baltica. Metasedimentary complexes represented by quartzite–schist Kiik Group quartzite–schists play a significant role in the Aktau–Mointy terrane structure, which is one of the largest Precambrian blocks in the western Central Asian Orogenic Belt (CAOB). U-Pb LA-ICP-MS dating of detrital zircons from quartzites in different Kiik Group areas and related magmatic age data establish that accumulation occurred in the late Mesoproterozoic to early Neoproterozoic (from 1150 to 920 Ma). Concordant age estimates of detrital zircons from Kiik Group quartzites predominately fell within intervals of 1149–2105 and 2272–2850 Ma with the prominent peaks at 1217, 1332, 1466, 1582, 1621, 1748, 1849, 2517 and 2689 Ma, and minor peaks at 1992, 2282, 2791 and 2823 Ma. Hf-in-zircon and Nd whole rock data suggest that the primary stages of continental crustal growth in the Aktau–Mointy terrane were associated with the tectono-magmatic processes of the Neoarchean, Paleoproterozoic, and Mesoproterozoic. Juvenile continental crust production occurred during the Neoarchean and Paleoproterozoic, with older complex tectono-magmatic reworking, and juvenile crust production finally prevailed in the mid-Mesoproterozoic. Based on the age patterns and Hf isotopic compositions of detrital zircons from the Aktau–Mointy terrane and other terranes in the western CAOB, the crustal basement of the Aktau–Mointy terrane exhibits certain similarities with those of the Northern Kazakhstan and Issyk–Kul terranes. They all exhibit a close tectonic affinity with the Chinese Central Tian Shan terranes, Yili Block, North and China Craton, while differing from the Tarim Craton. In the Rodinia supercontinent structure, the Aktau–Mointy terrane was probably located near the Sveconorwegian Orogen in the western Baltica with other Kazakhstan and Northern Tian Shan Precambrian blocks. [ABSTRACT FROM AUTHOR]