Western Central Asian Orogenic Belt Research Articles

Paleoproterozoic and Neoproterozoic Quartzites of the Kyrgyz North Tianshan: Age Determination according to the Results of Detrital Zircon Dating

LA-ICP-MS U–Pb dating of detrital zircons reveals two levels of accumulation of Proterozoic quartzites in the Kyrgyz North Tianshan. The quartzites of the Makbal Formation in the core of the Makbal anticlinorium have a maximum depositional age of 1.7 Ga and characterize the oldest stage of accumulation of quartzites in the western Central Asian Orogenic Belt. Quartzites of the Ovva Formation in the northeastern limb of the Makbal anticlinorium and Djeldysu Formation in the Burkhan anticlinorium have maximum depositional ages of ca. 1.0 Ga. Distributions of detrital zircon ages in these two formations are identical and indistinguishable from detrital zircon age distributions in the Neoproterozoic quartzites of the North Kazakhstan. The latter three quartzitic units have common provenance and, apparently, represent the same stratigraphic level. Distinctly different distributions of detrital zircon ages indicate that Precambrian microcontinents of the Kokchetav-North Tianshan zone represented independent continental massif with respect to Precambrian crustal blocks of the Middle Tianshan and Tarim Craton.

The Tekturmas ophiolite belt of central Kazakhstan: Geology, magmatism, and tectonics

The Tekturmas ophiolite belt of central Kazakhstan is located in the western Central Asian Orogenic Belt. It consists of three structural‐formational zones: Tekturmas (centre), Bazarbai (north), and Sarysui (south). The Tekturmas zone includes igneous and sedimentary rocks of Karamurun, Tekturmas, and Sarytau formations, which are dominated by volcanogenic, deep sea, and clastic deposits, respectively. The Bazarbai zone consists of Kuzek and Basarbai formations dominated, respectively, by basaltic and sedimentary rocks. The Sarysui zone includes terrigenous Airtau and olistostromic Ermek formations. The age of sedimentary rocks was constrained by microfossils. The igneous rocks of the Karamurun and Bazarbai formations are alkaline and subalkaline basalt/dolerite, andesibasalt, and andesite. There are three main groups of rocks: high‐Ti, mid‐Ti, and low‐Ti. The high‐Ti Karamurun volcanics and low‐Ti Bazarbai are variably enriched in light rare earth element (LREE) showing LREE enriched (high‐Ti) and LREE flat (low‐Ti) REE patterns. The high‐Ti group shows enrichment in Nb, Th, Zr, and Sm compared with the low‐Ti group. The low‐Ti group is special for the Nb troughs in primitive mantle‐normalized multi‐element diagrams, which are typical of supra‐subduction settings. Both groups yielded positive εNd values, although the averages are 4.6 and 7.5 for the high‐ and low‐Ti rocks, respectively. The geochemical features suggest formation of the igneous rocks in oceanic and supra‐subduction (intra‐oceanic arc) settings. The geological and geochemical features of the Tekturmas ophiolite belt accord well with the tectonic emplacement of ophiolites and formation of accretionary complex at a Pacific‐type convergent margin similar to those of the western Pacific.

Hydrocarbon potential of Late Palaeozoic residual basins in the Central Asian Orogenic Belt: Insights from the tectonic evolution of the Yinggen‐Ejinaqi Basin, Inner Mongolia, China

The Yinggen‐Ejinaqi Basin (YEB) is located in the central‐southern part of the Central Asian Orogenic Belt (CAOB). Tectonically, the basin is situated at the intersection between the eastern and western CAOB, where Late Palaeozoic strata were well exposed. Hence, it offers a great opportunity to study the deep strata of the eastern CAOB. In this paper, based on extensive studies of Upper Palaeozoic field outcrops and reanalysis of associated geochemical and geochronological data in previous research, the local stratigraphic correlation is established, and the tectonic evolution, sedimentary filling, sedimentary characteristics, and petroleum system of the YEB are discussed. It is revealed that: (a) The filling characteristics of the Upper Palaeozoic strata in YEB are closely related to the closure of the Paleo‐Asian Ocean (PAO). YEB underwent subduction‐related compression during the initial extensional phase of the fore‐arc basin from the Viséan to the Serpukhovian (late Early Carboniferous). During the Bashkirian–Sakmarian interval (Late Carboniferous to Early Permian), it was in subduction‐related fore‐arc and postarc rifting phases. During the Artinskian–Capitanian (Early‐Middle Permian) times, it was in an intracontinental rifting‐depression phase. During the Wuchapingian (Late Permian), it was in an intracontinental compressional phase. Tectonostratigraphy, tectonic evolution, and hydrocarbon potential assessments of the study area show that the Zhusileng Hangwula‐Tost Ul tectonic zone (ZHTUTZ) and southwestern part of Querrshan‐Atasbogd tectonic zone (QATZ) are identified as the main prospective areas for the exploration in the YEB. (b) According to the tectonostratigraphic analysis, tectonic evolution and hydrocarbon system of Carboniferous–Permian in the YEB, the YEB has similar attributes to the basin province in western CAOB (e.g., Santanghu and Junggar basins) and eastern CAOB (Erlian, Songliao Basin). To explore commercial oil and gas in the deeper strata (Carboniferous–Permian) of the eastern CAOB basins, further tectonostratigraphic analyses and comprehensive assessment on hydrocarbon potentials are necessitated in the near future.

Petrogenesis and tectonic setting of the Cambrian Kharly alkaline–carbonatite complex (Sangilen Plateau, Southern Siberia): Implications for the Early Paleozoic evolution of magmatism in the western Central Asian Orogenic Belt

The Cambrian Kharly alkaline plutonic complex composed mainly of foidolite and nepheline syenite makes up a small intrusive field in the Sangilen Plateau in Tuva (southern Siberia). The rocks show large ranges of major oxides (38–58 wt% SiO2; 1–18 wt% Na2O + K2O; 11–28 wt% Al2O3; 1.5–20 wt% CaO; 0.1–8 wt% MgO; 2–12 wt% Fe2O3) controlled by variable percentages of minerals: clinopyroxenes, calcic amphiboles, micas, nepheline and feldspars. Alkaline rocks are cut by carbonatite veins composed of predominant calcite coexisting with femic minerals (10–15% of aegirine-ferrosalite-hedenbergite, sodic-calcic amphiboles, ferrobiotite, Ti-garnet), Na–K feldspar and nepheline (up to 15–20%), fluorapatite (up to 20–25%), Sr-apatite, and accessory carbocernaite, titanite, Ti-magnetite and ilmenite. Carbonatites (4057–8859 ppm Sr, 426–1901 ppm Ba (Sr/Ba ≥ 2), 290–980 ppm REE + Y, 2 to 100 ppm Zr, and 0.5 to 15 ppm Nb) possibly originated at high (≥500–650 °C) temperatures as a result of liquid immiscibility. The isotope systematics of rocks and minerals (εNd(t) from ~2.9 to 6.5; 207Pb/206Pbin = 0.89; 208Pb/206Pbin = 2.15; 87Sr/86Sr(t) = 0.70567–0.70733, δ18OV-SMOW ≈ 7.2–19.5‰, and δ13CV-PDB from −6.0 to −1.4‰) suggest mixing of PREMA and EM 1 material during magma generation and crustal contamination of the evolving melts. The rocks bear signatures of interaction with “magmatic-equilibrated” fluids or heated meteoric waters. LILE/HFSE ratios indicate mixed magma sources that involved the material of IAB and OIB, as well as a crustal component, possibly, due to interaction of a mantle plume with rock complexes on the active continental margin.

Detrital zircon U-Pb ages and whole-rock geochemistry of early Paleozoic metasedimentary rocks in the Mongolian Altai: Insights into the tectonic affinity of the whole Altai-Mongolian terrane

Abstract The tectonic affinity of the terranes and microcontinents within the Central Asian Orogenic Belt (CAOB) remains controversial. The Altai-Mongolian terrane (AMT), as a representative tectonic unit in the Mongolian collage, plays a vital role in reconstructing evolution history of the CAOB. The well-preserved early Paleozoic sedimentary sequence covering in this terrane could be considered as a fingerprint to track its provenance and tectonic affinity. Here, we present new whole-rock geochemistry, detrital zircon U-Pb dating, and Hf isotopic analysis for the metasedimentary rocks from the Mongolian Altai in order to shed new light on the tectonic affinity of the AMT. The youngest detrital zircon ages and the regional intrusions constrain the depositional time of the Mongolian Altai sequence to between Late Silurian and Early Devonian, which is consistent with the Habahe group in the western Chinese Altai. The features of whole-rock geochemistry and the cumulative distribution curves of the detrital zircon age spectra indicate that the Mongolian Altai sequence was probably deposited in an active continental setting during early Paleozoic. The zircon age spectra of our samples are all characterized by a main age group in the early Cambrian (peak at 541 Ma, 522 Ma, 506 Ma and 496 Ma, respectively), subdominant age populations during the Tonian, as well as rare older zircons. The nearby Lake Zone of Ikh-Mongol Arc most likely provided plenty of early Paleozoic materials, the subdominant Neoproterozoic detrital zircons could be supplied by the felsic intrusions along the western margin of the Tuva-Mongol microcontinent, and the sparse older zircons may be derived from its basement material. The Precambrian age distribution of the AMT is quite similar to both the Tarim and Siberia cratons, but the Siberia Craton displays a closer resemblance in Hf isotopic composition with the AMT. Thus, we believe that the Siberia Craton contains a closer tectonic affinity with the AMT, and that the Tuva-Mongol microcontinent possibly rifted from the western margin of this craton after the Tonian. To the south of the AMT, recent studies indicated the Yili and Central Tianshan blocks in the Kazakhstan collage of the western CAOB likely have a closer affinity with Gondwana. Therefore, the microcontinents in the CAOB most likely derived bilaterally from both the Siberia Craton and the Gondwana supercontinent. Moreover, our Hf isotopic compositions indicate two significant continental growth events in the Tonian and early Paleozoic, respectively.

Early Silurian tholeiitic-boninitic Mailisu ophiolite, South Tianshan, Kyrgyzstan: a geochemical record of subduction initiation

ABSTRACTOphiolite assemblages of the South Tianshan fold-and-thrust-belt (STS) track the sea floor dynamics of the late Cambrian to Carboniferous Turkestan Ocean in the western Central Asian Orogenic Belt. We interpret new geochronological and geochemical data for the Mailisu ophiolite from western Kyrgyzstan as evidence for an early Silurian incipient subduction zone outboard the South Chatkal Arc of the Middle Tianshan. Igneous zircons from a leucogabbro yielded a U-Pb age of 440 ± 6 Ma indicating one of the oldest sea floor fragments in the STS. Metabasalt samples have N- to E-MORB trace element patterns and initial εNd values of ~5 to 9 support melting of depleted asthenosphere comprising a minor OIB component. Cross-cutting diabase dykes show geochemical characteristics of high-Ca basaltic boninites. Their low REE abundances and highly fractionated middle-heavy REE patterns indicate sources that were more melt-depleted than those of the metabasalts. Concave upwards LREE patterns are compatible with a residual peridotite source, fertilised by small-degree OIB-like melts with εNd of ~5 to 6. Positive Ba, Sr, and Pb anomalies support the involvement of slab-derived hydrous fluids that probably facilitated the melting of the residual mantle. The association of MORB-like metabasalts with younger dykes of boninite affinity suggests subduction initiation for the origin of the Mailisu ophiolite. Our data elucidate one of the major yet little understood episodes in the Paleozoic history of the Turkestan Ocean. Future identification of similar rock assemblages will be helpful to understand the insufficiently constrained history of ocean floor subduction in the Paleozoic Oceans of the western CAOB.

An overview of oceanic island basalts in accretionary complexes and seamounts accretion in the western Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest and long-lived accretionary collages worldwide, and provides an ideal opportunity to address the fundamental geological processes and continental growth of accretionary orogens. However, the tectonic history of the CAOB is still controversial. Previous studies mainly focused on the process and model of accretion and subduction, but did not pay much attention to oceanic intraplate volcanoes. In this paper, we provide an overview of the salient features of twelve OIB-hosting accretionary complexes with ages from Late Neoproterozoic to Early Carboniferous in western the CAOB, and distinguish rock association of typical seamounts from accretionary complexes and shear zones, including pillow basalt, volcanic breccia, limestone, olistostromes and terrestrial clastic rocks. Combined with unique structural features of seamount, the stratigraphic framework of seamount was preliminarily reconstructed in Paleo-Asian Ocean, but the distribution and scale of seamounts is still not clear. Geochemically, volcanic rocks from seamounts are mainly alkaline basalts with litte tholeiitic basalts. The alkaline basalts are characterized by LREE enrichment and HREE depletion, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The Nd, Sr and Pb isotopic features indicate that magma sources have enriched components and show fluid metasomatism. In short, this is consistent with geochemical features of Louisville, Ontong Java and Hawaiian seamounts which are related to mantle plumes, but do not contain high-Mg picrite and komatitic basalt. Based on our observations, and in combination with previous work, we present an innovative model in which the CAOB was involved multiple intra-oceanic arc with seamounts subduction and accretion, which is similar to the present-day Pacific and can better explain the tectonic evolution of the CAOB. However, further studies are needed to document the distribution, scale and mantle dynamics of seamounts in the CAOB.

The Late Carboniferous–Early Permian Ocean‐Continent Transition in the West Junggar, Central Asian Orogenic Belt: Constraints from Columnar Jointed Rhyolite

The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean‐continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time. Based on zircon U‐Pb dating, we determined that the ages of the newly‐discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO2 (75.78–79.20 wt%) and a moderate content of Al2O3 (12.21–13.19 wt%). The total alkali content (K2O + Na2O) is 6.14–8.05 wt%, among which K2O is 2.09–4.72 wt% and the rate of K2O/Na2O is 0.38–3.05. Over‐based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO2 (0.09–0.24 wt%), CaO (0.15–0.99 wt%) and MgO (0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi‐aluminum‐weak peraluminous high potassium calc‐alkaline and some calc‐alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE (Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE (Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2‐type granite, similar to the Miaoergou batholith, which indicates they both were affected by post‐orogenic extension. Combining petrological, zircon U‐Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean‐continent transition of the West Junggar is the Late Carboniferous–Early Permian.

The Overmaraat-Gol Alkaline Pluton in Northern Mongolia: U–Pb Age and Preliminary Implications for Magma Sources and Tectonic Setting

A new Wenlockian zircon U–Pb age (~426 Ma) of the Overmaraat-Gol nepheline syenite (foyaite, juvite) pluton in the SW Lake Hovsgol area (Northern Mongolia) prompts a long history of alkaline magmatism in the western Central Asian Orogenic Belt, exceeding the duration of the Devonian and Permian–Triassic events. The LILE and HFSE patterns of pluton samples analyzed by X-ray fluorescence (XRF) and inductively coupled plasma (ICP-MS) methods indicate intrusion in a complex tectonic setting during interaction of a mantle plume with accretionary-collisional complexes that previously formed on the active continental margin. As a result, the parent magma had a heterogeneous source with mixed mantle (PREMA and EM) and crustal components. This source composition is consistent with Nd–Sr isotope ratios of the Overmaraat-Gol alkaline rocks, from −0.1 to −1.2 εNd(t) and from ~0.706 to 0.707 87Sr/86Sr(t).

Petrology, mineral chemistry, and geochemistry of Late Triassic Ni–Cu ore-bearing mafic–ultramafic intrusions, Hongqiling, northeastern China: petrogenesis and tectonic implications

The Hongqiling magmatic Ni–Cu sulfide deposit, situated on the southern margin of the eastern Central Asian Orogenic Belt (CAOB), is composed of over 30 mafic–ultramafic intrusions. These ore-bearing intrusions are composed mainly of harzburgite, lherzolite, websterite, orthopyroxenite, and norite (gabbro). The constituent minerals are olivine, diopside, bronzite, calcic-hornblende, plagioclase, and spinel with orthopyroxene as a dominant mineral in these intrusions. These ore-bearing intrusions are not Alaskan-type complexes. Spinel and clinopyroxene both exhibit different chemical compositions from those in the Alaskan-type complexes. The rocks that make up the intrusions have high contents of MgO (average value = 25.20 wt.%) and low TiO2 (average value = 0.58 wt.%). The high MgO contents of the minerals and the high Mg# (71) of the calculated melt in equilibrium with olivine demonstrate that the parental magma of the Hongqiling mafic–ultramafic intrusions was a high-Mg tholeiitic magma. The Hongqiling ore-bearing mafic–ultramafic intrusions and the calculated “trapped liquids” for the olivine-orthopyroxene cumulate rocks are all enriched in large-ion lithophile elements and depleted in high field strength elements. The Ce/Pb, Ta/La, Th/Yb, and (La/Sm)PM values and the depletion of Nb and Ta suggest that the magma experienced crustal contamination. The Hongqiling ore-bearing intrusions display many similarities with mafic–ultramafic intrusions that formed in a post-collisional extensional environment in the western CAOB (e.g., Huangshanxi). Common features include their whole-rock compositions and mineral chemistry. Combined with the evolutionary history of the eastern segment of the CAOB, we believe that the Late Triassic Hongqiling mafic–ultramafic intrusions formed in a post-collisional extensional environment.

Contrasting granitic magmatism of the Kalba fold belt (East Kazakhstan): Evidence for Late Paleozoic post-orogenic events

The paper presents new original data and a review of previous studies on the Late Carboniferous – Early Permian granitoids of the Kunush and Kalguty intrusive complexes from the Kalba fold belt (East Kazakhstan). These rocks formed at the initial post-collisional stage of the Irtish-Zaisan orogen in the western Central Asian Orogenic Belt (CAOB). The granitoids form isometric or NW linear intrusions inside the Late Devonian - Early Carboniferous metasediments which overlap the accretionary lithology of the Kalba fold belt in front of the Altai active margin of the Siberian continent. These granitoids contain zircon grains with U-Pb ages of ca.308–291 Ma, synchronous with the peak post-orogenic magmatic event at ∼300 Ma in the CAOB. The Kunush high-Na granitoids, with high SiO2 (67–72%) and Al2O3 (15–18%) contents and the Na2O/K2O ratio (1.94–6.43), and low HREEs (Yb = 0.22–0.93 ppm) but moderate Sr/Y ratios (47–179), Mg# (35–55) and Ni, Cr, are generally common to high-Al TTG-series with non-subduction geochemical signatures. The Kalguty granitoids mostly belong to calc-alkalic to subalkalic high-K series, have non-corundum CIPW-norm compositions and mainly weakly peraluminous (ASI = 0.97–1.09) characteristics. Their Fe∗ = 0.7–1 at 62–74 wt.% SiO2 and relatively high Y/Nb (2–4) and Rb/Nb (9–18) ratios are similar to transitional compositions reported for fractionated I- and A2-type post-orogenic granites worldwide. The Kunush high-Na and Kalguty high-K granitoids formed at different depths in the crust and the parent melts were derived from metabasaltic (MORB; P = 10–15 kbar) and metagranitic (TTG-like; P < 10 kbar) protoliths, respectively. We propose that relaxation of tangential compression after oblique collision and the following thermal impact of mafic magma that acted simultaneously at different crustal depths, along with decompressional partial melting, may be a basic petrological scenario for synchronous formation of geochemically different granitoids.

Multi-stage metamorphic evolution and protolith reconstruction of spinel-bearing and symplectite-bearing ultramafic rocks in the Zheltau massif, Southern Kazakhstan (Central Asian Orogenic Belt)

Spinel-bearing ultramafic rocks, metamorphosed in high-pressure conditions, are described from the metamorphic complexes of the western part of the Central Asian Orogenic Belt (CAOB) in the Chu-Yili region in Southern Kazakhstan. They comprise small bodies of magnetite-bearing serpentinites, Cr-spinel-bearing serpentinized and amphibolitized dunites and peridotites enclosed by strongly retrogressed kyanite-bearing paragneisses. Apart from serpentinization, the ultramafic rocks were also overprinted by later rodingitization expressed in the development of the index-minerals hydrogrossular, prehnite and vuagnatite. Cr-spinel-bearing peridotites are characterized by extensive development of symplectitic and coronitic microtextures, which are interpreted to have been formed after garnet breakdown during decompression (i.e. exhumation). Calculated pressure (P) and temperature (T) obtained by phase diagram modeling and conventional geothermobarometry for the symplectites were P 11.5–14.5 kbar for a wide temperature range of T 600–850 °C, which followed the transition from garnet to spinel peridotite. Major and trace element whole-rock geochemical characteristics of the spinel-bearing ultramafic rocks as well as their structurally close relations indicate their mutual origin as parts of an oceanic cumulate complex of an arc-basin system. The protoliths were probably plagioclase-bearing (thus shallow, crustal) ultramafic rocks and troctolites, subducted to eclogite facies conditions and then exhumed along with other metamorphic complexes of the Zheltau massif. These spinel-bearing ultramafic rocks could be classified as crustal formations in accordance with the inferred depths of their origin (<5 kbar), in common with many HP ultramafic/mafic complexes in the CAOB. However, strongly depleted geochemical signatures of the rocks, together with the distinctive microtextural features observed in the spinel peridotites, are unique, and their occurrence in the western CAOB is described here for the first time.

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

AbstractThe South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (&gt;100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during Tmax conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the Tmax (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.

Supra-subduction igneous formations of the Char ophiolite belt, East Kazakhstan

There is a wide range of controversial tectonic concepts for the final development of the Paleo-Asian Ocean (PAO) in the western Central Asian Orogenic Belt (CAOB), largely due to scarcity of geochronological and geochemical data. Our paper presents first results of U-Pb zircon dating and detailed geochemical studies (major and trace elements, Sr, Nd and Pb isotopes) of subduction-related mafic to felsic igneous rocks of the Char ophiolite belt (East Kazakhstan). The new data shed light on the final stage of the PAO in this region. The igneous assemblages comprise mafic to felsic volcanic rocks and their plutonic varieties. The rocks yield Devonian and early Carboniferous U-Pb zircon ages, which match the previously reported micropaleontological ages of sedimentary rocks of oceanic origin. According to major element composition, the rocks belong to the tholeiitic and tholeiitic to calc-alkaline transitional igneous series. They are characterized by low TiO2 (0.7 wt% in average) and moderately enriched light rare-earth element (LREE) to flat chondrite normalized REE patterns (Lan = 3.5–34.6, La/Smn = 0.8–4.6, La/Ybn = 0.6–9.1). Primitive mantle normalized multi-element spectra show distinct troughs at Nb (Nb/Lapm = 0.1–0.38; Nb/Thpm = 0.04–0.6). Most of the samples have positive εNdt values ranging from +2.9 to +8.8 and 206Pb/204Pb ratios ranging from ~18.0 to ~19.7. The trace element and isotope data and the results of geochemical modeling in the Nb-Yb system suggest high degrees of melting of a strongly depleted mantle source in the spinel stability field. Clinopyroxene (cpx) monomineral thermometry and the calculations based on the composition of cpx hosted melt inclusions suggest that the melts from which the younger samples were derived crystallized at lower temperatures (below 1170 °С) at depths of 12, 6–3 km and in subsurface conditions, while the older samples derived at higher temperatures (up to 1193 °С) crystallized at a depth from 6 to 3 km. Our new data show that the Char ophiolite belt hosts fragments of at least two intra-oceanic arcs, middle Devonian and Early Carboniferous, which existed in the Irtysh-Zaisan branch of the Paleo-Asian Ocean, at the active margin of the Kazakhstan continent.

Structural styles and zircon ages of the South Tianshan accretionary complex, Atbashi Ridge, Kyrgyzstan: Insights for the anatomy of ocean plate stratigraphy and accretionary processes

The anatomy of an ancient accretionary complex has a significance for a better understanding of the tectonic processes of accretionary orogens and complex because of its complicated compositions and strong deformation. With a thorough structural and geochronological study of a fossil accretionary complex in the Atbashi Ridge, South Tianshan (Kyrgyzstan), we analyze the structure and architecture of ocean plate stratigraphy in the western Central Asian Orogenic Belt. The architecture of the Atbashi accretionary complex is subdivisible into four lithotectonic assemblages, some of which are mélanges with “block-in-matrix” structure: (1) North Ophiolitic Mélange; (2) High-pressure (HP)/Ultra-high-pressure (UHP) Metamorphic Assemblage; (3) Coherent & Mélange Assemblage; and (4) South Ophiolitic Mélange. Relationships between main units are tectonic contacts presented by faults. The major structures and lithostratigraphy of these units are thrust-fold nappes, thrusted duplexes, and imbricated ocean plate stratigraphy. All these rock units are complicatedly stacked in 3-D with the HP/UHP rocks being obliquely southwestward extruded. Detrital zircon ages of meta-sediments provide robust constraints on their provenance from the Ili-Central Tianshan Arc. The isotopic ages of the youngest components of the four units are Late Permian, Early-Middle Triassic, Early Carboniferous, and Early Triassic, respectively. We present a new tectonic model of the South Tianshan; a general northward subduction polarity led to final closure of the South Tianshan Ocean in the End-Permian to Late Triassic. These results help to resolve the long-standing controversy regarding the subduction polarity and the timing of the final closure of the South Tianshan Ocean. Finally, our work sheds lights on the use of ocean plate stratigraphy in the analysis of the tectonic evolution of accretionary orogens.

The Central Tianshan Block: A microcontinent with a Neoarchean-Paleoproterozoic basement in the southwestern Central Asian Orogenic Belt

The Central Tianshan Block (CTB) is one of the major continental constituents of the western Central Asian Orogenic Belt (CAOB). Its nature and origin is essential for understanding the accretionary processes and the reconstruction of the architectural framework of the CAOB. The Wuwamen Metamorphic Complex (WMC), which is believed to represent the basement of the CTB, was incorporated into the Wuwamen ophiolitic mélange during the Paleozoic evolution of the South Tianshan Ocean. The WMC is mainly composed of amphibolite-facies volcano-sedimentary rocks, including migmatites, orthogneisses, amphibolites, schists and locally marbles. Zircon U–Pb geochronology suggests that the protoliths of the interlayered biotite-two feldspar and biotite-plagioclase-hornblende gneisses in the Yangjuan area were formed between 2529 and 2513Ma and overprinted by amphibolite-facies metamorphism between 1815 and 1809Ma. Subsequently, the gneisses were intruded by ∼1724Ma meta-mafic dykes and ∼787Ma leucogranite dykes/veins. Geochemical data reveal that the biotite-two feldspar gneisses have high-silica adakite affinities with high SiO2 and Na2O contents as well as high Sr/Y ratios (61–117) and slightly evolved zircon Hf isotopic compositions. Consequently, they are interpreted to be the products of partial melting of relatively old subducted oceanic crust. In contrast, the interlayered biotite-plagioclase-hornblende gneisses have high MgO, Cr, Co and Ni contents and prominent negative Nb–Ta–Ti anomalies, indicating derivation from partial melting of an ancient sub-continental lithospheric mantle in a continental arc setting. The ∼1.72Ga meta-mafic dykes have geochemical similarities of typical Fe-Ti basalts and are characterized by incompatible element enrichments and primitive Sr–Nd–Hf isotopic signatures ((87Sr/86Sr)i=0.703446−0.703535); ɛNd(t)=+6.49 to +6.90; zircon ɛHf(t)=+0.57 to +10.6), which is consistent with partial melting of a relatively fertile, isotopically depleted asthenospheric mantle in a post-collisional extensional setting. In addition, the ∼787Ma leucogranites are trondhjemitic in composition and display extremely low total REE contents and highly evolved Nd–Hf isotopic signatures (ɛNd(t)=−4.71 to −4.53; zircon ɛHf(t)=−19.4 to −13.0). They are thought to have no petrogenetic relationship with the host orthogneisses and to be products of a foreign melt injection. Our new data provide robust evidence for a Neoarchean–Paleoproterozoic basement of the CTB. Comparison of the tectono-thermal-magmatic events reveals that the CTB and the Tarim Craton have a partly overlapping Precambrian crustal evolution, and thus the CTB could have originated from the Tarim Craton. However, a major difference between both is the presence of juvenile Mesoproterozoic magmatism in the CTB and its absence in the Tarim Craton and, therefore, further geological and geochronological research is required to determine the precise origin of the CTB and its tectonic affinity with adjacent terranes in the western CAOB.

Precambrian geology of the Kazakh Uplands and Tien Shan: An overview

A comprehensive review of new data on geology and geochronology of Precambrian terranes in the western Central Asian Orogenic Belt reveals new insights into its evolution. At the present surface, these terranes mostly consist of Meso- to Neoproterozoic sedimentary, magmatic and metamorphic assemblages, with insignificant Paleoproterozoic rocks. Archean material is represented exclusively by detrital and xenocrystic zircons in younger strata. Meso- to Neoproterozoic felsic magmatic rocks were mostly sourced from Neoarchean and Paleoproterozoic continental crust, indicating its reworking and potential wider presence at deeper crustal levels. Most Meso- to Neoproterozoic assemblages are of intraplate origin. The supra-subduction assemblages of Neoproterozoic and Mesoproterozoic ages are of limited extent.We propose to recognize the Issedonian and Ulutau-Moyunkum groups of terranes, separated by early Paleozoic Z-shaped ophiolitic suture, based on their different tectono-magmatic evolution in the Mesoproterozoic and Neoproterozoic. Distinctly different are the Mesoproterozoic and early Neoproterozoic assemblages, with lithological variations at the beginning of the late Neoproterozoic and practically no differences at the end of the Neoproterozoic.The Issedonian group of terranes could be part of a Mesoproterozoic (ca. 1100Ma) orogen between the Siberian, North China and Laurentian cratons. The pre-Mesoproterozoic crust of these terranes was completely reworked during the younger events. The Ulutau-Moyunkum group of terranes appear to be lithologically and geochronologically similar to the Tarim craton. Both the Issedonian and Ulutau-Moyunkum groups of terranes were metamorphosed during the Ulutau-Moyunkum event at 700±25Ma.The breakup into currently mappable Precambrian terranes took place during end-Ediacaran to early Paleozoic times after opening of oceanic basins, whose relics are preserved in numerous Paleozoic ophiolitic sutures.

Genesis of late carboniferous granitoid intrusions in the Dayinsu area, West Junggar, Northwest China: evidence of an arc setting for the western CAOB

ABSTRACTThe Dayinsu area is located in the northern part of the West Junggar district near the border between China and Kazakhstan and is an important component of the Central Asian Orogenic Belt (CAOB). The Dayinsu area hosts numerous granitoid plutons in Devonian–Carboniferous volcano–sedimentary strata. The older Laodayinsu and Kubei (345–330 Ma) plutons are located in the west with the younger Bayimuzha and Qianfeng (330–325 Ma) plutons in the east. The whole-rock SiO2 contents of the four granitoid plutons range from 52.22 to 68.42 wt.% and total alkaline contents (K2O + Na2O) range from 4.94 to 9.16 wt.%. The granites are enriched in large ion lithophile elements and light rare earth elements with depletions in Nb, Ta, Ce, Pr, P, and Ti. The plutons are metaluminous with I-type signatures. The geochemistry of the intrusions suggests that they formed in a subduction zone setting, and subsequently underwent fractional crystallization during emplacement, with higher degrees of fractionation in the eastern sector than in the west. Similarities in the geochronology and geochemical characteristics of the granitoid plutons in Dayinsu to those in the Tabei district (west to Dayinsu area) suggest that both districts are part of the Carboniferous Tarbagatay Mountain intrusive event. The early Carboniferous (345–324 Ma) granitoid intrusions in the Tarbagatay Mountain likely formed in an island arc subduction setting during the evolution of the CAOB.

TRACKING DEEP ANCIENT CRUSTAL COMPONENTS BY XENOCRYSTIC ZIRCONS OF PALEOZOIC FELSIC IGNEOUS ROCKS FROM THE ALTAI-EAST JUNGGAR TERRANE AND ADJACENT REGIONS AND ITS TECTONIC SIGNIFICANCE

The deep crustal continental components and architecture of the Central Asian Orogenic Belt have long been a matter of debate [Wang et al., 2009; Kroner et al., 2014; Xiao et al., 2015; Yang et al., 2017]. We present an integrated study of geochronological and Hf-inzircon isotopic data for xenocrystic zircons from the Paleozoic granitoid rocks and associated felsic volcanic rocks of the Chinese Altai, East Junggar and nearby regions. The aim is to trace the age spatial distribution of deep old crustal components in these key parts of the western Central Asian Orogenic Belt.

Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh shear zone (NW China): Implications for arc amalgamation and oroclinal bending in the Central Asian orogenic belt

The largest accretionary orogen in the world, the Central Asian orogenic belt, has evolved through the assembly of various oceanic and continental blocks. Understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes. One such major suture zone, which records the collisional history of the Siberian marginal arc system (Chinese Altai) with intra-oceanic arc systems (East/West Junggar), is the Irtysh shear zone in NW China. The spatial continuity and the tectonic nature of this shear zone are still poorly understood, but its development has supposedly made a significant impact on the architecture of the western Central Asian orogenic belt and the formation of the Kazakhstan orocline. Here, we provide new insight into the evolution of this shear zone based on detrital zircon ages, Hf isotope composition, structural data and Ar/Ar age constraints on the timing of deformation. Our results show a major discrepancy in detrital zircon populations and Hf model ages across the southern Chinese Altai and the northern East/West Junggar, thus allowing us to map the exact location of the tectonic boundary. Detrital zircon data constrain the initial closure of the Ob-Zaisan Ocean to the late Carboniferous (<323 Ma), and new structural and Ar/Ar geochronological data shed light on the subsequent collisional processes. We propose that the collisional zone between the Chinese Altai and the East/West Junggar was initially subjected to crustal thickening at ca. 323-295 Ma, followed by orogen-parallel extension (ca. 295 Ma) and transpressional deformation (ca. 286-253 Ma). The closure of the Ob-Zaisan Ocean in NW China postdates the initial phase of oroclinal bending in the western Central Asian orogenic belt, thus indicating that oroclinal bending initiated during subduction. Based on our new constraints and other available geological data, we suggest that the early stage of oroclinal bending was likely driven by slab rollback.