Paleozoic Evolution Research Articles

Early Paleozoic Magmatism and Crustal Evolution in the North Qinling Orogen

Early Paleozoic Magmatism and Crustal Evolution in the North Qinling Orogen

Geochronology and geochemistry of early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East

The Russian Far East and Northeast (NE) China are located in the eastern part of the Central Asian Orogenic Belt (CAOB), which consists of a series of micro‐continental massifs including the Erguna, Xing'an, Songnen–Zhangguangcai Range, Bureya, Jiamusi, and Khanka massifs. The Khanka Massif is located in the easternmost part of the CAOB, mainly cropping out in the territory of Russia, with a small segment in NE China. To the north and west of the Khanka Massif are the Jiamusi and Songnen–Zhangguangcai Range massifs, respectively. The boundary between these massifs is marked by the Dunhua–Mishan Fault. To the south lies the North China Craton, and to the east is the Sikhote–Alin Orogenic Belt separated by the Arsenyev Fault.However, the early Paleozoic evolution and tectonic attributes of the Khanka Massif are debated. These conflicting ideas result from the lack of systematic research on early Paleozoic igneous rocks from the Russian part of the Khanka Massif.It is generally accepted that the CAOB represents the largest known Phanerozoic accretionary orogenic belt. However, questions remain concerning the nature of the deep crust beneath the Khanka Massif, and whether Precambrian crust exists within the massif itself.In this paper, we report new zircon U–Pb ages, Hf isotopic data, and major‐ and trace‐element compositions of the early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East, with the aim of elucidating the early Paleozoic evolution and the tectonic attributes of the Khanka Massif, as well as the nature of the underlying deep crust.New U–Pb zircon data indicate that early Paleozoic magmatism within the Khanka Massif can be subdivided into at least four stages: ∼502 Ma, ∼492 Ma, 462–445 Ma, and ∼430 Ma.The ∼502 Ma pyroxene diorites show negative Eu anomalies, and the ∼492 Ma syenogranites, intruding the ∼502 Ma diorites, show positive Eu anomalies. These observations indicate that the primary parental magmas of these rocks were derived from different origins.The 462–445 Ma magmatism is made up of syenogranites and tonalites. The ∼445 Ma Na‐rich tonalites contain low REE concentrations, and are enriched in Eu and Sr. These observations, together with the positive εHf(t) values, indicate that they were derived from magmas generated by partial melting of cumulate gabbros.The ∼430 Ma I‐type granodiorites and monzogranites from the northern Khanka Massif, and the A‐type monzogranites from the central Khanka Massif display zircon εHf(t) values ranging from −5.4 to +5.8. This suggests that they formed from magmas generated by partial melting of heterogeneous lower crustal material.Zircon Hf isotopic data reveal the existence of Precambrian crustal material within the Khanka Massif. The geochemistry of the Middle Cambrian intrusive rocks is indicative of formation in an extensional setting, while Late Cambrian–middle Silurian magmatism was generated in an active continental margin setting associated with the subduction of a paleo‐oceanic plate beneath the Khanka Massif. Regional comparisons of the magmatic events indicate that the Khanka Massif has a tectonic affinity to the Songnen–Zhangguangcai Range Massif rather than the Jiamusi Massif.

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.

Dredge samples from the Chukchi Borderland: Implications for paleogeographic reconstruction and tectonic evolution of the Amerasia Basin of the Arctic

The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. We present new lithologic descriptions, along with U-Pb, 40Ar/39Ar, and apatite fission track analyses, for rocks dredged from the Chukchi Borderland. These new results provide constraints on the Chukchi Borderland9s crustal architecture, Paleozoic evolution, and pre-Cretaceous paleogeographic position prior to the formation of the Amerasia Basin. Based on the location and nature of dredged rocks, the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Early Paleozoic units recovered from Northwind Ridge in the eastern Chukchi Borderland consist of low-grade metasedimentary rocks with a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, high-grade gneisses and amphibolites dredged from the Chukchi Plateau in the central Chukchi Borderland yield U-Pb and 40Ar/39Ar ages that suggest an affinity with rock units of the Pearya terrane of Arctic Canada. U-Pb and 40Ar/39Ar ages reveal a Paleozoic tectonic history beginning in Early Cambrian to Early Ordovician time with subduction/arc-related high-T metamorphism. Intrusive relationships indicate that Late Ordovician to Silurian orthogneisses intruded the high-T units within an arc setting. Devonian exhumation of these high-grade units is recorded by U-Pb dating of sphene and 40Ar/39Ar thermochronology of micas and potassium feldspar. The exhumation of the high-grade rocks was coeval with the deposition of coarse-grained sediments bearing detrital zircon compositions similar to Silurian foreland basin sediments deposited in Pearya, as well as with greenschist facies metamorphism of metasedimentary rocks dredged from Northwind Ridge. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. This pre-rift position supports and refines the widely cited rotational models for opening of the Canada Basin.

Early–Middle Paleozoic ecosystem evolution and revolution (Part 2 of 2): an introduction

Papers included in this special issue have been derived and developed mostly from contributions presented at the 5th Annual Meeting of the International Geoscience Programme (IGCP) Project 591, held jointly with the 5th International Symposium on the Silurian System, sponsored by the International Subcommission on Silurian Stratigraphy (ISSS). The joint meeting was hosted by the Institut national de la recherche scientifique – Eau Terre Environnement (INRS–ETE) in Quebec City, Canada, including 3 days of scientific sessions, a pre-conference field excursion to the Gaspe Peninsula, a mid-conference excursion to the vicinity of Quebec City, and a post-conference excursion to Anticosti Island, Quebec. The goals of IGCP 591 and the joint meeting with ISSS were outlined in the first volume of this two-volume series (Jin and Melchin 2016). The papers in this volume cover a range of topics in Early–Middle Paleozoic paleontology, stratigraphy, and geochemistry. Microfossils, particularly conodonts, …

Early–Middle Paleozoic ecosystem evolution and revolution (Part 1 of 2): an introduction

Papers included in this special issue have been derived and developed mostly from contributions presented at the 5th Annual Meeting of the International Geoscience Programme (IGCP) Project 591, held jointly with the 5th International Symposium on the Silurian System sponsored by the International Subcommission on Silurian Stratigraphy (ISSS). The joint meeting was hosted by the Institut national de la recherche scientifique – Eau Terre Environnement (INRS–ETE) in Quebec City, Canada, including 3 days of scientific sessions, a pre-conference field excursion to the Gaspe Peninsula, a mid-conference excursion to the vicinity of Quebec City, and a post-conference excursion to Anticosti Island, Quebec. Built on the tremendous success and momentum gathered by its two predecessors, the IGCP Project 410 (The Great Ordovician Biodiversification Event, 1997–2002; Webby et al. 2004) and Project 503 (Ordovician Palaeogeography and Palaeoclimate, 2004– 2009; Harper et al. 2011), the current IGCP Project 591 (The Early to Middle Paleozoic Revolution) aims to enrich our knowledge about the geological interval from the Great Ordovician Biodiversification Event (GOBE) to the expansion of the terrestrial ecosystems in the Devonian. After the Cambrian Explosion that saw the origin of nearly all animal phyla, the GOBE was marked by an exponential increase in diversity in terms of ordinal, superfamilial, and familial taxonomic units. Life evolution from the Early Ordovician to the Early Devonian, the key geological interval targeted by IGCP 591, was associated with a large number of significant paleoenvironmental events in Earth history, leading to global ecosystem revolutions, …

Genesis of Vendian‒lower paleozoic sequences on the western slope of the Subpolar Urals (Sablya Range)

The geochemical and lithological analysis of sections in the Sablya Range (Subpolar Urals) made it possible to substantiate a model of spatial‒temporal and genetic interrelations of processes responsible for rock formation during the Vendian‒Early Paleozoic evolution stage in the northeastern margin of the East European continent. Different lithological types are defined among rocks at the transition between the basement and sedimentary cover with their description and discussion of the role of synchronous magmatism in the formation of the latter. The succession of geological events and unusual features in the evolution of the composition, stratigraphy, and genetic affinity of deposits constituting the section of the transitional interformation zone are established.

Early Paleozoic oceanic inliers and reconstruction of accretionary tectonics in the Middle Gobi region, Mongolia: Evidence from SHRIMP zircon U-Pb dating and geochemistry

The ophiolites that occur as inliers among the Late Paleozoic formations in the Middle Gobi area are crucial for understanding the tectonic evolution of South Mongolia. In this paper, we conducted detailed studies on the Namdain hundy ophiolite to provide some constraints on the Early Paleozoic evolution of the Middle Gobi region in Mongolia. The ophiolite mainly consists of ultramafic rocks (carbonatation), plagiogranite, metagabbro, basalt and chert. The metagabbro and plagiogranite from Namdain hundy ophiolite yielded SHRIMP zircon U-Pb ages of 528±7Ma and 519±5Ma, respectively. Though most of the volcanic rocks of this ophiolite show supra-subduction zone (SSZ) affinity, samples with OIB and N-MORB geochemical features were also identified, indicating genesis in a forearc setting. The granodiorite intruding into the Namdain hundy ophiolite yielded a SHRIMP zircon U-Pb age of 491±3Ma, which constrained the upper age limit of ophiolite emplacement. This granodiorite shows adakitic geochemical affinity, attesting to the existence of Cambrian paleo - subduction in South Mongolia. Based on the available data so far, we suggest the Middle Gobi area comprises of the Manlay accretion complex, the island arc and the Biluutiin ovoo back-arc basin. The spatial configuration of these three tectonic belts suggests that the polarity of the paleo-ocean subduction was from south to north in the Early Paleozoic, forming a trench-arc-basin system south of the Central Mongolia microcontinent.

Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S1), is locally recognized in low strain areas, and is commonly isoclinally folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW-SE tight-close upright macroscopic folds (F3) with the doubly plunging geometry. F3 folds are superimposed by ∼NNW-SSE gentle F4 folds. The F3 and F4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F3) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F4 folds.

A Late Paleozoic sill complex and related paleo-topography in the eastern North Sea analyzed using 3D seismic data

In this paper, we utilize large igneous intrusions as a key to a detailed analysis and understanding of the late Paleozoic evolution of the Ringkøbing-Fyn High, an important structural element in the North West European Craton. The study takes advantage of high-quality 3D seismic data and boreholes to map the geometry and lateral distribution of intrusive sills cross-cutting the sedimentary strata at a low angle (transgressive sills). Our analysis shows that the transgressive sills most likely sourced the vast extrusion of volcanics, which covered most of the Early Permian Northern Basin and which is associated to the Skagerak-Centered Large Igneous Province (SCLIP). Furthermore, a geometrical analysis of the sills demonstrates that the magmatic source for the sills was located SE of the studied area, suggesting a correlation with geophysically inferred lower crust intrusions. Hence, we are in this study able to constrain the full magmatic system from the lower crust intrusions to the surface volcanics. Intrusion of the sills occurred prior to an Early Permian faulting event, which created rotated fault blocks outlining the present Ringkøbing-Fyn High. The sills exposed for erosion at the crest of the footwall in turn controlled the Late Permian paleo-topography and the distribution of the Zechstein evaporites due to the fact that they are harder to erode. Hence, we are able to demonstrate a topography controlled thickness variation of the Zechstein evaporites. The study furthermore emphasizes that an understanding of the deepest parts of the North Sea Basin is crucial when evaluating the potential for yet unrecognized hydrocarbon plays.

The zircon U–Pb and Hf isotope constraints on the basement nature and Paleozoic evolution in northern margin of Yili Block, NW China

The northern margin of Yili Block in Xinjiang, China, consists of Precambrian basement, Early Paleozoic sedimentary rocks intersected with bimodal volcanic rocks, conglomerate and Late Paleozoic volcanic and sedimentary rocks. Five magmatic events are identified: ~2.5Ga, 1.5–1.4Ga, 1.1 to 0.9Ga, 480 to 450Ma and 410 to 350Ma based on SHRIMP zircon U–Pb ages of volcanic rocks and conglomerate in Tulasu Basin in west part of Northern Yili Block margin. The Precambrian ages record the evolution of the basement under Yili Block. The age population of 480 to 450Ma corresponds to the Early Paleozoic bimodal volcanic rock, that of 410 to 350Ma is documented by Late Paleozoic volcanic rocks.The εHf(t) value of ~2.5Ga zircon is 1.0, corresponding to TDM2 of 2.9Ga, indicating the old crust of Yili Block formed during 3.0 to 2.5Ga, which is also evidenced by TDM2 (3.2 to 2.6Ga) of 1.5–1.4Ga aged zircons. The old crust reworked at 1.5–1.4Ga, proved by εHf(t) (−16.3 to −1.3) value of the zircons. The 1.1 to 0.9Ga zircons show εHf(t) value of −7.6 to 2.7 and TDM2 of 2.3 to 1.7Ga, might be the remelting products of old crust with some contribution of mantle. The Precambrian age groups of Yili Block are coeval with Chinese Central Tianshan, Tarim Craton and Kyrgyz North Tianshan, suggesting that they may represent fragments of a single continent characterized by magmatic events at 1.5 to 1.4Ga and 1.1 to 0.9Ga.Bimodal volcanic rocks erupted at 480 to 450Ma, thus a rift-related tectonic setting was proposed in combination with reported geochemical characteristic. The Basaltic rocks with age of ~477Ma have εHf(t) value of 3.4 to 13.9, showing a magma source of depleted mantle with involvement of little crustal material. The felsic rocks with age of ~450Ma have εHf(t) value of −20.4 to 1.2, indicative of a continental crust source and little contribution of mantle magma. Their magma sources also show rift-related magma property. The volcanic rocks erupted during 410 to 350Ma were the products of subduction of the North Tianshan oceanic crust, which made contribution to crustal growth of northern Yili Block. The magma source of the Paleozoic volcanic rocks is depleted mantle contaminated by crust, constraint by their positive εHf(t) value (−0.2~12.8).

Zircon ages and Hf isotopic compositions of Ordovician and Carboniferous granitoids from central Inner Mongolia and their significance for early and late Paleozoic evolution of the Central Asian Orogenic Belt

We present zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the evolution of central Inner Mongolia, China, in the early and late Paleozoic. Zircons of a granodiorite yielded a 206Pb/238U age of 472±3Ma that reflects the time of early Paleozoic magmatism. Zircon ages were also obtained for a tonalite (329±3Ma), quartz-diorite (320±3Ma), and granite vein (297±2Ma). Our results, in combination with published zircon ages and geochemical data, document distinct magmatic episodes in central Inner Mongolia.The dated samples are mostly granodiorite, tonalite and quartz-diorite in composition with intermediate to high-silica, high Na2O (3.08–4.26wt.%), low K2O (0.89–2.86wt.%), and high Na2O/K2O and Sr/Y ratios. Their chondrite-normalized REE patterns are characterized by LREE enrichment. In mantle-normalized multi-element variation diagrams they show typical negative Nb and Ta anomalies, and all samples display positive εHf(t) and εNd(t) values, and low ISr. The Ordovician rocks, however, show higher Sr/Y and La/Yb ratios than the Carboniferous samples, implying that the older granitoids represent adakitic granitoids, and the Carboniferous granitoids are typical subduction-related arc granitoids but also with adakite-like compositions. The results are compatible with the view that the Central Asian Orogenic Belt (CAOB) in Inner Mongolia evolved through operation of several subduction systems with different polarities: an early–middle Paleozoic subduction and accretion system along the northern margin of the North China Craton and the southern margin of the Mongolian terrane, and late Paleozoic northward subduction along the northern orogen and exhumation of a high-pressure metamorphic terrane on the northern margin of the North China Craton.

Early Permian volcano-sedimentary successions, Beishan, NW China: Peperites demonstrate an evolving rift basin

The Lower Permian volcano-sedimentary Zhesi Group has been investigated in the Hongliuhe and Liuyuan areas in Beishan, China, which is significant for the reconstruction of Late Paleozoic evolution in the southern part of the Central Asian Orogenic Belt. A variety of volcanic facies were distinguished in the Upper Zhesi Group: pillow basalt with interstitial limestone, thin-interbedded limestone and basalt, closely packed pillows, pillow-fragmented hyaloclastite breccia, and peperite. Laser 40Ar/39Ar whole-rock dating of the basalt yielding an age of 277±11Ma, as well as Early Permian brachiopod fossils in the limestone interbedded with the basalt, indicate that basalt was erupted in the Early Permian. The identification of the peperite and other facies originating from magma-sediment mingling reveals that the basaltic lava flows were derived from autochthonous basaltic magmatism and formed as part of the Lower Permian succession. The peperite also indicates that these subaqueous basaltic lava flows are not dismembered ophiolitic components, but formed in an autochthonous extensional setting in the Early Permian. The clastic rocks in the Lower Zhesi Group underlying the basaltic flows and peperites in the Hongliuhe and Liuyuan areas show a general fining-upwards sequence, indicating that they were deposited in a progressively deepening basin overlying the Devonian Hongliuhe suture zone. Subaqueous volcanism in a rift basin or basins, accompanied by coeval deposition of carbonate sediment and mud, built up the peperite-bearing volcanogenic-sedimentary successions. From among the various tectonic hypotheses for the Beishan region, this study demonstrates that by Early Permian the region was developing post-collisional rift basins.

Late Carboniferous to early Permian sedimentary–tectonic evolution of the north of Alxa, Inner Mongolia, China: Evidence from the Amushan Formation

The late Paleozoic evolution of the Wulijishanhen (WSH)-Shangdan (SD) area near to the Chaganchulu Ophiolite belt is reinterpreted. Analysis of the upper Carboniferous to lower Permian sedimentary sequence, biological associations, detrital materials, sandstone geochemistry and volcanic rocks indicates that the SD area was an epicontinental sea and rift during the late Paleozoic rather than a large-scale ocean undergoing spreading and closure. This study reveals that the actual evolution of the study area is from the late Carboniferous to the early Permian. The fusulinids Triticites sp. and Pseudoschwagerina sp. in the limestones demonstrate that the Amushan Formation develops during the late Carboniferous to the early Permian. The limestones at the base of the SD section indicate that it is a stable carbonate platform environment, the volcanic rocks in the middle of the sequence support a rift tectonic background, and the overlying conglomerates and sandstones are characteristic of an epicontinental sea or marine molasse setting. The rift volcanism made the differences in the fossil content of the SD and WSH sections and led to two sections expose different levels within the Amushan Formation and different process of tectonic evolution. Moreover, the geochemical characteristics and detrital materials of the sandstones show that the provenance and formation of the sandstones were related to the setting of active continental margin. The quartz-feldspar-lithic fragments distribution diagram indicates that the material source for the sandstones was a recycled orogenic belt. Thus, the source area of the sandstones may have been an active continental margin before the late Carboniferous–early Permian. The characteristics of the regional tectonic evolution of the area indicate that the region may form a small part of the Gobi–Tianshan rift of southern Mongolia.

Correction of two Upper Paleozoic stratigraphic units in the Tianshan Mountains region, Xinjiang Uygur Autonomous Region and implications on the Late Paleozoic evolution of Tianshan tectonic complex, Northwest China

The present paper addresses two tectonostratigraphic concerns on the Late Paleozoic Tianshan tectonic complex (TTC), Xinjiang, Northwest (NW) China: (1) stratigraphic succession and age constraint of the Bayingou ophiolite mélange, eastern Tianshan Mountains and (2) timing of closure of the southern Tianshan ocean and accretion of the Siberian craton recorded in the Aiweiergou (AWEG) area, eastern Tianshan Mountains by integrating stratigraphy, palaeontology, tectonopalaeogeography and palaeobiogeography. In the Bayingou area, the detailed palaeontological survey denies the presence of brachiopod Gigantoproductus fauna, typical of the Early Carboniferous faunas in central–south Tianshan complex, in the Anjihai Formation. In contrast, the Anjihai brachiopod assemblage, as a whole, appears to have a high affinity with the Late Devonian faunas of the eastern Junggar Basin, northern Xinjiang, suggesting a Late Devonian age for the Anjihai Formation. The overlying Shadawang Formation yields the Early Carboniferous radiolarians. These two units form the main part of the Bayingou ophiolite mélange, which therefore is likely Late Devonian to Early Carboniferous in age. The Bayingou area has been likely part of the northern Tianshan-Junggar block since the Late Devonian, although it may have been part of the Central Tianshan tectonostratigraphic province prior to the Late Devonian. The topmost strata of the Bayingou ophiolite mélange are characterized by alternation of volcanics, conglomerate and mudstone, and are better re-assigned to the Taoxigou Group rather than the Keguqingshan Formation. The Bayingou ophiolite mélange comprises the Late Devonian Anjihai Formation, the Carboniferous Bayingou and Shadawang Formations, and the Early Permian Taoxigou Group. In the AWEG area, the Permian and Triassic rocks were previously misinterpreted as the Late Permian turbidites and Late Triassic red beds, respectively. In fact, the Permian successions in AWEG consist of the Early Permian Taoxigou Group and early Middle Permian Lucaogou Formation. The former represents a foreland molasse succession, while the latter yields abundant non-marine fossils of plants, bivalves, and gastropods, and represents typical lacustrine facies deposits. The unconformity between the Permian and the Triassic rocks cannot represent the closure of the Tianshan Ocean, but indicates tectonic uplifts in the foreland basins. In contrast, the molasse-type sediments of the Lower Permian Taoxigou Group may have resulted from the post-orogenesis uplifting and mark the closure of the Tianshan Ocean prior to the Early Permian. Thus, the closure of the Tianshan Ocean and the final tectonic accretion of the South Tianshan block might have taken place over the Permo-Carboniferous transition, strengthened by faunal assemblages obtained from both southern and northern sides of the TTC.

Collision-related Early Paleozoic evolution of a crustal fragment from the northern Gondwana margin (Slavonian Mountains, Tisia Mega-Unit, Croatia): Reconstruction of the P–T path, timing and paleotectonic implications

An orthogneiss from the oldest metamorphic complex at Mt. Papuk (Tisia Mega-Unit, Croatia) enables the quantification of the P–T evolution of Early Paleozoic rocks of the Panonian Basin basement in contrast to neighboring peri-Gondwanan terrains which are significantly overprinted by pre-Variscan, Variscan, and Alpine events. Two different groups of Ce-rich monazite within oval-shaped corona microstructures have been observed. Age dating of the corona cores yielded two populations with average ages of 528±7 (2σ) Ma and 465±7Ma, respectively. Furthermore, an Y-rich group, found inside garnet cores, was dated at 616±23Ma. Th-rich monazite included in garnet rims yielded an age of 491±6Ma. The youngest monazite group at 417±20Ma is located inside mica. The orthogneiss precursor was a calc-alkaline to high-K calc-alkaline igneous peraluminous crustal rock (diorite) from an active continental marginal setting. The calculated P–T pseudosection in the MnNCKFMASHTO system in combination with assemblage characteristics and mineral chemistry data provides good constraints on the P–T evolution: for stage I peak P–T conditions of 13kbar and 670°C were derived followed by stage II, which was characterized by moderate cooling accompanied by uplift to mid-crustal levels (5.2kbar and 610°C). Subsequently, the system cooled to 480°C at ~4.4kbar (stage III). Formation of titanite rims on ilmenite suggests further cooling to 4kbar and 400°C (stage IV). The clockwise P–T path implies exhumation from a tectonically thickened crustal setting (ca. 45km depth at a geothermal gradient of ~15°C/km) to mid-crustal levels (ca. 18km) followed by cooling at depths <14km. Crustal thickening was due to the collision of a continental plate (Gondwana) with a smaller plate, which was underthrust.

Late Paleozoic evolution of the South Tien Shan: Insights from P–T estimates and allanite geochronology on retrogressed eclogites (Chatkal range, Kyrgyzstan)

In the South Tien Shan range (Kyrgyzstan), the Late Paleozoic geodynamic evolution remains debated especially to the west of the Talas-Fergana fault (TFF) fault where suture-related high-pressure (HP) rocks are scarce. We provide new petrological and geochronological data on garnet amphibolites from the Chatkal range, to the west of the TFF, northwest of the South Tien Shan suture. These rocks are retrogressed eclogites. We used a micro-mapping approach combined with forward modeling and empirical thermobarometry to decipher the P–T path of these amphibolitized eclogites. The metamorphic peak conditions culminated at 490±50°C and 18.5±2kbar and were followed by higher temperature retrogression (∼560°C at 11–7kbar). In order to constrain the age of the HP stage, we dated allanite crystals texturally coeval to the HP mineral assemblage. Allanite grains dated in situ with a U–Pb LA-ICPMS methodology yield an age of 301±15Ma. Compared with previously published data for the east of the TFF, these P–T constraints allow improving the understanding of the Late Paleozoic geodynamic evolution of the South Tien Shan. To the east of TFF, the Turkestan Ocean closed around 320Ma with the collision of the Tarim Craton with the Kazakh microcontinent. To the west of TFF, the Turkestan Ocean closed around 300Ma, when the Alai block collided with the Kazakh microcontinent. This later collision involved nappe-stacking and intense subvertical folding in the western South Tien Shan. This complex folding explains the S-shape of the suture to the west of the TFF that cannot be observed in the eastern part. These new data allow us to propose a distinct tectonic evolution of the two sides of the TFF, which suggests that this fault was a major transform fault before being a strike-slip intra-continental fault.

Paleozoic magmatism and metamorphism in the Central Tianshan block revealed by U–Pb and Lu–Hf isotope studies of detrital zircons from the South Tianshan belt, NW China

As a major Precambrian microcontinent in the southernmost Central Asian Orogenic Belt (CAOB), the Central Tianshan block (CTS) in the Chinese Tianshan is essential for understanding the final assembly of the southern CAOB. It experienced multistage Paleozoic magmatism and metamorphism, but the detailed processes are still controversial and far from being completely understood. This paper reports coupled U–Pb and Lu–Hf isotopic data of detrital zircons from late Paleozoic (meta-)sedimentary strata in the South Tianshan belt, which can provide new insight into deciphering the Paleozoic evolution of the eastern segment of the CTS block. Characterized by typical oscillatory zoning and high Th/U ratios (>0.2), detrital zircons in the Permian sedimentary samples yield dominant age populations at ca. 505–490Ma, 475–440Ma, 430–400Ma and 340–250Ma, pinpointing the development of four episodes of magmatism in the eastern CTS block. Particularly, Ordovician–Silurian (475–440Ma) zircons, possessing low negative εHf(t) values, predominate in sedimentary strata in and surrounding the CTS block, indicating that the 475–440Ma magmatic rocks probably constitute the main body of the CTS block. The origin of this (early Paleozoic) episode of magmatism was most likely related to the southward subduction of the Junggar Ocean beneath the CTS block. Carboniferous–Triassic (340–250Ma) zircons have dominantly positive εHf(t) values, probably derived from the post-collisional juvenile rocks in the CTS block. Combined with previous studies, our data suggest that the single source terrane for the sampled strata was the CTS block, which had been a topographic high providing substantial detritus to the surrounding areas at least since the Early Permian.In the metasedimentary sample, detrital zircons mostly show partially/fully recrystallized internal textures and low Th/U ratios (<0.2), probably sourced from the amphibolite- to granulite-facies metamorphosed rocks in the eastern CTS block. The youngest zircons suggest that the sampled metasedimentary strata were Carboniferous (341–305Ma) in age, rather than Neoproterozoic (or Early Sinian in Chinese literatures) as previously interpreted. Based on U–Pb–Hf isotopes and Ti-in-zircon thermometer of metamorphic zircons in this study, as well as previous investigations, we suggest that the eastern CTS block has experienced (a) a 400–370Ma collisional event, related to the closure of the eastern segment of the South Tianshan Ocean, and (b) a 360–340Ma extensional event, as a result of post-collisional mantle upwelling. Therefore, the final assembly of the Eastern Tianshan took place during Late Carboniferous time.

The boundary between the Central Asian Orogenic belt and Tethyan tectonic domain deduced from Pb isotopic data

A detailed comparison of the tectonic features of Central Asian Orogenic belt (CAOB) and Tethyan Tectonic domain (TTD) is of great significance to our understanding of the origin of global orogenic systems. Currently, there are many uncertainties in the general framework to fully define the tectonic properties of the CAOB and TTD. The Pb isotope data from Paleo-Asian Ocean (PAO) ophiolites in the CAOB and Tethyan ophiolites in the TTD allow us to conduct a detailed comparative study between these two global orogenic systems. Results of the study show the presence of an isotopic boundary between the different mantle domains and tectonic properties of the CAOB and TTD, with the Xinjiang region in the former representing the transition between the two systems. The distinctive 208Pb/204Pb isotope compositions of the PAO and Tethyan mantles suggest the existence of a long time-integrated lower Th/U reservoir beneath the CAOB compared to that beneath the TTD throughout the Paleozoic. Results thus suggest the distinct Pb isotope compositions of the PAO and Tethyan mantles are intimately related to the different magma–tectonic processes that formed the CAOB and TTD. Based on plate tectonic reconstruction, the Neoproterozoic to Paleozoic evolution of the accretionary margins of the CAOB mimics the modern circum-Pacific Ocean rim. In this scenario, the PAO had a low Th/U mantle isotopic signature and the subduction of PAO crust gave rise to the circum-Pacific type accretionary orogen. On the other hand, the Tethys oceans produced the high Th/U mantle isotopic signatures in an evolving collisional orogen. Significantly, the generally radiogenic and juvenile Hf isotopic signature of the CAOB is consistent with an accretionary orogenic setting for PAO whereas the relatively more unradiogenic Hf isotopic signature of TTD is consistent with a collisional orogenic setting for Tethys oceans. Thus, our study sheds some light on the PAO evolution as well as the plate tectonic reconstruction of Central Asian and Tethyan orogens.

Paleozoic evolution of western Marie Byrd Land, Antarctica

We report geochemical data from (meta-)sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low-grade metaturbidite sequence, define distinct populations in the late Paleoproterozoic, late Mesoproterozoic, and Neoproterozoic–Cambrian. The late Paleoproterozoic group records magmatism derived from a mixed juvenile and crustal source. By contrast, the late Mesoproterozoic group yields Hf isotope values consistent with derivation from a juvenile Mesoproterozoic source inferred to be an unexposed Grenville-age orogenic belt beneath the East Antarctic ice sheet. For the Neoproterozoic–Cambrian population, Hf isotope values indicate reworking of these older materials during Ross–Delamerian orogenesis. New U-Pb ages from the Devonian–Carboniferous Ford Granodiorite suite across the Ford Ranges reveal an extended period of arc magmatism from 375 to 345 Ma. For four younger samples of Ford Granodiorite, Hf and O isotope values in zircon suggest involvement of a larger (meta-)sedimentary component in the petrogenesis than for two older samples. This contrasts with the secular trend toward more juvenile values documented from Silurian to Permian granite suites in the Tasmanides of eastern Australia and Famennian to Tournasian granite suites in New Zealand, pieces of continental crust that were once contiguous with western Marie Byrd Land along the Gondwana margin. The differences may relate to an along-arc change from the typical extensional accretionary mode in eastern Australia to a neutral or an advancing mode in West Antarctica, and to an across-arc difference in distance from the trench between the New Zealand fragments of Zealandia and western Marie Byrd Land. Upper Devonian anatectic granites in the Ford Ranges most likely record reworking of early Ford Granodiorite suite members during arc magmatism.