Evolution Of Belt Research Articles

Early Paleozoic Protoliths of Gneisses and Granitic Gneisses in the Eastern South Urals: Results of U–Th–Pb (SIMS) Geochronological Studies

The Early Paleozoic age of the protolith for gneisses in the East Uralian megazone (South Urals) is proved by zircon dating. Two metamorphic complexes have been identified in the eastern framing of the Middle–Late Paleozoic Suunduk granitoid pluton. The Zelenodolsk complex is represented by granitic gneisses, while the Kusakan complex (Kusakan Formation) is represented by metavolcanic gneisses, crystalline schists, and quartzites. U–Th–Pb analyses (SHRIMP II) of accessory magmatic zircons from the Zelenodolsk granitic gneiss yielded dates of 478 ± 5 Ma. The protolith age of the Kusakan gneiss (andesitic metatuff) is 529 ± 6 Ma. The origin of volcanic, volcanosedimentary rocks, and granitoids in the East Uralian megazone is associated with the evolution of the volcanoplutonic belt in the active continental margin.

Post-Orogenic Tectonic Evolution of the Jiangnan-Xuefeng Orogenic Belt: Insights from Multiple Geochronometric Dating of the Mufushan Massif, South China

The Mufushan massif, as continental intra-plate magmatites located in the Jiangnan-Xuefeng orogenic belt of the South China. The Mufushan massif constitutes the largest Mesozoic intrusive complex, intruded the Mesoproterozoic Lengjiaxi Formation. Multiple geochronometric dating was used to reconstruct their evolution from emplacement to exhumation. The Mufushan granitoids were emplaced at ∼150 Ma (U-Pb zircon) as post-orogenic magmatites contributing to Triassic crustal thickening. Onset of regional extension at ∼128 Ma (40Ar/39Ar white mica and biotite) manifests a tectonic regime switch. Intense exhumation prior to ∼55 Ma was followed by slow denudation and peneplanation for the next 37 Ma (∼55’18 Ma). Accelerated cooling since ∼18 Ma may have been caused by a far-field effect of the collision between India-Asia Plate or the Pacific-Plate subduction. Through a multi-geochronometric approach, this study provides a new comprehensive model for the cause of the intra-plate magmatism formation in the South China, and also established a reliable geochronological framework of the post-orogenic tectonic evolutions of the Jiangnan-Xuefeng orogenic belt.

A New Understanding on the Emplacement of Ophiolitic Mélanges and Its Tectonic Significance: Insights from the Structural Analysis of the Remnant Oceanic Basin‐Type Ophiolitic Mélanges

AbstractOphiolites represent on‐land fragments of paleo‐oceanic crust and have been recognized as one of the key markers of suture zones. Here, we provide new insights into the emplacement of ophiolitic mélanges based on detailed geological mapping and structural analysis in the West Junggar and Songpan‐Ganzi‐Bayan Har orogens (Fig.1 and Fig.2). The results show that some ophiolitic mélange belts cannot be regarded as suture zones. The distribution of these ophiolitic mélange belts are usually associated with the structural processes during the closure of remnant oceanic basins. After the remnant‐oceanic basin is filled with thick clastic deposit, the oceanic lithosphere material as the base of the remnant basin can be injected into the overlying sedimentary strata through various faultings under the regional compressive stress, forming the remnant oceanic basin‐type ophiolitic mélange system with dispersive distribution characteristics. Combining with previous researches, the emplacement mechanism of ophiolitic mélanges can be divided into four categories: subduction type which oceanic lithosphere subducted beneath active continental margin, obduction type which oceanic lithosphere obducted over passive continental margin, collision type between two continental lithospheres, and closure type of remnant‐oceanic basin (Fig.3). These different types of ophiolitic mélange belts will be superimposed and even re‐emplacement by the tectonic processes of post‐plate convergence, complicating their distribution. Therefore, identifying the emplacement mechanism type of ophiolitic mélange belts formed in different tectonic processes and backgrounds is of importance for understanding the process of ocean‐continental transition and the evolution of orogenic belts.

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo‐Asian Ocean

AbstractAs the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt (CAOB) have always been one of the hot topics among geologists (Sengör et al., 1993, 1996; Jahn et al., 2000a; Badarch et al., 2002; Windley et al., 2007; Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino‐Korean and Tarim paleo‐continents and the final collision between these continental margins because of the subduction of the Paleo‐Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite (serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the “ophiolite trinity”, were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U‐Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian (275–280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan (1997), indicate that the oceanic basin was not closed in early‐middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid‐ocean ridge basalt (N‐MORB), enriched mid‐ocean ridge basalts (E‐MORB), oceanic island basalt (OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo‐Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

Dating recurrent shear zone activity and the transition from ductile to brittle deformation: White mica geochronology applied to the Neoproterozoic Dom Feliciano Belt in South Brazil

Argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation, as temperature ranges for mineral crystallization and isotopic closure of different grain-size fractions overlap between ca. 275–425 °C. This study presents over 40 K-Ar ages of coarse-grained muscovite and multiple clay-sized white mica fractions extracted from mylonites and fault gouges in southernmost Brazil, complemented by detailed structural and XRD characterization. The widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low-grade ductile deformation into brittle faulting. The onset of the Ibaré Shear Zone is constrained in the Tonian, followed by reactivation during the Ediacaran evolution of the adjacent Dom Feliciano Belt, which records a remarkable long-term history of recurrent deformation for over 100 Myr. New data extends the geochronological constraints of the Dorsal do Canguçu Shear Zone into low-temperature conditions, estimating the onset of brittle activity at 525-470 Ma by dating early fault gouges formed shortly after final syn-kinematic white mica crystallization in the hosting mylonites. Dating of recurrent faulting events during the Paleo-Mesozoic complement the regional sedimentary and thermochronological record.

Exogenic basalt on asteroid (101955) Bennu

When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission5 that has been spectroscopically linked to the CM carbonaceous chondrite meteorites6. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7–9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt. Six bright boulders of exotic material on near-Earth asteroid (101955) Bennu stand out from the average asteroidal surface. This unexpected record of impactors offers clues to the formation history of Bennu.

Structural styles and evolution of a thin-skinned fold-and-thrust belt with multiple detachments in the eastern Sichuan Basin, South China

The structural deformation of the eastern Sichuan Basin, South China, is characterized by a thin-skinned fold-and-thrust belt with exposed, approximately parallel narrow anticlines and wide synclines. A 196-km long regional cross section has been constructed parallel to the shortening direction to analyze structural styles and evolution of the fold-thrust belt. The stratigraphic succession is composed of competent layers separated by three main incompetent layers which act as detachments. The basal detachment, the Cambrian evaporites, played a dominant role in the deformation; the middle and top detachments accommodated the displacement during deformation. Structural styles are dominated by faulted detachment folds with the breakthroughs of the fore- and/or backlimbs. Balanced restoration shows that the shortening between basal and top detachments is about 26 km with the shortening ratio of 12.4–18.6%. Integrated analyses of kinematics and balanced restoration reveal the evolution succession of kink bands, detachment folds, and faulted detachment folds. Kink bands initiated and evolved into detachment folds above the basal detachment as the shortening took place; with progressive shortening the back- and/or forelimbs of these folds developed thrusts.

How whistler mode hiss waves and the plasmasphere drive the quiet decay of radiation belts electrons following a geomagnetic storm

We show how an extended period of quiet solar wind conditions contributes to a quiet state of the plasmasphere that expands up to L ∼ 5.5, which creates the perfect conditions for wave-particle interactions between the radiation belt electrons and whistler-mode hiss waves. The correlation between the hiss waves and the plasma density is direct with hiss wave power increasing with plasma density, while it was generally assumed that these quantities can be specified independently. Whistler-mode hiss waves pitch angle diffuse and ultimately scatter freshly injected electrons into the atmosphere until the slot region is formed between the inner and outer belt and the outer belt is drastically reduced. In this study, we use and combine Van Allen Probes observations and Fokker-Planck numerical simulations. The Fokker-Planck model uses consistent event-driven pitch angle diffusion coefficients from whistler-mode hiss waves. Observations and simulations allow us to reach a global understanding of the variations in the trapped electron population with time, space, energy, and pitch angle that is based on the existing theory of quasi-linear wave-particle interactions. We show, for instance, the outer belt is pitch-angle homogeneous, which is explained by the event-driven diffusion coefficients that are roughly constant for equatorial pitch angle α 0∼<60°, E>100 keV, 3.5<L<Lpp∼6. The impact of this work is to bring an improved understanding of the belt evolution based on the integration of high quality and highly temporally and spatially resolved measurements that are integrated in modern computations. We also propose the event-driven method as an accurate method (within ×2) to predict the electron flux decay after storms.

Geochemical evolution of the Late Carboniferous to Early Permian igneous rocks from the Baolidao magmatic belt, eastern Central Asian Orogenic Belt

The Late Palaeozoic tectonic evolution of the eastern Central Asian Orogenic Belt (CAOB) and the genesis of magmatic rocks in the terrane have been topics of debate for decades. We have tried to understand the processes involved in the evolution of the Late Palaeozoic Baolidao belt and magma generation by compiling data from more than 200 samples along with new age estimates and geochemical data. Spinel stability field melting of the subduction hydrated juvenile depleted mantle (DM) is the dominant mafic magma generating process. The crustal granitoids from the Late Carboniferous suite (320 ~ 300 Ma) and the Early Permian suite (300 ~ 280 Ma) have contrasting characteristics. The Late Carboniferous suite is characterized by lower crustal AFC process at hornblende‐dominated and plagioclase‐free pressure, which resulted in considerable growth of continental crust, while the Early Permian suite is characterized by low pressure partial melting (plagioclase stable in residue) of a thinned juvenile crust. The process of the extension of hydrous mantle coupled with crustal thinning from the Late Carboniferous to Early Permian can be used to explain the chemical and petrogenetic features of the Late Palaeozoic magmatism along the Baolidao belt. We propose that lithosphere‐scale extension should have been a key process in the tectonic evolution of the eastern CAOB during the Late Palaeozoic.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of &amp;gt;300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Geochemistry of the Mesoproterozoic Intrusions, Geochronology and Isotopic Constraints on the Xiaonanshan Cu-Ni Deposit along the Northern Margin of the North China Craton

Mesoproterozoic magma events in the Bayan Obo rift belt have remained poorly constrained and as a result, the Late Paleoproterozoic-Mesoproterozoic tectonic evolution of the rift belt has remained unclear. By a multiple-facetted regional geological investigation of this belt, we have resolved the stratigraphic sequence and geochronology of the Bayan Obo Group and made new discoveries including a three-stage mantle-derived magmatic sequence. Zircon and baddeleyite dating of Xiaonanshan hornblende pyroxenite emplaced into the Bayan Obo Group yields 207Pb/206Pb ages of ca. 1.34 and 1.33 Ga. The geochronological, geochemistry, Hf isotopic analyses place an important constraint on ages of the Late Paleoproterozoic-Mesoproterozoic strata and the evolution of the rift belt. Our field observations and U-Pb dating results suggest that mineralization is genetically related to Mesoproterozoic magmatism in North China Craton, i.e., 1.33–1.34 Ga. The δ34SV-CDT values of sulphide from the ore-bearing ultra-/mafic samples are about 6.2‰, whereas the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values vary in the ranges of 17.598–18.115, 15.496–15.501, and 37.478–37.952, respectively. The Late Paleozoic mafic gabbro and acidic granite porphyry intrusions are possible to bimodal magmatic event related to the extensional tectonic setting of the Central Asia in this period.

Tectonic Evolution of Southeast Central Asian Orogenic Belt: Evidence from Geochronological Data and Paleontology of the Early Paleozoic Deposits in Inner Mongolia

In this study, we present detrital zircon U-Pb dating and paleontological data for the newly identified Ayadeng Formation in the northern margin of the North China Block (NCB) and Xibiehe Formation (molasse) in the Bainaimiao arc belt (BAB), which could provide strong evidence indicating the affinity of the BAB and the evolution of the southeast Central Asian orogenic belt (CAOB). Zircon U-Pb data of siltstone samples and paleontological data indicate the Ayadeng Formation dates back to the Early Ordovician. Although its location is near the NCB, its zircon age spectra and paleontology share a closer affinity with those of Tarim and NE Gondwana, as the U-Pb data suggest an age range of 490–2 192 Ma (peak age=629, 788, 965 and 1 935 Ma), and similar gastropod fossils are found in Tarim and NE Gondwana. The U-Pb ages of meta-sandstone samples in the Xuniwusu Formation indicate a shared inheritance with the Ayadeng Formation (before 440 Ma), and the U-Pb ages of sandstone samples in the Xibiehe Formation are concentrated, with age peaks centered at ca. 420 Ma. Fossil corals occur in these two formations, and their age components also indicate a collisional setting. Therefore, it is speculated that the BAB drifted away from Tarim or NE Gondwana during the Ordovician and became attached to northern NCB between 440–420 Ma as an exotic terrane. During the Early Paleozoic, there may have occurred a collision between an arc and a continental block.

Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China

The eastern Sichuan-western Hunan and Hubei belt (ESWHHB) is an important fold-thrust belt in the Middle-Upper Yangtze region of China, and it is also an important area for petroleum and gas prospect in China. The influence of mechanical stratigraphy on the deformation evolution of the ESWHHB is a hot problem that has received widespread attention. However, due to the complexity of geological conditions, this issue has not been sufficiently addressed. Previews geological exploration studies show that the deformation evolution of the belt is closely related to the mechanical stratigraphy. Physical simulation has proven to be effective for studying the deformation evolution of fold-and-thrust belt. Based on the geological conditions of the ESWHHB, six groups of physical models were designed to analyze the influences of the ductile layer and overlap configuration on the structural deformation of the ESWHHB. The results show that the mechanical stratigraphy has significant control on the deformation evolution of the fold-thrust belt. The ESWHHB evolution is related to the lower viscosity of the ductile layer and the larger thickness of the ductile layer, while only gradual propagated fold-and-thrust belt can be resulted from the higher viscosity of the ductile layer and the smaller thickness of the ductile layer. Additionally, the overlap between the stratigraphy at various structural belts leads to significant differences in their mechanical properties, and it critically influences the structural patterns of the ESWHHB.

Ca. 2.1 Ga Mahakoshal Supracrustal Belt: An allochthonous terrain in Central India Tectonic Zone

Back-arc magmatism is perhaps extremely rare in a Paleoproterozoic mobile tectonic regime. The present study provides a detailed account on the petrogenesis of basalts that are interleaved with marine sedimentary rocks in the Agori Formation, which overlies the Chitrangi Formation in the ~2.1 Ga Mahakoshal supracrustal belt within the Central India Tectonic Zone. The basalts are sub alkaline, tholeiitic in nature. They have moderate concentrations of TiO2 (~0.9 wt%), Nb (2.6–5.2 ppm), and Zr (59–105 ppm), Zr/Nb (18–24) and Zr/Y (3.1–3.6) ratios, mildly enriched chondrite normalized rare earth element patterns (LaN/YbN = 1.6–2.8), and negative Nb and Ti anomalies on a primitive mantle normalized trace element variation diagram. Post-magmatic low grade metamorphism, alteration and trace element mobility, and/or contamination by Archean continental crust, if any, cannot relate to these patterns. The positive initial εHf (2.1 Ga) + 2.24 to +5.15 and εNd(2.1 Ga) + 1.02 to +1.54 isotopic compositions in these basalts are variable, yet consistent with a juvenile mantle source. We ascribe the isotopic variability in the Agori basalts to reflect the general isotopic variability in the depleted mantle at 2.1 Ga. Consequently, the basalts were produced by decompression partial melting in the shallow upper mantle in a back-arc setting. Subduction zone magmatic processes produced the arc volcanics in the upper stratigraphic unit (Agori), whereas, a plume source generated the ferropicrites in the underlying Chitrangi Formation. The evolution of the Mahakoshal supracrustal belt predates the collision and interlocking of North and South India cratonic blocks, which resulted in the formation of Central India Tectonic Zone. Hence, it represents an allochthonous plume-arc accreted terrain in the CITZ.

Early Paleozoic Ocean Plate Stratigraphy of the Beishan Orogenic Zone, NW China: Implications for Regional Tectonic Evolution

AbstractThe Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well‐enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Paleozoic deposits. From the Cambrian to the middle Ordovician, this region was characterized by geographically extensive, flat‐bedded siliceous mudstone, indicating the existence of a large ocean basin. The oceanic plate entered the convergence phase in terms of the Wilson Circle during the Middle Ordovician, when numerous magmatic arcs formed along two opposite sides of the ocean. The magmatic arcs became the widest during the Silurian, suggesting that the Hongliuhe‐Niujuanzi‐Xichangjing Ocean (HNX; a southern branch of the Paleo Asian Ocean) was reduced to a small residual ocean in the central Beishan region by that time, and probably lasted till the Carboniferous or later by newly published data.

Zircon U–Pb Geochronology and Petrogenesis of Early–Midlle Permian Arc–Related Volcanic Rocks in Central Jilin: Implications for the Tectonic Evolution of the Eastern Segment of Central Asian Orogenic Belt

AbstractThis paper presents age and geochemical data of a recently identified Late Paleozoic volcanic sequence in central Jilin Province, with aims to discuss the petrogenesis and to constrain the tectonic evolution of the Central Asian Orogenic Belt in this area. Firstly, the volcanic rocks have zircon U‐Pb ages of 290–270 Ma. Secondly, they are characterized by (a) ranging in composition from the low‐K tholeiite series to high‐K calc‐alkaline series; (b) enrichment in light rare earth elements and depletion of heavy rare earth elements, with negative Eu anomalies; and (c) negative Nb, Ta, and Ti anomalies. Finally, the volcanic rocks yield ∊Hf(t) values of +7.1 to +17. These data suggest that the central Jilin volcanic rocks were possibly derived from predominant partial melting of a depleted lithospheric mantle that might have been modified by subducted slab–derived fluids. Combined with previous studies, the Late Paleozoic–Early Mesozoic magmatism in Central Jilin can be divided into two stages: (a) a volcanic arc stage (290–270 Ma) represented by low‐K to high–K, tholeiite to calc–alkaline plutons and (b) a syn–collisional stage (260–240 Ma) represented by high‐K calc–alkaline I‐type granites. Furthermore, the timing and the tectonic setting of the above magmatic rocks show that the arc was probably produced by the northward subduction of the Paleo‐Asian Ocean and that the final closure of the Paleo‐Asian Ocean occurred prior to the Early Triassic.

RETRACTED: Detrital zircon age distribution characteristics of Jurassic sandstone from the Southern Yili basin, China

The Central Asia orogenic belt contains a wide range of structural elements, including micro continent, back arc system, ocean island/plateau, ophiolite and subduction accretion complex. But its final closing time has been controversial. Based on the magmatic age of the surrounding orogenic belt, the source of this set of clastic sediments is determined, which provides new important evidence for the evolution of the South Tianshan orogenic belt. The results show that the Jurassic detrital zircons from the study area were mainly derived from magmatic zircons and are deposited in a proximal source. The detrital zircon age of the Lower Jurassic Badaowan and Sangonghe Formation are concentrated in 290–260 Ma, and in 350–290 Ma and 460–390 Ma, respectively. The detrital zircon age of the Middle Jurassic Xishanyao Formation concentrates in 370–320 Ma and 450–390 Ma. There are very few zircons from the Precambrian period. These ages are consistent with the timing, indicating these clastic sediments were mainly originated from the southern margin of the Yili - Middle Tianshan Block. The Late Permian - Middle Triassic detrital zircons almost do not exist, implying that there were no contemporary magmatism related to collision or post-collision in the South Tianshan district, its complex evolution and orogenic stage are still a challenging topic. In the ancient active plate margin, the sedimentary records in the pre-arc basin can provide more information about the magmatic arc and basin-orogen coupling than the present exposed arc itself. The rhyolite, trachyte, and trachyandesite of the Dahalajunshan Formation were widely developed in the Yili - Middle Tianshan Block during the Early Carboniferous. During the formation of Wulang Formation in the Early Permian, a large number of rhyolite were developed. The age data of 75 detrital zircons were obtained from the sandstone (J1s-5) of the Sangonghe Formation, of which 74 zircons have a concordance degree of over 90%, and their age data also fall on the harmonic curv. In addition, the age of the youngest zircons increased gradually from Early to Middle Jurassic, indicating that the sediments in this period had the feature of uncovering. Our study provides a good reference for the analysis of provenance and regional tectonic evolution.

Degrowth and the Blue Belt: Rethinking marine conservation in the British Overseas Territories

The UK Government has developed a ‘Blue Belt’, a network of large Marine Protected Areas involving seven British Overseas Territories. The Blue Belt is now one of the world's largest enclosures of space for conservation, enclosing four million km2 of ocean in some of the most remote spaces on earth. To be economically feasible, the UK's bold conservation targets are integrated with wider tourism, fishing, and economic growth-motivated governance agendas. This commentary argues for a degrowth alternative to the Blue Belt's development. The goal of degrowth is not to prevent increases in Gross Domestic Product, nor is degrowth the equivalent to recession in a growth economy. Sustainable degrowth provides a conservation framework for ensuring a just transition from neoliberal forms of governance that places local well-being and welfare needs above the interests of state actors, private investors, and holiday makers. In the current context of the Blue Belt, the commentary considers three nascent degrowth concepts for improving things: 1) blue degrowth, 2) degrowth tourism, and 3) degrowth environmental governance. The paper argues that instead of separating the UK from other spaces where biodiversity targets are realised, these targets should be used as opportunities to reconcile the UK's colonial relationships with the territories, to build local capacity, and resilience.

Detrital zircon provenance of Carboniferous sandstones of the Variscan Externides (SW Poland) \u2013 record of the eastern Variscides exhumation

The Variscan Externides in the area of south-western Poland (the basement of the Fore-Sudetic Monocline), are considered an extension of the Rheno-Hercynian Zone of the Variscan Orogen. The Externides are made of Carboniferous siliciclastic sediments, the exact stratigraphic age and provenance of which are not known in detail. To constrain the sedimentary evolution and detritus provenance of the Variscan external belt, eight sandstone samples from boreholes located in the area of SW Poland were selected for petrography, whole rock geochemistry and detrital zircon U–Pb dating. The results show abrupt change in detrital zircon U–Pb provenance, between strata that have been classified as Mississippian (L. Carboniferous) and Pennsylvanian (U. Carboniferous) based on previously published miospore zonation. The Pennsylvanian rocks (from the boreholes of Września-IG1 and Siciny IG1) bear a significant population of zircon grains with Lower Carboniferous ages (350–330 Ma), while such a zircon population is absent in Lower Carboniferous rocks (from the boreholes of Paproć-29 and Kobylin 1). The absence of Moscovian to Asselian zircons that are otherwise known from the adjacent areas with zircon-bearing volcanic activity, suggests that the studied Pennsylvanian sediments represent the lower part of Upper Carboniferous. The presented data indicate abrupt provenance change that occurred in the source areas at the Mississippian—Pennsylvanian turn, and that was probably caused by changes in the tectonic architecture of the Variscan Orogen. This tectonic changeover resulted in exhumation and erosion of the Lower Carboniferous rock complexes and manifested itself by recycling of the Lower Carboniferous zircon into Upper Carboniferous strata.