Orogenic Belt In Central China Research Articles

The 1.0 Ga S–type granite in the East Kunlun Orogen, Northern Tibetan Plateau: Implications for the Meso– to Neoproterozoic tectonic evolution

The East Kunlun Orogen (EKO) is characterized by widely distributed granitoids with different ages, which are keys to understanding the tectonic evolution of the Central China Orogenic Belt. Zircon U–Pb ages and Hf isotopic compositions, as well as the whole rock geochemistry of the gneissic granite from the basement rock, are carried out to elucidate the Meso– to Neoproterozoic tectonics of the EKO. The Al–rich minerals, including muscovite and tourmaline, and the A/CNK ratios (1.07–1.18) indicate S–type affinity of the granite. The granite displays high SiO2 and K2O contents, and slightly enrichment in LREE in chondrite normalized REE distribution pattern with strong negative Eu anomalies (δEu=0.10–0.15). The samples exhibit positive anomalies of Rb, Th, U and Pb, and depletion of Ba, Nb, Ta, Sr and Ti. Meanwhile, the high Rb/Ba and Rb/Sr ratios and low (CaO+FeO+MgO+TiO2) contents indicate that they were derived from pelitic material. Together with the negative εHf(t) values ranging from −5.97 to −2.34 and two–stage Hf model ages varying from 1968 to 1786Ma, the gneissic granite is suggested being originally derived from partial melting of the metasedimentary rocks of the Paleoproterozoic Jinshuikou Group in the central EKO due to the crust thickening. Most zircon grains from the gneissic granite show typical magmatic zircon morphology, and yield an U–Pb upper intercept age of 1006±20Ma (MSWD=1.5), representing the crystallization age. Integrated with the regional geology, our results suggest that the EKO has been probably involved into a Meso– to Neoproterozoic plate collisional event related to the assembly of the Rodinia supercontinent.

Early to Middle Paleozoic tectonometamorphic evolution of the Hongseong area, central western Korean Peninsula: Tectonic implications

The migmatized gneiss (viz. Gwangcheon gneiss) of the southern Hongseong area, central–western Korean Peninsula underwent intermediate to high granulite-facies metamorphism (ca. 9.2–12.0kbar and 770–870°C) during Early Silurian to Early Devonian in age. Field and petrographic evidences from the Gwangcheon migmatite provide a clear indication of partial melting. Particularly the migmatized paragneiss is juxtaposed with arc-related doleritic metagabbro, which might be related to the heat source of the migmatization. The migmatized paragneiss has similar arc-related geochemical characteristics, such as depletions in Ta–Nb, Sr, P, and Ti and enrichments in large ion lithophile elements (LILE) caused by the partial melting effect related to the doleritic metagabbro. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of the migmatized paragneiss yielded ages of 432±6Ma, 431±10Ma, 421±3Ma, and 403±3Ma. The U–Pb ages of inherited zircons from the migmatized paragneiss yielded an age spectra from Neoarchean to Early Paleozoic, with a dominant age population at Neoproterozoic. In contrast, new-growth and recrystallized equant zircons showed dominant concordant Early Silurian to Early Devonian ages that provide critical evidence on the timing of migmatization. These age results and the tectonic signatures are similar to those reported from the Central China Orogenic Belt in China, indicating their tectonic linkage at the northeastern margin of eastern Gondwana during Early to Middle Paleozoic in age.

Application of electron spin resonance (ESR) dating to ductile shearing: Examples from the Qinling orogenic belt, China

Shear zones are common structures in orogenic belts and elucidation of the tectonic evolution of these orogenic belts to a large degree depends on understanding the kinematics and timing of shear deformation. However, there is a lack of an accurate, fast and convenient way to determine the timing of deformation. In this paper, we apply the ESR (electron spin-resonance spectroscopy) dating method to three syntectonic quartz veins from the Funiushan tectonic belt in the Qinling orogenic belt in central China. The results agree well with the available ages of deformation in the area obtained through other dating methods. This demonstrates the accuracy and feasibility of using the ESR method to date quartz crystals formed during deformation. The method is fast and convenient, and satisfies the accuracy requirement. It is an effective means for determining the timing of deformation, especially in areas with intensive fluid activity during deformation.

Early to Middle Paleozoic arc magmatism in the Korean Peninsula: Constraints from zircon geochronology and geochemistry

The Hongseong area of the central southwestern Korean Peninsula has been tectonically correlated to the Qinling–Dabie–Sulu collisional belt of the central China Orogenic Belt. Here we have presented geochemical and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon data on the metaigneous rocks from the Early to Middle Paleozoic Gwangcheon gneiss, located in the middle of the Hongseong area. Our data show U–Pb ages of ca. 450Ma for quartz-dioritic gneiss, 442Ma for metagabbro, 409, 407 and 402Ma for tonalitic gneiss, 440Ma for intermediate dyke, and 401Ma for mafic intrusive. The rocks display geochemical features suggesting magma generation within Late Ordovician (ca. 450Ma) to Early Devonian (ca. 401Ma) subduction-related volcanic arc and subsequent collapsing environment. The ages and geochemical characteristics presented in this study correlate well with those of the Early to Middle Paleozoic orogenic belts in the Qinling orogenic belt. Our study provides further insights into the tectonic linkage among Paleozoic to Triassic East Asian continents, with evidence for a Paleozoic Pacific-type subduction prior to the Triassic Alpine-type collision.

The Middle Triassic Meiwu Batholith, West Qinling, Central China: Implications for the Evolution of Compositional Diversity in a Composite Batholith

An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.

Late Paleozoic provenance shift in the south-central North China Craton: Implications for tectonic evolution and crustal growth

U–Pb geochronologic and Hf isotopic results of seven sandstones collected from Late Carboniferous through Early Triassic strata of the south-central part of the North China Craton record a dramatic provenance shift near the end of the Late Carboniferous. Detrital zircons from the Late Carboniferous sandstones are dominated by the Early Paleozoic components with positive εHf(t) values, implying the existence of a significant volume of juvenile crust at this age in the source regions. Moreover, there are also three minor peaks at ca. 2.5Ga, 1.87Ga and 1.1–0.9Ga. Based on our new data, in conjunction with existing zircon ages and Hf isotopic data in the North China Craton (NCC), Central China Orogenic Belt (CCOB) and Central Asian Orogenic Belt (CAOB), it can be concluded that Early Paleozoic and Neoproterozoic detritus in the south-central NCC were derived from the CCOB. Zircons with ages of 1.9–1.7Ga were derived from the NCC. However, the oldest components can't be distinguished, possibly from either the NCC or the CCOB, or both. In contrast, detrital zircons from the Permian and Triassic sandstones are characterized by three major groups of U–Pb ages (2.6–2.4Ga, 1.9–1.7Ga and Late Paleozoic ages). Specially, most of the Late Paleozoic zircons show negative εHf(t) values, similar to the igneous zircons from intrusive rocks of the Inner Mongolia Paleo-Uplift (IMPU), indicating that the Late Paleozoic detritus were derived from the northern part of the NCC. This provenance shift could be approximately constrained at the end of the Late Carboniferous and probably hints that tectonic uplift firstly occurred between the CCOB and the NCC as a result of the collision between the South and North Qinling microcontinental terranes, and then switched to the domain between the CAOB and the NCC. Additionally, on the basis of Lu–Hf isotopic data, we reveal the pre-Triassic crustal growth history for the NCC. In comparison among the three crustal growth curves obtained from modern river sands, our samples, and the Proterozoic sedimentary rocks, we realize that old components are apparently underestimated by zircons from the younger sedimentary rocks and modern river sands. Hence, cautions should be taken when using this method to investigate growth history of continental crust.

Northward extrusion of the ultrahigh‐pressure units in the southern Dabie metamorphic belt, east‐central China

AbstractThe Dabie orogenic belt in central China extends along the Triassic suture between the Sino–Korean and Yangtze cratons. The structure of the metamorphic rocks have been investigated in the Shima–Wumiao area near the southeastern end of the Dabie orogenic belt. Detailed mapping revealed that the metamorphic sequence in this area can be subdivided into four subhorizontal lithotectonic units. These units are denoted by I, II, III, and IV in the order of structural bottom to top. Unit I is composed of felsic gneiss, eclogite, and minor pelitic gneiss. Unit II consists of felsic gneiss and eclogite with coesite pseudomorphs. Unit III consists of felsic gneiss, pelitic gneiss, metacarbonate, and eclogite with occasional coesite inclusion. Unit IV is composed of felsic gneiss, amphibolite, and minor pelitic gneiss. Those metamorphic rocks underwent penetrative ductile deformation which is denoted by gently south‐dipping or subhorizontal foliation and NNW‐trending mineral lineation. Deformation microstructures in oriented samples from Units I and II indicate consistent northward displacements of the hanging wall and those from Units III and IV indicate southward displacements. The subhorizontal structure and opposite shear directions between Units I–II and Units III–IV suggest northward extrusion of UHP metamophic units (II and III). The subduction polarity could be opposite to the extrusion trend. The UHP metamorphism in the Dabie orogenic belt is attributable to southward subduction of the Sino–Korean craton beneath the Yangtze craton.

Tectonic evolution of a composite collision orogen: An overview on the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt in central China

The formation of collisional orogens is a prominent feature in convergent plate margins. It is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision. The Paleozoic convergence between the South China Block (SCB) and the North China Block (NCB) is associated with a series of tectonic processes such as oceanic subduction, terrane accretion and continental collision, resulting in the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt. While the arc–continent collision orogeny is significant during the Paleozoic in the Qinling–Tongbai–Hong'an orogens of central China, the continent–continent collision orogeny is prominent during the early Mesozoic in the Dabie–Sulu orogens of east-central China. This article presents an overview of regional geology, geochronology and geochemistry for the composite orogenic belt. The Qinling–Tongbai–Hong'an orogens exhibit the early Paleozoic HP–UHP metamorphism, the Carboniferous HP metamorphism and the Paleozoic arc-type magmatism, but the three tectonothermal events are absent in the Dabie–Sulu orogens. The Triassic UHP metamorphism is prominent in the Dabie–Sulu orogens, but it is absent in the Qinling–Tongbai orogens. The Hong'an orogen records both the HP and UHP metamorphism of Triassic age, and collided continental margins contain both the juvenile and ancient crustal rocks. So do in the Qinling and Tongbai orogens. In contrast, only ancient crustal rocks were involved in the UHP metamorphism in the Dabie–Sulu orogenic belt, without involvement of the juvenile arc crust. On the other hand, the deformed and low-grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late Permian to early Triassic along the northern margin of the Dabie–Sulu orogenic belt, and it was developed on the passive oceanic margin during subduction in the early Paleozoic along the northern margin of the Qinling orogen.Three episodes of arc–continent collision are suggested to occur during the Paleozoic continental convergence between the SCB and NCB. The first episode of arc–continent collision is caused by northward subduction of the North Qinling unit beneath the Erlangping unit, resulting in UHP metamorphism at ca. 480–490Ma and the accretion of the North Qinling unit to the NCB. The second episode of arc–continent collision is caused by northward subduction of the Prototethyan oceanic crust beneath an Andes-type continental arc, leading to granulite-facies metamorphism at ca. 420–430Ma and the accretion of the Shangdan arc terrane to the NCB and reworking of the North Qinling, Erlangping and Kuanping units. The third episode of arc–continent collision is caused by northward subduction of the Paleotethyan oceanic crust, resulting in the HP eclogite-facies metamorphism at ca. 310Ma in the Hong'an orogen and low-P metamorphism in the Qinling–Tongbai orogens as well as crustal accretion to the NCB. The closure of backarc basins is also associated with the arc–continent collision processes, with the possible cause for granulite-facies metamorphism. The massive continental subduction of the SCB beneath the NCB took place in the Triassic with the final continent–continent collision and UHP metamorphism at ca. 225–240Ma. Therefore, the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc-type magmatism to arc–continent and continent–continent collision.

Geochemical Constraints On the Provenance and Depositional Setting of the Devonian Liuling Group, East Qinling Mountains, Central China: Implications for the Tectonic Evolution of the Qinling Orogenic Belt

Abstract The Qinling orogenic belt of Central China is one of the largest collision orogens in eastern Asia and is considered to mark the site where the eastern part of the Paleotethyan ocean, which separated the North and South China plates, was consumed. Many researchers have described the major units and tectonic framework of the Qinling orogenic belt, but there is little agreement over its Paleozoic history, particularly the tectonic setting of the Devonian assemblages. In this paper geochemistry is used to constrain the provenance and depositional setting of the Middle–Upper Devonian Liuling Group clastic sedimentary rocks from the East Qinling Mountains. Chondrite-normalized REE patterns are uniform with light REE enrichment (LaN/YbN c. 9.6), negative Eu anomalies (Eu/Eu* c. 0.62), and flat heavy REE patterns (GdN/YbN c. 1.6), indicating an upper-continental-crustal source and/or juvenile differentiated arc material. Trace-element discriminant diagrams also suggest a continental arc setting, albeit major-element chemistry can show some characters of passive-margin sandstones. Chemical index of alteration and A-CN-K relations, Nb-Ta negative anomalies (PAAS-normalized data), and high Cr-Ni-V-Ti anomalies all suggest that the source area was dominated by non-steady-state weathering regimes indicative of active uplift along an active continental convergent plate boundary, and not a passive margin. This indicates that the Liuling Group was probably related to subduction accretion with rapid uplift of its source areas. The minor recycling of older sedimentary components and oceanic-crust-related mafic inputs could be derived from the basement of the North Qinling arc and ophiolites or an accretionary complex, respectively. We conclude that the Middle to Upper Devonian Liuling Group was deposited at an active continental margin at the southern edge of the North China plate, and hence closure of eastern Paleotethys was post-Devonian.

Episodic tectonothermal events of the western North China Craton and North Qinling Orogenic Belt in central China: Constraints from detrital zircon U–Pb ages

Detrital zircon U–Pb geochronology serves as a proxy to study of crustal evolution and provenance discrimination. In order to unravel episodic tectonothermal events and their tectonic relationship between the North China Craton (NCC) and North Qinling Orogenic Belt (NQOB), detrital zircons from modern river sands and metasedimentary rocks were collected and dated by LA-ICPMS. Although the western NCC (Ordos block) is covered by Paleozoic–Cenozoic basin sediments, the U–Pb dating results show that the age populations of detrital zircons from the western NCC present prominent U–Pb age peaks at 2475Ma and 1850Ma, which indicates the western NCC (Ordos block) also has early Precambrian basement similar to the eastern and central craton. In addition, a significant number of early Paleozoic (520–400Ma) zircons have been found in the western NCC, which is quite different from the eastern NCC and is considered to be related to the collision between the NQOB and the NCC.The age spectra of detrital zircons from the NQOB presents a complex age pattern, which reveals four major age groups of Neoarchean (2.6–2.4Ga), Neoproterozoic (1.0–0.85Ga), early-middle Paleozoic (450–350Ma) and early Mesozoic (250–170Ma). As indicated by the U–Pb isotopic data that the NQOB could be an independent terrane at least prior to the Neoproterozoic and once a portion of the Grenville orogenic belt during the 1.2–0.8Ga with a peak of ∼1.0Ga. In other words, the NQOB has its unique geological evolution history obviously different from those of the NCC and the Yangtze Craton. The complete collision between the NQOB and the NCC perhaps took place at Paleozoic (450–400Ma).

Fractures in the low porosity and ultra-low permeability glutenite reservoirs: A case study of the late Eocene Hetaoyuan formation in the Anpeng Oilfield, Nanxiang Basin, China

The Naxiang Basin, located within the Qin-and-Dabie Mountains orogenic belt in central China, is a small intermountain faulted basin, in which the late Eocene Hetaoyuan Formation in the Anpeng Oilfield is the low porosity and ultra-low permeability glutenite reservoirs. The large-scale fractures are mainly developed in the thick-bedded conglomerates and gravel-bearing gritstones, the small-scale jointed-fractures bounded by layer are mainly developed in the medium-granular and fine sandstones between the conglomerates or gravel-bearing gritstones. There also developed three kinds of micro-fractures, namely intergranular fractures, intragranular fractures and transgranular fractures, in the tight glutenites. The fractures in the glutenite reservoirs are of chiefly high dip-angles. They assume chiefly the E-W strikes, next are the NE-SW and NW-SE strikes. On the log curve, fractures usually show such responses as a decrease of the deep and shallow lateral resistivities, an increase of the acoustic time difference, a reduction of the density, an increase of the compensation neutrons, an occurrence of middle and high gamma ray and an expansion of the calipers. Fractures make contributions to the low porosity and ultra-low permeability glutenite reservoirs by improving the permeability and then porosity. Micro-fractures are the important channels connecting intergranular and intragranular solution pores, thereby making better the connectivity within the tight glutenite reservoirs. The small- and medium-scale fractures serve as the flow channels within the layers of fine and medium-granular sandstones, while the large-scale fractures serve as the main flow channels for the whole reservoir. Under the impact of the present-day stress, the NE-SW fractures perpendicular to the minimum principal stress assume a tensile state and have good connectivity, large apertures and high permeability, and are the major direction for fluid flow, thereby providing a major basis for a further development.

The formation of foliated (garnet‐bearing) granites in the Tongbai‐Dabie orogenic belt: partial melting of subducted continental crust during exhumation

AbstractFoliated (garnet‐bearing) (FGB) granites are associated closely with and are usually the major wall rocks of the high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Tongbai‐Dabie region, the mid segment of the Qinling‐Dabie‐Sulu orogenic belt in central China. These granites appear either as small plutons or as veins, which commonly intrude into or surround the HP and UHP metamorphic eclogites or gneisses. The veins of FGB granites usually penetrate into the retrograded eclogites or gneisses along the foliations. Condensation rims can occasionally be found along the margins of granite veins. These granites are rich in Si and alkali with high Ga/Al ratios, and depleted in Ca, Mg, Al, Ti, Sc, V, Ni, Co, Cr and Sr, which are similar to A‐type granites. In a chondrite normalized diagram, the samples are light rare earth elements enriched with different extent of negative Eu anomaly. Moreover, Rb, Nb, Ta, Sr, P and Ti show different degrees of negative anomalies, whereas Ba, K, La, Zr and Hf show positive anomalies in the primitive mantle normalized diagram. Negative anomalies of Eu and Sr indicate strong influence of plagioclase. In conventional discrimination diagrams, these FGB granites belong to the A‐type granite, with geochemical characteristics affinitive to post‐collisional granites. The εNd (230 Ma) values (−15.80 to −2.52) and TDM values (1.02–2.07 Ga) suggest that magma for the FGB granites were derived from a heterogeneous crustal source. Therefore, the FGB granites may provide clues for deciphering the formation of post‐collisional granites. It is proposed that the magma of the FGB granites both in the HP and UHP units was formed in an extensional tectonic setting slightly post‐dating the HP and UHP metamorphism, most likely as a result of decompressional partial melting of UHP retrograded eclogites during exhumation.

New 40Ar– 39Ar age constraints on the deformation along the Machaoying fault zone: Implications for Early Cambrian tectonism in the North China Craton

The Machaoying fault zone extends along the southern margin of the North China Craton (NCC) and controlled the regional structures and hydrothermal mineral systems in this area. The fault underwent at least two major deformational phases, as revealed by macro- and micro-structural observations from a well-developed segment of the fault in the Hongzhuang–Baitu area, located south of the Xiong'er Mountains. Early ductile deformation is characterized by thrusting from north to south, which was subsequently overprinted by late brittle faulting. Syntectonic strain shadows of biotite are preserved around rotated porphyroclasts of quartz amygdales in mylonite. The biotite yields a 40Ar– 39Ar plateau age of 524.9 ± 1.9 Ma, which is interpreted as the time of regional thrusting along the Machaoying fault zone. The thrusting may be temporally correlated with an Early Cambrian discontinuity in sedimentation observed in the rocks sequences of the NCC, suggesting a compressional regime in this area and a craton-wide tectonic event. Many 540–500 Ma tectonic events have been previously identified in the Qinling–Qilian–Kunlun Orogenic Belt of central China and in massifs in northeastern China, both of which surround the NCC, and some of these were interpreted to be associated with assembly of Gondwana. However, paleomagnetic data indicate that the NCC was unlikely to have been connected with Gondwana in the Early Cambrian and thus our new biotite date cannot record deformation along the Gondwanan margin. Dating of K-feldspar from a quartz–K-feldspar vein formed along one of the brittle faults of the Machaoying fault zone yields a much younger 40Ar– 39Ar plateau age of 119.5 ± 0.7 Ma. This is a minimum age for the brittle deformation along the southern margin of the NCC, which also overlaps the age of widespread gold and molybdenum mineralization in the region.

Reply to comment by M. A. Kendrick and D. Phillips (2009) on “The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions” by Hua-Ning Qiu and J. R. Wijbrans (2008) [Earth Planet. Sci. Lett. 268 (2008) 501–514

The 40Ar/ 39Ar apparent ages found by Qiu and Wijbrans [Qiu, H.N. and Wijbrans, J.R., 2008. The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions. Earth Planet. Sci. Lett., 268, 501–514. doi: 10.1016/j.epsl.2008.01.042; Qiu, H.N. and Wijbrans, J.R., 2006. Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/ 39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta, 70, 2354–2370. doi: 10.1016/j.gca.2005.11.030.] for crushing in vacuo experiments on Dabieshan eclogite garnets are best explained by a simple mixing model with radiogenic 40Ar ⁎ and air argon for the final plateau data, and excess 40Ar released in the initial crushing steps. More intricate models such as the hypothesized mixture of phengite, K-feldspar, amphibole and omphacite solid impurities by Kendrick [Kendrick, M.A., 2007. Comment on ‘Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/ 39Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans’. Geochim. Cosmochim. Acta, 71, 6040–6045. doi: 10.1016/j.gca.2007.01.029.] and Kendrick and Phillips [Kendrick, M.A. and Phillips, D., 2009. Discussion of ‘The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions by Qiu Hua-Ning and Wijbrans J.R.’. Earth Planet. Sci. Lett.] cannot explain the consistently observed 40Ar/ 39Ar age plateaux and isochrons because mixing of more than two K-containing end members each with different ages, different initial 40Ar/ 36Ar ratios and different proportional argon release patterns cannot define an isochron and age plateau, and the solid inclusion mixing model fails to address the consistently observed correlation between the radiogenic signal and chlorine derived 38Ar.

Discussion of ‘the Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions by Qiu Hua-Ning and Wijbrans J.R.’

The in vacuo crushing procedure employed by Qiu and Wijbrans (2008) is identical to that of Qiu and Wijbrans (2006) and has produced a similar result. The authors provide no description of the sample material analysed and the display of data is highly selective (see also Kendrick, 2007). The new step-heating data produced for crushed garnets are readily explained in terms of degassing from mineral inclusions (Fig. 1) and the argon isotope systematics preclude the ~ 450 Ma 'plateau' or 'isochron' ages being related to primary metamorphic fluid inclusions. The range of apparent ages determined for garnets, phengite and biotite by Qiu and Wijbrans (2006, 2008) extend from ~ 150 Ma to over 1000 Ma and only a subset of garnets yield in vacuo crushing apparent ages of ~ 450 Ma (see Fig. 2). The range of apparent Ar/Ar ages reported by Qiu and Wijbrans (2006, 2008) is similar to that determined for Dabie Shan phengite previously (Giorgis et al., 2000). Such variability is typical of UHP metamorphic rocks where it is well understood that argon has limited mobility and that the prevalence of extraneous Ar makes application of the Ar/Ar technique challenging (Arnaud and Kelley, 1995; Scaillet, 1996; Boundy et al., 1997; Giorgis et al., 2000). In summary, the in vacuo crushing data presented by Qiu and Wijbrans (2006, 2008) appear to add little to our understanding of Dabieshan geochronology, the prevalence of extraneous Ar in UHP terranes, or the in vacuo crushing technique. The data are poorly constrained and remain compatible with the majority of published data, that constrain Dabieshan UHP metamorphism to a Triassic age of 210-230 Ma (see Ames et al., 1993; Chavagnac and Jahn, 1996; Zheng et al., 2003; Chen et al., 2006; Zhao et al., 2007).

Emplacement age of the Songshugou ultramafic massif in the Qinling orogenic belt, and geologic implications

The Songshugou ultramafic massif is located to the north of the Shang‐Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline‐type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA‐ICP‐MS U−Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled ∼1 m from the massif, the other ∼5 m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (<0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10 ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 506±7 and 510±7 Ma, suggesting that the Songshugou ultramafic massif was emplaced at ∼510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif. To better understand the cooling history of the massif, 40Ar/39Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20 m away from the massif were determined. The 40Ar/39Ar ages increase from 372±15 Ma (JSM‐01) near the massif to disturbed, unreliable ‘plateau’ ages of 474±8 Ma (JSM‐03) and 781±146 Ma (JSM‐04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at ∼1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang‐Dan fault.

The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions

The pressure–temperature–time evolution of the UHP eclogites of Dabie–Sulu, in the eastern sector of the Central Orogenic Belt of China shows a complex pattern of predominantly Triassic, and to a lesser extent Early Paleozoic ages. 40Ar/ 39Ar stepwise-crushing analysis of eclogitic garnets yielded consistently Paleozoic ages. Because our initial results from the Bixiling eclogite were of limited areal extent, their significance required further testing. Data for garnet-bearing gneiss and eclogites from new locations in northern and southern Dabieshan (Shuanghe, Ganghe and Hongmiao), confirm the Paleozoic garnet results obtained from Bixiling eclogite. We also tested the technique on a garnet sample of known Paleozoic age from Altyn Tagh in the western Central Orogenic Belt. These new results indicate that light-colored garnets from Dabieshan and Altyn Tagh yield concordant 40Ar/ 39Ar ages ranging from 445–465 Ma by in vacuo crushing, and confirm that a Paleozoic (U)HP metamorphic event took place in the Central Orogenic Belt from Altyn Tagh in western China to Dabieshan in eastern China. In contrast, dark-colored garnets from mafic metavolcanics do not yield 40Ar/ 39Ar plateaux or isochrons ages.

Pb Isotope Mapping in the Tongbai‐Dabie Orogenic Belt, Central China

Abstract: Tongbai‐Dabie orogenic belt in Central China is a part of the collisional belt between the Yangtze and North China cratons. It represents one of the most extensive ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic rocks in the world. The Pb isotope mapping in this area is a significant method to constrain the crustal structure and tectonic evolution and to identify the tectonic boundaries within the vertical tectonic stack. Based on the Pb isotope compositions of the Dabie complex (DBC), the Tongbai complex (TBC), UHP and HP metamorphic rocks and associated foliated granites, the lower metamorphosed rocks from North Huaiyang (NHY) tectonic belt, and Cretaceous granites in the Tongbai‐Dabie orogenic belt, we determined the Pb isotope geochemical map of the Tongbai‐Dabie orogenic belt. The Pb isotope map shows that the Pb isotope compositions are similar within each geological body or lithotectonic unit, but the Pb isotope compositions of different lithotectonic units show systematic variations in the Tongbai‐Dabie orogenic belt. The NHY tectonic belt contrasts strongly with the Tongbai‐Dabie UHP‐HP metamorphic belt in Pb isotope compositions. It is suggested that the line along the Xiaotian‐Mozitan fault, the north limit of the Tongbai‐Dabie UHP and HP metamorphic rocks, represents an important tectonic boundary. Within the Tongbai‐Dabie HP‐UHP metamorphic belt, to the south of Xiaotian‐Mozitan fault, the vertical variations of Pb isotope compositions in different lithotectonic units and the spatial relationship among different major lithotectonic units have been constrained.

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

ABSTRACT An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou ( ) and Xiangfanggou ( ) slices exposed in the eastern Qinling ( ) orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine–grained coesite and quartz pseudomorphs after coesite- and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South Altun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age (∼500–400 Ma). The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie ( )-Sulu ( ) ultrahigh/high pressure metamorphic belt of Triassic age (∼250–220 Ma). The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping ( ) rock group. The East Qinling UHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These two ultrahigh metamorphic belts are separated by the suture-like Shangnan ( ) Danfeng ( ) fault system or Guishan ( )-Meishan ( ) fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultra-high/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam-North Qinling ultrahigh metamorphic belt probably represents an intercontinental deep subduction/collision belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of ∼200–300 Ma, responsible for the formation of the two ultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.

Phanerozoic high-pressure eclogite and intermediate-pressure granulite facies metamorphism in the Gyeonggi Massif, South Korea: Implications for the eastward extension of the Dabie–Sulu continental collision zone

Petrological analysis, zircon trace element analysis and SHRIMP zircon U–Pb dating of retrogressed eclogite and garnet granulite from Bibong, Hongseong area, SW Gyeonggi Massif, South Korea provide compelling evidence for Triassic (231.4 ± 3.3 Ma) high-pressure (HP) eclogite facies (M1) metamorphisms at a peak pressure–temperature ( P– T) of ca. 16.5–20.0 kb and 775–850 °C. This was followed by isothermal decompression (ITD), with a sharp decrease in pressure from 20 to 10 kb and a slight temperature rise from eclogite facies (M1) to granulite facies (M2), followed by uplift and cooling. Granitic orthogneiss surrounding the Baekdong garnet granulite and the ophiolite-related ultramafic lenticular body near Bibong records evidence for a later Silurian (418 ± 8 Ma) intermediate high-pressure (IHP) granulite facies metamorphism and a prograde P– T path with peak P– T conditions of ca. 13.5 kb and 800 °C. K–Ar ages of biotite from garnet granulites, amphibolites, and granitic orthogneisses in and around the Bibong metabasite lenticular body are 208–219 Ma, recording cooling to about 310 °C after the Early Triassic metamorphic peak. Neoproterozoic zircon cores in the retrogressed eclogite and granitic orthogneiss provide evidence that the protoliths of these rocks were ∼ 800 and ∼ 900 Ma old, respectively, similar to the ages of tectonic episodes in the Central Orogenic Belt of China. This, and the evidence for Triassic HP/UHP metamorphism in both China and Korea, is consistent with a regional tectonic link within Northeast Asia from the time of Rodinia amalgamation to Triassic continent–continent collision between the North and South China Blocks, and with an eastward extension of the Dabie–Sulu suture zone into the Hongseong area of South Korea.