North Qilian Orogen Research Articles

Tectono-thermal events of the North Qilian Orogenic Belt, NW China: Constraints from detrital zircon U-Pb ages of Heihe River sediments

The ∼800km long Heihe River in northwestern China originates from the North Qilian Mountain, and passes through the Beishan Mountain before it reaches the Juyanhai Lake. Geologically, the river crosscuts the North Qilian Orogenic Belt, the Alax Terrane, and the Beishan Orogenic Belt. LA-ICP-MS U-Pb dating of detrital zircon from Heihe River sediments shows four age groups: 2700–1700Ma, 1700–650Ma, 650–358Ma, and 358–230Ma, and age peaks at ∼2450Ma, ∼1850Ma, ∼950Ma, ∼520Ma, 505Ma, ∼480Ma, ∼450Ma, ∼430Ma, ∼380Ma, ∼320Ma, and ∼295Ma. The age distribution patterns indicate that Heihe River sediments were mainly sourced from the North Qilian Orogenic Belt. The detrital zircon ages demonstrate that the North Qilian Orogenic Belt may have Archean to late Paleoproterozoic rocks. Several tectonothermal events are also identified: opening of the North Qilian Ocean (part of the Paleo-Tethys Ocean) at early Neoproterozoic, subduction of oceanic lithosphere and formation of a trench-arc-basin system at late Neoproterozoic to late Ordovician, closure of the oceanic basin at late Ordovician, and collision of the Qaidam and Alax terranes at early Devonian. Our study indicates that the formation of the North Qilian Orogenic Belt was mainly related to the formation and closure of the Paleo-Tethys Ocean.

Detrital zircon U–Pb ages of Paleozoic sedimentary rocks from the eastern Hexi Corridor Belt (NW China): Provenance and geodynamic implications

The Paleozoic tectonic framework of the eastern Hexi Corridor Belt is ambiguous. However, thick Paleozoic sediments from the eastern Hexi Corridor Belt can provide clues to this problem. In this paper, we deal with the detrital zircon provenance of Middle Ordovician to Late Devonian strata using LA-ICP-MS U–Pb dating, documenting the temporal and spatial changes of provenance. Detrital zircon U–Pb ages indicate that the primary provenance was the Qilian Orogenic Belt and the Alxa Block. However, two samples from Late Devonian strata show different provenance characteristics, from the North Qilian Orogenic Belt, and the Alxa Block and the North China Block, respectively. Different age distributions through time reflect provenance changes due to continuous convergence of the Central Qilian Block and the Alxa Block from the Middle Ordovician to Late Devonian. A significant age cluster ranging from 2000 to 1800Ma, which is a detrital zircon signature of the North China Block, appeared in the Late Devonian Zhongning Formation. Our preferred interpretation is that the amalgamation of the Alxa Block and the North China Block likely occurred at that time. Moreover, similar Paleozoic fossils, paleolatitudes and detrital zircon distribution indicate that the Hexi Corridor Belt has an affinity to eastern Gondwana.

Coupled Lu–Hf and Sm–Nd geochronology constrains blueschist-facies metamorphism and closure timing of the Qilian Ocean in the North Qilian orogen

The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6GPa and 530–550°C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3±2.0Ma and 462.2±5.6Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous 40Ar/39Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466Ma.

Paleozoic magmatism and porphyry Cu-mineralization in an evolving tectonic setting in the North Qilian Orogenic Belt, NW China

The NWW-striking North Qilian Orogenic Belt records the Paleozoic accretion–collision processes in NW China, and hosts Paleozoic Cu–Pb–Zn mineralization that was temporally and spatially related to the closure of the Paleo Qilian-Qinling Ocean. The Wangdian Cu deposit is located in the eastern part of the North Qilian Orogenic Belt, NW China. Copper mineralization is spatially associated with an altered early Paleozoic porphyritic granodiorite, which intruded tonalites and volcaniclastic rocks. Alteration zones surrounding the mineralization progress outward from a potassic to a feldspar-destructive phyllic assemblage. Mineralization consists mainly of quartz-sulfide stockworks and disseminated sulfides, with ore minerals chalcopyrite, pyrite, molybdenite, and minor galena and sphalerite. Gangue minerals include quartz, orthoclase, biotite, sericite, and K-feldspar. Zircon LA-ICPMS U–Pb dating of the ore-bearing porphyritic granodiorite yielded a mean 206Pb/238U age of 444.6±7.8Ma, with a group of inherited zircons yielding a mean U–Pb age of 485±12Ma, consistent with the emplacement age (485.3±6.2Ma) of the barren precursor tonalite. Rhenium and osmium analyses of molybdenite grains returned model ages of 442.9±6.8Ma and 443.3±6.2Ma, indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite. Rhenium concentrations in molybdenite (208.9–213.2ppm) suggest a mantle Re source. The tonalities are medium-K calc-alkaline. They are characterized by enrichment of light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), depletion of heavy rare-earth elements (HREEs) and high-field-strength elements (HFSEs), and minor negative Eu anomalies. They have εHf(t) values in the range of +3.6 to +11.1, with two-stage Hf model ages of 0.67–1.13Ga, suggesting that the ca. 485Ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere. In contrast, the 40-m.y.-younger ore-bearing porphyritic granodiorite is sub-alkaline and peraluminous. They are enriched in LREEs and LILEs, depleted in HFSEs, and show weak negative Eu anomalies. They display εHf(t) values of captured or inherited zircons in the range of +8.5 to +10.0, and younger two-stage Hf model ages of 0.78Ga and 0.86Ga, similar to those of ca. 485Ma tonalite. The ca. 445Ma zircons have εHf(t) values of −2.1 to +9.9, with two-stage Hf model ages of 0.75–1.27Ga. Moreover, they have relatively high oxygen fugacity than that of the precursor barren tonalite. The ca. 445Ma magmas at Wangdian thus formed in a subduction setting, and incorporated melts from the subduction-modified lithosphere that had previously been enriched by additions of chalcophile and siderophile element-rich materials by the earlier magmatism and metasomatism during the Paleo Qilian-Qinling Ocean subduction event.

Tectonic affinity of the Alxa Block, Northwest China: Constrained by detrital zircon U–Pb ages from the early Paleozoic strata on its southern and eastern margins

The tectonic affinity of the Alxa Block is important in the reconstruction of the paleogeographical evolution of China. The early Paleozoic strata (the Dahuangshan Formation and Xiangshan Group) of the southern and eastern Alxa Block have consistent rock compositions, similar depositional ages, paleocurrents, detrital zircon age distributions, and cumulative probability curves of crystallization ages for detrital zircon grains relative to the depositional ages, and were deposited in similar slope basins. All these data indicate that the early Paleozoic strata of the Alxa Block were sourced predominantly from Neoproterozoic orogenic belts in the eastern Gondwana continent, instead of the Alxa Block to the north, the North China Block to the east and the North Qilian Orogenic Belt to the south. During the early Paleozoic, the Alxa Block was an independent block with the South China Block (SCB) to the west and the North China Block (NCB) to the east and situated to the northwest of the eastern Gondwana with its long axis trending north northwest–south southeast, and it belonged to a passive continental margin dipping to the north northwest, with the eastern part of the margin located closer to the eastern Gondwana. The Hexi Corridor is part of the Alxa Block and a part of the same slope basin during the early Paleozoic. The southern boundary of the Alxa Block is now the front thrust of the North Qilian Shan.

Syn-collisional adakitic granodiorites formed by fractional crystallization: Insights from their enclosed mafic magmatic enclaves (MMEs) in the Qumushan pluton, North Qilian Orogen at the northern margin of the Tibetan Plateau

The Qumushan (QMS) syn-collisional granodiorite, which is located in the eastern section of the North Qilian Orogen at the northern margin of the Greater Tibetan Plateau, has typical adakitic characteristics and also contains abundant mafic magmatic enclaves (MMEs). This recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed MMEs. The MMEs and their host granodiorites share many characteristics in common, including identical crystallization age (~430Ma), same mineralogy, similar mineral chemistry and whole-rock isotopic compositions, indicating their genetic link. The MMEs are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the QMS adakitic granodiorite. Subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as “MMEs” dispersed in the adakitic granodiorite host during emplacement. The geochemical data and petrogenetic modeling of trace elements suggest that the QMS adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the MMEs. The parental magma for the QMS granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision, which may have also experienced interaction with mantle peridotite during ascent.

Geology, geochronology and isotopic geochemistry of the Xiaoliugou W–Mo ore field in the Qilian Orogen, NW China: Case study of a skarn system formed during continental collision

The Xiaoliugou W–Mo ore field lies within the mid-Paleozoic North Qilian Orogen, NW China, and hosts a W resource of 48.8Mt @ 0.4% and 412.6Mt of Mo @ 0.075%. It contains five deposits, including Xiaoliugou, Qiqing, Guishan, Qibao and Shiji. The main mineralization styles at Xiaoliugou are skarn and veins in which the mineral sequence is scheelite>molybdenite>chalcopyrite and occurs in the endo- and exo-contact zones of granite intrusions. The scheelite-dominated orebodies are overprinted by molybdenite-dominated quartz veins. Two molybdenite samples yielded Silurian Re–Os model ages of 427.4±6.0Ma and 428.2±6.0Ma. Three muscovite samples coexisting with molybdenite yielded Middle Devonian Ar–Ar ages of 392.0±2.7Ma, 391.1±2.7Ma and 391.4±2.8Ma. The Re–Os and Ar–Ar ages indicate that the W–Mo mineralization and alteration occurred at ca. 428–391Ma, which corresponds with regional continental collision within the Qilian Orogen. δ34S for the sulfides molybdenite and pyrite are 7.70–11.67‰ and 4.98–13.17‰, respectively. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb of the sulfides are 17.98–21.73, 15.34–18.81, and 37.18–38.63, respectively. The granites yield similar corrected (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i, ranging 16.14–19.35, 15.44–15.63, and 37.41–38.31, respectively. We calculated δ18O of the fluid inclusions in quartz range from −3.38–2.34‰, whereas the δD of the hydrothermal fluids ranges from −94 to −47‰. The S-, Pb-, O- and D-isotopic data imply that the metals originated from the granite intrusion with a minor component sourced from the host sediments, and that the ore-forming fluids were dominated by magmatic-hydrothermal fluids mixed with minor meteoric water.

Timing of amalgamation of the Alxa Block and the North China Block: Constraints based on detrital zircon U–Pb ages and sedimentologic and structural evidence

Detrital zircon LA-ICP-MS U–Pb ages of the Paleozoic strata in the southeastern Alxa Block indicate that the primary provenance did not change between the Devonian and the Early Carboniferous. Except for a sample from the Cambrian strata, the Early Paleozoic North Qilian Orogenic Belt may have been a major source of Paleozoic sediments in the study area. The main source of Lower–Middle Devonian sediments was the Early Paleozoic North Qilian Orogenic Belt. The source of the Upper Devonian sediments was the Alxa Block, while the North China Block was a minor contributor. The deformation of the Cambrian and Devonian Systems in the southeastern Alxa Block indicates that a strong east–west compression event occurred in the study area before the Early Carboniferous. The paleocurrents of the Upper Devonian in the southeastern Alxa Block indicate that the source was located to the north and was not the North Qilian Orogenic Belt to the south. Moreover, the deposition of the Upper Devonian in the southeastern Alxa Block was a response to a strong deformation event that occurred along the eastern boundary of the Alxa Block during the Late Devonian and Early Carboniferous. The North China Block became the primary source during the Early Carboniferous, and the Alxa Block was a minor source. A regional stratigraphic comparison also indicates that similar depositional environments were present until the Carboniferous and Permian on the Alxa Block and North China Block. All of these data indicate that amalgamation of the Alxa Block and the North China Block occurred between the Late Devonian and the Early Carboniferous.

Geochemical constraints on the protoliths of eclogites and blueschists from North Qilian, northern Tibet

An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-temperature eclogites and blueschists from the North Qilian orogen in northern Tibet. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts rather than mid-ocean ridge basalts as commonly thought. These high pressure metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts-like, mid-ocean ridge basalts-like to oceanic island basalts-like, respectively, in trace element distribution diagrams. Such variable compositions are comparable with those of backarc basin basalts at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, and steep HREE patterns, with protolith U–Pb ages of 486–496Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U–Pb ages of 463±10Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to a localized fluid source for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that a backarc basin was transformed to an oceanic subduction zone where eclogite-facies metamorphism occurred.

Deep structures and surface boundaries among Proto-Tethyan micro-blocks: Constraints from seismic tomography and aeromagnetic anomalies in the Central China Orogen

The Qinling–Qilian orogen preserves the records of Early Paleozoic convergence among the Proto-Tethyan micro-blocks. In this study, we analyze the seismic velocity structure and the aeromagnetic anomalies in the Qinling–Qilian orogen and its adjacent areas, showing that the northernmost boundaries of these Proto-Tethyan micro-blocks are defined by the Guyuan–Longshoushan Fault in the Qilian orogen and by the Luonan–Luanchuan Fault in the Qinling orogen, respectively. The lithosphere north of the Qinling–Qilian orogen subducted southward under the Qinling–Qilian orogen. The boundaries of the micro-blocks in the Qinling–Qilian orogen of the Proto-Tethys domain are mostly EW- or NE-trending. Combined with Early Paleozoic geological records, our results indicate that the south-dipping low-velocity anomaly under the Alxa block and the south-dipping high-velocity ones under the North Qilian orogen and the North China block might indicate that the Alxa block, the North Qilian Oceanic slab and the North China block subducted southward. The south-dipping high-velocity anomaly under the North Qilian orogen is interpreted as the remnant of the Proto-Tethys Ocean in that area. However, the north-dipping low-velocity anomaly under the South Qinling and the North Qinling orogens, the Qaidam block and the north-dipping high-velocity one under the Yangtze and the Bikou blocks suggest that the South Qinling and the North Qinling orogens, the Qaidam, the Yangtze and the Bikou blocks subducted northward. The present spatial framework of the Qinling–Qilian orogen is related to Early Paleozoic convergence among the Proto-Tethyan micro-blocks.

On the origin of mafic magmatic enclaves (MMEs) in syn-collisional granitoids: evidence from the Baojishan pluton in the North Qilian Orogen, China

Mafic magmatic enclaves (MMEs) are abundant in Baojishan syn-collisional granitoids located in the eastern section of the North Qilian Orogen. Zircon U-Pb ages of the host granodiorite (433.7 ± 3.4 Ma) and their MMEs (431.6 ± 2.8 Ma) are the same as the time of the Qilian ocean closing and continental collision at ∼440–420 Ma, indicating that the granitoids represent a magmatic response to the collision between the Qilian-Qaidam block and the Alashan block. The MMEs have the same mineralogy as the host granodiorite except that they are more abundant in mafic phases (e.g., amphibole and biotite) and thus have higher heavy rare earth element (HREE) abundances. Both the host granodiorite and the MMEs have light REE-enriched patterns and relatively flat HREE patterns (i.e., [Dy/Yb]N = 1–1.1). They are enriched in large ion lithophile elements (LILEs; e.g., Rb, K, Pb) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and show a varying Sr anomaly (i.e., Sr/Sr* = 0.9–2.2) for the host and a negative Sr anomaly (i.e., Sr/Sr* = 0.4–0.6) for the MMEs. Both the host granodiorite and the MMEs have overlapping and indistinguishable Sr-Nd-Hf isotopic compositions (87Sr/86Sr(i) = 0.7067–0.7082, εNd(t) = −3.9–−3.2, εHf(t) = 1.0–14.7). The extremely high εHf(t) = 14.7 of sample BJS12-06MME likely results from the calculation due to nugget effect of zircons because of the unexpectedly high Hf (3.53 ppm) and too high Zr (128 ppm). All these characteristics are fully consistent with the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing model. The radiogenic Sr and unradiogenic Nd (εNd(t) <0) indicate the contribution of mature continental crust, while variably radiogenic Hf (εHf(t) > 0) for both the MMEs and their host granodiorite manifest the significant mantle input. The apparent decoupling between Nd and Hf isotopes are likely caused by partial melting of recycled terrigenous sediments and the remaining part of the North Qilian ocean crust under the amphibolite facies conditions during the collision.

Late Devonian closure of the North Qilian Ocean: evidence from detrital zircon U–Pb geochronology and Hf isotopes in the eastern North Qilian Orogenic Belt

The closure age of the North Qilian Ocean, which lay between the Alashan Terrane and Central Qilian Block during the early Palaeozoic, is intensely debated. This article presents a provenance study of detrital materials from three Upper Devonian sedimentary formations in the Pingchuan area of Gansu Province in the eastern North Qilian Orogenic Belt (NQOB), to constrain the tectonic evolution of the belt. U–Pb dating and Lu–Hf isotopic studies were conducted on detrital zircons from samples of sedimentary rocks. Four age populations of U–Pb zircon ages are defined as: 0.5–0.4 Ga (peak at 472 Ma); 0.8–1.3 Ga (peaks at 960 Ma and 1102 Ma); 1.8–2.1 Ga (two prominent peaks at 1963 Ma and 2045 Ma, and two subordinate peaks at 1816 Ma and 2168 Ma); and 2.4–2.5 Ga (peak at 2495 Ma). Zircons with U–Pb age spectra of 1.8–2.1 Ga and their corresponding εHf(t) values are markedly different from those of the North China Block, Dunhuang Terrane, and Central Qilian Block, but have a strong similarity to zircons from the Alashan Terrane. Furthermore, zircons with ages ranging from 1.8 to 2.1 Ga are reported for the first time in early Palaeozoic strata of the eastern NQOB. The presence of Alashan-derived clasts in the Upper Devonian strata in the NQOB indicates that the sediments were deposited after amalgamation between the Alashan and Qilian terranes. Combined with previous geochronological and geochemical data from the early Palaeozoic igneous and sedimentary rocks, we propose that the closure of the North Qilian Ocean occurred in the Late Devonian. The similar age spectra of detrital zircons from sediments of the Alashan, Cathaysia, and Australia imply that the Alashan–Qilian–Qaidam block was probably located in the periphery of Gondwana in the early Palaeozoic.

The 600–580 Ma continental rift basalts in North Qilian Shan, northwest China: Links between the Qilian-Qaidam block and SE Australia, and the reconstruction of East Gondwana

We report a sequence of thick, well-preserved basaltic lavas interlayered with shallow marine dolomitic carbonates, mudstones and siltstones of the Zhulongguan Group, in the western segment of the North Qilian orogen, northwest China. Two new zircon SIMS ages show that this sequence formed at ∼600–580Ma. The mafic volcanics can be subdivided into tholeiitic and alkaline basalts, and have compositions similar to present-day ocean island basalt (OIB) or continental flood basalts. The occurrence, geochemical features and age data suggest that the Zhulongguan basalts originated at a continental rift setting in the latest Neoproterozoic, within the north margin of the Qilian-Qaidam block. This volcanic-sedimentary formation exhibits close affinity to the passive continental margin in southeastern Australia. Our observations favor a link of the Qilian-Qaidam block with SE Australia (also south China) during the breakup of Rodinia, thereby filling a void in existing reconstructions of the region.

Compositional and Sr–Nd–Hf isotopic variations of Baijingsi eclogites from the North Qilian orogen, China: Causes, protolith origins, and tectonic implications

Eclogites from the Baijingsi area in the North Qilian orogen at NW China were analyzed for major and trace element abundances as well as Sr, Nd, and Hf isotope ratios to evaluate the impacts from subduction processes on these compositional parameters and to reveal the complexity in protolith characteristics. The major element compositions of the Baijingsi eclogites are within the ranges for basalts. However, the absence of systematic variations among major oxides indicates varying extents of metamorphic modifications. Seven samples are characterized by profound Nb and Ta depletions. Six of them form coherent trends in the La versus Th, HFSE, and LREE plots. They are classified as the Group 1 samples. Other eight samples, referred to as the Group 2 samples, define distinct La–Zr and La–Sm trends and have flat variation patterns with slight Th–Nb–Ta–LREE depletions in the primitive mantle-normalized multiple-element diagram. Group 1 dominates the 87Sr/86Sr–87Rb/86Sr, 143Nd/144Nd–147Sm/144Nd and 176Hf/177Hf–176Lu/177Hf trends for relatively larger variations in the isotope and abundance ratios. Based on the errorchron ages from the samples, closure temperatures of the isotope systems, and isotope ratio versus 1/X plots (e.g., 87Sr/86Sr–1/Sr), it is inferred that the Rb–Sr isotope system was subjected to metamorphic modification, whereas the Sm–Nd and Lu–Hf isotope systems mainly reflect protolith characteristics. In addition to the Nb and Ta depletions, the Group 1 samples generally have εHf(510) values (protolith initial at 510Ma) higher than those of MORB at a given εNd(510), reflecting derivation from relatively high Lu/Hf ratio sources. They also plot within the fields for arc lavas in the Ta/Yb–Th/Yb and Th–Ta–Hf/3 diagrams. All these features are consistent with derivation from protoliths of an arc origin. In contrast, the Group 2 samples have εNd(510) values slightly higher than those of the Group 1 samples at a given εHf(510), plotting on the low εNd(510) margin of the MORB field. They also deviate from the Ta/Yb–Th/Yb trend defined by MORB and OIB toward higher Th/Yb values within the fields of back-arc basin basalts. Therefore, it is inferred that the protoliths of the Group 2 samples were generated as back-arc basin basalts possibly associated with the protoliths of the Group 1 samples in a common supra-subduction zone. The εNd(510)–εHf(510) relationship of the Baijingsi eclogites shows an affinity to the Indian ocean type mantle, implying protolith generation at the southern hemisphere on the margin or a fragment of the Gondwana continent, most likely in the Qilian Block. Based on the current tectonic configuration, a southward subduction is required for protolith generation and might eclogitize the arc protoliths by subduction erosion. Alternatively, the northward subduction in a bidirectional subduction system might become dominant during the amalgamation of the Qilian Block and the North China Craton to eclogitize the arc protoliths in the Qilian Block. Resolving these two possibilities requires more petrological, geochemical, and structural evidence from the North Qilian orogen.

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab

A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian orogen. It is geochemically characterized by high SiO2, Na2O (high Na/K), elevated Sr/Y and (La/Yb)N, positive Sr anomaly, relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted Nb, Ta, Ti, resembling the high-silica adakite. Zircon U-Pb SHRIMP dating yields a weighted mean age of 438±3 Ma. This age is significantly younger than eclogitization ages of 460–490 Ma in the North Qilian orogen, suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean. Whole-rock Sr and Nd isotopic analyses give initial ratios of ISr=0.7044–0.7047 and ɛNd(t)=3.0–4.1, indicating that they are derived from partial melting of the juvenile oceanic crust. In view of the tectonic evolution of the North Qilian orogen, the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of ∼60 km.

Tectonics of the North Qilian orogen, NW China

The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥710Ma as a consequence of breakup of supercontinent Rodinia.Initiation of the subduction in the Qilian Ocean probably occurred at ~520Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~517–490Ma. In the beginning of Ordovician (~490Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of ~490–445Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~6–7°C/km.The Qilian Ocean was closed at the end of the Ordovician (~445Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~100–200km at about 435–420Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials.Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation.

Detrital zircon U–Pb ages of Middle Ordovician flysch sandstones in the western ordos margin: New constraints on their provenances, and tectonic implications

The southwestern Ordos Margin is located between the Alax massif to the northwest, the North China Craton (NCC) to the east, and the North Qilian Orogenic Belt (NQOB, part of the Chinese Central Orogenic belt) to the southeast. Controversy surrounds the tectonic setting of the Western Ordos Margin in the Early Paleozoic; key questions are whether the Alax massif connected to the NCC in the Early Paleozoic, when exactly this event did happen, where the Lower Paleozoic sedimentary rocks along the southwestern Ordos had their provenances, and whether the so-called Helan aulacogen formed in the Ordovician. Laser-ablation inductively coupled plasma mass spectroscopy U–Pb dating of detrital zircons in four samples from Middle Ordovician sandstones in the Xiaoluo Shan and Niushou Shan located in the Western Ordos yield a majority of 206Pb/ 238U ages between 800 and 1000 Ma, a smaller group yields 206Pb/ 238U ages between 500 and 600 Ma; there are a few zircons with other ages, including some older than 2400 Ma. The youngest 206Pb/ 238U ages of the four samples are similar and range from 533 ± 3 Ma to 556 ± 4 Ma. These zircon U–Pb ages combined with several lines of sedimentary evidence imply that the so-called Helan aulacogen did not exist in the Ordovician; a peripheral foreland basin related to the NQOB developed instead. The detritals mainly came from the Alax massif, the North Qilian Arc, and the Dunhuang massif; and the NCC can be excluded as an important provenance. This study also suggested that the Alax massif did not connect with the NCC before the Middle Ordovician, and that the Alax is similar to the Yangtze Craton or the Tarim Craton with a majority of intrusions in the range of 800–1000 Ma and containing Precambrian tillites, which are characteristics that differ from the NCC.

Provenance record of a foreland basin: Detrital zircon U–Pb ages from Devonian strata in the North Qilian Orogenic Belt, China

Devonian strata in the North Qilian Belt of the Qilian Orogen accumulated in a foreland basin formed through collision between the North China Craton and the Central Qilian Belt. U–Pb age data for 393 detrital zircons from four sandstone samples from the Lower to Middle Devonian Laojunshan Formation and Upper Devonian Shaliushui Formation range in age from 3.6 Ga to 0.4 Ga and indicate input from a multi-component source. Detrital zircons older than the early Mesoproterozoic (1.4 Ga) were likely derived from the North China Craton, whereas late Mesoproterozoic to Neoproterozoic zircons were derived from the Central Qilian Belt. Early Paleozoic zircons were derived from the North Qilian continental arc and associated ophiolites. The occurrence of Neoproterozoic zircons indicates that their source, the Central Qilian Belt, was likely once part of the Yangtze Block of the South China Craton prior to its separation from the craton in the late Neoproterozoic.

Sedimentary geochemistry and provenance of the Lower and Middle Devonian Laojunshan Formation, the North Qilian Orogenic Belt

The Laojunshan Formation is a suite of molasse formed during the rapid uplift of the North Qilian Orogenic Belt (NQOB). Forty-one samples of sandstone have been collected from the Sunan and Minle sections in the western sector and the Gulang and Jingyuan sections in the eastern sector of the NQOB belt. Geochemical analyses of those samples indicated: 1) The MgO+Fe2O3 T and Al2O3/SiO2 values are higher, and K2O/Na2O ratios are lower in the western sector than those in the eastern sector. 2) All of them are depleted in Nb and Ta elements. The samples from the western sector are depleted in Rb element and enriched with Sc, Co, Ni, V, and Cr elements in the Upper Crust-normalized patterns. However, those from the eastern sector are depleted in Sr without enrichments of Sc, Co, Ni, V, and Cr. 3) All of the samples display a right-inclined REE pattern after Chondrite-normalized REE pattern. But LaN/YbN and Eu/Eu* ratios of the samples from the western sector are lower than those of the samples from the eastern sector. These geochemical characteristics suggest the prominent input of mafic clast with minor granitic rocks into the Sunan area, felsic clast into the Gulang and Jingyuan area, and both mafic and felsic clast into the Minle area. The angular shapes of gravels imply that these ill-sorted sediments were deposited near their sources without recycling. Geochemical features above also demonstrated that no major chemical weathering occurred for the western provenance, but deposits in the eastern sector resulted from low or middle degree chemical weathering. Evidences combining tectonic discriminations and comparisons with potential provenances revealed that sediments in the Sunan area were derived mainly from the North Qilian Continental arc, whereas sediments in the Minle, Gulang, and Jingyuan areas were derived not only from the North Qilian Continental arc but also from the basement of the Middle Qilian block. Integrated with the characteristics of development of Silurian and Devonian, these imply that the orogeny of NQOB is diachronous in the trending direction due to the oblique collision.

An early Palaeozoic double-subduction model for the North Qilian oceanic plate: evidence from zircon SHRIMP dating of granites

The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic mélange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO2, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U–Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane.