Qilian Ocean Research Articles

Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints

The Datong–Menyuan Complex, located in the northern margin of the Qilian Block, is composed dominantly of high‐grade metamorphic rocks covered by a Paleozoic–Mesozoic sedimentary sequence. In this study, we present petrographic observation, conventional thermobarometry and P–T pseudosection modelling, whole‐rock major and trace geochemistry, zircons U–Pb dating and Hf isotopic data together, to reveal the relationship between the magmatism and metamorphism along the northern margin of the Qilian Block. The anticlockwise P–T paths of gneiss and amphibolites are obtained using the garnet isopleths thermobaromety combined with phase equilibria modelling. The P–T pseudosection also shows that the gneiss and amphibolite underwent similar retrograde metamorphism with slightly different peak metamorphic conditions at ~720 °C and ~6.4 kbar for the former and ~700 °C and ~7.1 kbar for the latter. This indicates the Datong–Menyuan Complex recorded an amphibolite–granulite‐facies metamorphism. The zircon U–Pb analyses of 3 intrusive rocks including granitic dike, diorite, and granite are dated at ca. 499.8 ± 4.3, 495.9 ± 3.3, and 505.5 ± 3.2 Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 ± 4.1, 504.4 ± 3.9, and 499.3 ± 2.9 Ma. Zircon Hf isotopic analyses show that 505‐Ma zircons from the granitoid have positive εHf(t) values ranging from +8.5 to +12.8, indicating a depleted mantle source. Based on the penecontemporaneous magmatic and metamorphic event, we suggest that the southward subduction of the north Qilian Ocean triggered the activity of mantle‐derived magma and coeval metamorphic event. A subsequent compression is attributed to crustal thickening, indicating counter clockwise P–T paths. Combining these data with previous studies, it suggests paired metamorphic belts: the penecontemporaneous high‐temperature metamorphic belt related to an arc along the northern Qilian Block and high‐pressure/low‐temperature metamorphic belt existed in the north Qilian suture.

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.

Petrology and Geochemistry of the lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt: An eclogite facies metamorphosed oceanic slice

Although lawsonite eclogite is predicted to be common at depth of ~45–300km in subduction zones, it is rarely exposed at Earth's surface. In this study, we identified a lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt. It is mainly composed of garnet, amphibole, clinozoisite, quartz and rutile with minor omphacite relics and talc. Petrographic investigation identified abundant rectangular- or rhombic- shaped mineral aggregates composed of polycrystalline clinozoisite or polyphase clinozoisite+paragonite±phengite±quartz and clinozoisite+amphibole±quartz±Na-rich plagioclase occurring in garnet porphyroblasts. The peculiar shape, mineral assemblages and reconstructed compositions of these aggregates show that they are pseudomorphs after lawsonite. Phase equilibrium modeling predicts that the lawsonite (pseudomorph)-bearing eclogite experienced a clockwise P–T evolution with a peak stage at 570°C, 27kbar, near the quartz–coesite transition boundary. Geochemical data show that the lawsonite (pseudomorph)-bearing eclogite has a MORB-like affinity with flat Chondrite–normalized rare earth element (REE) distribution patterns. Zircons from this sample have similar δ18O values (5.38±0.10‰) with the normal mantle (5.3±0.3‰), indicating that the protolith of the lawsonite (pseudomorph)-bearing eclogite is derived directly from mantle rock without continental crust contamination. The MORB-like character and zircon O isotopic composition of the lawsonite (pseudomorph)-bearing eclogite indicate that its protolith is likely to be an oceanic crustal slice. SIMS and LA–ICP–MS zircon U–Pb dating yielded a 436–438Ma eclogite-facies metamorphic age for the lawsonite (pseudomorph)-bearing eclogite, similar to the continental subduction age of the North Qaidam UHPM belt, but did not obtain its protolith age. These data indicate that the protolith of the lawsonite (pseudomorph)-bearing eclogite may be either a piece of Proterozoic ocean that was emplaced in the active continental margin of the Qaidam block during the amalgamation of the Rodinia supercontinent, or the early Paleozoic oceanic crust in the transition zone between the Qilian Ocean and the Qaidam block.

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.

Geochemistry, zircon U–Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U–Pb–Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072°C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845°C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U–Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4±3.7Ma and 361.7±4.6Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (−11±0.5 to −9±0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from −8±1.0 to +10±0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence after a period of extensional post-collisional intraplate evolution, strong compressive forces affected the area.

Closure time in the East Qilian Ocean and Early Paleozoic ocean–continent configuration in the Helan Mountains and adjacent regions, NW China

Lithofacies and faunal assemblage affinities of the Helan Mountains and its adjacent blocks are explored using field investigations and analysis of previous results. The tectonic affinity between the Alxa Block and the North China Block is analyzed by means of LA-ICP-MS detrital U–Pb zircon age spectrums. The Early Paleozoic stratigraphic framework is constructed and stratigraphic contact relationships between the study area and its adjacent regions are assessed. The ocean–continent configuration during the Caledonian period is discussed and the closure time in the east of North Qilian Ocean is determined. Finally, the basin type and its evolutionary process during the Early Paleozoic are analyzed. The carbonate platform facies of the Alax Block and the abyssal flysch deposition on the margin of the Central Qilian Block, established the coexistence of ocean and continent configuration during the Cambrian. During the early Ordovician, the stable shallow water platform-shelf facies was distributed over all the study area and the biofacies was also unified. The east of the North Qilian Ocean closed at that time. The North Qilian Ocean and the North Qinling Ocean did not affect the study area, but the North China Sea was connected intermittently to the research area. The east Central Qilian Block, the Alax Block, the North China Block and the Qinling Microblock collided together to form a unified megablock, which resulted in the formation of a complex foreland depression during the early Ordovician. The eastern boundary of the basin is the Aselang–Chedao Transform Fault, which lies between the Alax Block and the North China Block. The southern boundary of the basin is marked by the Guyuan–Qingtongxia Fault which is located between the Alax Block and the Central Qilian Block. The western boundary of the basin is an overlapping boundary toward the Alax Block. During the late Ordovician, when the Yangtze Block approached toward the North China Block, the Shangdan Ocean began to influence the study area and the turbidite fans were formed because of the large-scale transgression. Thus, the basin type is a complicated foreland depression influenced by transgression during the late Ordovician.

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.

Petrogenesis of the early Paleozoic low-Mg and high-Mg adakitic rocks in the North Qilian orogenic belt, NW China: Implications for transition from crustal thickening to extension thinning

The petrogenesis and geodynamic implications of the early Paleozoic adakitic rocks in the North Qilian orogenic belt remain topics of debate. In this study, petrology, zircon U–Pb age and Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on both low-Mg and high-Mg adakitic rocks from the North Qilian orogenic belt. These results will provide significant constraints on the evolution of the North Qilian orogenic belt. In general, the low-Mg adakitic granites mainly consist of plagioclase (50–55%), alkali feldspar (20–25%), quartz (20–25%) and biotite (5%). In contrast, the high-Mg adakitic granodiorites are generally composed of alkali feldspar (30–35%), plagioclase (30–45%), quartz (20–25%) and amphibole (∼10%). The low-Mg adakitic granitoids (446–457Ma) are characterized by high SiO2 (69–72wt.%), low Mg# (39–42) and low Cr and Ni contents. However, the high-Mg adakitic granitoids (430–431Ma) have relatively lower SiO2 (65–66wt.%), higher Mg# (50–59) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial 87Sr/86Sr ratios (0.7068–0.7080), negative εNd(t) (−2.3 to −3.3) and εHf(t) values (−6.8 to 1.9) with old zircon Hf model ages (1.2–1.6Ga). However, the high-Mg adakitic rocks show lower initial 87Sr/86Sr ratios (0.7044–0.7066), higher εNd(t) (−1.8 to 3.0) and positive εHf(t) values (2.3–7.7) with younger zircon Hf model ages (0.9–1.1Ga). These results suggest that the low-Mg rocks were generated by partial melting of thickened crust, whereas the high-Mg rocks were derived from anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Qilian Ocean. The continent–continent collision and thickening probably occurred during 440–467Ma with formation of low-Mg adakitic rocks; and the transition of the tectonic regime from compression to extension probably occurred at ∼430Ma with formation of high-Mg adakitic rocks.

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 nature and history of the Qilian Block in the context of the development of the Greater Tibetan Plateau

The Palaeozoic granitoids in the Qilian Block are important for understanding the tectonic evolution of the Northern Tibetan Plateau. We choose granitoids from Huangyuan (HY) and Gangcha (Gcha) for a detailed study. The granitoids are S-type and I-type, and record different magmatisms from the Neoproterozoic to the Palaeozoic. Most samples have an emplacement age of ~450Ma with three samples being significantly older (924Ma, 797Ma and 503Ma). The ~924Ma and ~797Ma magmatisms represent crustal growth and crustal reworking, respectively. The 503Ma plagiogranite-like granite carries mantle isotope signatures and resulted from extensive fractional crystallization of mantle-derived melt in a back-arc setting. The ~450Ma granitoids have various chemical compositions, but most of them share similar trace element patterns resembling the bulk continental crust composition. Despite their large compositional and age variations, significant correlations on SiO2-variation diagrams and in isotope spaces suggest that these granitoids are different products essentially derived from common sources. The significant mantle contributions (70%–80%, apart from QL09-02) required by whole rock Sr–Nd–Pb–Hf isotopic compositions strongly suggest the “mantle source” as last fragments of the subducted/underthrusting oceanic crust at the onset of collision. Based on all the petrology, geochronology, geochemical data and adjacent tectonic associations, we suggest that the Qilian Ocean started opening in the Neoproterozoic between the Qaidam Block and the Qilian Block. A back-arc basin was developed between the Qilian Block and the Alashan Block shortly after. The 450Ma granitoids are the products in response to the closure of the Qilian Ocean and the onset of the Qilian–Qaidam continental collision.

The Age and Origin of the Jinfosi Biotite Granite, North Qilian, NW China: Evidence from U–Pb Zircon Age Data, Geochemistry, and Nd–Sr–Pb Isotopes

New geochemistry, Nd–Sr–Pb isotopes and U–Pb zircon data from the Jinfosi Biotite granite provide important constraints on the evolution of the crust in this part of the North Qilian, NW China. The Jinfosi Biotite granite have the following properties: SiO2 &gt; 65%, A/CNK(Molar Al2O3/(CaO + Na2O + K2O) ratios generally &gt; 1.1, Na2O generally &lt; 3.2%, Sm/Nd values between 0.17 and 0.27, and high Rb/Sr values. A chondrite-normalized rare earth element (REE) pattern shows negative Eu anomalies and depletion in heavy REEs. 143Nd/144Nd values are relatively low, and values of εNd(t) and εSr(t) are indicative of continental lithosphere. (87Sr/86Sr)i values are between 0.69952 and 0.70962, corresponding to continental crust mixed with a minor component of mantle material. Values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.9–19.0, 15.59–15.85, and 38.00–and 39.00, respectively, corresponding to S-type collision-related granites. The Jinfosi Biotite granite yield a SHRIMP zircon U–Pb age of 416.7 ± 4.1 Ma. R1–R2 and Rb versus (Yb + Nb) discrimination diagrams indicate that the Jinfosi biotite granite was produced during continental collision following closure of the paleo-North Qilian Ocean.

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 record of continental collision: Assembly of the Qilian Orogen, China

The Qilian Orogen, Northwest China, preserves a record of Early to Middle Paleozoic subduction and collision between the Central Qilian Block and North China Craton through closure of the intervening Late Neoproterozoic to Early Paleozoic North Qilian Ocean. Ordovician strata in the Hexi-Corridor Basin document the initial collisional history with a change in tectonic setting from retro-arc to foreland basin. Ages of zircons from the Upper Ordovician strata range from Archean to early Paleozoic with prominent peaks at around 2.5 Ga, 1.6 Ga, 1.0 Ga, and 0.47 Ga. The early Paleozoic ages are consistent with the time of magmatic arc activity related to northward subduction of the Qilian Ocean. Precambrian grains are restricted to the upper part of the sampled sections, with the proportion of Paleozoic grains descending up the section. This trend is interpreted to reflect the gradual termination of subduction zone magmatism and the swamping of the basin by material from the colliding Central Qilian Block and North China Craton. Archean and the early Proterozoic detritus were likely derived from the North China Craton to the north, whereas the Central Qilian Block to the south provided the late Proterozoic grains. Silurian and Devonian strata within the Hexi-Corridor Basin continue to show this mixed source provenance with depositional environment changing from a flysch to molasse setting.

Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust?

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO 2 (2–5 wt%), medium TiO 2 (1–2 wt%) and low TiO 2 (<1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative εNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction. Geochronological data indicate that the UHPM occurred about 500–440 Ma ago, and that there were two eclogite protoliths with ages of 800–750 and ∼1000 Ma, respectively. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, north Qaidam mountains, which have Sr and Nd ages of 768±39 and 780±22 Ma. The age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites. A later series of calc-alkaline, island arc volcanic rocks was formed at about 500 Ma ago, essentially contemporaneous with the UHP metamorphism. We propose the following tectonic model for the evolution of the north Qaidam UHP belt. At about 1000 Ma ago, a number of continents were amalgamated to form the Rodinian continent in this general area, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800–750 Ma ago to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the north Qaidam mountains. In the early Paleozoic another Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma ago and the subduction-related granites (470–450 Ma). Once the oceanic lithosphere was consumed, the subsequent continent–continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites, to cause the ultrahigh pressure metamorphism.

40Ar/39Ar ages of blueschist facies pelitic schists from Qingshuigou in the Northern Qilian Mountains, western China

Abstract Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the high‐grade blueschists at Qingshuigou. White mica 40Ar/39Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1–453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The 40Ar/39Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high‐grade metamorphism during the same subduction event. From this chronological evidence and the presence of well‐developed Silurian remnant‐sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.