North Qilian Research Articles

Genesis of W–Mo mineralization in the Xiaoliugou and Ta’ergou ore fields, North Qilian Orogen (NW China): Constraints from fluid inclusions and S–Pb[sbnd]H[sbnd]O[sbnd]Nd isotopes

Tungsten–molybdenite (W–Mo) polymetallic mineralization are intensely developed in the western section of the North Qilian Orogen (NW China), represented by the Xiaoliugou and Ta’ergou ore fields. In this study, fluid inclusion microthermometric and S–PbHONd isotopic analyses were conducted on the W–Mo ores to shed insights into the genesis of the mineralization belt. The ore fields are characterized by abundant liquid-vapor two-phase inclusions which have medium–high temperature-salinity (200–320 °C and 4–10 wt% NaCl equiv.). The consistent δ18OV-SMOW (7.7‰ to 13.1‰) and δ2HV-SMOW (−88.1‰ to −49.4‰) values of quartz and muscovite indicate that the ore-forming fluid originated from granitic intrusion but mixed with meteoric water. Typical δ34S values of their sulfide ores range from 6 to 18‰, and the ore-related granites (206Pb/204Pb = 18.18–26.61, 207Pb/204Pb = 15.51–16.02, 208Pb/204Pb = 37.60–39.77) and sulfides (206Pb/204Pb = 17.97–20.06, 207Pb/204Pb = 15.42–16.64, 208Pb/204Pb = 37.39–41.06) have variable Pb/Pb ratios, indicating both magmatic rocks and meta-sedimentary rocks contributed to the mineralization. The negative Nd values (εNd(t) = −9.0 to −3.2) and old two-stage Nd model ages (TDM2 = 1517 to 1901 Ma) of the scheelites suggest that the ore-forming materials may have been derived from old crust together with the granitic magma. An isochron age of 422 ± 42 Ma (initial 143Nd/144Nd = 0.511701 ± 0.000046) has been yielded by the scheelite Sm-Nd dating from the Ta’ergou ore field, implying the 450–420 Ma magmatic–hydrothermal event resulted in the intense W–Mo mineralization in the North Qilian, especially in the western section of this orogen. Consequently, we propose a multi-metal coupled mineralization model to illustrate the mineralization process of the ore fields, which involving the formation of ore source beds (wall-rocks) in Proterozoic and the intrusion of granites in Early Paleozoic. This research highlights the use of multiple isotopic methods in revealing complex fluid evolutionary process of large-scale W–Mo deposits.

Petrogenesis and tectonic implications of cambrian Nb-enriched I- and aluminous A-type granites in the North Qilian suture zone

ABSTRACT Granitoids in active continental margins are probes of sub-arc magmatic processes and are crucial for the understanding of crust–mantle interaction during subduction. Here we report data for the middle to late Cambrian Tuole (TL) and Dabaishitou (DBST) granitic plutons in the western segment of the North Qilian suture zone (NQSZ). The TL-DBST plutons comprise monzonite, alkali-feldspar granite, gabbroic diorite, monzonitic porphyry and biotite monzogranite. Zircon U‒Pb ages reveal that these granitoids crystallized at ca. 509‒492 Ma, consistent with the early subduction of the North Qilian Ocean. Mineralogical, petrological, and geochemical features indicate that the TL monzonite and gabbroic diorite are both high-K calc-alkaline I-type granitoids but the latter is Nb-enriched; the TL alkali-feldspar granites and DBST granitoids belong to aluminous A-type and transitional I- and A-type granites, respectively. The I- and aluminous A-type granites show similar and comparable isotopic compositions, e.g., εHf(t) values of ‒5.6 to +2.7 versus ‒2.4 to +3.5, but different melting conditions, e.g., high temperature and low oxygen fugacity for aluminous A-type granites as indicated by zircon trace elements. Geochemical and isotope data suggest that all these granitoids were mainly derived from the sub-arc mantle metasomatized by slab-derived fluids/asthenospheric mantle-derived melts/both prior to melting with minor input of LCC-derived melts (ca. 10–20%). The HFSE enrichment feature was inherited by granites and further enhanced by protracted crystal fractionation at low oxygen fugacity conditions to produce the TL aluminous A1-type granites but weakened by fractionation at relatively high oxygen fugacity conditions to produce the DBST I-A-transitional-type granite. Partial melting of the sub-arc lithospheric mantle was induced by upwelling of the asthenospheric mantle and their interactions in response to slab rollback during subduction initiation (ca. 509–506 Ma), and by addition of slab-derived fluids again as subduction proceeded (ca. 493–492 Ma).

Tectonic evolution of the Alxa Block and its affinity: Evidence from the U-Pb geochronology and Lu-Hf isotopes of detrital zircons from the Longshoushan Belt

The NW-SE trending Longshoushan Belt is a significant transitional belt between the Alxa and Qilian blocks. Its basement rocks were predominated by greenschist- to amphibolite-facies metamorphosed volcanic-sedimentary rocks, which were named the Longshoushan Complex and sporadically exposed in the middle and eastern Longshoushan Belt. The U-Pb ages of detrital zircons from the middle Longshoushan are scattered in the period of 3006–1981 Ma, predominated by Paleoproterozoic (>95%) with a single peak of 2.0 Ga. These zircons have the 176Hf/177Hf ratios of 0.281150–0.281687, corresponding to hafnium model ages (TDMC) of 3.04–2.15 Ga. These age features are in accordance with those of the Alxa Block and the western North China Craton. In contrast, the protoliths of the eastern Longshoushan reveal a wide age range from 3389 Ma to 529 Ma, which can be apparently divided into four age populations of 2.5 Ga, 1.6 Ga, 1.0 Ga and 0.8 Ga. This is consistent with recent reported age data of the Hexi Corridor, indicating that the eastern Longshoushan is not a part of the Longshoushan Complex, but has affinity with the Hexi Corridor. The cumulative probability curves of measured crystallization ages and depositional ages of detrital zircons imply that the middle Longshoushan could be deposited in a convergent basin (e.g., back-arc basin) in the Paleoproterozoic, but the eastern Longshoushan was typical of tectonic setting of a collisional basin (e.g., a foreland basin) in the Cambrian. The closure of the North Qilian Ocean formed the North Qilian Orogen between the Central Qilian and Alxa blocks, resulting to a foreland basin in the Hexi Corridor (i.e., eastern Longshoushan).

Provenance and Implication of Carboniferous–Permian Detrital Zircons from the Upper Paleozoic, Southern Ordos Basin, China: Evidence from U-Pb Geochronology and Hf Isotopes

Carboniferous–Permian detrital zircons are recognized in the Upper Paleozoic of the whole Ordos Basin. Previous studies revealed that these Carboniferous–Permian zircons occurred in the Northern Ordos Basin mainly originated from the Yinshan Block. What has not been well documented until now is where this period’s zircons in the Southern Ordos Basin came from, and very little discussion about their provenance. To identify the provenance of the detrital zircons dating from ~350 to 260 Ma, five sandstone samples from the Shan 1 Member of Shanxi Formation and eight sandstone samples from the He 8 Member of Shihezi Formation were analyzed for detrital zircon U-Pb age dating and in situ Lu-Hf isotopic compositions. The results indicate that the two age clusters of 520–378 Ma and ~350–260 Ma in the Southern Ordos Basin most likely derived from the North Qinling Orogenic Belt–North Qilian Orogenic Belt and the North Qinling Orogenic Belt, respectively. Furthermore, we propose that the zircons aging ~320–260 Ma are representative of the important tectonothermal events occurred in the North Qinling Orogenic Belt during the Late Paleozoic.

Structural anatomy of the early Paleozoic Laohushan ophiolite and subduction complex: Implications for accretionary tectonics of the Proto-Tethyan North Qilian orogenic belt, northeastern Tibet

Abstract Recognition of accretionary tectonics in ancient orogenic collages is important for reconstructing the long-term subduction, accretion, and erosional history of fossil convergent margins, and for understanding crustal growth and supercontinent assembly. The North Qilian orogenic belt (NQOB), located between the Alxa block and the Central Qilian–Qaidam block in northeastern Tibet, is a typical Phanerozoic accretionary-to-collisional orogenic belt that represents the termination of the northern branch of the Proto-Tethys Ocean. It contains two subparallel ophiolitic belts, arcs, and subduction complexes; the ophiolitic rocks in the northern belt have generally been considered to have formed in a back-arc setting. However, the subduction-accretion-collision history, subduction polarity, and timing of closure of the back-arc ocean remain equivocal. To address these problems, we conducted detailed field, structural, and geochronological investigations of the Laohushan ophiolite–accretionary complex and related sedimentary rocks in the eastern NQOB. The Laohushan Complex is divisible into (1) a northern sedimentary forearc, and a supra-subduction zone-type ultramafic-mafic forearc (ca. 450 Ma) composed of serpentinized harzburgite, gabbro, basalt, and plagiogranite; and (2) a southern accretionary complex, which consists of relatively coherent basalt-chert-mudstone ocean plate stratigraphy that is structurally repeated many times, trench-fill turbidites, mélanges, and widespread thrust imbricates and duplexes, block-in-matrix and asymmetric structures. Kinematic analysis indicates that the accretionary complex underwent southward thrusting and shearing; coupled with the spatial architecture of the different tectonic units, which suggests northward subduction beneath the northern forearc on the southern margin of the Alxa block. Detrital zircon ages of forearc clastic sandstones, pelagic mudstones, trench-fill turbidites, and the matrix of mélanges, together with the zircon ages of igneous ophiolitic rocks and post-accretionary intrusions, indicate that the terminal accretion and tectonic stacking of the Laohushan subduction complex was between ca. 447 and 430 Ma. We propose a geodynamic model involving back-arc basin opening (ca. 517–449 Ma), intra-oceanic subduction-accretion (ca. 449–430 Ma), and final obduction of the northern forearc to account for the evolutionary processes of the North Qilian back-arc basin. The anatomy of the forearc ophiolite and structurally lower accretionary complex indicates the complicated origins and mechanism of emplacement of the ophiolitic rocks. Field-based reconstruction of accretionary complexes and upper plate ophiolites, together with provenance analysis of related sedimentary sequences, provide crucial constraints on the prolonged evolution of paleo-ocean basins and accretionary-to-collisional orogens.

Mineral Compositions of Syn-collisional Granitoids and their Implications for the Formation of Juvenile Continental Crust and Adakitic Magmatism

AbstractContinentalcollision zones have been proposed as primary sites of net continental crustal growth. Therefore, studies on syn-collisional granitoids with mafic magmatic enclaves (MMEs) are essential for testing this hypothesis. The Baojishan (BJS) and Qumushan (QMS) syn-collisional plutons in the North Qilian Orogen (NQO) on the northern margin of the Tibetan Plateau have abundant MMEs in sharp contact with host granitoids, sharing similar constituent minerals but with higher modal abundances of mafic minerals in MMEs. The QMS host granitoids have high Sr/Y and La/Yb ratios, showing adakitic compositions, which are differentfrom the BJS granitoids. Based on bulk-rock compositions and zircon U-Pb age-dating, recent studies on these two plutons proposed that MMEs represent cumulates crystallized early from the same magmatic system as their host granitoids, and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies. Here, we focus on the trace-element geochemistry of the constituent mineral phases of both MMEs and their host granitoids of the QMS and BJS plutons. Weshow that different mineral phases preferentially host different trace elements; for example, most rare earth elements (REEs and Y) reside in titanite (only found in the QMS pluton), amphibole, apatite, epidote and zircon (mostly heavy-REEs); and high-field-strength elements (HFSEs) reside in biotite, titanite, amphibole and zircon. Based on the mineral chemical data, we show that for these two plutons, MMEs are of similar cumulate origin, crystallized from primitive andesitic melts in the early stage of granitoid magmatism. The primitive andesitic melts for these syn-collisional granitoids are most likely produced by the partial melting of the oceanic crust, supporting the hypothesis of continental crustal growth considering the syn-collisional granitoids represent juvenile continental crust. As evidenced by distinct mineral compositions, the two plutons have different parental magma compositions, for example higher TiO2 content and higher Sr/Y and La/Yb ratios in the QMS parental magmas, a signature best understood as being inherited from the source. The higher TiO2 content of the parental magma for the QMS pluton leads to the common presence of titanite in the QMS pluton (absent in the BJS pluton), crystallization of which in turn controls the trace-element (REE, Y, Nb, Ta and others) systematics in the residual melts towards an adakitic signature. Therefore, parental magmas with high TiO2 content and high Sr/Y and La/Yb ratios, as well as their further fractionation of titanite, are important factors in the development of adakitic compositions, as represented by the QMS host granitoids. This model offers a new perspective on the petrogenesis of adakitic rocks. The present study further demonstrates that, in general, mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks.

Iron Isotope Behavior During Melt‐Peridotite Interaction in Supra‐subduction Zone Ophiolite From Northern Tibet

AbstractMelt‐peridotite interaction has the ability to modify the δ56Fe of peridotite. However, iron isotopic fractionation during melt‐peridotite interaction is not well understood at present. In this study, we present high‐precision iron isotopic data for serpentinized and carbonated peridotite from the Yushigou supra‐subduction zone ophiolite in the North Qilian orogen of northern Tibet, to provide insights into iron isotope behavior during melt‐peridotite interaction in the mantle wedge. High‐Cr and high‐Al dunites coexist with harzburgite in the Yushigou ophiolite. The high‐Cr and high‐Al dunites were produced by interaction of peridotite with boninitic melt and tholeiitic melt, respectively. Serpentinization and carbonation have negligible influence on δ56Fe of the peridotite. The harzburgite mostly has δ56Fe ranging from −0.055 ± 0.029‰ to 0.056 ± 0.030‰, which overlaps with δ56Fe of abyssal peridotite. The high‐Al dunite is enriched in FeO, but it has δ56Fe (0.004 ± 0.034‰–0.083 ± 0.064‰) similar to the harzburgite. In contrast, the high‐Cr dunite displays a large range in δ56Fe from −0.173 ± 0.017‰ to 0.173 ± 0.057‰. Moreover, δ56Fe of the high‐Cr dunite is negatively correlated with its FeO content, which is ascribed to kinetic iron isotopic fractionation during melt‐peridotite interaction. Surprisingly, distinct iron isotope behavior during melt‐peridotite interaction recorded by the Yushigou peridotite can well explain the large variation of FeO and δ56Fe in abyssal peridotite and mantle wedge peridotite on a global scale.

Petrogenesis and tectonic setting of Early Paleozoic granites and high-Mg diorites in the Northern Qilian Orogen, China

The Northern Qilian Orogenic Belt (N-QOB) was formed by oceanic subduction and collision between the Alashan and Central Qilian Belt in the Early Paleozoic. New zircon U–Pb ages, Hf isotopic and whole-rock elemental data of the intrusive rocks in the central N-QOB have been conducted to constrain the tectonic evolution of N-QOB. Zircon dating results revealed that the Ladonggou, Suyougou granitic plutons and Xichagou dioritic pluton were emplaced at 505 ± 3 Ma, 471 ± 3 M and 425 ± 3 Ma, respectively. The Ladonggou monzogranite displays depletion of Nb, Ta, Ti, P and negative εHf(t) values (−6.5 to −0.3), and old Hf crustal model ages (1.5–1.9 Ga), suggesting generation from melting of ancient crust above subducted North Qilian oceanic slab. The Suyougou granite possesses geochemistry of I-type granite. The relative enrichment of LILEs, depletion of HFSEs and negative Eu anomalies, as well as prominent Hf isotope heterogeneities and old Hf crustal model ages (2.1–2.5 Ga) suggest a magma source of ancient crust mixed with minor mantle-derived melts in a subduction-related setting. Diorites from the Xichagou plutonare characterized by high Mg# values, enrichments in Cr, Ni, Y, Yb and high Sr/Y ratios, and convincing subduction-related features, suggesting generation from partial melting of subducted sediments mixed with mantle components. Taking all these geochemistry and geochronology together with the regional geology, we interpreted the Ladonggou monzogranite, Suyougou granite and Xichagou diorite were related to the long-term northward subduction of North Qilian oceanic lithosphere during ca. 505 and 425 Ma.

Formation and metamorphism of sedimentary rocks beneath the Dingxi Basin: Record of the evolution from subduction to collision in the North Qilian Orogen

AbstractThe North Qilian Orogen (NQO) in northwest China underwent oceanic subduction and subsequent continental collision. Metasedimentary rocks from a deep borehole in the Dingxi Basin, NQO, contain garnet, biotite, plagioclase, quartz and minor cummingtonite and chlorite in the matrix, with inclusions of kyanite and staurolite in garnet. The mineral textures and compositions define clockwise pressure–temperature evolution with peak conditions of ~10.5 kbar and ~670°C, followed by isothermal decompression down to ~6.5 kbar. Age and Hf isotope data of detrital zircon support the formation of the sedimentary protolith in an arc setting at ~460 Ma, and the age and rare earth element characteristics of metamorphic monazite reflect exhumation at ~425 Ma. These results indicate a complete cycle of deposition–burial–exhumation for the sedimentary rocks, and directly constrain the continental collision process in NQO to yield a geotherm of ~21°C/km and to culminate before 425 Ma.

Petrogenesis of Early Paleozoic high Sr/Y intrusive rocks from the North Qilian orogen: Implication for diachronous continental collision

Abstract A combination of U-Pb zircon ages and geochemical and Sr-Nd-Hf isotopic data are presented for the Early Paleozoic granodiorites from the Haoquangou and Baimawa plutons in order to probe the crustal thickness variation of the eastern North Qilian and the diachronous evolution of the North Qilian orogen. The granodiorites formed at 436–435 Ma and have high Sr/Y ratios (63–117). Elemental and isotopic data combined with geochemical modeling and comparisons with experimental data suggest that they were produced from the melting of relatively juvenile mafic rocks in the thickened lower crust. Together with other petrological and geochemical data and the calculation of variation in crustal thickness, this indicates that the eastern North Qilian experienced clear crustal thickening and thinning from the Late Ordovician to Late Silurian. Based on available data, we suggest that diachronous collision from east to west, which probably resulted in the distinct intensity of orogenesis between eastern and western North Qilian, can well account for the differential distribution of Early Paleozoic high Sr/Y magmatism and other geological differences between the eastern and western parts of the North Qilian. Our study also implies that diachronous collision may lead to, apart from distinct metamorphic, structural and sedimentary responses, the large differences in magmatism and deep crustal processes along the orogenic strike.

Petrography and geochemistry of the Lower Silurian sandstones from the Angzanggou Formation in the North Qilian Belt, China: implications for provenance, weathering and tectonic setting

AbstractThe North Qilian Orogenic Belt is on the northeastern margin of the Tibetan Plateau. It is connected with the Alxa Block in the north and the Tethyan orogenic assemblage in the south. The Lower Silurian Angzanggou Formation, deposited in the northern area of the North Qilian Orogenic Belt, is the most important for revealing the architecture and orogenic processes of the North Qilian Belt. Provenance analysis of the Angzanggou Formation can reveal not only the tectonic evolution of the central orogenic belt of China but also Palaeozoic Asia plate reconstructions. Petrographic analysis indicated that the compositional and textural maturity of the sandstones was low. The detrital composition of the Angzanggou Formation samples consists of quartz (8–14 %), feldspar (6–29 %) and lithic fragments (56–86 %). The sandstones could be classified as litharenites or feldspathic litharenites. The detrital modal composition suggests that these sandstones were probably deposited in a fore-arc basin. The element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks. The SiO2/Al2O3 ratio, the index of chemical variability and the Th/Sc versus Zr/Sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low. The index of alteration (CIA) and the A–CN–K diagram indicate the intensity of weathering was moderate. The discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the Angzanggou Formation rocks were derived from a continental island arc, and a sedimentary cover probably overlaid the volcanic arc. Therefore, we infer that during Early Silurian time the North Qilian Belt sediments accumulated in a fore-arc basin.

Cambrian mafic and granitic intrusions in the Mazar-Tianshuihai terrane, West Kunlun Orogenic Belt: Constraints on the subduction orientation of the Proto-Tethys Ocean

The West Kunlun Orogenic Belt (WKOB) carries key information about the evolution of the Proto-Tethys Ocean. In order to have a better understanding of the early Paleozoic tectonic evolution of the WKOB and to unravel the process of the Tarim assemblage to Gondwana, we report field observations, petrography, ages, chemical and Sr-Nd-Hf isotopic compositions of the newly identified Cambrian mafic and granitic intrusions in the Mazar Terrane-Tianshuihai terrane (MZT-TSHT) at the interior of the WKOB. Our zircon U–Pb dating reveals that the mafic and granitic rocks in Tianshuihai Terrane (TSHT) emplaced at ca. 530 Ma, and that the mafic rocks in the Mazar Terrane (MZT) emplaced at ca. 500–490 Ma. Whole-rock major and trace element geochemistry defines a tholeiitic signature for these mafic rocks. Isotopically, the TSHT mafic rocks have depleted Nd isotope compositions with initial εNd(t) isotopes ranging from 2.74 to 3.60, whereas the MZT mafic rocks exhibit enriched whole-rock Nd isotope compositions with εNd(t) values ranging from −12.90 to −9.71. The distinct chemical and isotopic signatures argue that the Cambrian mafic rocks in the TSHT were derived from a depleted mantle source in an initial subduction setting, whereas the mafic rocks from MZT were derived from an enriched sub-continental lithospheric mantle source in an arc setting. The Cambrian granites in the TSHT show typical fractionated I-type granite elemental signatures, in line with their Nd-Sr-Hf isotope compositions. These Cambrian granites were most likely generated by partial melting of Mesoproterozoic (0.9–1.3 Ga) crustal sources due to underplating of the coeval mafic magma. Our results in combination with previous studies suggest that the Cambrian mafic and granitic rocks in the MZT-TSHT were generated by the southward subduction of the Proto-Tethys Ocean initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive early Paleozoic accretionary wedge in WKOB. The Proto-Tethys Ocean finally closed at ca. 440 Ma as demonstrated by the amphibolite facies metamorphism of the accretionary complex and the middle to late Devonian molasses unconformably overlying the accretionary complex. This process caused the blocks in East Asia, including the Tarim, Qaidam and North Qilian, Yangtze and Indochina, to dock at the northern fringe of the Eastern Gondwana.

Whole-rock and zircon geochemistry of the Xiaoliugou granites, North Qilian Orogen (NW China): Implications for tectonic setting, magma evolution and W–Mo mineralization

Ordovician W–Mo polymetallic mineralization-related granites are widely developed in the western section of the North Qilian Orogen, NW China. In order to shed light on the tectonic setting, magma evolution and genesis of mineralization in the orogeny, highly evolved granitic rocks (monzogranites) from the Qibao and Qiqing deposits in the Xiaoliugou ore field were studied. Typical of subduction-related A-type granites, the Xiaoliugou granitic rocks are enriched in SiO2, Rb, Th, U, Ta, Nd, Zr and Hf but relatively depleted in Ba, Sr, P and Eu. The variable initial whole rock 87Sr/86Sr ratios (0.62937–0.70543) and wide range of zircon εHf(t) values (−18 to 6) suggesting mantle–crust interaction and mixing of depleted mantle and deep crust in the source area. Whole-rock elemental and Rb–Sr isotopic geochemistry suggest slow cooling, significant fractional crystallization, weak wall-rock assimilation/contamination and intense fluid differentiation/metasomatism during the genesis of the Xiaoliugou granitic rocks. Zircon U–Pb dating yielded three age clusters: Precambrian (>540 Ma) (n = 32), Ordovician (~450 Ma) (n = 40), and Cretaceous (~135 Ma) (n = 18). Zircon internal textures indicate a range of sources including residual/detrital zircons from the magma source region and local sediments, magmatic and fluid-altered zircons, and late stage hydrothermal zircons. Zircon trace element and Lu–Hf isotopic compositions suggest fluid metasomatism during granite emplacement in the Ordovician and overprinting by a more evolved, oxidized fluid in the Cretaceous. Involvement of a late, high-temperature, highly fractionated, F-rich fluid could have enhanced W–Mo mineralization. Consequently, a two-stage genetic model is proposed for the Xiaoliugou granitic-minerogenic system involving primary mineralization in the Ordovician (~450 Ma), followed by a Cretaceous magmatic hydrothermal overprint (~135 Ma).

Paleoclimatic record from the clay mineralogy of Quaternary sediments of drill core DWJ from Jiudong subbasin (western Hexi Corridor), NW China

AbstractThe thick sedimentary deposits of the Jiudong subbasin in the western Hexi Corridor of NW China can potentially provide important records of climate change in arid and semiarid areas subject to the interaction of the East Asian summer monsoon (EASM) and the westerlies. Here we present the results of a study of the clay mineralogy of the sediments of drill core DWJ from the Jiudong subbasin. The clay minerals consist mainly of illite, followed by chlorite, kaolinite, and smectite. The clay mineral assemblages are used to define four paleoclimatic stages since ~1.8 Ma: (1) During ~1.8–1.2 Ma, the climate was relatively cold and arid; (2) during ~1.2–0.9 Ma, it was relatively warm and wet, with strongly seasonal precipitation; (3) during ~0.9–0.3 Ma, there was a long-term trend of cooling and drying; and (4) during ~0.3–0.1 Ma, the climate was warmer and wetter than during the previous period, with strongly seasonal precipitation. The paleoclimatic record of the Jiudong subbasin since ~1.8 Ma is consistent with records from the Tengger Desert and typical monsoonal areas. This suggests that the EASM penetrated into the central regions of the North Qilian Shan and Zhangye–Jiudong subbasin during ~1.2–0.9 and ~0.3–0.1 Ma.

Partial melting of thickened lower crust in post‐collisional setting: Evidence from high silicon adakitic granites in the North Qilian orogen

During orogenic extension, partial melting of the continental crust usually produces post‐collisional granitoids, and the characteristics of their petrogenesis can provide crucial constraints on the geodynamics of tectonic regimes and post‐collisional lithospheric processes. A case study for this issue is the high‐silicon adakitic granites from the Jingtieshan area, North Qilian orogenic belt. In this study, we present a range of evidences on petrology, zircon U‐Pb dating, zircon Lu‐Hf isotopic composition, whole‐rock geochemistry, and Sr‐Nd isotopes from the studied high‐silicon adakitic granites to better understand the geodynamic processes of the North Qilian orogenic belt. The adakitic rocks consisted of plagioclase (55–60 %), K‐feldspar (5–10 %), quartz (20–25 %), and a small amount of biotite and accessory minerals. Zircon grains from the studied sample yielded a weighted mean 206Pb/238U age of 430 ± 2 Ma (MSWD = 0.96) with positive εHf(t) values (−1.4 to +6.3, with an average of +4.2). Geochemical results suggested that the adakitic rocks were enriched in LREE without evident Eu anomalies (except for one sample; δEu = 0.32–0.41), and these samples exhibited high Sr/Y ratios with negative εNd(t) values (−4.9 to −3.1). A combined study of petrology and geochemical arguments lead us to propose that the high‐silicon adakitic granites from the Jingtieshan area within the North Qilian orogenic belt were generated by partial melting of the thickened crust in a post‐collisional tectonic setting and that the adakitic melts were derived from the partial melting of the metabasite that was emplaced into the crust during the late Proterozoic‐early Neoproterozoic.

Geology, Fluid Inclusions and Stable Isotopes of the Xialiugou Polymetallic Deposit in North Qilian, Northwest China: Constraints on its Metallogenesis

The Xialiugou polymetallic deposit is located in the North Qilian Orogenic Belt, Northwest China, of which the main ore-bearing strata are the Middle Cambrian Heicigou Group. The mineralization is zoned with “black” orebodies (galena–sphalerite), which are stratigraphically above the “yellow” orebodies (pyrite–chalcopyrite–tennantite) at the lower zone, corresponding to the alteration assemblages of quartz–sericite in the ore-proximal zone and chlorite in the ore-distal zone. The Xialiugou mineralization can be divided into three stages: (1) Stage I (pyrite); (2) Stage II (chalcopyrite–tennantite–sphalerite); and (3) Stage III (galena–sphalerite). Fluid inclusions data indicate that the physicochemical conditions that lead to ore formation were the medium–low temperature (157–350 °C) and low salinity (0.17–6.87 wt % NaCleqv), and that the ore-forming temperature tended to decrease with the successive mineralization processes. Taking the H–O isotopic compositions (δDV-SMOW = −51.0‰ to −40.5‰, δ18OH2O = −0.4‰ to 8.6‰) into consideration, the ore-forming fluids were most likely derived from seawater with a small amount of magmatic- and meteoric-fluids input. In addition, the combined S (−3.70‰ to 0.10‰) and Pb isotopic (206Pb/204Pb = 18.357 to 18.422, 207Pb/204Pb = 15.615 to 15.687, 208Pb/204Pb = 38.056 to 38.248) data of pyrite indicate that the ore-bearing volcanic rocks may be an important source of ore-forming materials. Finally, we inferred that the Xialiugou deposit shares similarities with the most important volcanogenic massive sulfide (VMS) deposits (Baiyinchang ore field) in China and typical “black ore” type VMS deposits worldwide.

Discrepancy between bulk-rock and zircon Hf isotopes accompanying Nd-Hf isotope decoupling

Abstract Zircon is an important accessory mineral for studying the crust-mantle interaction and crustal growth through time because zircon crystals not only allow precise dating but also record initial Hf isotope ratios of the host magma. Our study on a suite of gabbronorite, mafic diorite cumulate, diorite and granite from the Kekeli Batholith in the North Qilian Orogenic Belt, Northern Tibetan Plateau, shows (1) a significant Hf isotope discrepancy between zircon and their bulk rocks; and (2) bulk-rock Nd-Hf isotope decoupling. These observations thus demonstrate that zircons do not always capture the full history of magmatic system. The significant positive correlation between bulk-rock Hf isotope ratios and TiO2 content (R2 = 0.94) indicates that Ti-rich minerals (e.g., ilmenite, amphibole) are likely important Hf hosts. The early-formed Ti-rich minerals possibly record different Hf isotopes from those of zircons crystallized subsequently, thus causing discrepancy between zircon and bulk-rocks and leading to bulk-rock Nd-Hf isotope decoupling. Correlations between bulk-rock TiO2 content, Mg isotopes and Hf isotopes indicate a mixing process, with granite and gabbronorite representing two compositional endmembers. Because Ti minerals have higher crystallization temperatures than zircons, when the mixing melts have contrasting isotopes (or from heterogeneous sources/have a strong crustal contamination), the bulk-rock and zircon Hf isotope discrepancies reflect mineral crystallization sequence during mafic and felsic magma mixing. It is thus imperative to consider early formed minerals such as Ti-rich minerals and the bulk rock composition, not just zircons, when using Hf isotopes to track melt evolution and precisely constrain mantle contribution to granitoid petrogenesis.

Petrogenesis of Early Silurian granitoids in the Longshoushan area and their implications for the extensional environment of the North Qilian Orogenic Belt, China

Analyses of the zircon U—Pb age, mineralogical chemistry and whole-rock geochemistry of the Jiling pluton along the southern margin of the Alxa Block were conducted. The results of these analyses will provide crucial information for investigations of Paleozoic crust-mantle interactions in the North Qilian Orogenic Belt (NQOB) and explorations of the transition age from a compressional to extensional environment during the collision between the Alxa Block and the Qilian-Qaidam Block. The zircon dating results reveal that the Jiling pluton is composed of Early Silurian granitoids (435–442 Ma) with contemporaneous mafic microgranular enclaves (MMEs). Most granitoids contain amphibole and titanite and have A/CNK values <1, illustrating that they belong to amphibolite-rich calc-alkaline granitoids (ACGs) and K-rich and K-feldspar porphyritic calc-alkaline granitoids (KCGs). Both the granitoid and MME samples display variable initial Sr and Nd isotopic compositions. We suggest that the Jiling pluton was formed by crust-mantle-derived magma mixing. The high HFSE content of the MMEs indicates that the mantle-derived magmas were derived from melting of the mantle wedge metasomatized by adakitic magma, and the crustal-derived magmas were derived from melting of Paleoproterozoic meta-basaltic rocks in the Longshoushan Group. The discovery of the Jiling granitoids indicates that the NQOB was dominated by a compressional environment during Middle Ordovician-Early Silurian in response to the collision between the Qilian-Qaidam Block and the Alxa Block, and the initial extensional environment began before 442 Ma.

Iron isotopic composition of supra-subduction zone ophiolitic peridotite from northern Tibet

Abstract We present high-precision iron isotope data for harzburgites and constituting minerals from the Yushigou supra-subduction zone ophiolite in the North Qilian orogen of northern Tibet to provide insights into iron isotope behavior in subduction zones. The Yushigou harzburgites represent typical melting residues of silica-enriched forearc mantle wedge with significant melt extraction and minimal melt percolation. The δ56Fe ranges from −0.096‰ to 0.110‰ in olivine, from −0.111‰ to 0.105‰ in orthopyroxene and from −0.141‰ to 0.054‰ in spinel. Coexisting minerals fall on a line with a slope of 1 in the δ-δ plot, indicating iron isotope equilibrium between mineral pairs. Iron isotopic fractionation between olivine and orthopyroxene is minor, which is consistent with theoretical predictions and previous observations. Good iron isotopic equilibrium between olivine and orthopyroxene is a further indicator that silica enrichment in arc peridotites is inherited from their mantle source. In contrast, significantly negative iron isotopic fractionation between spinel and olivine contradicts with most previous constraints, which may be ascribed to Cr substitution in spinel. Bulk rock harzburgites have δ56Fe ranging from −0.097‰ to 0.076‰, the average of which (0.002 ± 0.020‰) is similar to average δ56Fe determined for abyssal peridotites. Based on this inference, the lower δ56Fe of arc magmas than MORB should be due to smaller discrepancy in Fe3+/ΣFe between arc magmas and arc peridotites when compared with MORB and abyssal peridotites. Our modelling demonstrates that this can be achieved if arc magmas are produced by redox-buffered melting of silica-enriched mantle wedge while MORB are produced by redox-unbuffered melting of the depleted mantle.

Geological Characteristics, Metallogenic Age and Genesis of Niumaoquan East Gold Deposit in the Western North Qilian

Geological Characteristics, Metallogenic Age and Genesis of Niumaoquan East Gold Deposit in the Western North Qilian