Qilian Orogenic Belt Research Articles

The origin and emplacement of the Lajishankou ophiolite complex in the South Qilian belt: evidences from geological mapping

AbstractMany ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.

The Constraints of the Neotectonic Movement of the Northweast Region of China on Earthquae Activity in Qinghai Province

Indian Plate on the south, Eurasian Plate on the north, and the Northwest region of China have been squeezed together since the Himalayan tectonic movement, while many faults have been formed. The research was based on the study of direction of active faults lied at the Qinghai-Tibet Plateau, which were formed in the Neotectonics since the Late Pleistocene epoch, the distribution characteristic of the stress release in the crustal deformation and epicenter of the earthquake on the both side of the Aerjin fault, Qilian Orogenic Belt and Eastern Kunlun. The intersection region of NNW-striking faults and the NWW-striking faults was under the maximum tectonic compression, it showed the most complicated geometric configuration and the stressed concentration site, so it can easily result in an earthquake, and precautionary measures must be made in this region.

The nature and history of the South Qilian orogenic belt: Constraints from compositions of rivers' sediments and their detrital zircon U–Pb geochronology, Lu‐Hf isotopic compositions

Detrital zircon U–Pb dating, Hf isotopic data, and Nd isotopic compositions of river sediments are used to constrain crustal evolution processes of their provenance in the South Qilian orogenic belt. The detrital zircons from the Yuka, Tataleng, and Bayinguole rivers range in age from Neoarchean (2,772 Ma) to Permian (261 Ma), with age peaks at ca. 2.4 Ga, ca. 1.95 Ga, and ca. 0.45 Ga. Hf isotopic compositions of Paleoproterozoic, Pan‐African, and Caledonian zircon age groups display a wide range ofεHf(t) values, suggesting diverse sources of host magma of the zircon grains. The basement formation of the South Qilian orogenic belt from depleted mantle occurred at two stages; ca. 3.1–2.5 Ga and ca. 2.0–1.6 Ga. Meanwhile, the Nd model ages of silt‐mud from the region are between 1.98 Ga and 2.27 Ga with an average value of 2.12 Ga. Our study indicates that the Qilian oceanic plate formed between the Qaidam and Alxa blocks from late Neoproterozoic to Silurian. The South Qilian orogenic belt is a product of the Qilian Ocean closure and subsequent collision between the Qaidam Block and central Qilian belt during Ordovician–Silurian times. Combining regional tectonothermal events with new results of river sediments suggests that rifting from a continent and opening of the Qilian Ocean occurred at ca. 780–520 Ma, subduction of the oceanic crust at ca. 520–440 Ma, ocean closure and continental subduction at ca. 440–420 Ma, exhumation of subducted crust at ca. 420–400 Ma, and orogenic collapse and extension at ca. 400–360 Ma.

Geochronology and geochemistry of early Palaeozoic intrusive rocks in the Lajishan area of the eastern south Qilian Belt, Tibetan Plateau: Implications for the tectonic evolution of South Qilian

The Lajishan area, located in the South Qilian orogenic Belt, north‐eastern Tibetan Plateau, contains a compositionally diverse range of Cambrian to early Ordovician felsic intrusions that reflect the early Palaeozoic tectonic evolution of the Central Qilian Belt. This paper presents new geochemical, Sr─Nd and Lu─Hf isotopic, and zircon U─Pb data for the Tadong, Huangcaoshan, and Yindonggou plutons in the eastern Lajishan area. LA‐ICPMS U─Pb ages of 517.4 ± 1 Ma for the Tadong pluton which is the first find Late Cambrian granitoids in this area, 467 ± 1 Ma for the Huangcaoshan pluton and 446 ± 1 Ma for the Yindonggou pluton. The new data indicate that the Tadong pluton and Huangcaoshan pluton belong to I‐type granite. Their A/CNK values range from 0.82 to 1.03, suggesting a close relationship with I‐type granitoids. Their trace‐element geochemical characteristics include enrichments in the LILEs, pronounced depletions in Nb and Ta, enrichment of the LREEs relative to the HREEs, and Eu anomalies. These features are similar to those of I‐type volcanic‐arc magmas. Combined with Hf and Sr─Nd isotopic data, The Tadong pluton generated by the mixing of melts was derived from the partial melting of the Precambrian basement and mantle‐derived magmas. The Huangcaoshan pluton was derived from juvenile crust with contamination. The Yindonggou pluton shows A‐type characteristics, their A/NCK is 0.88–1.07, and 10000 × Ga/Al is higher than 2.6. They were characteristic with high Sr, low Y, low Yb, and enriched LREE. Combined with Hf and Sr─Nd isotopic data, the Yindonggou pluton was generated by the partial melting of oceanic slab material and was crustally contaminated associated with slab breakoff. Combined previous research suggests that Tadong pluton and Huangcaoshan pluton formed from southern subduction of the South Qilian ocean into the Central Qilian Belt; the Yindonggou pluton formed in collisional setting.

Hydrothermal copper mineralization in the Mesoproterozoic Huashugou banded iron formation, Northwest China: Characteristics, timing of formation and genesis

Precambrian banded iron formation (BIF)-related copper deposits constitute a rare category in the world with their origins remaining equivocal. Here we investigate the Huashugou Fe-Cu deposit in the North Qilian Orogenic Belt (NQOB), Northwest China, containing approximately 428 Mt iron ore @ 40 wt% Fe and 0.2 Mt Cu (metal) @ 1.82 wt% Cu. The deposit consists of the upper BIF and the lower copper orebodies, which are hosted in the dominantly clastic-carbonate sedimentary Mesoproterozoic Jingtieshan Group. The distribution of the copper orebodies is controlled by NNW-SSE-trending faults. The BIF and phyllite hosted mineralization includes abundant Cu-sulfide minerals distributed along foliation planes, as transgressive veinlets, fissures, and irregular lenses and aggregates, with or without quartz and siderite. Sulfides (chalcopyrite and pyrite) from the copper ores show large variation in δ34S values (−2.7‰ to 32.2‰), which partly overlap with the δ34S values of pyrite and slightly lower than δ34S of barite from the Jingtieshan Group rocks. Most of the sulfides from copper ores have high 207Pb/204Pb and 208Pb/204Pb ratios relative to the country rocks although there are minor overlaps. The lead isotope data show linear correlation between 207Pb/204Pb and 206Pb/204Pb. The sulfur and lead isotope compositions suggest that the ore-forming materials were derived from the host strata. Fluid inclusion studies constrain the trapping temperature of copper mineralization as 180–200 °C, and are related to low salinity (<10.2 wt% NaCl equiv) fluids. The δ18O and δD values of quartz (13.3‰–17.3‰ and −82‰ to −63‰, respectively) and δ13C and δ18O of siderite (−6.7‰ to −4.3‰ and 15.4‰–18.7‰, respectively) intergrown with sulfides, along with the characteristics of the wall rock alteration in the Huashugou area, suggest that strata-sourced ore-forming fluids with a contribution of meteoric water remobilized sulfur and copper from the country rocks. Chalcopyrite from the ores yield Re–Os isotope age of 583.6 ± 7.9 Ma (2σ, MSWD = 1.6), providing tight constraints on the timing of the copper mineralization. We classify the late Neoproterozoic Cu deposit associated with Huashugou BIF as an epigenetic hydrothermal-type deposit.

Triassic tectonic interactions between the Alxa Massif and Ordos Basin: Evidence from integrated provenance analyses on sandstones, North China

The Helanshan Tectonic Belt is situated among the Qilian Orogenic Belt to the southwest, the Alxa Massif to the west and the Ordos Block to the east. The Helanshan Tectonic Belt has received increasing attention since it was argued as the plate boundary separating the Alxa Massif from the Ordos Block, and hence Alxa Massif was isolated from the North China Craton during Triassic. The Middle to Upper Triassic strata in the northwestern Ordos and the Helanshan Tectonic Belt witnessed the tectonic evolutionary processes between the Alxa Massif and the Ordos Basin, hence are pivotal to understand the relationship between the Alxa Massif and the North China Craton. In this work, integrated paleocurrent orientation, gravel component, sandstone modal composition and detrital zircon U–Pb isotopic analyses were applied to trace the source area and reconstruct the source-to-sink relations between the Alxa Massif and Ordos Basin. The southeast orientated paleocurrent directions suggest detritus in the northwestern Ordos were mostly sourced from the northwest. Clastic compositions reveal a mixed orogeny provenance, including magmatic arc and continental block sources. Detrital zircons from four sandstones in the Helanshan Tectonic Belt yields four major LA-ICPMS U-Pb age groups of 2600–2200 Ma, 2000–1700 Ma, 480–400 Ma and 350–200 Ma, suggesting multiple-sourced provenance. Comparing to the magmatic records in the Alxa Massif and the Qilian Orogenic Belt, the Triassic detritus in the Helanshan Tectonic Belt were mainly from the Alxa Massif with additional supplements from the Qilian Orogenic Belt, which finally debouched to the lacustrine in the interior Ordos Basin to the southeast. Triassic basin evolution of the northwestern Ordos was further associated with post-collision in the Alxa Massif after the closure of the Paleo-Asian Ocean.

Geochronology and petrogenesis of Neoproterozoic S‐type granites from the Qilian Block, north‐west China, and their geological implications

Early Neoproterozoic granites rarely outcrop in the Qilian Orogenic Belt (QOB) at the north‐eastern margin of the Qinghai–Tibet Plateau; however, they are vital to understanding the Precambrian evolution of the QOB. Thus, we present an integrated study, comprising petrology, geochronology, and whole‐rock geochemistry of two Early Neoproterozoic granitic plutons (Baihuagou and Yamacha plutons) in the eastern Qilian Block (QB). The results provide three key findings. First, their magma emplacement age is ca. 900 Ma (i.e., 918 Ma for Baihuagou and 910 Ma for Yamacha), and the two plutons have similar high‐K calc‐alkaline, strongly peraluminous characteristics, with slight enrichment in light rare earth elements; weak negative Eu anomalies; enrichment in large‐ion lithophile elements (e.g., K, Th, Ba, and Rb); strong depletion in Ti; and weak depletion in Nb, Ta, and, Sr. These characteristics indicate that the granites from these plutons are S‐type granites. Second, the two‐mica granite and gneissic granite samples may have been formed by partial melting of ancient crustal material (mainly psammitic rocks), with contributions from a meta‐igneous component, under high‐temperature and low‐pressure conditions. Third, new data from this study, combined with previously reported dates, indicate a continental margin arc setting for the QB during the Early Neoproterozoic (ca. 900 Ma).

Detrital zircon record of rivers' sediments in the North Qilian Orogenic Belt: Implications of the tectonic evolution of the northeastern Tibetan Plateau

The North Qilian Orogenic Belt (NQOB) is an important element of the northern edge of the Tibetan Plateau, and several rivers run along it. Zircon U–Pb dating, trace elements, and Hf isotopic data derived from the sediments carried by the rivers have been used to deduce the crustal evolution of the NQOB. U–Pb ages obtained from detrital zircons found in the sediments of the Zamu, Zhuanglan, and Beida rivers can be divided into five major age groups, that is, 2,600–1,500, 1,500–1,100, 1,100–650, 550–400, and 360–150 Ma, and the corresponding peaks occur at ~2,450, ~1,750, ~950, ~450, and ~250 Ma. Archean components, which may be derived from the ancient continental nucleus in the region, are rare in the river sediments. The zircon grains with ages of 360–150 Ma are most likely related to crustal thickening and decompression melting in a post‐collisional tectonic setting. The age distribution patterns indicate that the sediments carried by the Zamu, Zhuanglang, and Beida rivers are derived from the NQOB. Furthermore, the Hf isotopic compositions of Meso‐ and Neoarchean, Mesoproterozoic, Grenvillian, Pan‐African, Caledonian, and Hercynian zircon age groups exhibit a wide range of εHf (t) values, suggesting diverse sources. The existence of strongly negative εHf (t) values among the Grenvillian and Paleoproterozoic zircons indicates that the source magma included reworked Palaeoarchean crustal materials. New results indicate that continental rifting and the opening of the Qilian Ocean occurred at 775–520 Ma; the subduction and closure of the Qilian Ocean occurred at 520–440 Ma; arc–continent collision and continental subduction occurred at 440–420 Ma; and orogen collapse and extension occurred at 400–360 Ma. Our study indicates that the formation of the NQOB was mainly related to the evolutionary history of the Proto‐Tethys Ocean.

Early Silurian adakitic high‐Mg diorite from the Longshan area: Implication for melting of mantle lithosphere in the south‐eastern Qilian Orogenic Belt

The Longshan area is located in the junction zone between the Qinling and eastern Qilian orogenic belts. Early Palaeozoic igneous rocks in this area can provide important insights into the evolution of Shangdan Ocean. This paper reports new petrology, chronology, major and trace element geochemistry, Sr–Nd isotope, and zircon Lu–Hf isotopic compositions for the Early Silurian pyroxene diorite in the Longshan area. The pyroxene diorite has a zircon U–Pb age of 432 ± 1 Ma (MSWD = 0.25, n = 27). The pyroxene diorite intrusion displays low SiO2 (53.34 to 56.32 wt.%), high Na2O (3.11% to 3.79 wt.%) and Sr (1112 to 1234 ppm) contents, and moderate Mg# (52.70 to 55.00) values, high Sr/Y (67 to 84) ratios, depleted in Y and HREE; these features are identical with the low‐Si adakite or sanukitoids that derived from melting of metasomatized mantle wedge. The pyroxene diorite intrusion has evolved Sr–Nd isotopic compositions, (87Sr/86Sr) i = 0.705286 to 0.705310, εNd(t) = −3.4 to −3.9. Zircons from the pyroxene diorite display variable εHf(t) values of −3.6 to +0.1. These features suggest that the pyroxene diorite intrusion was derived from a hydrous melting metasomatized mantle wedge which has been modified by slab melts. The occurrence of pyroxene diorite indicates an Early Silurian active continental margin setting in the south‐eastern Qilian Orogenic Belt.

Geochemical composition and provenance of aeolian sands in the Ordos Deserts, northern China

Identifying the provenance of aeolian sediments in deserts is of great importance for understanding the Earth surface processes. In this context, we conducted detailed fieldwork in the Ordos Deserts (Maowusu and Kubuqi) in the middle portion of the desert belt in northern China, and measured the major, trace and rare earth elements (REE) of aeolian sands and their potential source rocks and sediments. Our results show that aeolian sands in the eastern (northeastern) and western (southwestern) Maowusu (Mu Us) Sandy Land exhibit different degrees of mineralogical maturity and Eu/Eu* values. Thus, we interpret that these aeolian sands have different provenances, though in the same sandy land. Our data suggest that the local lacustrine sediments and sandstones are the main sources of aeolian sands in the eastern province of the Maowusu Sandy Land, while aeolian sands in the western Maowusu Sandy Land and the Kubuqi Desert have the same external sources. The comparison of geochemical compositions of sediments in the Ordos Deserts with their potential sources in adjacent regions indicates that there is no genetic linkage between the Helan Mountains, the Yinshan Mountains and the Ordos Deserts although they are not far apart. The Qilian Orogenic Belt in the northeastern margin of the Tibetan Plateau is, however, the most likely original provenance for the western Maowusu (Mu Us) Sandy Land and the Kubuqi (Hobq) Desert, but with fluvial sediments in the Ningxia-Inner Mongolian section of the Yellow River as the immediate source. In one side, our results demonstrate that dune fields that are close to each other can have significantly different source sediments. On the other hand, our work suggests that some dune fields and landforms that are far apart from one another, e.g., the Badain Jaran Desert in western Inner Mongolia, the Ordos Deserts and the fluvial sediments in Ningxia-Inner Mongolian section of the Yellow River, can share the same ultimate sources.

Geochronology, petrogenesis and tectonic significance of Huashugou granitoids in North Qilian, NW China

North Qilian Orogenic Belt (NQOB) is a typical early Paleozoic orogenic belt in China. However, the tectonic setting of NQOB remains controversial, especially the post-subduction processes. In this study, we report whole-rock geochemistry, zircon U–Pb ages, and Hf–Nd isotopic compositions of Huashugou granitoids in the western segment of the NQOB, NW China. LA-ICP-MS U–Pb dating of zircon from Huashugou granitoids indicate magmatic crystallization ages of ca. 420 Ma, with late-Paleoproterozoic captured zircons of ca. 1700 Ma. The Huashugou granitoids show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Th and U), depletion of heavy rare earth elements and high-field-strength elements (e.g., Nb, Ta and Ti), negative Eu anomalies, and A/CNK < 1, showing I-type granite features. The zircon grains from these rocks display negative to positive εHf(t) values (−2.0 to 6.2) and slightly negative εNd(t) values (−5.2 to −1.7). The two-stage zircon Hf model ages range from 1010 to 1533 Ma, overlapping with the two-stage Nd model ages of 1302–1559 Ma. These features could reflect the source magma of the Huashugou granitoids was derived from partial melting of crustal components of the Paleoproterozoic–Mesoproterozoic basement with input of mantle-derived material. The data obtained in this study, in conjunction with those from previous studies, suggest the transition of the tectonic regime of NQOB from compression to extension probably occurred before 420 Ma.

Microsampling Lu–Hf geochronology on mm‐sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt

AbstractThis study presents Lu–Hf geochronology of zoned garnet in high‐P eclogites from the North Qilian orogenic belt. Selected samples have ~mm‐sized garnet grains that have been sampled with a micro‐drill and analysed for dating. The Lu–Hf dates of bulk garnet separates, micro‐drilled garnet cores and the remnant, rim‐enriched garnet were determined by two‐point isochrons, with cores being consistently older than the bulk‐ and rim‐enriched garnet. The bulk garnet separates of each sample define identical garnet–whole rock isochron date of c. 457 Ma. Consistent U–Pb zircon dates of 455 ± 8 Ma were obtained from the eclogite. The Lu–Hf dates of the drilled cores and rim‐rich separates suggest a minimum garnet growth interval of 468.9 ± 2.4 and 452.1 ± 1.6 Ma. Major and Lu element profiles in the majority of garnet grains show well‐preserved Rayleigh‐style fractionated bell‐shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P–T path from ~470–525°C and ~2.4–2.6 GPa. The exceptional high‐Mn garnet core in one sample indicates an early growth during epidote–blueschist facies metamorphism at &lt;460°C and &lt;0.8 GPa. Therefore, the Lu–Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu–Hf dates of rim‐rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet‐bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm‐sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469 Ma) and the ultimate closure (earlier than c. 452 Ma) of the fossil Qilian oceanic basin.

Petrogenesis and tectonic implications of the Early Paleozoic granites in the western segment of the North Qilian orogenic belt, China

Early Paleozoic granitic magmatism in the North Qilian orogenic belt records a complete Wilson cycle and provides critical geological clues for unraveling the regional tectonic history. In this study, we report the results of zircon U-Pb ages, Hf isotopic analysis and systematic whole-rock geochemical data for the Late Ordovician Hongliuhe granite and Early Silurian Qingshan monzogranite in the western segment of the North Qilian orogenic belt to constrain their emplacement ages, petrogenesis, and regional evolution history. U-Pb dating reveals that the Hongliuhe granite was emplaced around 453–452 Ma, and the Qingshan monzogranite was emplaced about 440–438 Ma. A geochemical study shows that the two granites belong to the calc-alkaline to high-K calc-alkaline series. The Hongliuhe granite shows adakitic and peraluminous features, while the Qingshan monzogranite belongs to metaluminous to weak peraluminous granites. Zircons in the Hongliuhe granite show εHf(t) values ranging from −15.1 to +11.7 with two-stage Hf model ages (tDM2) of 687–2398 Ma, whereas zircons in the Qingshan monzogranite show εHf(t) values ranging from +5.7 to +11.0 with two-stage Hf model ages from 814 to 1057 Ma. The geochemical characteristics indicate that the Hongliuhe granite was a transitional I/S-type granite and was generated from a thickened lower crust with the addition of minor Paleo- to Mesoproterozoic crustal materials, which left a rutile + garnet + pyroxene ± plagioclase residue. The Qingshan monzogranite formed from the partial melting of mafic crust with minor mantle-derived materials, and the fractionation of Ti-bearing phases, apatite and pyroxene occurred during the magma's evolution, which left an amphibole and plagioclase residue. We infer that the Hongliuhe granite formed during the northward subduction of the North Qilian Ocean, while the Qingshan monzogranite was generated during the post-collision stage between the Qilian and Alxa blocks. This observation indicates that syn-collision stage of the North Qilian orogenic belt began before 453 Ma, and that the extension regime began prior to 444 Ma.

Reply to “Enrichment of Bi-Be-Mo-Cd-Pb-Nb-Ga, REEs and Y in the Permian coals of the Huainan Coalfield, Anhui, China: Discussion”

Precambrian banded iron formation (BIF)-related copper deposits constitute a rare category in the world with their origins remaining equivocal. Here we investigate the Huashugou Fe-Cu deposit in the North Qilian Orogenic Belt (NQOB), Northwest China, containing approximately 428 Mt iron ore @ 40 wt% Fe and 0.2 Mt Cu (metal) @ 1.82 wt% Cu. The deposit consists of the upper BIF and the lower copper orebodies, which are hosted in the dominantly clastic-carbonate sedimentary Mesoproterozoic Jingtieshan Group. The distribution of the copper orebodies is controlled by NNW-SSE-trending faults. The BIF and phyllite hosted mineralization includes abundant Cu-sulfide minerals distributed along foliation planes, as transgressive veinlets, fissures, and irregular lenses and aggregates, with or without quartz and siderite. Sulfides (chalcopyrite and pyrite) from the copper ores show large variation in δ34S values (−2.7‰ to 32.2‰), which partly overlap with the δ34S values of pyrite and slightly lower than δ34S of barite from the Jingtieshan Group rocks. Most of the sulfides from copper ores have high 207Pb/204Pb and 208Pb/204Pb ratios relative to the country rocks although there are minor overlaps. The lead isotope data show linear correlation between 207Pb/204Pb and 206Pb/204Pb. The sulfur and lead isotope compositions suggest that the ore-forming materials were derived from the host strata. Fluid inclusion studies constrain the trapping temperature of copper mineralization as 180–200 °C, and are related to low salinity (<10.2 wt% NaCl equiv) fluids. The δ18O and δD values of quartz (13.3‰–17.3‰ and −82‰ to −63‰, respectively) and δ13C and δ18O of siderite (−6.7‰ to −4.3‰ and 15.4‰–18.7‰, respectively) intergrown with sulfides, along with the characteristics of the wall rock alteration in the Huashugou area, suggest that strata-sourced ore-forming fluids with a contribution of meteoric water remobilized sulfur and copper from the country rocks. Chalcopyrite from the ores yield Re–Os isotope age of 583.6 ± 7.9 Ma (2σ, MSWD = 1.6), providing tight constraints on the timing of the copper mineralization. We classify the late Neoproterozoic Cu deposit associated with Huashugou BIF as an epigenetic hydrothermal-type deposit.

Geochemistry of Sandstones from the Silurian Hanxia Formation, North Qilian Belt, China: Implication for Provenance, Weathering and Tectonic Setting

The North Qilian orogenic belt is the key to figure out the evolution and assembly of Asia. The Upper Silurian Hanxia Formation which is deposited in the north area of North Qilian Orogenic Belt is of utmost important to reveal the architecture and orogenic process of the North Qilian belt. So provenance analysis of the Hanxia Formation is of significance to reveal not only that the tectonic evolution of the central orogenic belt China, but also that Paleozoic Asia plate reconstruction. The ratios of elements and some discrimination diagrams based on geochemistry indicate that felsic rocks constitute their main source rocks. The chemical index of alteration is less than 80, indicating that the source rocks are relatively fresh and of low maturity. On the Th/Sc versus Zr/Sc scatter plot, samples from Hanxia Formation occur along the magmatic compositional variation trend of rocks, indicating that the rocks did not undergo obvious sedimentary sorting and recycling. The major and trace elements discrimination diagrams imply that Hanxia Formation rocks were derived from continental island arc. Recent studies show that the North China plate subducted southwards and produced subduction-related arc magmatism along the southern margin of the North Qilian Terrane during the Silurian. Therefore, we infer that in the late Silurian period the subduction-related arc became accreted to the Central Qilian terrane to the south, forming a composite continental arc, and the North Qilian belt accumulated in a fore-arc basin.

Geochemistry of Early Paleozoic boninites from the Central Qilian block, Northwest China: Constraints on petrogenesis and back-arc basin development

Early Paleozoic boninites occur in the Central Qilian orogenic belt, Northwest China. Their petrogenesis provides insights into lithosphere process and tectonic evolution of the Qilian block. In this paper, we carry out a study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic boninites in the Lajishan area of the Central Qilian block. The Lajishan boninites (∼483 Ma) have high Al2O3/TiO2 (36.7–64.7) and CaO/TiO2 (31.1–49.6) ratios, and high MgO (7.86–10.47 wt%), Cr (439–599 ppm) and Ni (104–130 ppm) contents, indicating that the boninites result from a refractory mantle source. They are depleted in high field-strength elements (HFSE) and enriched in large ion lithophile elements (LILE), coupled with slightly high initial 87Sr/86Sr values of 0.7059–0.7074 and low εNd(t) values of −1.05 to +2.66, indicating that the mantle source was metasomatized by subducted slab-derived components. We found that an assemblage of low-Ca group and high-Ca group boninites occurred in the Lajishan belt. The high-Ca group boninites were derived from relatively fertile mantle with slightly higher melting degree, whereas the low-Ca group boninites were generated by partial melting of more refractory mantle wedge peridotites with slightly lower melting degree. The assemblage of low-Ca group and high-Ca group boninites reveals that the low-Ca group boninites were generated by the further melting of the more refractory mantle source after the segregation of the high-Ca group boninitic magmas in response to the back-arc basin opening. In the light of reported boninites worldwide, a diagram of Zr/Y vs. CaO/Al2O3 is used to identify boninites in fore-arc and back-arc regions. We suggest that the Lajishan boninites represent the products of back-arc basin development in response to the northward subduction of the Qaidam-West Qinling ocean slab.

Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age (~430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation

The western Ordos Basin (WOB), situated in a tectonic transition zone in the North China Craton, acts as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia. In this study, U-Pb ages for 1203 detrital zircons of 14 sandstone samples collected from 11 sections are presented to unravel the sediment source locations and paleogeographic environments of the Early–Middle Jurassic coal-bearing Yan'an Formation in the WOB. Data show that there are five prominent age groups in the detrital zircons of the Yan'an Formation, peaking at ca. 282 Ma, 426 Ma, 924 Ma, 1847 Ma, and 2468 Ma. Samples from the northern, middle, and southern parts of the WOB contain these five age categories in various proportions. In the northern region, the Yan'an Formation exclusively contains Early Permian detrital zircons with a single age group peaking at 282 Ma, matching well with the crystallizing ages of the widespread Early Permian granites in the Yinshan Belt to the north and the Alxa Block to the northwest. While in the southern region, the Yan'an Formation mainly contains three groups of detrital zircons, with age peaks at 213 Ma, 426 Ma, and 924 Ma. These zircon ages resemble those of the igneous rocks in the Qilian-Qinling Orogenic Belt to the south-southwest. Samples in the middle region, characterized by a mixture age spectrum with peaks at 282 Ma, 426 Ma, 924 Ma, 1847 Ma and 2468 Ma, are previously thought to have mixed derivations from surrounding ranges. However, by referring to the detrital-zircon age compositions of the pre-Jurassic sedimentary successions and combining with paleontological and petrographic analysis, we firstly propose that the sediments of the Yan'an Formation in the middle region were partly recycled from the Triassic and Paleozoic sedimentary strata in the WOB. The occurrence of recycled sedimentation suggests that the Late Triassic–Early Jurassic intraplate compressional deformation was very intense in the WOB, especially for regions in front of the Qilian Orogenic Belt.

Petrology, Geochemistry and Zircon U‐Pb Geochronology of the Xiangshan Group in the Eastern Hexi Corridor Belt: Implications for Provenance and Tectonic Evolution

AbstractThe eastern Hexi Corridor Belt (HCB) is located in the transitional belt among the Alxa Block, the Qilian Orogenic Belt and the North China Block. Because of its unique tectonic location, the tectonic setting, provenance, and even the age of the sedimentary strata in the eastern HCB during the Early Paleozoic remain controversial. This study analyzes the provenance of the poorly studied Xiangshan Group, discusses its age of development and tectonic setting in the eastern HCB using a combination of petrological, geochemical and LA‐ICP‐MS U‐Pb zircon dating methods. Based on the youngest age peaks and the fossil evidence, we suggest that the Xiangshan Group is Middle Cambrian to Late Ordovician in age. The complexity of the geochemical characteristics and associated diagrams suggests that the early stage of the Xiangshan Group developed in a passive continental margin environment, late in the back‐arc basin of the eastern HCB. Based on the sandstone detrital composition, whole‐rock geochemistry and detrital zircon ages, we conclude that the Xiangshan Group had an early provenance that was mainly from the Qilian Block and a late provenance from the Qilian Block and the western Alxa Block. The eastern HCB and its northern and southern blocks have similar palaeontology, lithology and basement age characteristics to the South China Block. This indicates that the eastern HCB might not have formed in the intra‐continental aulacogen of the North China Block during the Early Paleozoic but has a close affinity to eastern Gondwana.

Crustal S-velocity structure and radial anisotropy beneath the southern part of central and western North China Craton and the adjacent Qilian Orogenic Belt from ambient noise tomography

The crustal S-velocity structure and radial anisotropy along a dense linear portable seismic array with 64 broadband seismic stations were investigated from ambient noise tomography with about one-year-long ambient noise recordings. The array transverses the southern part of the central North China Craton (CNCC) and western NCC (WNCC) from east to west and reaches the adjacent Qilian Orogenic Belt (QOB). The phase velocity structures of Rayleigh waves at 5–35 s and Love waves at 5–30 s were measured. The crustal S-velocity structures ( V sv and V sh) were constructed from the dispersion data (Rayleigh and Love waves, respectively) from point-wise linear inversion with prior information of the Moho depth and average crustal V p/ V s ratio. The radial anisotropy along the profile was calculated based on the discrepancies between V sv and V sh as 2×( V sh− V sv)/( V sh+ V sv). The results show distinct structural variations in the three major tectonic units. The crustal architecture in the southern CNCC is complicated and featured with wide-distributed low-velocity zones (LVZs), which may be a reflection of crustal modification resulting from Mesozoic-Cenozoic tectonics and magmatic activities. The pronounced positive radial anisotropy in the lower-lowermost crust beneath the Shanxi-Shaanxi Rift and the neighboring areas could be attributed to the underplating of mantle mafic-ultramafic materials during the Mesozoic-Cenozoic tectonic activation. In southern Ordos, the overall weak lateral velocity variations, relative high velocity and large-scale positive radial anisotropy in mid-lower crust probably suggest that the current crustal structure has preserved its Precambrian tectonic characteristics. The low-velocity westward-dipping sedimentary strata in the Ordos Block could be attributed to the Phanerozoic whole-basin tilting and the uneven erosion since late Cretaceous. Integrated with previous studies, the systematic comparison of crustal architecture was made between the southern and northern part of CNCC-WNCC. The similarities and differences may have a relation with the tectonic events and deformation histories experienced before and after the Paleoproterozoic amalgamation of the NCC. The nearly flat mid-crustal LVZ beneath the southern QOB weakens gradually as it extends to the east, which is a feature probably associated with crustal vertical superpositionand ductile shear deformation under the intensive compressional regime due to the northeastward growth and expansion of the Tibetan Plateau.