North Qilian Mountains Research Articles

Effects of deep magmatic degassing and shallow seawater circulation on trace element and sulfur cycling in submarine hydrothermal systems: Insights from the Shijuligou analog, North China

Limited access to modern subseafloor sulfides hampers our understanding of the link between magmatic volatile influx and the cycling of trace elements and sulfur, as well as the effect of the subsequent shallow seawater circulation on these processes. Hence, we studied a well-preserved fossil analog of submarine hydrothermal systems – the Shijuligou volcanogenic massive sulfide (VMS) deposit from the North Qilian Mountains in North China to examine variations in elements and isotopes of subseafloor sulfides vertically.The vertical distribution of trace elements in subseafloor sulfides is strongly controlled by temperature gradients and redox states during the interaction between hot fluids and seawater beneath the paleo-seafloor. While the enrichment of elements like As, Sb, and Au (median: 1335, 43.7, and 0.30 ppm, respectively, n = 21) and negative δ34S values (mean: −3.07 ‰, n = 7) of euhedral pyrites in the jasper, along with the precipitation of high sulfidation minerals (e.g., enargite), suggest the input of magmatic volatiles into hydrothermal systems. During the shallow seawater-hydrothermal circulation, pyrites in the veined and stockwork zones exhibit distinctly elevated δ34S values (up to 15.74 ‰), accompanied by increased concentrations of wall-rock-derived elements (e.g., Cu, Ni, Si, and Ti) and low-temperature-responsive elements (e.g., Pb, Zn, and Cd). Sulfur isotopes of sulfides vary significantly from the surface to the deep ore zones, ranging from −3.36 to 19.84 ‰ (mean: 9.25 ‰, n = 37). The negative δ34S values of pyrites at the paleo-seafloor are due to the addition of H2S derived from the disproportionation of magmatic SO2. The increased δ34S values of stockwork and disseminated sulfides at depth are attributed to the progressive reduction of seawater sulfates by ferrous iron released from the alteration of fresh basalts. The trace elemental and isotopic characteristics of sulfides suggest the Shujuligou VMS deposit resembles the fossil analog of immature, subduction-related submarine hydrothermal systems.

Katian (Late Ordovician) trilobites of the North Qilian Mountains and their palaeogeographical implications for the Proto‐Tethys Archipelagic Ocean

AbstractTrilobites from the middle Koumenzi Formation (Katian, Upper Ordovician) of the North Qilian Mountains, Menyuan, northeastern Qinghai Province are systematically documented for the first time. The fauna consists of five families, seven genera and seven species, one of which is new (Remopleurides zhangi), showing a close relationship to those of the Kazakh terranes (such as the Chu‐Ili terrane, Chingiz‐Tarbagatai area and the Karatau‐Naryn and North‐Tien Shan microcontinents), and the North China and Laurentia palaeoplates during the Katian (Late Ordovician). The cluster and non‐metric multidimensional scaling analyses of the Middle–Late Ordovician (late Darriwilian–Katian) trilobite faunas with 299 genera or subgenera from 46 horizons of 37 areas, provide valuable information for the palaeogeographical reconstruction of the Proto‐Tethys Archipelagic Ocean (PTAO) of this interval. The Qilian terrane and adjacent areas are essential components of the PTAO, some of which include the Qilian terrane, the North Qilian Mountains area, the Altun faulted terrane, the Hexi Corridor area and the East Qinling terrane. Their relative positions in the PTAO are inferred by the palaeobiogeography of trilobite faunas. Furthermore, based on discussions of the spatiotemporal distribution of those faunas, the Pliomerina and/or Sinocybele Province of the Middle–Late Ordovician (late Darriwilian–Katian) age is defined as a trilobite faunal province of the PTAO. Moreover, a distinct faunal subprovince, essentially comprising the South China Palaeoplate and its neighbours (e.g. Tarim, Annamia, Sibuma, East Qinling, Turkestan‐Alai and probably Talesh), might be surrounded by the equatorial cold‐water tongue.

An early Paleozoic Subduction Channel in the North Qilian Mountains, Northwestern China: Structural, Petrological and Chronological Constraints

AbstractDetailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel.

Two contrasting accretion v. collision orogenies: insights from Early Paleozoic polyphase metamorphism in the Altun–Qilian–North Qaidam orogenic system, NW China

Abstract The Altun–Qilian–North Qaidam (AQQ) orogenic system in northern Tibet is considered to be the northernmost orogenic collage of the Prototethyan domain. It is regarded as resulting from collisions between various continental terranes derived from the northern margin of Gondwana, although the AQQ orogenic system also includes abundant ophiolites, arc magmatic rocks and subduction–accretion complexes. Some researchers regard the orogenic system of north Tibet as a typical accretionary orogen built by the development of an evolving arc–accretion complex growing southwards along the margin of the Tarim and North China cratons during the Paleozoic. We propose, based on both published data and our new data, that two distinct accretion and collision orogenies developed in the AQQ during the early Paleozoic. The diagnostic marks are HP–LT metamorphic rocks in the North Altun–North Qilian Mountains and UHP metamorphic rocks in the South Altun–North Qaidam Mountains. A review of metamorphic, geochronological, geochemical and structural data indicates that the North Altun–North Qilian HP–LT metamorphic belt is related to early Paleozoic subduction–accretion and, together with ophiolite mélanges and arc metamorphic–magmatic complexes, forms an early Paleozoic accretionary orogen. By contrast, the South Altun–North Qaidam UHP metamorphic belt is associated with continental subduction and collision, accompanied by Barrovian-type metamorphic overprinting and collision-related magmatism, reflecting an early Paleozoic collisional orogeny.

Origin of the Mesoproterozoic Jingtieshan bedded barite deposit, North Qilian Mountains, NW China: Geochemical and isotope (O, S, Sr) evidence

The bedded Jingtieshan barite deposit, which occurs in the western part of North Qilian, NW China, is intimately associated with banded iron formation (BIF) and is hosted in weakly metamorphosed Mesoproterozoic clastic–carbonate rocks. Although numerous studies have focused on the BIF, the origin of the barite mineralization is poorly understood. The barite orebody occurs as a lenticular/stratiform unit overlying and locally interlayered with the BIF. In this contribution, we present a comprehensive and integrated study of the geochemistry and S–O–Sr‐isotope characteristics of the Jingtieshan barite deposit, thereby constraining its origin and the depositional conditions in the Mesoproterozoic ocean. The ore is composed of barite, minor carbonate minerals, specularite, and other minerals and is characterized by low Sr concentrations (711–1469 ppm), total rare earth element concentrations (7–28 ppm), and La/Ce values, relative enrichment in light rare earth element, positive Gd anomalies, and elevated Lan/Cen, which indicate that mineralizing fluids were derived from seawater and submarine hydrothermal solutions. Barite separates yield δ34S = 19.5–29.1‰ and δ18O = 12.7–23.6‰, which are consistent with a coeval seawater origin for SO42− modified bacterial sulfate reduction. Moreover, the 87Sr/86Sr values suggest that Sr, as well as Ba, was derived from contemporaneous seawater and hydrothermal solutions, with additional radiogenic Sr derived from late‐stage hydrothermal alteration. The redox‐sensitive trace element ratios of V/(V + Ni), V/(V + Cr), and V/Cr, combined with negative Ce anomalies (0.11–0.82), indicate that the barite formed under oxic marine conditions in a redox stratified ocean. Geological, geochemical, and isotope data support a sedimentary–exhalative origin for the Jingtieshan barite deposit, where barite was deposited under oxic conditions in a shallow marine environment when ascending Ba‐bearing submarine hydrothermal solutions encountered sulfate‐rich seawater.

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.

Study of the minerogenetic mechanism and origin of Qinghai nephrite from Golmud, Qinghai, Northwest China

Electronic microprobe analysis showed that all QN samples are mainly composed of tremolite and minor accessory minerals, such as diopside, calcite, serpentinite, and magnetite. According to the cation coefficients, the crystallo-chemistrygenesis illustration demonstrates that all QN deposits are contact metasomatic. Depending on the mole percent of Fe2+(3+)/(Mg2++Fe2+(3+)) and the content of Cr, Co, and Ni in all QN samples measured by X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), green and azure-green QNs are characterized as serpentinite-related contact metasomatic deposit (S-type), whereas white, green-white, brown, blue-violet, yellow, and viridis QNs are dolomite- related contact metasomatic deposit (D-type). The assemblages and chemical composition of accessory minerals of the eight-color QN samples show evident characteristics, which reveal four possible ore-forming processes. We also measured trace and rare earth elements (REEs) in these samples through ICP-MS to deduce the origin of and the changes in metallogenic conditions. The chondrite-normalized REE patterns of D-type QN exhibit moderately negative Eu anomalies with moderate light REE enrichment, flat heavy REE (HREE), and low ƩREE concentrations, similar to dolomitic marble. Green QN samples of S-type show enrichment in HREE and moderately negative Eu anomalies, which is consistent with characteristics of dunite. Whereas azure-green QN samples of S-type exhibit a right-dipping V-type curve with severe depletion in Eu (δEu=0.36–0.47), in accordance with the characteristics of gabbro from Yushigou ophiolite in North Qilian mountains. Furthermore, this finding is consistent with the content of trace elements and the petrographic analysis results. On the basis of several significant differences in the characteristic elements, which may have been affected by the metallogenic environment, we inferred the differences in oxygen fugacity and basicity of mineralization environments in different-colored QNs.

Geochemistry and Re–Os geochronology of the organic-rich sedimentary rocks in the Jingtieshan Fe–Cu deposit, North Qilian Mountains, NW China

The Jingtieshan Group in the North Qilian Mountains, NW China, is dominantly composed of banded iron formations (BIFs), copper deposits and organic-rich sedimentary rocks (ORS, carbonaceous phyllite). X-ray diffraction analysis of the ORS shows the mineral assemblage to be quartz+clay minerals. The total organic carbon contents show a range of 0.44–1.72%. Here we present the results of the geochemistry and Re–Os geochronology of the ORS from the Jingtieshan Group. The high values of Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), and Th/U ratio, indicate intense weathering. The Al2O3/TiO2, Zr/Sc, Th/Sc, La/Th ratios, high rare earth elements abundances, light rare earth elements enrichment (normalized to chondrite), and distinctly negative Eu anomalies, suggest that the Jingtieshan Group ORS were derived mainly from felsic volcanic units. The new Re–Os isochron age of 1308±100Ma (2σ, n=6, MSWD=23) broadly overlap with the previous published ages determined using Sm–Nd and U–Pb isotope systems. The new age represents the depositional age of the Jingtieshan Group, as well that of BIF in the Jingtieshan area. Furthermore, the initial 187Os/188Os ratios (0.44±0.07) indicate that the Os in the seawater was dominantly derived from hydrothermal fluids (∼75%). The Ce anomaly (Ce/Ce∗=0.95–1.00) and V/(V+Ni) ratios (0.71–0.86), as well as the lack of enrichment in redox-sensitive trace elements such as U, V, Zn, Pb, Cu, Ni, Cr, Co and Mn, together with the presence of overlying BIF, suggest that the Jingtieshan area represents a ferruginous deep-water succession. This, and intense submarine hydrothermal activities contributed to the deposition of the Jingtieshan BIF.

Geochronology and Geochemistry of Andesites in Langlike Cooper Deposit, North Qilian Mountain, China

Geochronology and Geochemistry of Andesites in Langlike Cooper Deposit, North Qilian Mountain, China

Apatite fission track constraints on the pattern of faulting in the north Qilian Mountain

The Qilian (祁连) Mountain is an active orogenic belt located at the northeastern margin of the Tibetan Plateau. During the process of continuous convergence between Indian and Eurasian plates, the Qilian Mountain grow correspondingly by means of reaction of old faults and generation of new ones. Here we present apatite fission-track data along a river profile crossing three minor fault (the Minle (民乐)-Damaying (大马营) fault, the Huangcheng (皇城)-Taerzhuang (塔尔庄) fault and the Kangningqiao (康宁桥) fault) which compose the North Qilian fault (NQF) to test the timing and patterns of the fault activities. Apatite fission-track (AFT) results indicate that these minor faults experienced two active phases in the Cretaceous and the Oligocene-Miocene. Further research indicate that the initiation timing of faulting became younger northward in both active periods and the later phase probably more active than the former phase. These tectonic activities might be highly related to the docking of the Lhasa Block to the south in the Cretaceous and uplift and expansion of the Tibetan Plateau in the Cenozoic.

高寒退化草地狼毒与赖草种群空间格局及竞争关系

PDF HTML阅读 XML下载 导出引用 引用提醒 高寒退化草地狼毒与赖草种群空间格局及竞争关系 DOI: 10.5846/stxb201112031850 作者: 作者单位: 西北师范大学地理与环境科学学院,西北师范大学地理与环境科学学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金 (91125014,40971039);甘肃省生态经济重点学科资助 Spatial pattern and competition relationship of Stellera chamaejasme and Aneurolepidium dasystachys population in degraded alpine grassland Author: Affiliation: College of Geography and Environment Science,Northwest Normal University,College of Geography and Environment Science,Northwest Normal University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:种群空间格局的尺度转换效应反映了植物种内、种间竞争关系的变化过程。采用草地群落学调查与点格局分析方法,在祁连山北坡阿尔泰针茅草原,分析了干扰状态下狼毒(Stellera chamaejasme)和赖草(Aneurolepidium dasystachys)种群的空间格局及竞争关系。结果表明:随着草地退化过程加剧,狼毒种群空间格局由聚集分布转变为非聚集分布,反映了协作转向竞争的种内个体间关系;赖草种群以聚集格局为主,体现了种内相互协作的个体间关系;二者空间关联性存在正相关、不相关和负相关的转换过程,正相关尺度逐渐缩小,负相关仅在重度退化草地出现,种群间关系表现为由协作转向竞争且竞争强度逐渐增大。干扰状态下,种群空间格局的尺度转换效应影响植物种内、种间的竞争关系及强度,反映了退化草地物种应对干扰的更新途径。 Abstract:Human activities are affecting the habitat, which decrease the number of habitats and recede the quality. The biggest threat to species diversity worldwide is the loss of suitable habitat. Human activities affect species dynamics and further influence species diversity by changing habitat. Species evolvement at different types of habitat is very complex, so there is an urgency to understand species responses to human-caused habitat degradation. Spatial pattern and competition relationship are important in plant ecology as they affected the dynamics of plant populations, communities and ecosystem processes, which can reveal the dynamic and stability of population and community structure. Stellera chamaejasme and Aneurolepidium dasystachys are the main species in alpine degraded grassland, have a great significance for understanding reproductive strategies of grassland plant resources with studies on spatial pattern and competition relationship of S. chamaejasme and A. dasystachys. In this study for two main aims: First, to describe the scale transformation of population's spatial pattern and spatial association along with the different plot; Second, to analyze the competition relationship and competition intensity of S. chamaejasme and A. dasystachys. Study sites locates in the Ma Changtan grassland in SuNan country of the north Qilian Mountain (38°47'22.1″-38°52'11″ N, 99°45'11.7″-99°57'41.2″ E). In early August, 2011, four experimental plots were established by the community characteristics and S. chamaejasme population characteristics. At the same time, we recorded fine-scale (0-100 cm) spatial pattern and spatial association of S. chamaejasme and A. dasystachys population from 2 m×2 m grids (2 cm×2 cm quadrats) in different plot. The point pattern analysis and Monte Carlo stochastic simulation method were used to quantitatively analyze spatial pattern and competition relationship. The results showed that: (1) In the process of natural pasture degeneration, the spatial patterns of S. chamaejasme were aggregated at 53-58 cm and 63-100 cm scales under no degradation and light degradation conditions, and subsequently random or regular under moderate degradation and heavy degradation conditions, which meant that the intraspecific relationship converted mutualism into competition within different plot. (2) The spatial patterns of A. dasystachys were aggregated within four plots, which meant that intraspecific mutualism was the most important competition relationship of A. dasystachys in the process of natural pasture degeneration. (3) The spatial association of S. chamaejasme and A. dasystachys population changed on the different scale and degradation gradient. Under no degradation and light degradation conditions, the positive correlation was shown at 8-20 cm and 66-71 cm scales; under moderate degradation conditions, there was no significant correlation as a whole and the scale ranges was from 0 cm to 100 cm; under heavy degradation conditions, negative correlation was shown at 16-28 cm scales. It reflected that the interspecific relationship converted mutualism into competition. The scale transformations of population's spatial pattern, as well as the spatial association, are not only the antecedent conditions affecting changes of competition relationship, but also the critical factors of population's competition intensity. It may reflect the adaptive approach of regeneration and reproduction when S. chamaejasme and A. dasystachys population face the outside interference. 参考文献 相似文献 引证文献

Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM.On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.

Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain, NW China

The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain. It has been drawn much attention and extensively studied in recent years. In this study, ion microprobe (SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite. Eighteen analyses yield a relatively consistent apparent 206Pb/238U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma (MSWD=1.06), which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite. Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures. The petrological, geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician, while the ancient Qilian oceanic plate subducted northwards. The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era.

Occurrence of Alaskan‐type mafic‐ultramafic intrusions in the North Qilian Mountains, northwest China: Evidence of Cambrian arc magmatism on the Qilian Block

AbstractField relationships, mineralogy and petrology, whole‐rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan‐type. The mineral chemistry of the chromian spinels (Cr‐spinels) and clinopyroxenes, and the variation trend of the whole‐rock compositional plot in the (Na2O + K2O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass‐resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H2O‐rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform‐type layered sequence, followed by diapiric re‐emplacement. The occurrence of the Alaskan‐type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.

Tectonic evolution of early Paleozoic HP metamorphic rocks in the North Qilian Mountains, NW China: New perspectives

The North Qilian Mountain range is one of the three oldest orogenic belts preceded with a cold oceanic subduction-zone so far recognized on Earth with a subduction history from 490 Ma to 440 Ma in rock record. This orogenic belt has received much attention over the past decades. A significantly improved understanding of the tectonic evolution of the orogenic belt is now emerging thanks to detailed mineralogical, petrologic, geochemical and geochronological studies as well as field observations carried out over the past few years. In this paper, we offer new perspectives on the tectonic evolution of the North Qilian orogenesis in terms of subduction-zone metamorphism by integrating existing observations and our new data. We provide fairly comprehensive documentation of various observations to support our interpretations, which will also serve as a convenient guide for the field excursion of the 8th International eclogite conference. In the North Qilian subduction-zone complex (suture zone), two sub-belts of high-pressure (HP) metamorphism have been distinguished in terms of mineral assemblages, i.e., the low-grade blueschist belt and high-grade blueschist–eclogite belt. The low-grade blueschist belt consists of lawsonite–pumpellyite–glaucophane schist, lawsonite–glaucophanite and minor lawsonite-bearing felsic blueschist with a typical mineral assemblage of Lws + Pmp + Gln + Ab + Chl ± Arg. This assemblage constrains the metamorphic conditions of T = ∼250–350 °C and P = 0.6–1.1 GPa. Protoliths of the lawsonite-bearing mafic blueschist are similar to present-day N-type MORB. The high-grade blueschist belt occurs as three slices within the island-arc volcanic complex and consists of blueschist- to eclogite-facies metamorphosed greywacke, basite, serpentinite, meta-pelite, chert and marble. Lawsonite and Mg-carpholite occur in eclogites and meta-pelites, respectively. P− T estimation yields peak metamorphic conditions of P = 2.2–2.6 GPa and T = 460−540 °C for eclogite and carpholite meta-pelite. Geochemical data suggest that protoliths of mafic blueschist and eclogite in both belts have features of present-day MORB or OIB. These observations indicate that the North Qilian HP metamorphic rocks have experienced a history of seafloor subduction in a cold subduction-zone with a geotherm of ∼6–7 °C/km in the early Paleozoic before exhumed at the Late Silurian and Devonian in response to continental collision.

Cenozoic basin development and its indication of plateau growth in the Xunhua-Guide district

The Xunhua, Guide and Tongren Basins are linked with the Laji Mountain and the northern West Qinling thrust belts in the Xunhua-Guide district. Basin depositional stratigraphy consists of the Oligocene Xining Group, the uppermost Oligocene-Pliocene Guide Group and the Lower Pleistocene. They are divided into three basin phases by unconformities. Basin phase 1 is composed of the Xining Group, and Basin phase 2 of the Zharang, Xiadongshan, Herjia and Ganjia Conglomerate Formations in the Guide Group, and Basin phase 3 of the Gonghe Formation and the Lower Pleistocene. Three basin phases all develop lacustrine deposits at their lower parts, and alluvial-braided channel plain depositional systems at upper parts, which constitute a coarsening-upward and progradational sequence. Basin deposition, paleocurrent and provenance analyses represent that large lacustrine basin across the Laji Mountain was developed and sourced from the West Qinling thrust belt during the stage of the Xining Group (Basin phase 1), and point-dispersed alluvial fan-braided channel plain deposition systems were developed beside the thrust and uplifted Laji Mountain and sourced from it, as thrusting migrated northwards during the stage of the Guide Group (Basin phase 2). Evolution of basin-mountain system in the study area significantly indicates the growth process of the distal Tibetan Plateau. The result shows that the Tibetan Plateau expanded to the northern West-Qinling at Oligocene (29–21.4 Ma) by means of northward folded-and-thrust thickening and uplifting and frontal foreland basin filling, and across the study area to North Qilian and Liupan Mountain at the Miocene-Pliocene (20.8–2.6 Ma) by means of two-sided basement-involved-thrust thickening and uplifting and broken foreland basin filling, and the distant end of Tibetan Plateau behaved as regional erosion and intermontane basin aggradational filling during the Pliocene and early Pleistocene (2.6–1.7 Ma).

The Dongcaohe ophiolite from the North Qilian Mountains: A fossil oceanic crust of the Paleo-Qilian ocean

The Dongcaohe ophiolite, located at the south of the North Qilian subduction complexes, is a tectonic block with an exposed area of about 3 km×6 km. It consists of an intrusive section overlain by an extrusive section. The lower part of the intrusive section consists of cyclic layers of cumulate dunites, troctolites, anorthosites, anorthositic gabbros, and gabbros with small discordant dunite and troctolite bodies. This layered sequence grades upward to isotropic gabbros and gabbronorites, which are overlain by the extrusive sequence of diabasic sheeted dikes and basaltic lavas. The overall mineral crystallization sequence was olivine±Cr-spinel, plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. The Cr-spinel (Mg#: 42–66, Cr#: 41–57) in these layered cumulates and present-day abyssal peridotites have similar compositions. Also, the compositional variations of the plagioclase and clinopyroxene in the intrusive section reflect crystallization from melts compositionally similar to the present-day ocean basalts. Moreover, the rare earth element (REE) and multi-element distribution patterns of the intrusive and extrusive lithologies in the Dongcaohe ophiolite are consistent with crystallization of mid-ocean ridge basalts. The zircon grains separated from the gabbronorite have an SHRIMP average 206Pb/238U weighted age of 497 ± 7 Ma, which is considered as the tectonic emplacement age of the Dongcaohe ophiolite. The field occurrence, mineral and whole-rock compositions indicate that the Dongcaohe ophiolite represents a well-persevered oceanic crustal fragment composed of a complete oceanic crustal section of layered cumulates at bottom upgrading through isotropic cumulates to sheeted dikes and lava flows.

THE SILICATE MINERAL INCLUSIONS IN THE CHROMIAN SPINEL FROM THE DONGCAOHE OPHIOLITE, NORTH QILIAN MOUNTAINS, NORTHWESTERN CHINA: RECORD OF SYNTEXIS OF LOWER OCEANIC CRUST

Diopside, enstatite, pargasite, aspidolite, analcime, chlorite, serpentine, titanite, andradite, and grossular are found as mineral inclusions in grains of chromian spinel in the troctolites and dunites of the layered cumulate series and discordant layered bodies, Dongcaohe ophiolite, in the North Qilian Mountains, Qinghai Province, northwestern China. The mineral inclusions in the layered cumulate series are remarkably different in species and composition from those in the discordant layered bodies. Diopside, the liquidus phase, was enclosed by the growing crystals of chromian spinel. Enstatite, pargasite and aspidolite were products of the closed-system reaction between the trapped mineral phases and trapped residual melt. Analcime crystallized or precipitated directly from the evolved entrapped melt or hydrothermal solution in the inclusions. Chlorite, serpentine, titanite, andradite, and grossular are of secondary or metamorphic origin and were produced by alteration of olivine, pyroxene, or amphibole during postmagmatic hydrothermal alteration or subsequent seafloor metamorphism of the ophiolite. Syntexis of the lower oceanic crust was involved in the formation of a modified magma, which solidified as the discordant layered bodies scattered within the layered cumulate series of the Dongcaohe ophiolite.

Sedimentary geochemistry of chert from the Middle-Upper Ordovician in Shihuigou area, North Qilian orogenic belt and its tectonic implication

The North Qilian orogenic belt is an elongate tectonic unit that lies between the North China plate to the north and the Middle Qilian microplate to the south, and is formed by a collision of the two plates in the Caledonian. The Shihuigou Section from Yongdeng County, Gansu Province, is in the eastern sector of the North Qilian Mountains, spanning the Ordovician island-arc zones. The Zhongpu Group is distributed in the Shihuigou area and composed of medium-basic volcanic rocks and volcanic clastic rocks interspersed with cherts, limestones, slates, and metamorphic sandstones. The geochemistry of chert from the Zhongpu Group reveals that all cherts coexisting with island-arc volcanic rocks formed in a continental margin basin environment. Research results of the rare earth elements reveal that these cherts formed in a relatively deep-water basin with no significant terrestrial interference. Therefore, it is inferred that the North Qilian orogenic belt was previously an archipelagic ocean in the Ordovician.

A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: petrological and U‐Pb geochronological constraints

AbstractThe north Qilian high‐pressure (HP)/low‐temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite‐bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite‐phengite‐garnet assemblages were 2.1–2.4 GPa at temperatures of 420–510 °C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 °C km−1, consistent with metamorphism in a cold subduction zone. SHRIMP U‐Pb dating of zircon from two lawsonite‐bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph‐bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.