A new ophiolitic mélange containing boninitic blocks in Alxa region:Implications for Permian subduction events in southern CAOB

The Alxa region, located in the southernmost part of Central Asian Orogenic Belt, is a key region for understanding the tectonic processes associated with the closure of the Paleo-Asian Ocean. Issues of late Paleozoic tectonic settings and tectonic unit divisions of the Alxa region still remain controversial. In this study, we report a new ophiolitic mélange named the Tepai ophiolitic mélange in the northern Alxa region, northwest of Alxa Youqi. The tectonic blocks in the Tepai ophiolitic mélange are mainly composed of serpentinized peridotites, serpentinites, mylonitized gabbros, gabbros, basalts, and quartzites, with a matrix comprising highly deformed clastic rocks. A gabbro exhibits a zircon LA-ICP-MS Ue Pb age of278.4 ± 3.3 Ma. Gabbros exhibit high Mg O and compatible element contents, but extremely low TiO_2,totally rare earth element and high field strength element contents. These rocks exhibit light rare earth element depleted patterns, and display enriched in large-ion lithophile elements and depleted in high field strength elements. Boninite-like geochemical data show that they were formed in a subductionrelated environment, and derived from an extremely depleted mantle source infiltrated by subduction-derived fluids and/or melts. The Tepai ophiolitic mélange exhibits similar zircon U-Pb-O isotopic compositions and whole-rock geochemical characteristics to those of the Quagan Qulu ophiolite.Therefore, we propose that the Tepai ophiolitic mélange may have been the western continuation of the Quagan Qulu ophiolite. Our new finding proves the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.

On Palaeozoic Tectonics in the Alxa Region, Inner Mongolia, China

Abstract: Two ophiolitic mélange belts in the Late Carboniferous formations have been discovered recently in the Alxa region. One is in the Engger Us fault and possesses properties of oceanic crust. The other is in the Badain Jaran fault and shows properties of a back-arc basin. These two faults, together with the Yagan fault, constitute the important boundaries of tectonic units in the Alax region. The four tectonic units delimited by these faults are different in rock assemblages, metamorphism and geochemistry. They reflect the nature of tectonic environments in which they are found. The tectonic units may be traced and correlated to the eastern and western neighbouring areas. The formation and evolution process of the units and their interaction in the Alxa region may be described in terms of the evolution of the Palaeo-Mongolian Ocean and its continental margins.

Diurnal characteristics of geoelectric fields and their changes associated with the Alxa Zuoqi M_S5.8 earthquake on 15 April 2015(Inner Mongolia)

In China, efforts are being made to monitor geoelectric fields through a large network of stations deployed and managed by the China Earthquake Administration. The diurnal variations in the geoelectric field waveforms were similar in the quiet magnetic periods when K〈5 （generally, K〈3 indicates a quiet time）. The arrival time points of the maxima in the geoelectric field waveforms exhibited differences in local time related to geographic longitude. The amplitude of diurnal variation was several to 16.6 mV/km and decreased with increasing latitude. Further, the amplitude of diurnal variation, which was related to seasonal changes, was larger in summer and autumn than in spring and winter. The periods of diurnal changes during quiet days were 24, 12, 8, 6, 4 hours and several minutes over large areas. Finally, the observed diurnal variations in geoelectric field prior to the Alxa Zuoqi Ms5.8 earthquake on 15 April 2015 were studied, and pronounced changes in the spectral values of the geoelectric fields were found to be associated with the Alxa Zuoqi earthquake in Inner Mongolia.

Tectonic Units and Their Fundamental Characteristics on the Northern Margin of the Alxa Block

The northern margin of the Alxa block is the junction of a tectonic units. Four first-order tectonic units are distinguished: 1. the Yagan structural zone characteristic of an immature island arc; 2. the Zhusileng-Hangwula structural zone, which was a passive continental margin in the Early Palaeozoic and was transformed into an active continental margin in the Late Palaeozoic;3. the Shalazha structural zone characteristic of a mature island arc; 4. the Nuru-Langshan structural zone, which was a Proterozoic orogenic belt and later evolved into an extensional transtional crust in the Palaeozoic. The above-mentioned tectonic units differ remarkably in sedimentary formations, magmatic rock associations, metamorphism and geochemistry and are bounded by faults between one another.

Late Carboniferous to early Permian sedimentaryetectonic evolution of the north of Alxa, Inner Mongolia, China： Evidence from the Amushan Formation

The late Paleozoic evolution of the Wulijishanhen（WSH）-Shangdan（SD） area near to the Chaganchulu Ophiolite belt is reinterpreted. Analysis of the upper Carboniferous to lower Permian sedimentary sequence, biological associations, detrital materials, sandstone geochemistry and volcanic rocks indicates that the SD area was an epicontinental sea and rift during the late Paleozoic rather than a large-scale ocean undergoing spreading and closure. This study reveals that the actual evolution of the study area is from the late Carboniferous to the early Permian. The fusulinids Triticites sp. and Pseudoschwagerina sp.in the limestones demonstrate that the Amushan Formation develops during the late Carboniferous to the early Permian. The limestones at the base of the SD section indicate that it is a stable carbonate platform environment, the volcanic rocks in the middle of the sequence support a rift tectonic background, and the overlying conglomerates and sandstones are characteristic of an epicontinental sea or marine molasse setting. The rift volcanism made the differences in the fossil content of the SD and WSH sections and led to two sections expose different levels within the Amushan Formation and different process of tectonic evolution. Moreover, the geochemical characteristics and detrital materials of the sandstones show that the provenance and formation of the sandstones were related to the setting of active continental margin. The quartz-feldspar-lithic fragments distribution diagram indicates that the material source for the sandstones was a recycled orogenic belt. Thus, the source area of the sandstones may have been an active continental margin before the late Carboniferouseearly Permian. The characteristics of the regional tectonic evolution of the area indicate that the region may form a small part of the Gobie Tianshan rift of southern Mongolia.

Driving Force and Ecosystem Service Values Estimation in the Extreme Arid Region from 1975 to 2015:A Case Study of Alxa League,China

The research on ecosystem service values(ESVs)estimation in arid region is weak.We took the Alxa League of China’s Inner Mongolia Autonomous Region,an extreme arid region,as an example and constructed an equivalent coefficient method to assess its ESVs from 1975 to 2015,by determining the standard unit of ESVs and the basic equivalent of the value of different ecosystem services per unit area based on the regional characteristics,literature research,expert knowledge and land use data.The results show that the ESVs first decreased from 83170.4 million yuan(RMB)in 1975 to 82337.8 million yuan(RMB)in 2000 and then increased to84033.6 million yuan(RMB)in 2015,and the ESV of sparse grassland and desert account for about 33%and 29%of the total ESVs,respectively.Among the four service types,the regulating services,support services,supply services and cultural services account for66.5%,22.8%,6.0%and 4.7%,respectively.The changes of ESVs in Alxa League are determined by the socio-economic development and ecological changes.This study provides a new method to estimate the ESVs in arid region by integrating existing methods and regional characteristics,such as the cost of water for arid ecosystems.

Precambrian Tectonic Affinity of the Southern Langshan Area, Northeastern Margin of the Alxa Block: Evidence from Zircon U-Pb Dating and Lu-Hf Isotopes

The Alxa Block is the westernmost part of the North China Craton(NCC), and is regarded as one of the basement components of the NCC. Its geological evolution is of great significance for the understanding of the NCC.However, the Precambrian basement of the Alxa Block is still poorly studied. In this study, we present new in situ LA-ICPMS zircon U-Pb and Lu-Hf isotope data from the Diebusige Metamorphic Complex(DMC) which located in the eastern Alxa Block. Field and petrological studies show that the DMC consists mainly of metamorphic supracrustal rocks and minor metamorphic plutonic rocks and has experienced amphibolite-granulite facies metamorphism. Zircon U-Pb dating results suggested that the amphibolite sample yields a crystallization age of 2636 ± 14 Ma and metamorphic ages of 2517–2454 Ma and 1988–1952 Ma, proving the existence of exposed Archean rocks in the Langshan area and indicating that late Neoarchean to Paleoproterozoic metamorphic events existed in the Alxa Block. Two paragneiss samples show that the magmatic detrital zircons from the DMC yield 207Pb/206Pb ages ranging from 2.48 Ga to 2.10 Ga with two youngest peaks at 2.13 Ga and 2.16 Ga, respectively, and they were also overprinted by metamorphic events at 1.97–1.90 Ga and 1.89–1.79Ga. Compilation of U-Pb ages of magmatic detrital and metamorphic zircons suggested that the main part of the DMC may have been formed at 2.1–2.0 Ga. Zircon Lu-Hf isotope data show that the source materials of the main part of the DMC were originated from the reworking of ancient Archean crust(3.45–2.78 Ga). The Hf isotope characteristics and the tectonothermal event records exhibit different evolution history with the Khondalite Belt and the Yinshan Block and the other basements of the Alxa Block, indicating that the Langshan was likely an independent terrain before the middle Paleoproterozoic and was subjected to the middle to late Paleoproterozoic tectonothermal events with the Khondalite Belt as a whole.

Geochemistry,geochronology and Hf isotope of granitoids in the northern Alxa region:Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial.The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa.In this paper,we present new zircon U-Pb ages,Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region.The zircon U-Pb data could be grouped as two phases:Late Devonian granite and diorite(ca.373-360 Ma),and Late Carboniferous granodiorite(ca.318 Ma).The Late Devonian granites and diorites are metaluminous to slightly peraluminous,with A/CNK and A/NK ratios of 0.90-1.11 and0.95-2.19,respectively.The Late Devonian diorites are characterized by high MgO,Cr and Ni contents and MgO#values,together with variableεHf(t)values from-1.0 to+1.3 and old TDM2 ages varied from 1283 Ma to 1426 Ma,indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust.Even though the Late Devonian granites yielded most positive and minor negative eHf(t)values between-1.1 to+5.7(three grains are negative)with two-stage model ages(TDM2)of 1003-1438 Ma,they display low MgO,Cr and Ni contents and MgO#values,suggesting that they were mainly derived from juvenile crustal materials,mixed with a small amount of ancient crust.The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series,with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and1.75 to 1.90,respectively.These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle,as evidenced by their highεHf(t)values(+11.6 to+14.1)and young TDM2 ages(427 Ma to 586 Ma),as well as high Mg#values,and MgO,Ni and Cr contents.Our data,along with available sedimentary evidence and previous researches,indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting.The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean,but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.

Provenance of the Permo-Carboniferous sediments in the northern Alxa and its tectonic implications for the southernmost Central Asian Orogenic Belt

As the southernmost segment of the Central Asian Orogenic Belt(CAOB),the northern Alxa orogenic belt(NAOB)connects the southeastern and southwestern segments of the CAOB.The NAOB amalgamated with the closure of the Paleo-Asian Ocean;however,the closure time of the Paleo-Asian Ocean is still on great debate.In this study,we reported new detrital zircon U-Pb geochronology and Hf-O isotopes for the Permo-Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications.The Permo-Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment.Based on the zircon U-Pb geochronology,the Amushan Formation was deposited in the late Carboniferous to early Permian,but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian.The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic,Mesoproterozoic(with a peak age at 1458 Ma),early Neoproterozoic(with peak ages of 988 Ma and 929 Ma),early Paleozoic(with a peak age at 447 Ma) and late Paleozoic(with a peak age at 294 Ma).Combined with the zircon Hf-O isotopes,the provenance was considered to be the Alxa Block,the Shalazhashan terrane and the Zhusileng-Hangwula block(and the southern Beishan orogenic belt).The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous.The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins.Combined with the intra-plate volcanism,the deposits were considered to be formed in extensional settings.Accordingly,after the closure of the Paleo-Asian Ocean,the NAOB stepped into an extensional stage.

Geochronology and Tectonic Implications of Diabase Intruded Into Xiangshan Group in the Southeastern Alxa Block, NW China

The southeastern Alxa Block(i.e.,the Hexi Corridor)is a tectonic junction between the North China Block(NCB)to the east,the Alxa Block to the north,and the western Qinling–North Qilian Orogenic Belt to the southwest(Fig.1).The southeastern Alxa Block had undergone

Seasonal dynamics of N:P ratio stoichiometry and Ca fraction for four dominant plants in the Alxa Desert

Desert plants take on unique physiologically adaptive mechanisms in response to an adverse environment. In this study, we determined the concentrations of leaf nitrogen(N), phosphorus(P), and calcium(Ca) fraction for dominant species of Artemisia ordosica, A. frigida, Calligonum mongolicum, and Oxytropis aciphylla in the Alxa Desert and discussed seasonal changes of their leaf N:P ratio and Ca fraction. The results showed that, from May to September, the N:P ratios of A. ordosica and C. mongolicum gradually and significantly increased, while those of A. frigida, and O. aciphylla had an increase trend that was not significant; the physiologically active Ca of A. ordosica and A. frigida increased significantly,while that of C. mongolicum and O. aciphylla decreased significantly. The physiologically inert calcium of C. mongolicum increased extremely significantly, while that of others was not significant. There was a significantly positive correlation between the N:P ratio and physiologically active Ca for A. ordosica, and the N:P ratio was significantly and negatively correlated with physiologically active Ca for O. aciphylla. These findings revealed that the physiological regulation mechanism was different for the plants either in earlier stage or later stage of plant-community succession.

Early Carboniferous High Ba-Sr Granitoid in Southern Langshan of Northeastern Alxa: Implications for Accretionary Tectonics along the Southern Central Asian Orogenic Belt

Voluminous granitoids are widely distributed in the Langshan region,northeast of the Alxa block,and record the evolutionary processes of the southern Central Asian Orogenic Belt.The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex.Early Carboniferous zircon LA-ICP MS U-Pb ages were from 327 Ma to 346 Ma.The Dabashan pluton can be classified as monzogranite and syenogranite,and exhibits high K2 O contents and K2 O/Na2 O ratios,which reveal a high-K calc-alkaline nature.The samples display strongly fractionated REE patterns,and are enriched in large ion lithophile elements(LILE)relative to high field strength elements(HFSE).The Dabashan plutons display unusually high Ba(823–2817 ppm)and Sr(166–520 ppm)contents and K/Rb ratios(315–627),but low Rb/Ba ratios(0.02–0.14),and exhibit fertile zircon Hf isotopic compositions[εHf(t)=-14 to-20],which are comparable to those of typical high Ba–Sr granitoids.Based on the geochemical compositions of the samples,we suggest that subducted sediments and ancient crustal materials both played important roles in their generation.Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment-related melts with residual garnet in the source,which caused partial melting of ancient lower crust.Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths.The resultant magmas experienced some degree of fractional crystallization during their ascent.Given these geochemical characteristics,together with regional tectonic,magmatic,and structure analysis data,an active continental margin environment is proposed for the generation of these rocks.

Multiple Deformation in the Northeastern Alxa Block: Implications for the Southern Central Asian Orogeny and Its Subsequent Intraplate Evolution

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic-early Mesozoic deformation events have been distinguished:(1) nearly north-south-striking ductile thrusting along the eastern Alxa Block in the Late Devonian;(2) nearly east-west-trending brittle top-to-the-south thrusting in the Mid-Late Permian;(3) nearly east-west-trending initially ductile then brittle dextral transtension with 30-40 km of displacement in the Late Permian;and(4) northeast-trending ductile sinistral shearing with 120-125 km of displacement in the Mid-Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri-eastern Gondwana block. Parallel east-west-trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the PaleoAsian Ocean in the Mid-Late Permian. The Late Permian eastwest-trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west;this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt(CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far-field tectonic effects related to plate margin tectonic activities(e.g., the closure of the Mongolian-Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India-Eurasia collision). The large-scale brittle and ductile strike-slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo-Asian Ocean.

Global Comparisons of Geological Heritages in Alxa and Establishment of Global Geopark

Alxa,located at the junction of the North China plate and the Tarim plate,is characterized by complicated geological conditions and obvious climate changes.Unique geological and geographic conditions formed a variety of landscapes in Alxa, including desert landscape,Gobi landscape,wind -erosion landform,canyon landform,etc.This paper compares and evaluates the scientific interests

The Construction of Natural Disaster Database of Alxa and Periodicity Analysis

A compilation of all meaningful historical data of natural-disasters taken place in Alxa of inner-Mongolia is used here for the construction of a 65 Ma high precision database.The data in the database are divided into subsets according to the types

Variation of gravity before the Alxa Zuoqi M5.8 earthquake

In this study, a classic survey adjustment computation method was used for data obtained in the Inner Mongolia and Ningxia gravimetric networks between September 2013 and April 2015 so as to investigate the variation of gravity before the Alxa Zuoqi M5.8 earthquake. The relationship between gravity variation and the Alxa Zuoqi M5.8 earthquake was analyzed. The results showed that： （1） the severe variation in gravity field at the test sites before the Alxa Zuoqi M5.8 earthquake, as well as the subsequent accelerated rising, might be an earthquake precursor; （2） the Alxa Zuoqi M5.8 earthquake occurred at the turning point where the high-gravity gradient zone changed from the NE direction to NW.

The First Discovery of the Late Cretaceous Protoceratopsid Fauna from Alxa, Inner Mongolia, China

Objective Alxa,located in western Inner Mongolia,is covered by desert in many places with few Cretaceous outcrops.Only some Early Cretaceous dinosaur fossils have been discovered from several areas in Alxa Zuoqi and Ejin Qi,but no Late Cretaceous dinosaur fauna has been formally reported in Alxa.Fortunately,

阿拉善地块晚古生代构造演化新模式:来自岩浆岩和沉积岩的记录

本文根据阿拉善地块岩浆岩和沉积岩的记录分析,结合造山带岩石圈演化,提出阿拉善地块随着古亚洲洋在晚志留世末闭合,发生弧-陆或陆-陆碰撞、以及与相邻地块的碰撞,使陆壳得到广泛加厚;晚古生代时期以伸展为特征,早石炭世造山带岩石圈初始垮塌,致使阿拉善地块北部珠斯楞—杭乌拉带由泥盆纪相对稳定的被动陆缘,转为活动陆缘,并形成一套前弧碎屑岩-火山岩沉积;晚石炭世乌力吉—宗乃山—沙拉扎山带岩石圈初始垮塌、诺尔公—红古玉林带造山带岩石圈初始垮塌,由此构成阿拉善地块岩石圈部分垮塌,使中南部广泛区域沉降,形成一套后弧环境的海相碳酸盐-碎屑岩沉积岩石组合,并伴随同时期的辉长岩等基性岩体侵入;二叠纪造山带岩石圈完全垮塌,大规模软流圈上涌物质和热,使俯冲的洋壳物质、加厚下地壳混合物质、交代岩石圈地幔衍生新生物质等被加热熔融,形成高(超)钾岩浆储库。早二叠世,因侧向强迫重力翻转或受塔里木早二叠世地幔柱活动的触发,导致高(超)钾质岩浆沿珠斯楞、乌力吉、诺尔公陆壳中的岩石圈不连续挤出,以气球膨胀式短时、快速侵位,形成以珠斯楞、乌力吉、诺尔公为代表的二叠纪巨量花岗岩带,该时期也应该是阿拉善地块最重要的大规模成矿时期。阿拉善地块是华北克拉通的组成部分,至今在其南部依然保留着大于200 km厚的岩石圈。阿拉善地块在早古生代时期汇聚,伴随古亚洲洋、祁连洋的形成、演化与消亡及与华北地块相互作用,使阿拉善地块被“破坏”;晚古生代时期伸展,伴随造山带岩石圈垮塌被“肢解”,总体上属于西部“华北克拉通破坏”范畴,显生宙以来的构造-岩浆事件,是西部“华北克拉通破坏”的响应。

阿拉善仿古地毯图案及文化因素分析

阿拉善仿古地毯作为一种传统手工艺品,具有丰富多样的图案和浓厚的文化内涵。图案是地毯艺术的核心,承载着丰富的象征意义,其中的文化因素更是值得探究,可以更好地帮助人们理解和传承这一传统艺术。

内蒙古阿拉善右旗拜兴图地区土壤地球化学异常特征及找矿预测

为优选阿拉善右旗拜兴图地区找矿预测靶区,在查明该区成矿地质条件的基础上开展1∶1万土壤地球化学测量。采用元素变异系数、ILR变换、ILR-PCA多元统计分析法对研究区Au、Ag、Co、Cu、Pb、Zn和As等7种元素进行分析,结果表明:(1)成矿元素主要赋存于断裂与矿化蚀变带交汇部位,Au、Ag呈现强分异富集特征,成矿潜力大;Pb、Zn、As、Co、Cu呈现低分异富集特征,成矿潜力较小。(2)ILR变换可消除原始数据的闭合效应,使元素呈现标准正态分布;ILR-PCA第一主成分能最大限度提取原始数据的综合信息,即PC1正端载荷(Au-Ag-As)为成矿指示元素,元素异常强度和形态受断裂与矿化蚀变带控制,负端载荷(Co-Cu-Pb-Zn)为多期次岩浆热液活动引起。(3)对Ⅰ号找矿靶区进行工程验证,查明靶区数条金(银)矿体,金、银含量均达到或超过最低工业品位,表明拜兴图地区具有较大的金、银找矿潜力。