Discovery of~2.5 Ga zircon U-Pb age from the diabase,Ar Horqin Banner Area,Inner Mongolia:Implications for Neoarchean crystalline basement of Bainaimiao Arc Belt

1.Objective The Bainaimiao Arc Belt(BAB),one of the most important units of the southeastern Central Asian Orogenic Belt(CAOB),locates along the northern margin of the North China Craton(NCC).There is a lack of understanding on the existence,scale,and affinity of the crystalline basement of the BAB.Based on the recent geochronologic studies.

Basement Characteristics and Crustal Evolution of the Copper-Gold Metallogenic Belt in the Middle and Lower Reaches of the Yangtze River:Some Isotope Constraints

Studies of the Pb, Sr and Nd isotopic composition of Mesozoic intrusive rocks indicate that the basement of the copper-gold metallogenic belt of the middle and lower reaches of the Yangtze River has “two-layer structure” and partly has “multi-layered structure”, and is inhomogeneous and shows the distinct feature of E-W provincialism. The calculated model lead ages (t1) are mostly greater than 2600 Ma, and the model neodymium ages (TDM) vary from 953 to 2276 Ma and concentrate in two time intervals: 1800–2000 Ma and 1200–1600 Ma. It is concluded that the basement of the MBYR is composed of the Late Archaeozoic to Middle Proterozoic metamorphic series and that the crust was initiated in the Archaean and continued to grow in the Early and Middle Proterozoic, and the proportion of new crust formed by mantle differentiation during the Late Proterozoic is low.

Forming Proterozoic basement within eastern Central Asian Orogenic Belt:Evidence from zircon U-Pb-Hf-O isotopes

As part of the mosaic of micro-continents within the Central Asian Orogenic Belt(CAOB), the Xing’anAirgin Sum Block(XAB) features increasingly-recognized Meso-Neoproterozoic geological records. However, the origin, temporal-spatial distribution of ancient materials, and their roles in crust evolution remain to debate. This paper presents an integrated study of zircon U-Pb ages and Hf-O isotopes for Mesoproterozoic and Paleozoic granites from the Erenhot region of central Inner Mongolia, along eastern CAOB. The intrusion of 1450 Ma syenogranite denotes that the Precambrian basement of XAB extends from Sonid Zuoqi westward to Erenhot. The 384 and 281 Ma monzogranites containing Mesoproterozoic xenocrystic zircons possess Proterozoic-dominant two-stage Hf model ages, further suggesting the wide existence of Proterozoic crust beneath western XAB. Cyclic Proterozoic crustal growth and reworking seem to show close linkages with the orogenesis during relevant supercontinent cycles. 1450-1360 Ma juvenile crustal growth at Erenhot and synchronous ancient crust reworking at Sonid Zuoqi and Abagaqi were likely resulted from retreating subduction involved in Columbia breakup, while 1.2-1.0 Ga reworking and 0.9-0.7 Ga growth events within the Erenhot basement might respond to assembly and breakup of Rodinia, respectively. Besides, our work confirms that reworking of Neoproterozoic crust played important roles during Paleozoic multi-stage accretion of CAOB.

SEGMENTATION OF FORELAND BELTS AND RELATIONSHIP WITH BASEMENT DEFORMATION IN THE SOUTHWEST TARIM BASIN

In the foreland belts of Pamir\|West Kunlun (PWK) and Southern Tienshan (ST), the thin\|skinned thrusting systems (tsts) from mountains thrusting to the Southwest Tarim Basin (STB) and the back\|thrusting systems (bts) from basin thrusting to the adjacent mountains were developed. The different active structural systems can be segmented: Kazike\|Arte deformed bts and tsts, Yigeziya tsts, Qimugen arcurate bts, Qipan thrusting and faulted\|folded system, Kekeya tsts, Keliyang—Yuliqun bts, Sangzhu\|Duwa tsts and Cele arcurate thr usting system along the PWK foreland belts, and Kalajun—Kalpin tsts, Atushi\|Bapanshuimo bts along the foreland belts of ST.Through the study of aerial magnetic anomalies and many seismic profiles, we found that all bts and their triangle zones were formed over the NE—SW basement depressions of STB (thickness of sedimentary covers >10km). We also found that all tsts were formed over the basement uplift belts of STB (thickness of sedimentary covers <10km, 6～10km in general).In the basement depression of the basin, the bts and their triangle zones could be transformed to tsts due to continuous compression. The largest displacement of the tsts or bts is located in the convex part but the shortest displacement is located in the conjunction area of two segments. The arcurate tsts and bts display different displacements along foreland belts.

塔里木盆地东北缘吐格尔明背斜和阳北断裂带构造分析

吐格尔明背斜和阳北断裂位于塔里木盆地北缘,南天山山前库车褶皱冲断带的东段,两者均为基底卷入型构造。阳北断裂是一个反转构造,其变形历史可以追溯到侏罗纪—白垩纪的正断层;新生代构造反转,发生了多期冲断变形加速期,分别发生于白垩纪末—古近纪初、古近纪末—新近纪初、中新世早期、上新世和第四纪。吐格尔明背斜构造带是阳北断裂中新世早期及以后的冲断作用派生出来的一个次级基底卷入型构造变形带。它由吐格尔明背斜及其南、北两条呈背冲关系的逆冲断层组成。背斜核部元古宇变质岩出露地表;中、新生界直接不整合于变质岩之上,缺失全部古生界,说明研究区可能属于一个长期存在的古生代古隆起。

塔里木盆地西北缘乌什西次凹的地层系统和构造特征

乌什西次凹位于塔里木盆地西北缘,隶属于库车坳陷的乌什凹陷。它位于南天山主山脉(哈尔克山)和塔里木盆地柯坪—温宿凸起之间。区域构造格局上,这里是塔里木克拉通向西北自然延伸的部分。次凹北缘的阿合奇断裂是南天山造山带和塔里木克拉通的分界。乌什西次凹内发育与柯坪—温宿凸起相似的晚前寒武纪—古生代地层系统,包括寒武系、奥陶系和石炭系的烃源岩。新近系—第四系碎屑岩建造直接不整合于变形的古生代被动大陆边缘碎屑岩—碳酸盐岩沉积建造之上;剖面上,向南天山方向加厚,向塔里木克拉通方向减薄,呈现典型的前陆盆地剖面结构特征。这是一个晚新生代陆内前陆盆地,叠加在晚海西期—燕山期古隆起之上。这里的构造变形主要有3期,分别是中海西期、晚海西期—印支期和晚喜山期。变形以厚皮冲断构造及其相关褶皱为主,薄皮构造不发育。平面上,主构造线走向NE-SW。剖面上,以南天山向塔里木冲断为主。

塔里木盆地库车褶皱冲断带东段依奇克里克背斜的构造特征和形成演化

依奇克里克背斜位于塔里木盆地北缘,库车褶皱冲断带北部构造带东段。它是一个基底卷入型开阔直立背斜,是在晚海西期古隆起和印支期库车周缘前陆盆地前渊带背景下形成的一个含油气构造。背斜核部出露的最老地层是白垩系下统,向两翼方向依次出露古近系、新近系和第四系。钻井资料揭示,之下还有三叠系和侏罗系含煤碎屑岩建造,不整合于石炭系灰岩之上。中新统吉迪克组至更新统西域组是背斜的生长地层,古近系及以下地层是前生长地层。背斜位于库车褶皱冲断带根带,发育厚皮构造;向前锋方向,逐渐演变为薄皮构造。褶皱冲断带的两个主滑脱断层分别发育于侏罗系煤层和吉迪克组膏盐层。作为印度—亚洲碰撞的远程效应,背斜的变形起始于古近纪末—新近纪初,经多个变形加速期,脉式冲断,直至第四纪晚期才最终定型。

鄂东南铁铜金钨钼多金属矿集区基底属性:来自继承锆石年龄和Hf同位素的约束

长江中下游成矿带鄂东南矿集区的基底存在统一的川中式基底和北部川中式基底、南部江南式基底的争议。笔者对灵乡岩体西段闪长玢岩开展锆石U-Pb定年和Hf同位素分析,结果显示闪长玢岩的锆石具有复杂的组成与来源:最年轻的4颗锆石加权平均年龄为(141±4)Ma,结合前人测年结果推测其可作为闪长玢岩的成岩年龄;其余16颗锆石具有较为宽泛的年龄(217~2550 Ma)和Hf同位素组成(εHf(t)=-11.2~11.7)。进一步结合前人获得的鄂东南矿集区岩浆岩继承锆石数据,分别对比川中式基底崆岭杂岩、江南式基底梵净山群及下江群的锆石数据,发现其与崆岭杂岩具有明显差异,但和梵净山群及下江群具有高度相似性。并且鄂东南矿集区岩浆岩继承锆石和梵净山群、下江群均记录了~1.5 Ga地壳增生事件。结合区域地球物理特征,笔者认为鄂东南矿集区南部为江南式基底,北部为川中式基底,两者的分界线大致为灵乡-大冶-网湖一线。

竖向后浇带预制加强型钢板式模板免拆施工技术研究

文中以某住宅项目地下室为例,探讨了一种新型的竖向后浇带防护技术。该技术采用竖向后浇带预制加强型钢板式模板免拆施工技术,通过焊接连接钢板和方通等材料形成主体框架,并以钢板作为防护板,起到有效防护作用。针对传统方法存在的施工周期长、成本高、质量不稳定等问题,本研究提出的技术具有施工周期短、成本低、施工质量稳定等优势。实践证明,该技术在地下室竖向后浇带防护方面具有良好的应用前景和推广价值。

抗浮作用下大底盘底板后浇带施工影响研究

为研究抗浮水位改变对沉降后浇带施工质量的影响,依托实际工程,利用Midas GTS建立结构施工阶段抗浮水位计算模型,研究不同水位高度对结构的影响情况,提高了地下水的降水限值。后浇带封闭时,利用传感器得到后浇带浇筑后28 d内温度、应变的变化数据,结合理论分析,得到以下结论:提高降水安全高度后,根据规范要求,待沉降后浇带两侧结构沉降稳定后浇筑混凝土,后浇带的浇筑质量可以满足设计要求;此外在施工时应采取加强防水措施,避免钢筋锈蚀。

Tectonic evolution of the West Kunlun Orogenic Belt along the northern margin of the Tibetan Plateau:Implications for the assembly of the Tarim terrane to Gondwana

The West Kunlun orogenic belt(WKOB) along the northern margin of the Tibetan Plateau is important for understanding the evolution of the Proto-and Paleo-Tethys oceans. Previous investigations have focused on the igneous rocks and ophiolites distributed mostly along the Xinjiang-Tibet road and the China-Pakistan road, and have constructed a preliminary tectonic model for this orogenic belt. However, few studies have focused on the so-called Precambrian basement in this area. As a result, the tectonic affinity of the individual terranes of the WKOB and their detailed evolution process are uncertain. Here we report new field observations, zircon and monazite U-Pb ages of the "Precambrian basement" of the South Kunlun terrane(SKT) and the Tianshuihai terrane(TSHT), two major terranes in the WKOB. Based on new zircon U-Pb age data, the amphibolite-facies metamorphosed volcanosedimentary sequence within SKT was deposited during the late Neoproterozoic to Cambrian(600-500 Ma), and the flysch-affinity Tianshuihai Group, as the basement of the TSHT, was deposited during the late Neoproterozoic rather than Mesoproterozoic. The rock association of the volcano-sedimentary sequence within SKT suggests a large early Paleozoic accretionary wedge formed by the long-term lowangle southward subduction of the Proto-Tethys Ocean between Tarim and TSHT. The amphibolitefacies metamorphism in SKT occurred at ca. 440 Ma. This ca. 440 Ma metamorphism is genetically related to the closure of the Proto-Tethys Ocean between Tarim and the Tianshuihai terrane, which led to the assembly of Tarim to Eastern Gondwana and the final formation of the Gondwana. Since the late Paleozoic to early Mesozoic, the northward subduction of the Paleo-Tethys Ocean along the HongshihuQiaoertianshan belt produced the voluminous early Mesozoic arc-signature granites along the southern part of NKT-TSHT. The Paleo-Tethys ocean between TSHT and Karakorum closed at ca. 200 Ma, as demonstrated by the monazite age of the paragneiss in the Kangxiwa Group. Our study does not favor the existence of a Precambrian basement in SKT.

Timing of Metamorphism and Provenance of the Metamorphic Basement of the Xiangshan Uranium Orefield, Jiangxi Province, China

The origin, age and evolution of the Precambrian metamorphic basement of southern China provide useful insights into early crustal development. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry （LA-ICP-MS） U-Pb age data for detrital zircons from five samples of the Precambrian metamorphic basement of the Xiangshan uranium orefield. Two of these samples, from the northern Xiangshan volcanic basin, yielded a total of 140 U-Pb ages that cluster within the Neoproterozoic （773-963 Ma; 79.3% of data points）, with the rest being scattered through the Paleoproterozoic and Mesoproterozoic, along with a single Archean age. These ages indicate that this basement material is associated with the Cathaysia Block. In comparison, the 172 concordant ages from the other three samples from the southern part of the Xiangshan volcanic basin cluster within the Neoproterozoic （767-944 Ma; 59.8%） as well as the Proterozoic （37.8%） and the Archean （2502-2712 Ma; 14.5%）. These samples are also free of zircons with Grenvillian ages, indicating that these units are associated with the southeastern Yangtze Block. Combining these data with the geochemistry of these units, which suggests that the metamorphosed sedimentary rocks within the northern and southern parts of the Xiangshan basin have a common component from a magmatic island arc that formed during the early Neoproterozoic, we infer that the basin was located along the boundary between the Cathaysian and Yangtze blocks. In addition, the zircons within the samples from the southern and northern parts of the Xiangshan basin show different pre-Neoproterozoic （963 Ma） age populations but similar post- Neoproterozoic zircon populations, indicating that the amalgamation of the Cathaysian and Yangtze blocks occurred after the Neoproterozoic （960 Ma）, with magmatism peaking at 830 Ma and rifting starting at -770 Ma, leading to the subsequent deposition （from bottom to top） of the Shenshan, Kuli, and Shangshi formations.

Reinterpretation of the northern South China Sea pre-Cenozoic basement and geodynamic implications of the South China continent: constraints from combined geological and geophysical records

The pre-Cenozoic northern South China Sea(SCS)Basin basement was supposed to exist as a complex of heterogeneous segments,divided by dozens of N-S faulting.Unfortunately,only the Hainan Island and the northeastern SCS region were modestly dated while the extensive basement remains roughly postulated by limited geophysical data.This study presents a systematic analysis including U-Pb geochronology,elemental geochemistry and petrographic identification on granite and meta-clastic borehole samples from several key areas.Constrained from gravity-magnetic joint inversion,this interpretation will be of great significance revealing the tectono-magmatic evolution along the southeastern margin of the Eurasian Plate.Beneath the thick Cenozoic sediments,the northern SCS is composed of a uniform Mesozoic basement while the Precambrian rocks are only constricted along the Red River Fault Zone.Further eastern part of the northern SCS below the Cenozoic succession was widely intruded by granites with Jurassic-to-early Cretaceous ages.Further western part,on the other hand,is represented by meta-sedimentary rocks with relatively sporadic granite complexes.To be noted,the western areas derived higher-degree and wider metamorphic zones,which is in contrast with the lowerdegree and narrower metamorphic belt developed in the eastern region.Drastic collisions between the Indochina Block and South China continent took place since at least late Triassic,resulting in large-scale suturing and deformation zones.At the westernmost part of the northern SCS,the intracontinental amalgamation with closure of the Meso-Tethys has caused fairly stronger and broader metamorphism.One metamorphic biotite granite is located on the suturing belt and yields a Precambrian U-Pb age.It likely represents the relict from the ancient Gondwana supercontinent or its fringes.Arc-continental collision between the Paleo-Pacific and the southeast China Block,on the other hand,results in a relatively narrow NE–SW trending metamorphic belt during the late Mesozoic.Within the overall geological setting,the Cenozoic SCS oceanic basin was subsequently generated from a series of rifting and faulting processes along the collisional-accretionary continental margin.

Structural Control of Ore Mineralization in the Southeastern Margin of Western Nigeria Basement

Structural analyses are often ignored in mineral prospecting endeavor of any geological terrain despite the importance of geological structures in the formation of ore deposits. This paper correlates the results of mineral prospecting campaign to those of the structural analysis in the southeastern margin of the western Nigeria basement. The mineral prospecting was executed by essentially geochemical-exploration methods, while the structural analysis was achieved by lineament analysis using Landsat-8 imagery. The mineral prospecting campaign eventually led to the discovery of gold bearing marble deposit in the Dagbala area;silver and copper bearing silicified sheared rock in the Dagbala, Ojirami, Erurhu and Atte areas;lead and zinc bearing metaconglomerate around Egbigele;uranium and thorium bearing pegmatite in the Dagbala area. The Landsat-8 lineament analysis showed the presence of a shear zone in the central, folds in the SW, and fractures in the NE parts of the study area. The relation between the two is such that the gold and silver-copper mineralization is associated with the shear zone, the lead-zinc mineralization to the folds, and the uranium-thorium mineralization to the fractures. Indeed, geological structures guide mineralization and their analysis can be employed for mineral prospecting.

松辽盆地北部古中央隆起带基底岩石弹性参数测试及特征分析

古中央隆起带基底上覆地层缺失断陷期沉积地层,基岩暴露地表时间长,风化壳普遍发育。基底岩性复杂,主要发育花岗岩、浅变质沉积岩及浅变质火山岩3大类岩石;基底主要发育花岗岩、变质岩储层,不同类型储层的裂缝发育程度不同。钻井揭示:花岗岩的脆性大,易形成裂缝,花岗岩储层物性最好,属于孔隙—裂缝型储层;基底花岗岩风化壳储层非均质性强,储层物性较差,以裂缝为主。目前有关基底花岗岩和变质岩的岩石物理弹性参数特征的实验室研究较少。为此,采用超声频段和地震频段弹性参数测试方法,系统分析了围压及孔隙压力对岩石纵、横波速度的影响,研究了不同孔隙度和饱和状态下岩石弹性参数与微裂缝密度及孔隙纵横比的关系;应用弹性参数交会分析,明确了基底风化壳储层及裂缝敏感参数,建立了符合基底孔隙—裂缝型储层特征的岩石物理模型,获得以下成果和认识:(1)对松辽盆地北部古中央隆起带基底33块岩性样品,采用超声频段和地震频段实验测试,测得的标准铝块的超声频段纵波速度(6430 m/s)与参考值(6400 m/s)基本相同,相对误差为0.5%,精度较高。(2)随着围压增加,在水饱和和气饱和状态下纵、横波速度均增大,且横波速度变化不大。随着孔压增加,在水饱和和气饱和状态下纵、横波速度均呈降低趋势,且在气饱和状态下纵波速度的变化量大于水饱和状态,在水饱和和气饱和状态下横波速度的下降趋势相似。(3)不同压力的超声频段和地震频段速度差异表明微裂隙是影响频散绝对变化量的主要因素,随着压力增加,微裂隙逐渐闭合,孔隙间的流体流动效应减弱,喷射效应降低。(4)地震频段和超声频段储层敏感参数优选结果相同,即v_(P)/v_(S)(纵横波速度比)、ν(泊松比)、λ(拉梅系数)、λ/μ(拉梅系数/剪切模量)、λ_(ρ)(拉梅系数×密度)等参数对流体最敏感,可作为识别基底风化壳的储层敏感参数。(5)在裂缝密度为3条/m、裂缝倾角为50°的条件下建立了饱和度—孔隙度解释图板,在含水饱和度为50%、裂缝倾角为50°的条件下建立了裂缝密度—孔隙度解释图板,为风化壳孔隙—裂缝型储层预测奠定了理论基础。

中亚造山带西南缘微地块前寒武纪基底构造属性与大陆亲缘性

中亚造山带西南缘分布有众多条带状展布的大陆微地块,它们是哈萨克斯坦微大陆的重要组成部分,其构造属性、大陆亲缘性和地壳演化历史对于揭示中亚造山带古生代增生造山历史及其动力学过程具有重要意义。长期以来,对这些微地块的基底物质组成、形成背景和构造亲缘性的认识,一直存在争议。文章通过总结近年来陆续发表的地质和同位素年代学数据,梳理了各大陆微地块前寒武纪基底的岩石组合、形成时代和构造背景,进一步分析了各地块的构造属性及其与周缘主要大陆板块之间的亲缘性。已有结果显示,中亚造山带西南缘主要微地块中,除了境内外中天山、伊塞克、哲尔套和科克契达夫地块发育有中元古代—古元古代的结晶基底以外,其它微地块(如伊犁南缘、伊犁北缘—阿克陶、楚伊犁、卡拉套、拜克努尔、额热蒙套和乌卢套等)均没有可靠的中元古代原地结晶基底。所有微地块中均发育类似的早新元古代变质作用和岩浆作用,形成了微陆块统一的结晶和变质基底。经过综合对比分析,笔者认为这些微地块可能具有不同的起源,但在新元古代经历了相似的构造—岩浆演化,处于相似的古地理环境,记录了新元古代早期大陆汇聚造山、晚期大陆裂解和古亚洲洋扩张过程中被动陆缘形成的过程。

建筑地下室顶板及后浇带防水施工技术的应用

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