Chronology and Geochemistry of the Nadingcuo Volcanic Rocks in the Southern Qiangtang Region of the Tibetan Plateau:Partial Melting of Remnant Ocean Crust along the Bangong-Nujiang Suture

The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source and peterogenesis are still debated.^（40）Ar-^（39）Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene.In addition,the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios,similar with the adakite derived from partial melting of oceanic crust.They can be further classified as high Mg~#（Mg~#=48-57） and low Mg~# （Mg~#=33-42） subtypes.The Nadingcuo adakitic rocks have relatively low（^（87）Sr/^（86）Sr）_i and highε_（Nd）（t）, showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%-20%crustal material of Lhasa terrane.On these bases we suggest that the low Mg~# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment,and the high Mg~# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge.Therefore,the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.

Early Cenozoic Mega Thrusting in the Qiangtang Block of the Northern Tibetan Plateau

Recent mapping and seismic survey reveal that intensive compression during the Early Cenozoic in the Qiangtang block of the central Tibetan Plateau formed an extensive complex of thrust sheets that moved relatively southward along several generally north-dipping great thrust systems. Those at the borders of the ~450 km wide block show it overrides the Lhasa block to the south and is overridden by the Hohxil-Bayanhar block to the north. The systems are mostly thin-skinned imbricate thrusts with associated folding. The thrust sheets are chiefly floored by Jurassic limestone that apparently slid over Triassic sandstone and shale, which is locally included, and ramped upward and over Paleocene-Eocene red-beds. Some central thrusts scooped deeper and carried up Paleozoic metamorphic rock, Permian carbonate and granite to form a central uplift that divides the Qiangtang block into two parts. These systems and their associated structures are unconformably overlain by little deformed Late Eocene-Oligocene volcanic rock or capped by Miocene lake beds. A thrust system in the northern part of the block, as well as one in the northern part of the adjacent Lhasa block, dip to the south and appear to be due to secondary adjustments within the thrust sheets. The relative southward displacement across this Early Cenozoic mega thrust system is in excess of 150 km in the Qiangtang block, and the average southward slip-rate of the southern Qiangtang thrusts ranged from 5.6 mm to 7.4 mm/a during the Late Eocene-Oligocene. This Early Cenozoic thrusting ended before the Early Miocene and was followed by Late Cenozoic crustal extension and strike-slip faulting within the Qiangtang block. The revelation and understanding of these thrust systems are very important for the evaluation of the petroleum resources of the region.

THE MESOZOIC QIANGTANG FORELAND BASIN IN QINGHAI—XIZANG PLATEAU,CHINA

Mesozoic Qiangtang foreland Basin is located between Jinshajiang suture belt and Bangong—Nujiang suture belt and an important petroliferous basin in the north of Qinghai—Xizang plateau. This foreland basin is E—W elongated basin with 800km in length and 200km to 300km in width. Both margins of the basin are large suture belt and linked to thrusting of large suture belt, so Mesozoic Qiangtang foreland basin belongs to composite foreland basins according to Jordan’s classification of the foreland basin(1988). The foreland basin is filled with 5000～8000m thick late Triassic to Cretaceous marine sediments. The spacial changes of the stratigraphy indicates that the basin texture looks like a symmetric body, and it can be subdivided into three tectonic geomorphic units from north to south ,such as northern basin, center uplift, and southern basin. The depth in the north varies from 5000 to 8000m,the depth in center uplift varies from zero to 1000m, the depth in the south varies from 5000 to 7000m, Which show that the occurrence of the Mesozoic filling stratigraphy are thicker in the north and the south of the basin, but thinner in the center uplift of the basin. There are two center of subsidence of the basin, both of them are located in south and north foredeep belt, lying in the front of suture belt. The basin is one of common foreland basins between suture belt and belongs to typical symmetric foreland basin.This kind of basin geometry allows large thickness of synsedimentary molasse sediments to be preserved and related to basement uplifts and thrusts in the cratonic edge of the mountain belt.From late Triassic to Cretaceous the foreland basin is filled by four tectonic sequences, including late Triassic tectonic sequence(TS\-1),early Jurassic tectonic sequence(TS\-2), middle Jurassic to early Cretaceous tectonic sequence(TS\-3) and middle to late Cretaceous tectonic sequence (TS\-4).A tectonic Sequence is a body of genetically related strata isolated by unconformity ,deposited in a basining stage responding to a thrusting episode. Late Triassic tectonic sequence(TS\-1) is bounded by Ta and Tb and composed of the Xiaochaka formation which is more than 2500m in depth, it is a coarsing\|upward tectonic sequence,the lower is fan delta sediments with debris conglomerate,the middle is carbonate ramp sediments,the upper is delta sediments; early Jurassic tectonic sequence(TS\-2) is bounded by Tb and Tc and composed of the Nadigangri formation which is more than 1000m in depth, it is a thinning\|upward tectonic sequence,the lower is fan delta sediments with debris conglomerate,the upper is subaquatic detrital sediments; middle Jurassic to early Cretaceous tectonic sequence(TS\-3) is bounded by Tc and Td and composed of the Quemocuo formation,Buqu formation, Xiali formation, Suowa formation and Xueshan formation, which is more than 3000m in depth; middle to late Cretaceous tectonic sequence (TS\-4) is bounded by Td and Tf and composed of the Abushan formation, it is a thinning—upward alluvial fan sediments with more than 1000m in depth.

Petrogenesis of high-Mg# Cenozoic volcanic rocks of southern Qiangtang area, Tibetan Plateau： geochemical and Sr-Nd isotopic evidence

The Nadingcuo volcanic rock suite is the most voluminous Cenozoic volcanic suite in the southern Qiangtang area of the northern Tibetan Plateau. These high-K calc-alkaline volcanic rocks were formed between36 and 34 Ma,characterized by high Mg#values,high concentrations of Ti O2 and P2O5,87 Sr /86 Sr ratios of0. 704682--0. 706 112,and εNd( t) values of- 1. 2 to 1. 6. There is a lack of reasonable explanations for sourcing and origin of magmas that formed the rocks with high Mg#values and Ti O2 and P2O5enrichments,which makes the previous research results still controversial. This study reviews the geochemical characteristics of Nadingcuo volcanic rocks and the data we have newly found in our fieldwork. We give some new interpretation to the magmatic evolution of the basaltic magmas in the discussed area dominated by fractional crystallization.The geochemistry of trachyandesite and trachyte units in the studied area is indicative of formation from mantlederived magmas that mixed with crustal materials. The high values of Mg#and Ti O2 and P2O5enrichment in these units are evident to show the mixing between mantle-derived magmas with ~ 30--40 wt.% rhyolitic melt or assimilation of a similar amount of felsic rocks. The geochemistry of basaltic rocks in the area also suggests that the Nadingcuo basalts may have been derived from an ocean island basalt( OIB)-type source that contained and was mixed with ancient mantle wedge derived material,indicating that a 36--34 Ma asthenospheric upwelling event in the Qiangtang area may relate to the northward subduction of Indian lithospheric mantle and the southward subduction of Asian lithospheric mantle. This upwelling of asthenospheric material was centered in the southern Qiangtang area between 36 and 34 Ma,while the northward movement of the Indian Craton caused this upwelling mantle flow to continuously migrate northward,resulting in the current centering of this upwelling in the Hoh Xil-Kunlun region.

TECTONIC CHARACTERISTICS AND EVOLUTION OF THE QIANGTANG BASIN IN NORTHERN TIBET PLATEAU

The Qiangtang basin is located in the north of Qinghai—Tibet plateau and sandwiched by Nianqingtangula continental block and Kekexili\|Bayuankal continental block. Its southern boundary is the Bangongfu—Nujiang suture zone and its northern boundary is the Xijinwulan\|Jinshajiang suture zone.The basement of Qiangtang basin is composed f metamorphic rock of Proterozoic age, which can be divided into two parts. The competent lower part with isotope age of 2056～2310Ma experienced multi\|stage deformation and the soft upper part is dated 1111～1205Ma. Within the basin, it groups into Northern Qiangtang Depression, Central Rise and Southern Depressions and are complicated by a number of subdepressions and subuplifts.The strata of Middle Devonian\|Tertiary are overlain on the basement and composed of marine carbonate rocks, clastic rocks and terrestrial sandstone and conglomerate. Several sets of faults and folds have developed in the cover sequence and the deformation is very strong, characterized by orientation, zonation and equidistance in space and by diversity and disharmony in the profile. The major deformation occurred in Yanshan\|Himalayan period.

玉树北部二叠纪镁铁质岩的成因及其对大陆弧后盆地形成的指示

系统地调查与研究弧后盆地岩浆作用能为探究汇聚板块边缘的壳幔相互作用、板块俯冲动力学和弧后盆地形成-演化机制等提供重要信息。本文以最近在青海玉树北部地区发现的二叠纪镁铁质岩类为研究对象,开展了岩相学、锆石U-Pb年代学、岩石地球化学与同位素地质学等研究,以期为深入认识大陆弧后盆地形成机制及有关镁铁质岩浆的起源提供关键线索。锆石LA-ICP-MS U-Pb年代学测定结果表明,玉树北部辉长岩的结晶年龄为260±1 Ma。玄武岩样品具有相对低的TiO2和Nb/Y值以及相对高的Ce/Nb值,其相对于原始地幔要显著富集轻稀土和Th等元素,亏损Nb、Ta、Ti和P等元素,可与世界上典型弧后盆地玄武岩(例如Okinawa Trough BABBs)成分特征类比。而辉长岩样品具有相对高的TiO2和Nb/Y值以及相对低的Ce/Nb值,其相对于原始地幔要显著富集轻稀土和Th等元素,轻微亏损Nb、Ta和Ti,总体上具有与OIB类似的成分特征。玄武岩样品具有变化较大的ISr(0.706~0.709)和εNd(t)值(-1.8~+1.3),而辉长岩样品则具有相对均一的ISr(0.709)和εNd(t)值(-1.8~-1.5)。综合本次研究成果可以确定,玄武岩应该起源于一个受俯冲组分改造的岩石圈地幔,而辉长岩应该来自于一个与地幔柱相关的源区。结合区域最新研究资料可以进一步判断研究区镁铁质岩应该形成于大陆弧后盆地扩张的背景中,并且该弧后盆地的产生很可能与洋壳深俯冲导致的板块回撤、峨眉山地幔柱活动密切相关。

考虑水力连通的羌塘内流区洼地单元提取与分类

羌塘内流区是青藏高原面积最大的内流区,气候变化正在加剧其湖泊水系结构与水量的演变,开展内流区流域调查并科学估算其水资源的变化具有重要意义。本文提出一种新型内流区流域划分方法,该方法通过引入高程-面积联合阈值,以解决由气候变化与数字高程模型分辨率等因素引起的洼地单元提取难题。基于已监测的羌塘内流区流域重组与湖泊水位的变动幅度,确定了高程-面积联合阈值分别为10 m和50 km^(2),并对洼地单元进行划分,共识别出163个具有合理集水面积、永久性分水岭的封闭内流区洼地单元。引入多个相关数据集及已有算法评估本方法,结果表明:该方法适用于大尺度内流区洼地单元提取,提取精度优于现有算法及数据产品;依据洼地单元间水力连通特征,将全区163个洼地单元划分为5种主要类型,其中以“上下游互通型”(Ⅱ型)、“高山型”(Ⅳ型)洼地单元为主,“凹陷型”(Ⅴ型)洼地单元则多与其他类型复合存在,受气候条件的影响更加显著。

未来情景羌塘高原野牦牛栖息地分布及其影响因素

近30年来,羌塘高原野牦牛(Bos mutus)种群数量虽缓慢恢复,但仍为野生有蹄类中仅有的易危物种。由于其对人为活动规避明显且具有极强的攻击性,野牦牛栖息地分布和质量数据仍很匮乏。把野外调查与最大熵(Maxent)、土地利用模拟模型(FLUS)、InVEST三种模型相结合,系统分析羌塘高原野牦牛栖息地分布及其影响因素,并通过未来气候、未来土地覆被和未来食物情景构建2050年不同温室气体排放浓度(RCPs)情景下羌塘高原生境状况,预测栖息地变化状况,以期为青藏高原生物多样性维护提供数据支撑。结果发现:2020年前后野牦牛栖息地总面积为25.1万km^(2),集中分布在那曲市北部,阿里地区分布零散。栖息地以草原和荒漠为主,部分位于冰川区,野牦牛对气候条件反应敏感,偏好生活在暖季降雨量约在200mm,冷季降雨量约10mm,年最低温度-30℃的区域,坡度耐受性高。约92%的野牦牛栖息地位于羌塘国家自然保护区内,仅有南部约1.2万km^(2)栖息地与人类活动交叠明显。2050年前后羌塘高原暖湿化明显,草原面积增加,野牦牛栖息地将向西北部无人区扩张,目前栖息地分布较零散的阿里地区也将出现大面积高质量栖息地,三种RCPs情境下栖息地面积分别为28.2万km^(2)(RCP2.6)、28.4万km^(2)(RCP4.5)和28.0万km^(2)(RCP8.5),新增栖息地以极重要栖息地为主,边界与羌塘自然保护区范围更加吻合,自然保护地体系建设将有力支撑野牦牛的保护。

The Cretaceous tectonic event in the Qiangtang Basin and its implications for hydrocarbon accumulation

The tectonic event during Cretaceous and its relationship with hydrocarbon accumulation in the Qiangtang Basin is discussed based on zircon U-Pb dating and the study of deformation, thermochronology and hydrocarbon formation. LA-ICPMS zircon U-Pb dating indicates that the tectonic event took place during the Early-Late Cretaceous （125-75Ma）. The event not only established the framework and the styles of structural traps in the basin, but also led to the cessation of the first hydrocarbon formation and the destruction of previous oil pools. The light crude oil in the basin was formed during the second hydrocarbon formation stage in the Cenozoic, and ancient structural traps formed during the Cretaceous event are promising targets for oil and gas exploration.

Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2>59%, enrichment in A12O3(15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g) and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg<0.35), weak Eu anomaly (Eu/Eu=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magma resulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and the petrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.

DEFINITING AND ITS GEOLOGIC MEANING OF SOUTH-NORTH TREND FAULTED STRUCTURE BELT IN QIANGTANG BASIN, NORTH PART OF TIBET

There were more expounding to north—west (west) trend fault and north\|east trend fault within Qiangtang Basin, North Part of Tibet, in the past literature. With increasing of geophysical exploration data, nearly east\|west trend structure began to be taken note to. Since the year of 1995, by a synthetic study to geophysical and geological data, that south\|north trend faulted structures are well developed. These structures should be paid much more attention to, because they have important theoretical meaning and practical significance.1 Spreading of south\|north faulted structure belt According to different geological and geophysical data, the six larger scale nearly south\|north faulted structure belt could be distinguished within the scope of east longitude 84°～96° and near Qiangtang Basin. The actual location of the six belts are nearly located in the west of the six meridian of east longitude 85°,87°,89°,91°,93°,95° or located near these meridian. The six south\|north faulted structure belts spread in the same interval with near 2° longitude interval. The more clear and much more significance of south\|north trend faulted structure belts are the two S—N trend faulted structure belts of east longitude 87° and 89°. There are S—N trend faulted structure belts in the west of east longitude 83°,81°, or near the longitudes. The structure belts spreading features,manifestation,geological function and its importance, and inter texture and structure are not exactly so same. The structure belts all different degree caused different region of geological structure or gravity field and magnetic field. There is different scale near S—N trend faulted structure belt between the belts.

ANISOTROPIC FEATURE OF QIANGTANG MASSIF TEXTURE

Qiangtang Massif is located in the hinterland of Qinghai—Tibet plateau, which belong to the mid\|east section of Tethys Tectonic Domain.1 Features of the whole texture and structure of Qiangtang massif By synthetic analysis of gravity,magnetic field,MT,seismic surveying,etc. Geophysical data, the massif, lied in the tectonic setting and geodynamic setting mingled by the south,north tectonic belts, have the features of massif,basin and tectonic belt three forming an organic whole,multi\|degree coupling in plane and section with division of region in south\|north trend,division of block\|fault in east\|west trend,division of sphere\|layer in vertical direction. (1) Belting in south\|north trend: Qiangtang massif could be divided into four units from north to south, that is north edge doming zone, west part doming area,Qiangtang Basin and south edge doming zone. Qiangtang Basin also can be divided into four tectonic units—north Qiangtang down\|warping region, middle downing zone, south Qiangtang down\|warping and east part slope region. The near east\|west trend tectonic zones are well developed. There is aero\|magnetic anomaly distributed in belting with east\|west trend but also concentrated in section. Gravity anomaly is high in the south\|west part and low in the northeast part. Inter\|crust low resistance layer alternately distributed with high and low belting of sou th\|north trend in plane.

BIOLOGICAL MARKER OF MIDDLE JURASSIC OIL SHALE SEQUENCE FROM SHUANGHUI AREA,NORTHERN TIBETAN PLATEAU

The oil shale with marine origin was first reported in 1987 from Shuanghui of the Qiangtang region. Its depositional sequence consists of brown\|black oil shale interbedded massive to thin limestone. Eleven oil shale beds occur and aggregated thickness is up to 47 38m. It deposit age is confined in middle Jurassic by fossils identification. Nine samples selected from horizons with high\|organic contents have been examined by organic geochemistry approach. The oil\|shale range widely in organic carbon content (Toc), average in 8 34%, maximum values reaching 26.12%. Toc are markedly varied in vertical section. The Upper and lower members are slightly low and increase in the middle. The oil\|shale sediments are characterized by high concentration in chloroform bitumen“A”(608～18707)×10 -6 )and total hydrocarbon ((311～5272)×10 -6 ).The Rock\|Eval T \|max data (434～440℃) and vitrinite reflectance values (0.88%～1.26%) indicate that oil\|shale sequence are mature in all samples. The organic matter is predominantly made up of typeⅡ kerogen.

The northern Qiangtang Block rapid drift during the Triassic Period:Paleomagnetic evidence

As one of the pivotal Gondwana-derived blocks,the kinematic history of the northern Qiangtang Block(in the Tibetan Plateau)remains unclear,mainly because quantitative paleomagnetic data to determine the paleoposition are sparse.Thus,for this study,we collected 226 samples(17 sites)from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block(NQB).Stepwise demagnetization isolated high temperature/field components from the samples.Both Early and Late Triassic datasets passed field tests at a 99%confidence level and were proved to be primary origins.Paleopoles were calculated to be at 24.9°N and 216.5°E with A95=8.2°(N=8)for the Early Triassic dataset,and at 68.1 N,179.9 E with A(95)=5.6°(N=37)for the Late Triassic,the latter being combined with a coeval volcanic dataset published previously.These paleopoles correspond to paleolatitudes of14.3°S±8.2°and 29.9 N15.6°,respectively.Combining previously published results,we reconstructed a three-stage northward drift process for the NQB.(1)The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic;(2)thereafter,the block rapidly drifted northward from southern to northern hemispheres during the Triassic;and(3)the block converged with the Eurasian continent in the Late Triassic.The^4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of^11.85 cm/yr.The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.

西藏羌塘中部猫耳山石榴角闪岩的岩石学、地球化学及年代学研究

青藏高原腹地羌塘地体中部猫耳山增生杂岩内保留有早古生代变质记录,对研究冈瓦纳大陆北缘的早古生代构造格架和演化具有重要意义。猫耳山石榴角闪岩呈透镜状产出于斜长角闪岩内,主要由石榴石、角闪石、单斜辉石、斜长石、白云母、黝帘石、绿泥石、榍石和钛铁矿组成。石榴角闪岩的全岩地球化学分析表明其具有俯冲带上(SSZ型)蛇绿岩特征,与区域内寒武-奥陶纪蛇绿岩残片一致。根据岩相学、矿物化学、相平衡模拟以及锆石定年结果,得出石榴角闪岩的成因及变质过程如下:(1)石榴角闪岩原岩为~477Ma的特提斯洋蛇绿岩残片;(2)在394~383Ma发生麻粒岩相变质过程,峰期变质温压条件为~900℃、~1.55GPa,矿物组合为石榴石+角闪石+透辉石;(3)早期的折返导致其降温降压至~800℃、~1.0GPa,矿物组合为石榴石+角闪石+透辉石+斜长石;(4)进一步的角闪岩相退变质发生于358~348Ma,矿物组合为角闪石+斜长石+透辉石+钛铁矿,在石榴石周围和港湾处形成斜长石+角闪石的后成合晶结构。石榴角闪岩的P-T-t演化轨迹结合区域蛇绿岩和岩浆岩记录,指示早古生代特提斯洋壳俯冲、泥盆纪特提斯洋弧后扩张和石炭纪特提斯洋内岛弧增生的过程。

羌北地块上志留统龙木措上组岩石磁学特征

开展羌北地块早古生代古地磁学研究,定量约束出其古生代以来的古地理位置,可为研究青藏高原古构造格局、显生宙特提斯演化和古地理重建等提供重要基础和关键制约。在进行古地磁研究之前,首先进行岩石磁学特征的研究,明确岩石中载磁矿物的组合类型和特征,为后续退磁实验方案的选择以及剩磁的原生性的讨论提供岩石磁学基础。对羌北地块上志留统龙木措上组灰岩和砂岩进行岩石磁学特征研究,包括等温剩磁获得曲线、磁化率随温度变化(χ-T)曲线、三轴等温系统热退磁实验、低温磁学性质测试以及扫描电镜(SEM)和能谱分析(EDS)等。实验结果表明,龙木措上组灰岩样品中的磁性矿物以磁铁矿为主,另外还有少量的磁赤铁矿和磁黄铁矿,砂岩样品中的磁性矿物较为复杂,主要为磁铁矿,可能还含有磁黄铁矿等其他磁性矿物。研究结果表明龙木措上组地层中灰岩样品可以分离出稳定的高温剩磁分量,适宜开展进一步古地磁学研究。

羌塘高原典型矿区水系沉积物地球化学特征与区域化探扫面方法

羌塘高原是目前区域化探全国扫面工作最大的空白区和地质找矿工作的新区,这里的化探扫面和地质找矿工作正在推进中。水系沉积物测量是该区主要区域化探扫面方法。在羌塘高原上,风成沙广布,其粒级主要集中在–40目,比例高达90%以上,是影响区域化探找矿效果的最大因素。通过四个矿区水系沉积物地球化学特征研究发现:1)水系沉积物不同粒级中,大部分矿化指示元素含量都呈不对称的"反S"型或"U"型分布,富集在+40目粒级和–160目粒级中。以–10^+40目为采样粒级,很大程度上可以消除风成沙的干扰。2)大型矿床形成的指示元素异常沿水系迁移距离为4~8 km,异常面积>25 km2;小型矿床形成的异常迁移距离1 km左右,异常面积1 km2左右。3)确定区域化探扫面最佳技术指标为:采样粒级–10^+40目;采样密度1点/4 km2(勘查目标定位到大型以上矿床时)或1点/km2(勘查目标定位到小型以上矿床时)。

青藏高原羌塘地区几个关键地质问题的思考

羌塘地区是青藏高原最大的一块中比例尺地质调查空白区,许多关键地质问题集中于此。综合多年羌塘地区的地质实践,结合近年来1∶25万地质调查的初步成果,提出龙木错-双湖板块缝合带、蛇绿岩、高压变质带、羌塘的基底、南北羌塘地层序列及地层格架、盆地成因类型、羌塘盆地与油气资源、羌塘地区的深部地质结构、羌塘地区生态与环境等方面存在的问题,期望起到抛砖引玉的作用,提供给在羌塘地区从事地质调查的同行们参考。

认识青藏高原的重要窗口──羌塘地区近年来研究进展评述(代序)

2003年以来,中国地质调查局在羌塘地区部署了30多个1∶25万图幅和少量1∶5万图幅的地质填图工作,各种类型的科学研究不断开展,获得了大量重要的地质调查和研究成果。在古生界广泛分布的羌塘中部取得了基底组成、时代和性质方面的重大进展;冈瓦纳和扬子大陆边缘古生代沉积建造、地层序列和古生物区系研究更加深入,地层格架的建立臻于完善;龙木错-双湖缝合带蛇绿岩的研究更是令人鼓舞,不但识别出了二叠纪蛇绿岩,早古生代完整蛇绿岩的发现和精确的定年研究还使古特提斯洋的演化历史至少上溯到奥陶纪;羌塘中部高压变质带已控制的长度达到500km,榴辉岩的发现和研究丰富了高压变质带的研究内容,根据高压变质矿物学和变质作用的研究已经可以建立羌塘中部板块碰撞过程模型;岩浆岩与岩浆作用研究取得的成果揭示,羌塘中部自奥陶纪以来发生了多期重大的岩浆事件;构造运动分期的研究对冈瓦纳板块与扬子板块界线的确定起到了关键作用。如果把青藏高原喻为打开地球奥秘的金钥匙,那么把羌塘喻为打开青藏高原地学奥秘的钥匙是恰当的。

青藏高原羌塘早古生代蛇绿岩--堆晶辉长岩的锆石SHRIMP定年及其意义

羌塘中部果干加年山西段发现完整的蛇绿岩组合,由变质橄榄岩、堆晶辉长岩、辉长岩岩墙群、玄武岩、硅质岩等组成。对堆晶辉长岩进行地球化学和定年初步研究,堆晶辉长岩具有洋中脊玄武岩的地球化学特征,获得锆石SHRIMP U-Pb谐和年龄为438±11 Ma,是羌塘地区乃至青藏高原南部首次发现完整早古生代的蛇绿岩。结合角木日地区二叠纪蛇绿岩、双湖以东的晚泥盆世法门期和二叠纪-三叠纪放射虫硅质岩的研究,羌塘中部古特提斯洋盆可能形成于早古生代,持续演化到三叠纪,并且可能是一个连续过程,使我们对青藏高原古特提斯洋的历史及其演化阶段的划分需要重新思考。