青藏高原双湖地区二叠系玄武岩地球化学及其大地构造意义

对青藏高原羌塘地块中部双湖地区发育的二叠系碱性系列和拉斑系列玄武岩进行了详细的地球化学研究。碱性系列玄武岩富集LILE和LREE,其La/Nb比值和OIB相近,Ti/V比值明显高于典型MORB,在微量元素构造环境判别图解上位于OIB区域。拉斑系列玄武岩具有相对平坦的稀土元素配分模式,和典型MORB相比,其Ti/V比值也明显偏高,在微量元素构造环境判别图解上位于MORB和OIB重合的区域,表明岩石起源于一个低度富集的地幔源区。结合区域地质背景,认为双湖二叠系拉斑系列-碱性系列玄武岩组合可能形成于陆间裂谷到小洋盆环境,这套玄武岩的产出可能代表古特提斯洋沿龙木错—双湖构造带在不同地区的发育程度不同,双湖地区在二叠纪应为一个陆间裂谷到小洋盆环境。     

Economic Sampling and Extraction of Undisturbed,High Quality Samples in Normally Consolidated Lacustrine Clays Using a Large Diameter Tube

This paper describes the development, design and use of a large diameter sampling tube. High quality test specimens are essential for the investigation of mechanical properties of a soil for high risk projects and when complex and expensive testing methods are to be used. Block sampling is recommended to give the highest sample quality for clayey soils, however, extracting blocks of normally consolidated lacustrine silty clay without excessive disturbance was challenging due to the inherent structure of the soft varved silty clay and difficulty in maintaining K_o conditions, as well as no vertical strain, in the sample. A new sample tube, with an inner diameter of 196 mm, an area ratio of 4% and an outer cutting-edge angle of 11° was designed to offer a larger cross sectional area than conventional thin walled sampling tubes, to provide the necessary side support and to prevent water ingress at the sides of the sample. The length-diameter aspect ratio was 1.275 to optimise the amount of clay sampled for subsequent testing and in an attempt to minimize the pressure in front of the tube. Samples were taken in initially newly excavated trenches at a depth of c. 1 m with this new sampler and with conventionally sampled soil specimens, prior to the main testing programme with samples from 6 m depth. A comparative study was then performed including preliminary unconsolidated unconfined compression tests followed by anisotropically consolidated undrained triaxial compression tests. It was important to establish whether this approach had led to an improvement in sample quality prior to embarking on an extensive triaxial stress path testing programme on this varved soil (Messerklinger, Non-linearity and small strain behaviour in lacustrine clay, 2006; Messerklinger and Springman, Geotech Test J 30(6), 2007; Messerklinger and Springman, Geotech Geol J, 2008). The results showed that the undrained shear strength of the specimens from the new sampler was consistently around 20% higher than that of specimens extruded from conventional thin walled tube samplers. This confirmed that samples with a significantly higher quality could be extracted from normally consolidated, fine grained, varved lacustrine deposits with this large diameter ‘block’ sampling tube.     

旁那石榴蓝闪片岩特征及其构造意义

在拉萨市墨竹工卡县旁那村附近的二叠系(变质)玄武岩之下发现了石榴蓝闪片岩,与二叠系海相灰岩、石英岩、白云母片岩共生.该片岩包括石榴石和角闪石变斑晶及由(多硅)白云母、斜黝帘石、石英、金红石、锆石、钠长石、绿泥石、榍石、钛铁矿组成的基质.角闪石变斑晶中可见两期蓝闪石,早期蓝闪石富铁、贫钠,晚期蓝闪石包裹早期蓝闪石,其成分接近纯的蓝闪石端员,并常具有冻蓝闪石或阳起石的冠状体.从核部至边部,钾质白云母单位分子中的硅含量快速降低;含有霓辉石、金红石和石英等包体的石榴石的锰含量急剧降低,镁含量则升高.运用视剖面(pseudosection)方法模拟该片岩p-t演化轨迹,视剖面温压计算范围为250～650℃和0.4～2.8 GPa.模拟结果表明该片岩的变质温压始于450℃和2.5 GPa,先经历近等压升温过程至500℃,再经历近等温降压过程至0.6 GPa,表明该片岩系冷洋壳快速俯冲与折返的产物.拉萨地块中部早三叠世查曲浦组安山岩和晚三叠世过铝质花岗岩的产出以及在与该变质杂岩共生的碳酸盐岩中二叠纪化石的发现共同限定了冷洋壳的俯冲应发生在二叠、三叠纪之交.区域地质分析表明,在中生代早期冈瓦纳大陆裂解之前,其东北边缘的冷洋壳向冈瓦纳大陆之下俯冲,随后冈瓦纳大陆向外增生,然后再裂解,形成现今的拉萨地块.     

拉萨地块北缘尼玛地区晚白垩世古地磁结果及其构造意义

通过对青藏高原拉萨地块北缘尼玛县城附近晚白垩世红层8个采点的系统古地磁测定,获得了研究区晚白垩世可靠的古地磁数据。通过系统热退磁获得了采样剖面高温特征剩磁方向平均值为:Dg=9.9°,Ig=5.5°,κg=30.3,α95=10.2°,N=8(地理坐标下);D_s=8.2°,I_s=27.4°,κs=37.6,α_(95)=9.2°,N=8(层面坐标下);对应古地磁极为:71.2°N,241.9°E,dp/dm=5.5°/10.0°。该高温剩磁分量在地理坐标下与现代地磁场方向有显著差别,具有正、反极性,并通过了倒转检验,由此认为其很可能代表岩石形成时的原生剩磁。本次研究结果表明拉萨地块北缘在晚白垩世位于~14.5°N的古纬度位置(参考点:31.8°N,87.2°E)。对比稳定亚洲大陆参考极显示晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间发生了1200±630km(10.9°±5.7°)的南北向构造缩短,但并未发生明显的相对旋转作用(0.8°±5.9°)。综合地质证据提出印度与亚洲大陆碰撞所造成的亚洲大陆内部南北向构造缩短主要集中在拉萨地块以北的区域。     

Early Jurassic metamorphism of the eastern segment of the Lhasa terrane in South Tibet and its tectonic significance

The metamorphic belt in the Dongjiu area is located in the eastern segment of the Lhasa terrane in South Tibet. The Dongjiu metamorphic rocks are primarily composed of schist and gneiss, with minor amounts of marble, and the protoliths are sedimentary rocks with Precambrian and early Palaeozoic zircons probably deposited during the Palaeozoic or late Neoproterozoic. On the basis of petrology and phase equilibria modelling, this study shows that the Dongjiu metamorphic belt has experienced a kyanite-grade metamorphism, which is characterized by a decompressional vector with slight cooling from a peak of 9.6 kbar and 745°C to medium-pressure amphibolite-facies metamorphic overprinting at 5–6 kbar and 600–630°C. This P–T path was well recorded and recovered by garnet zoning profiles. Laser ablation inductively coupled plasma mass spectrometry in situ U–Pb analyses on metamorphic zircons and zircon rims yielded concordant ²⁰⁶Pb/²³⁸U ages of c. 194–192 Ma, suggesting that the Dongjiu metamorphic rocks were formed during the Early Jurassic. Therefore, the Dongjiu metamorphic belt, together with the western Nyainqentanglha, Basongco, and Zhala metamorphic belts, constitutes a nearly continuous tectonic unit with an E–W extension of at least 500 km between the northern and southern Lhasa terranes. The metamorphic ages of these belts, ranging from 230 to 192 Ma, show a younger trend from west to east, indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the Late Triassic to Early Jurassic.     

扬子地块西北缘碑坝地区白玉~1.79 Ga A型花岗岩的发现及其对构造演化的制约

本文首次对扬子地块西北缘碑坝地区白玉花岗岩进行了锆石U-Pb年代学、全岩地球化学和锆石Hf同位素研究。锆石LA-ICP-MS U-Pb年龄测定结果表明,白玉花岗岩形成于~1790 Ma。在全岩地球化学组成上,白玉花岗岩高Si,富碱,高FeO~T/(FeO~T+MgO)比值,低Mg、Ca、Mn和P,A/CNK介于0.92~0.96之间,显示为准铝质的特点;微量元素中富集Ga、Th、Zr、Y,贫Sr、P、Ti;稀土元素总量较高,轻重稀土之间分异较明显,并表现出强烈的负Eu异常;这些特点与A型花岗岩一致。锆石负的εHf(t)值(-12.66~-10.69)显示,白玉花岗岩来源于古老地壳物质的部分熔融。地球化学和区域研究成果分析表明,白玉花岗岩最有可能形成于陆内裂谷盆地;结合前人研究成果我们认为,华南存在与Columbia超大陆有关的俯冲、拼合和裂解记录:碑坝地区与弧有关的后河群花岗岩类岩石证实了扬子地块西北缘在~2.10Ga为活动大陆边缘环境;华南与Columbia超大陆拼合有关的碰撞发生于2.00~1.88Ga,宜昌崆岭地区发生陆陆碰撞作用的时间(2.00~1.94Ga)要早于浙西南地区(1.93~1.88Ga),扬子地块本身(西、东扬子地块)及华夏地块(至少是华夏地块西部)在古元古代Columbia超大陆汇聚时期就已经拼贴形成统一的块体;1.87~1.82Ga期间,华南的A型酸性岩形成于后碰撞的构造背景,是由碰撞后的大洋岩石圈拆沉所致;而在~1.79Ga,华南进入了陆内裂解阶段,与华北及其他大陆记录的Columbia超大陆陆内裂解时限较为一致。     

Late Cretaceous granitoids in Karakorum,northwest Tibet: petrogenesis and tectonic implications

New zircon LA-ICP-MS U–Pb age, zircon Hf isotope, and whole-rock major and trace elemental data of the Late Cretaceous Ageledaban complex in the Karakorum Terrane (KKT), northwest Tibet, provide new constraints on the tectonic processes of the collision and thickening of the terrane between the Lhasa and Qiangtang terranes. The granitoids from the Ageledaban complex have a variable SiO₂ content, from 62.83 to 73.35 wt.% and A/CNK<1.1 (except for YM61-2). They have rare earth element and trace element patterns that are enriched in light rare earth elements, Rb, Pb, Th, and U, and are depleted in Ba, P, Sr, Ti, and Nb, indicative of weakly peraluminous-metaluminous I-type affinity. Zircon U–Pb dating reveals that the Ageledaban complex was emplaced at ca. 80 Ma. Zircons from the monzogranite and monzonite samples with concordant ²⁰⁶Pb/²³⁸U ages about 80 Ma have a zircon ε_Hf(t) of ?6.6 to ?1.1, corresponding to the Mesoproterozoic Hf crustal model ages (T_DM^C = 1.2–1.6 Ga); the remaining inherited zircons from the monzonite with concordant ²⁰⁶Pb/²³⁸U ages of about 108.1 Ma have ε_Hf(t) values that range from ?8.3 to ?5.0, corresponding to the Mesoproterozoic Hf crustal model ages with an average of 1.6 Ga. These signatures indicate that the Ageledaban granitoids may have been derived from the partial melting of a mixed mantle-crust source. Together with the age and geochemical data in the literature, we propose that the collisional event in the KKT in northwestern Tibet would postdate the northern Lhasa–southern Qiangtang collision, which occurred first in the Amdo in the east and later in the Shiquanhe in central Tibet. Our results support the previous view that the collision of the Bangong–Nujiang suture zone (BNSZ) may be diachronous.     

上扬子东南缘民乐-古丈地区锰矿地质特征及找矿方向

湖南省民乐-古丈地区锰矿位于上扬子地块东南缘之武陵台陷的半局限浅海盆地中,为典型的海相沉积型锰矿床。区内主要有民乐和古丈2个锰矿产地,位于松桃成锰盆地的中东部,与贵州道坨锰矿、杨立掌锰矿处于同一断陷槽中,成矿地质条件较好,找矿潜力较大。通过对区内锰矿床（点）成矿地质特征、古构造、岩相古地理等综合分析,总结区域锰矿成矿规律和找矿方向,为上扬子地块东南缘南华系锰矿找矿突破提供依据。     

西藏班公湖-怒江成矿带主碰撞期成矿作用:荣嘎钼矿岩石地球化学及同位素年龄的证据

荣嘎钼矿是新近在班公湖-怒江成矿带发现并证实的首例斑岩型钼矿床。为了查明班公湖-怒江成矿带主碰撞期的成岩成矿作用,探讨荣嘎钼矿的富Mo机制,对该矿床进行岩石地球化学以及同位素年代学的研究。本次研究获得荣嘎含矿二长花岗斑岩的锆石SHRIMP206Pb/238U加权平均年龄为99. 8±1. 9Ma (MSWD=2. 0),属晚白垩世早期。7件辉钼矿样品的模式年龄较为一致,介于98. 0±1. 6Ma～101. 8±1. 7Ma之间,加权平均年龄为99. 7±1. 1Ma。荣嘎含矿二长花岗斑岩样品主量元素表现为富硅(SiO_2=72. 83%～75. 49%)、富碱(Na_2O+K_2O=8. 12%～8. 55%)、过铝质(A/CNK=1. 08～1. 13)的高钾钙碱性系列岩石。样品明显富集Rb、K、Th、U等元素,中等亏损Nb、Ta,强烈亏损Ba、Sr、P、Ti等元素,具中等程度的负Eu异常(δEu=0. 53～0. 63),稀土元素配分曲线呈轻稀土相对较陡,重稀土较为平坦的右倾型。荣嘎含矿二长花岗斑岩样品的(~(87)Sr/~(86)Sr)i值为0. 7042～0. 7055,ε_(Nd)(t)值介于0. 03～0. 68之间,锆石ε_(Hf)(t)值介于+12. 4～+17. 9之间,两阶段模式年龄t_(DM2)较为年轻(31～314Ma)。基于以上同位素以及岩石地球化学特征研究,荣嘎含矿二长花岗斑岩很可能是幔源岩浆诱发了富Mo洋底沉积物的熔融并与壳源熔体混合形成母岩浆,再经历结晶分异作用而形成。结合本文以及区域上的研究资料,荣嘎钼矿可能形成于拉萨地块与南羌塘地块碰撞的大地构造背景,富Mo大洋沉积物熔体的加入及随后的结晶分异是荣嘎含矿岩浆中金属Mo富集的主要原因。班公湖-怒江成矿带南缘侏罗-白垩纪海相地层分布的地区,可能为勘查钼矿床的有利靶区,应重点评价晚白垩世早期高分异中酸性岩体的含矿性。     

西藏中拉萨地块西段左左乡晚侏罗世-早白垩世花岗岩年代学、地球化学与岩石成因

近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素,以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期(163Ma和160Ma)的年龄,属于晚侏罗世;南侧岩体花岗岩年龄为142～147Ma,属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩,富集大离子亲石元素和轻稀土元素,亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分(ε_(Hf)(t)=-16.8～-13.6),可能来源于古老下地壳的部分熔融;包体具有相似的Hf同位素成分(ε_(Hf)(t)=-15.7～-13.6),结合晚侏罗世寄主岩与包体较高的MgO(4.13%～6.90%)、Cr(146×10~(-6)～370×10~(-6))和Ni(31×10~(-6)～113×10~(-6))含量,说明包体代表的中基性岩浆可能源于古老富集地幔的熔融,并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的ε_(Hf)(t)值(-8.2～-4.8和-3.8～+0.2),指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入,并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合,经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶,最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。