喀喇昆仑地体甜水海地区102Ma辉长岩的发现及其对区域中生代构造演化的约束

喀喇昆仑地体是青藏高原的重要构造单元。近来,我们对其中的林济塘中生代盆地地层进行了调查,并在喀喇昆仑地体甜水海地区褶皱的中侏罗统龙山组灰岩中发现了辉长岩侵入体。辉长岩具有高Al2O3(15.07%~15.32%)与TiO2(1.31%~1.34%)、贫P2O5(0.19%~0.21%)和低碱(Na2O+K2O=3.49%~3.72%)的特征,明显富集轻稀土(La/Yb=6.17~7.39),富集Th、La、Nd与Tb,亏损Rb、Sr、P与Y,辉长岩Sr-Nd同位素表现出富集的特征(εNd(t)值分别为-5.8与-5.9)。LA-ICP-MS锆石U-Pb测年结果显示辉长岩岩浆锆石206Pb/238U年龄为102±1Ma(MSWD=0.92;n=14),指示岩体形成于早白垩世晚期。辉长岩中含有大量继承锆石,38颗继承锆石年龄分布在2450~232Ma之间,其中16颗为前寒武纪年龄锆石,意味着该地区可能存在前寒武纪基底。这些特征意味着辉长岩在形成的过程中经历壳源物质的强烈混染(Nb/U、Ta/U、Ce/Pb分别为26.6~29.1、1.69~1.76、9.72~11.3)。这种经历壳源物质的强烈混染岩浆的形成可能与软流圈上涌有关,即102Ma辉长岩形成时甜水海地区处于伸展背景。结合区域地质特征,指出102Ma时研究区进入新的伸展与海盆发育阶段,并建立了喀喇昆仑地体中生代三阶段构造演化模型,即侏罗纪-早白垩世早期的正常俯冲与弧盆演化、早白垩世的平板俯冲与造山阶段及晚白垩世的正常俯冲与弧盆演化。     

反倾岩质边坡变形特征的三维数值模拟研究--以龙滩水电站工程边坡为例进行三维变形特征分析

以龙滩水电站工程边坡三维变形为例,分析了层状反倾岩质边坡当边坡与岩层夹角变化时,边坡的变形特征。并通过数值模拟根据变形曲线对不同范围边坡与岩层走向夹角的变形特征进行了定性描述。     

玲珑金矿区成矿后花岗斑岩脉的锆石SHRIMP U-Pb年龄对胶北地体演化的制约

采用SHRIMP测年技术对玲珑金矿区成矿后花岗斑岩脉的锆石进行了U Pb年龄测定,获得岩脉岩浆锆石的206Pb/238U平均年龄为119.4 Ma,同时获得大量的继承锆石年龄,范围从3 114~127 Ma。继承锆石年龄显示,胶北地体前寒武纪经历了中太古代的陆核生成阶段;新太古代花岗岩绿岩地体增生阶段;古元古代孔兹岩系的沉积阶段;新元古代蓬莱群盖层形成阶段。整个古生代,地体处于稳定的隆升受剥蚀状态,没有明显的岩浆活动。三叠纪,华南与华北克拉通碰撞,郯庐断裂带形成。晚三叠世—中侏罗世,后碰撞的巨大挤压作用导致华北克拉通南缘地壳增厚,玲珑和蚌埠荆山等花岗岩生成。中晚侏罗世,郯庐断裂带发生了巨大的左行平移,胶北地体从蚌埠—五河的东部推移到现在的位置。早白垩世,郯庐断裂由左旋压扭向右行拉张转化,导致胶北地体上郭家岭岩体生成。中晚白垩世,郯庐断裂发生了大规模的拉张,导致沂沭裂谷、胶莱盆地、青山组火山岩、金矿床形成和隆起区大量脉岩侵入。     

西藏拉萨地体南部晚白垩纪石榴石二辉麻粒岩的发现及其构造意义

拉萨地体位于青藏高原南部,其中-新生代构造演化过程与新特提斯洋的俯冲、消亡和印度与欧亚大陆的碰撞密切相关,是揭示青藏高原形成与演化历史的关键之一。本文报道了在拉萨地体东南部林芝岩群中发现的石榴石二辉麻粒岩。这种岩石的共生矿物为石榴石、单斜辉石、斜方辉石、拉长石、富钛角闪石和黑云母,其原岩相当于基性岩,变质作用的温压条件为T=747~834℃和P=0.90~1.35GPa,形成深度可达45km。与麻粒岩伴生的石榴石斜长角闪岩和大理岩中锆石的U-Pb定年研究表明,它们的变质作用发生在85~90Ma。结合在林芝岩群中广泛发育的同时代岩浆作用和区域变质作用,我们认为拉萨地体南部在晚中生代发生了强烈的安第斯型造山作用。     

北苏鲁超高压变质带中斜长角闪岩的成因

在北苏鲁超高压变质带的威海地区的正片麻岩中,存在大量规模不一的含石榴石斜长角闪岩的透镜体或不规则团块。锆石中矿物包体的激光拉曼测试、阴极发光图像分析、稀土元素和微量元素以及Lu-Hf同位素的LA-(MC)-ICP-MS测试等综合研究结果表明,含石榴石斜长角闪岩 (WH17) 只存在一种成因类型的锆石,即变质锆石。该类锆石自核部到边部均保存了典型的超高压包体矿物组合：柯石英 (Coe)+石榴石 (Grt)+绿辉石 (Omp)+多硅白云母 (Phe)+金红石 (Rt)+磷灰石 (Ap),相应的阴极发光图像自核部到边部十分均匀,具有典型变质锆石的特点。锆石核部和边部的稀土元素含量特征和配分模式也十分相似,主要表现为轻稀土元素相对亏损,而重稀土元素相对平坦,无Eu异常 (Eu/Eu^*=0.94~1.04), 具较明显的正Ce异常 (Ce/Ce^*=83.4~111.0),低的Th/U (<0.02) 和 Lu/Hf (<0.00010) 比值以及极低的 Th (1×10^-6~4×10^-6) 含量。锆石核部到边缘的重稀土元素亏损与石榴石稳定有关,而无Eu异常则与斜长石分解关系密切。上述特征表明,含石榴石斜长角闪岩中的锆石均是在超高压变质阶段形成的变质锆石,而研究样品中继承性岩浆结晶锆石的缺乏,是由于含石榴石斜长角闪岩在原岩形成时Zr的成分明显匮乏所致。SHRIMP U-Pb定年结果表明,含柯石英锆石自核部到边部记录了十分一致的²⁰⁶Pb/²³⁸U 年龄,变化于 240.2±3.3Ma 到220.2±2.8Ma 之间,加权平均年龄为229.8±2.0Ma, 这组年龄与前人对苏鲁超高压变质带中榴辉岩、正片麻岩、副片麻岩和大理岩等含柯石英锆石微区记录的年龄完全吻合,应代表苏鲁地体超高压变质时代。Lu-Hf同位素研究结果表明,含石榴石斜长角闪岩中变质锆石的Hf同位素特征与苏鲁超高压变质带中榴辉岩的继承性岩浆锆石和变质锆石均存在本质差别,其中¹⁷⁶Hf/¹⁷⁷Hf=0.00001~0.00003, ¹⁷⁶Hf/¹⁷⁷Hf (_t)=0.282052~0.282107, 相应的ε_Hf(t)=-21.1~-18.8, 模式年龄t_DM2=2.14~2.24Ga,这些Lu-Hf同位素特征与围岩正片麻岩含柯石英锆石微区的Hf同位素性质十分相似, 充分表明了北苏鲁含石榴石斜长角闪岩在超高压变质阶段形成的变质锆石所必须的Zr和Hf成分不是来自体系本身,而是来源于围岩正片麻岩。     

北苏鲁荣成地区超高压变质带的形成与折返动力学

山东省荣成地区位于苏鲁高压-超高压变质带的东北部,以花岗质片麻岩、副片麻岩为主,夹有少量的榴辉岩、石榴辉石岩、麻粒岩、超基性岩、石英岩、大理岩和斜长角闪岩等,各类岩石的锆石中普遍含有柯石英包裹体,表明荣成地区岩石曾经历超高压变质作用过程。荣成地区区域构造格架表现为面理产状总体为NNE-SSW走向,向南转为由NE-SW走向,呈弧形展布,倾向SE或SSE,超高压变质岩石由一系列近平行的剪切岩片组成,岩片之间的分界线为一些韧性剪切带,是折返阶段角闪岩相-绿片岩相的产物。结合野外宏观变形现象、显微构造分析以及糜棱岩中石英的优选方位EBSD （electron backscatter diffraction,电子背散射技术）测量结果,可以判断北苏鲁荣成地区韧性剪切带主要形成于中、低温（550~350℃）条件,并具有由NW向SE“斜向正滑”的剪切指向。根据韧性剪切带中所含的榴辉岩透镜体的显微构造和绿辉石与金红石的优选方位的EBSD的研究,重塑超高压变质阶段的流变学特征：绿辉石和金红石都具有高温的滑移系组构模式。横穿苏鲁高压-超高压变质带的地震反射剖面揭示了苏鲁高压-超高压变质带呈厚10km以上的穹形板片,位于苏鲁高压-超高压变质板片下部的荣成及江苏刘山以韧性正断裂系列为主。Ar-Ar测年结果表明,发生这种伸展韧性剪切作用的时间在117~130Ma。位于板片上部的南苏鲁以韧性逆冲性断裂系列为主。基于以上各方面的研究,进一步验证了苏鲁超高压-超高压变质带折返动力学的挤出模式。     

Records of Indosinian Orogenesis in Lhasa Terrane, Tibet

Based on the deformation characteristics of the ductile shear zones in Sumdo (松多) Group, the quartz fabric by EBSD (electron backscatter diffraction), the data of muscovite 40Ar-39Ar geochronology (220-230 Ma) from ductile shear zones and the zircon SHRIMP U-Pb chronology (190 Ma) of granites in Snmdo region, Lhasa (拉萨) terrane is thought to have experienced an important Indosinian orogenic event at 220-230 Ma, which caused the closure of the paleo-Tethys Ocean along the tectonic zone of eclogite and the collision between northern part and southern part of the Lhasa terrane. The zircon SHRIMP U-Pb chronology of 190 Ma for biotite adamellite, with the distributing characteristics of the granite massif intruding in Sumdo Group, indicates that the biotite adamellitc should be the late orogenic or post-orogenic granite resulting from the lndosinian orogenesis. The discovery of Indosinian orogenic belt in Lhasa terrane expansed the southern boundary of lndosinian orogenic belt in Qinghai (青海)-Tibet plateau to Lhasa terrane from Qiangtang (羌塘) terrane, which changed the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet plateau and extended the "T" type lndosinian orogenic belt in China. The study is very important for the formation and distribution of paleo-Tethys Ocean in Tibet. The ancient terrane framework and evolution of Qinghai-Tibet plateau need further research.     

Statistical analysis of soil variability in a humid forest landscape of Central Cameroon

The quantitative evaluation of key factors of soil variability together with the spatial pattern of underlying variables are important steps in understanding the functioning of soilscapes. Fifteen physical and chemical variables describing 24 pedons were used to elucidate processes of soil differentiation in a humid forest environment in Southern Cameroon. Landscape positions delineated on the basis of remote-sensed imageries included flat convex summits (FCS), linear valley slopes (LVS), colluvial inland valleys (CIV), and alluvial floodplain. Ordination by principal component analysis (PCA) resulted in a three-factor model, which efficiently summarized the dataset explaining 81.4% of total soil variance (TSV). The pedogenetic factor of ferralitization emerged as the main factor accounting for 43.7% of TSV. Identified underlying processes included basic cation leaching, solum acidification, and in situ clay build up. Cation exchange properties as influenced by organic constituents and soil available P were the second and third most important factors associated with 26.7 and 11.4% of TSV respectively. Organic constituents and related biological processes had strong control on nutrient availability and cation-exchange properties, while soil available phosphorus (AP) showed a strong dependence on potential acidity and organic matter mineralization status. The analysis of variance highlighted significant differences for most soil properties across landscape position and soil depth except for organic carbon (OC), total nitrogen (TN), and total phosphorus. All the soils, however, have low pH, low basic cation saturation, and correlatively high exchangeable Al irrespective of their position in the landscape.     

Paleogene magmatism and gold metallogeny of the Jinping terrane in the Ailaoshan ore belt,Sanjiang Tethyan Orogen (SW China): Geology,deposit type and tectonic setting

The Jinping terrane is situated in the southern segment of the Ailaoshan ore belt, Sanjiang Tethyan Orogen (SW China). The Paleogene intrusions in Jinping consist of syenite porphyry, fine-grained syenite and biotite granite stocks/dikes, and contain relatively low TiO₂ (0.21–0.38 wt%), P₂O₅ (0.01–0.35 wt%), and high Na₂O (2.00–4.62 wt%) and K₂O (4.48–7.06 wt%), belonging to high-K alkaline series. Paleogene gold mineralization in Jinping comprises four genetic types, i.e., orogenic, alkali-rich intrusion-related, porphyry and supergene laterite. The NW–NNW-trending faults and their subsidiaries are the major ore-controlling structures. The orogenic Au mineralization, dominated by polymetallic sulfide-quartz veins, occurs in the diorite and minor in Silurian-Devonian sedimentary rocks. It contains a CO₂-rich mesothermal fluid system generated from the mixing of mantle-derived fluids with crustal-derived metamorphic fluids, and the ore-forming materials were upper crustal- or orogenic-derived. The alkali-rich intrusion-related Au mineralization is hosted in the Ordovician-Silurian sedimentary rocks and minor in the Paleogene alkaline intrusions, and the Au orebodies occur predominantly in the alteration halos. It contains a CO₂-bearing, largely metamorphic-sourced mesothermal fluid system, and the ore-forming materials were derived from the ore-hosting rocks and minor from the alkali-rich intrusions. The porphyry Cu-Mo-Au mineralization occurs in the granite/syenite porphyries and/or along their contact skarn, with the mineralizing fluids being magmatic-hydrothermal in origin. The former two hypogene Au mineralization types in Jinping were mainly formed in the late Eocene (ca. 34–33 Ma) and slightly after the porphyry Cu-Mo-Au mineralization (ca. 35–34 Ma), which is coeval with the regional Himalayan orogenic event. Subsequent weathering produced the laterite Au mineralization above or near the hypogene Au orebodies.     

中国大陆克拉通地体地壳密度结构特征

本文根据重力场小波变换的尺度—源深度转换律,进行地面重力异常场多尺度分解,取得了反映中国地壳不同埋藏深度的小波细节,揭示了克拉通地体地壳的密度结构。中国大陆克拉通地体地壳密度结构的总体特点是:地壳密度总体偏高,尤其是在经历了岩浆底垫作用后,下地壳形成了粗大的和高密度的壳根,而造山带的下地壳通常有低密度的山根。现今的克拉通地体有几个演化的阶段,由陆地核演化为一定规模的克拉通地体可能经历漫长的时间。不同演化的阶段地体的物理性质有明显的变化,要赋予大地构造单元以演化阶段的内涵,才能准确理解它们的行为和属性。同时,根据地壳密度结构成像的结果,可以了解克拉通地体的成熟度。中国大陆克拉通地体或者内部地块的地壳密度结构大致可以分为三类。上中下地壳密度都偏高的属于第一类,称为典型的克拉通,塔里木、阿拉善、华北东部、上中扬子等属于第一类。第二类克拉通地体上地壳密度偏低,而下地壳密度偏高,称为弱结晶基底的克拉通,鄂尔多斯和佳木斯属于第二类。第三类上中地壳密度偏高,而下地壳密度偏低,称为无壳根的克拉通残片,柴达木和准噶尔盆地属于此类,它们可能是古克拉通地体的残片。