广东南雄盆地晚白垩系古地磁研究

对广东南雄盆地大凤一河南街剖面和古城一坪岭剖面上白垩统南雄组（Ｋ２ｎ）２５个采点、古新统上湖组（Ｅ１ｓ）２个采点的古地磁研究，经系统热退磁及倒转检验，分析出南雄组的特征剩磁方向为Ｄ＝３２．７°，Ｉ＝２７．４°，Ｋ＝１６．１，α９５＝８．４°，Ｎ＝２０；显示晚白垩世时期华南大陆位于１４．５°Ｎ的位置，同时晚白垩世以后华南大陆可能曾发生较大规模的顺时针旋转。     

鲁西下常庄中三叠世磁铁角闪岩的岩石地球化学和Sr-Nd同位素:对岩石成因和构造背景的制约

下常庄中三叠世(242Ma)磁铁角闪岩的岩相学、岩石地球化学和Sr-Nd同位素的研究结果表明,下常庄磁铁角闪岩主要由角闪石(40%~45%)、磁铁矿(35%~45%)和斜长石(10%~15%)及少量的单斜辉石(1%~3%)组成;具低的SiO_2(24.25%~38.40%)、K_2O(0.02%~1.09%)和Na_2O(0.16%~1.68%)含量,较高的MgO(6.86%~11.81%)、Fe_2O_3~T(21.69%~36.62%)和TiO_2(7.37%~13.46%)含量;轻稀土元素(LREE)相对富集、重稀土元素(HREE)相对亏损(∑LREE/∑HREE=7.09~8.97、(La/Yb)_N=10.67~16.62),无明显的Eu异常(δEu=0.83~1.01);富集大离子亲石元素(Rb、Ba、Sr),高场强元素(Nb、Ta、Zr、Hf)不亏损,具Pb和Ti的正异常;全岩~(87)Sr/~(86)Sr初始比值介于0.702 645~0.706 534之间,ε_(Nd)(t)值变化于-7.51~-12.59之间,T_(DM1)=2 374~2 413 Ma。上述结果暗示,下常庄磁铁角闪岩的原岩可能为角闪辉长岩,起源于受下地壳物质混染的华北地块富集岩石圈地幔的部分熔融。下常庄中三叠世磁铁角闪岩原岩的形成时代与大别—苏鲁高压-超高压变质的峰期时间一致,暗示它们可能形成于中生代早期洋壳俯冲-消减向陆-陆碰撞造山转换的构造背景。     

福建上杭盆地发现3.4Ga碎屑锆石

碎屑锆石已经成为研究区域地质演化的一种重要手段。本文对华夏地块东部上杭盆地底部产出的含砾石英砂岩中的碎屑锆石开展了LA-MC-ICP-MS U-Pb测年和Hf同位素测量。结果表明,这些碎屑锆石主要是岩浆锆石,锆石年龄变化于328~3 403 Ma,至少记录了10期的岩浆活动事件,表明该地区曾经历了长期复杂的地质演化历史。与华夏地块其他地区的碎屑锆石或捕获锆石一样,上杭盆地这些碎屑锆石年龄数据记录下了诸如晚太古代—早元古代地壳生长事件、哥伦比亚超大陆和Rodinia超大陆的汇聚与裂解、冈瓦纳大陆的聚合等全球主要地质事件,以及具有华夏地块特征的早古生代强烈的板内造山运动(武夷—云开运动)。最高峰值年龄出现在450 Ma,表明研究区附近最为强烈的事件是加里东运动(当地称作武夷—云开运动,或者广西运动)。Hf同位素数据表明,?Hf(t)=-27.2~19.8,绝大多数εHf(t)值小于0,两阶段亏损地幔Hf模式年龄(tDM2)介于1 124~3 979 Ma,峰值1 800~1 900 Ma,表明其早期岩浆活动主要与大规模的新生地壳有关,而晚期主要表现为对早期形成地壳的大规模改造,古老物质的再循环,很少有新生地壳增生。     

A New Species of Amsassia from the Ordovician of Korea and South China: Paleobiological and Paleogeographical Significance

A new species of the probable calcareous alga Amsassia, A. koreanensis, is recognized from the Duwibong Formation (Middle Ordovician, Darriwilian) of the Taebaeksan Basin in mid-eastern Korea. This is the first report of the genus from the Korean Peninsula, expanding its geographical range to the eastern Sino-Korean Block. The new species also occurs in the Xiazhen Formation (Upper Ordovician, Katian) at Zhuzhai in the South China Block. Amsassia koreanensis is the smallest species of this modular genus, having a maximum module diameter of 0.28 mm. Module increase is by bipartite, tripartite and quadripartite types of longitudinal axial fission, but unlike other species of the genus, quadripartite fission is common. The types of fission are comparable to those in some Tetradiida (now Prismostylales, florideophycean rhodophyte algae), although the processes of fission are different. The distribution of A. koreanensis further strengthens the biogeographical connection between the Sino-Korean and South China blocks, suggesting that these two paleocontinents were located closer together during the Middle to Late Ordovician than previously speculated.     

Tectonic Framework of Late Paleozoic Intrusions in Xingxingxia: Implications for Final Closure of South Tianshan Ocean in East Tianshan

This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during(304±3)–(278±3) Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during(268±5)–(259.9±2.6) Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities(i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian) and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of(268±5)–(259.9±2.6) Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around(268±5)–(260±3) Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.     

扬子地块西南缘前寒武纪铜多金属矿床及成矿系列

文章在分析扬子地块西南缘与前寒武纪铜多金属矿床成矿特征及与有关的重大地质事件的基础上,研究了区域成矿作用与重大地质事件的响应,划分了区域金属成矿系列,较详细地论述了主要成矿系列的组成及成矿规律,总结了扬子地块西南缘前寒武纪铜多金属矿床及成矿系列。认为本区成矿作用属于多期次成矿,至少可以分为前期预富集作用及两次大的成矿作用(古元古代成矿作用和中元古代成矿作用),以及若干个小的成矿作用;1.7Ga—1.5Ga、1.1Ga—1.0Ga、中新生代是3次规模、强度较大的铜矿成矿作用,形成了研究区内规模较大的诸多铜多金属矿床。     

华北地块中部早白垩世富碱侵入岩的成因:锆石U-Pb年代学和Sr-Nd-Hf同位素制约

本文对华北地块中部早白垩世富碱侵入岩:山西的二峰山霓辉二长岩、塔儿山二长闪长斑岩、狐偃山石英二长岩、万荣石英二长闪长岩和河南的黄龙垴霓辉正长岩、水冶霓霞正长岩、东冶角闪闪长岩进行了岩石地球化学和Sr-Nd同位素分析以及4个侵入岩中锆石LA-ICP-MS U-Pb年代学和Hf同位素的测试。结果表明,二峰山、狐偃山、万荣和黄龙垴侵入岩中锆石有两种,多数为自形-半自形晶、内部结构均匀、发育震荡环带和条纹状吸收以及具有相对高Th/U比值(0.13~4.40)的岩浆锆石,其206Pb/238U加权平均年龄分别为130±1Ma、130±3Ma、127±1Ma和127±2Ma,即富碱侵入岩形成于早白垩世;少量为继承锆石,发育核-边结构,核部呈浑圆状和弱的震荡环带,给出了三叠纪(207~226Ma)和古元古代-新太古代(1791Ma、1979Ma、2088Ma、2370Ma和2559Ma)的年龄;岩石的SiO 2=55.33%~65.00%、MgO=0.21%~7.66%、Mg#=19~62、(Na2O+K2O)=5.89%~14.38%,属于弱碱性-过碱性岩石;富集大离子亲石元素(Rb、Ba、Sr)和轻稀土元素,亏损高场强元素(Nb、Ta、Ti)和重稀土元素,具有明显的Pb正异常和弱的Eu异常;其87Sr/86Sr初始值多数介于0.7049~0.7062之间,εNd(t)值变化于-20.1~-13.3之间,岩浆锆石的Hf同位素组成为εHf(t)=-28.9~-17.4。上述结果结合符山高镁闪长岩及其中地幔橄榄岩包体的研究暗示,华北地块中部早白垩世富碱侵入岩起源于受拆沉陆壳物质熔融的熔体改造的富集岩石圈地幔物质的部分熔融,其形成于古太平洋板块西向俯冲动力学背景下的岩石圈伸展环境。     

Influence of Stochastic Discontinuity Network Parameters on the Formation of Removable Blocks in Rock Slopes

Summary  We study the effects of discontinuity network parameters on the formation of removable wedges in rock slopes. Discontinuities are simulated using the Poisson disk model, and removable wedges are identified using block theory. The formation of removable wedges of different sizes is assumed to follow a Poisson process. Poisson regression and Monte Carlo simulation are then used to identify statistically relevant parameters of the model, and to study the effects that variations in their values have on formation of removable blocks. The sensitivity of the results as a function of the mean orientations of the discontinuity sets forming the blocks is also studied by means of a parametric study. The volumetric intensity of discontinuities in the rock mass is found to have a significant impact on the computed estimates of removable block formation. As predicted by theory, our results indicate that, everything else being equal, the expected rate of formation of removable wedges is proportional to the square of the intensity measure. Estimates are also sensitive to changes in discontinuity size, especially in cases in which discontinuities are smaller than one to two times the height of the slope. The interaction between the mean size of discontinuities and the coefficient of variation of discontinuity sizes is found to be significant as well. Finally, results of our sensitivity analysis suggest that the orientation of discontinuity sets significantly affects the rate of formation of removable blocks in rock slopes. Author’s address: Dr. Rafael Jimenez-Rodriguez, ETS Ing. de Caminos, Canales y Puertos. Universidad Politecnica de Madrid, Spain     

川西龙门山彭灌杂岩地球化学特征:岩石成因与构造意义

对位于扬子地块西缘龙门山逆冲推覆构造带中的彭灌杂岩进行了岩石地球化学研究,重点讨论了岩石成因、成岩物质来源及其构造意义。彭灌杂岩为一新元古代侵入的杂岩体,主要由中酸性侵入岩组成,其地球化学特征表现为SiO2含量变化范围较大(57.70%~71.90%),Na2O>K2O(K2O/Na2O=0.31~0.85),里特曼指数δ=1.33~2.28,A/CNK=0.93~1.08,彭灌杂岩主体属于准铝质到弱过铝质的钙碱性花岗岩类;稀土总量不高(∑REE=76.25×10-6~139.80×10-6),轻重稀土分异程度差别较大(LaN/YbN=3.86~35.23),弱负到正Eu异常(δEu=0.70~1.86);富集大离子亲石元素(LILE)Rb、Sr、Ba、K等,亏损高场强元素(HFSE)Nb、Ta、P和Y。与正常壳源钙碱性花岗岩相比,彭灌杂岩显示出更高的Mg#值(多数>45)和Sr/Y比值(多数20.03~111.34)。通过与其毗邻的埃达克质雪隆包杂岩的比较,表明彭灌杂岩可能是底侵的幔源岩浆与下地壳熔融产物混合的结果。结合其它研究成果,认为扬子西缘新元古代大面积分布的岩浆岩形成于俯冲汇聚和火山弧构造环境。     

The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes

The Abbotsford Landslide of 8 August 1979 occurred in an urban area of Dunedin, New Zealand, causing much damage to houses and urban infrastructure. Rapid failure occurred after weeks of preliminary movements, resulting in the formation of a approximately 5 million m³ block slide. It caused the loss of 69 houses, with an overall cost of about NZ $10–13 million. After several months of investigations, a commission of inquiry found that unfavorable geology (weak clay layers in a 7°-dip slope) was the underlying cause of the landslide. An old sand quarry at the toe of the slope and a leaking water main above the slide area were found to be man-made factors that contributed to the failure. Slope stability analysis showed that after sand excavation (approximately 300,000 m³), the water table had to rise 0.3 m less for failure to occur. Because the quarry closed 10 years before the landslide occurred, it is concluded that a long-term rise in groundwater levels because of the increased rainfall over the previous decade and leakage from the water main controlled the timing of the failure and, in this sense, are considered to have triggered the landslide.