Buckling of ring stiffened domes under external hydrostatic pressure

The paper reports on the buckling of three ring-stiffened prolate domes under external hydrostatic pressure. The study was partly theoretical and partly experimental, where in the case of the latter, the finite element was used. Comparison between experiment and theory was good. The effect of ring stiffening the domes was to increase their buckling resistances by factors varying from 4.43 to 5.72.     

藏南北喜马拉雅穹窿高Sr/Y二云母花岗岩中磷灰石地球化学特征及其岩石学意义

北喜马拉雅穹窿最东部的雅拉香波穹窿发育两套高Sr/Y比值二云母花岗岩,分别形成于始新世(约43～44Ma)和中新世(约18～20Ma)。虽然在Sr-Nd同位素系统特征和形成时代上存在明显差异之外,但无论在矿物组成,还是在元素地球化学(高CaO,高Na/K和Sr/Y比值等)特征上,这两套花岗岩都存在高度相似性。为探讨在这两套花岗质岩浆形成和演化过程中,磷灰石的地球化学行为特征,应用LA-ICP-MS分析了磷灰石的微量元素地球化学组成。测试结果揭示(1)在这两套花岗岩中,微量元素在磷灰石与熔体之间的配分行为相似;(2)始新世二云母花岗岩中包含残留的磷灰石;(3)在同一件样品中,在磷灰石颗粒之间,存在一定程度的微量元素地球化学特征的不均一性,反映了局部熔体地球化学特征;(4)在花岗质岩浆演化过程中,富钙长石组分的斜长石的分离结晶作用,不仅导致熔体的Ca和Sr含量降低,Na含量和Eu负异常幅度增大,同时导致熔体的LREE含量升高。     

藏南马拉山高钙二云母花岗岩的年代学特征及其形成机制

马拉山片麻岩穹窿位于特提斯喜马拉雅带内,由马拉山二云母花岗岩、错布二云母花岗岩和派枯错复合淡色花岗岩组成。马拉山二云母花岗岩东西展布约10km,锆石U-Pb分析表明,马拉山二云母花岗岩的结晶时间较长,从17.6Ma到16.9Ma,或者至少是两次深熔作用的产物,分别发生在17.6Ma和16.9Ma。全岩主量元素、微量元素和Sr、Nd、Hf同位素分析表明马拉山二云母花岗岩是一个较均一岩体,具有以下特征:(1)高SiO2,Al2O3和相对较高的CaO(1.2%～2.0%);(2)较高的Sr,较低的Rb和Rb/Sr比值(<1.3),且随着Ba浓度的增加,Rb/Sr比值保持不变;(3)高度变化的Zr/Hf比值(25.9～39.9);(4)富集轻稀土,亏损重稀土,几乎无或弱的负Eu异常;(5)较一致的Sr和Nd同位素组成;(6)锆石岩浆增生边和继承性锆石的Hf同位素比值高度变化,εHf(t)分别为-20.4～-8.0和-27.2～-9.5。这些特征暗示马拉山二云母花岗岩是变泥质岩在较高温压条件下水致白云母部分熔融的产物,与藏南裂谷系的东西向伸展作用密切相关。     

Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast)

The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic “weak type” orogen.     

Vibration of a thin-walled prolate dome under external water pressure

The paper reports on a theoretical and an experimental investigation carried out on a thin-walled hemi-ellipsoidal prolate dome in air and also under external water pressure. The investigation found that there was good correlation between experiment and theory. The theoretical investigation was carried out using the finite element analysis to model both the structure and the fluid. The theoretical investigation used two different programs, one of which was the giant computer program ANSYS and the other was an in-house program developed by Ross. For the shell structure, the ANSYS program used 2 different doubly curved thin-walled shell elements, while the in-house program used a simpler axisymmetric thin-walled shell element. This axisymmetric element allowed a sinusoidal variation of the displacements in the circumferential direction, thus, decreasing preparation and computational time. Agreement between the 3 different finite elements was found to be good. This was found particularly encouraging for the in-house software, as it only took a few hours to set up the computer model, and a few seconds to analyse the vessel, whereas the ANSYS software took several weeks to set up the computer model, and several minutes to analyse the shell dome. The ANSYS software, however, did have the advantage in producing excellent graphical displays in both the pre-processing and post-processing modes.