Vibrations of a rectangular orthotropic plate with a free edge: a comparison of analytical and numerical results

Transverse vibrations of a rectangular orthotropic plate with a free edge while the others are supported according to four different combinations of boundary conditions are considered in the present paper. The presence of a concentrated mass is also taken into account. In view of the difficulty of satisfying exactly the natural boundary conditions at the free edge, the fundamental frequency of vibration is determined by means of the optimized Rayleigh-Ritz method and a pseudo-Fourier expansion which contains, in its argument, the optimization parameters γ_i. The results are in excellent agreement with those obtained by means of a finite element algorithmic procedure.     

龙泉关韧性剪切带的新认识

龙泉关韧性剪切带五台山花岗岩－绿岩地体与阜平高级变质地体的接触部位，它不仅改造了“阜平群”而且也改造了“五台群”宏观构造，显微组构和矿物组合研究表明，龙泉关韧性剪切带经历了两期变形作用，早期发生于恒山古陆块－车厂弧地质体－阜平古陆块碰撞过程中，形成低角度逆冲型韧性剪切带，使地壳显著增厚，晚期发生于增厚同的纵向伸展过程中，使地壳得以均衡发展，形成低角度伸展型韧性剪切带，龙泉关韧性剪切带的变形特征反映     

同位素地球化学填图与化学地球动力学在东秦岭造山带研究中的应用

总结了应用同位素地球化学填图和化学地球动力学研究东秦岭造山带的初步经验，并以较成功的实例来说明，内容包括：（１）华北和扬子克拉通幔源和壳源岩石化学和Ｎｄ、Ｐｂ同位素组成及壳幔演化差异的确定；（２）南秦岭前寒武纪基底应归属于扬子陆块构造－地球化学省的地球化学论证；（３）关于东秦岭蛇绿岩铅同位素的Ｄｕｐａｌ型特征及其同三江地区（属古特提斯范围）蛇绿岩的相似性的揭示；（４）北秦岭元古宙基底可能为古洋岛型微陆块的地球化学证据；（５）东秦岭新元古代和早古生代洋壳俯冲消减及聚汇带壳－幔再循环的地球化学证据；（６）关于陆－陆碰撞过程中杨子陆块边缘（南秦岭）俯冲于华北陆块边缘（北秦岭）之下，从碰撞型花岗质岩浆源区地球化学研究获得的直接证据。这些初步成果说明同位素填图与化学地球动力学在造山带研究中是具有重要前景的技术途径。     

月球起源研究进展

月球是地球唯一的天然卫星,早期学者认为地月系统是普遍存在的行星一卫星系统的一员,但是月球又有许多特征,如质量异常大,挥发分和Fe元素亏损等,传统理论难以解释这些特征.因此,针对月球的起源提出了四种学说:捕获理论,共增生理论,分裂理论和大碰撞假说.目前对大碰撞假说研究较多,但研究者们始终无法较好解释地月系统氧同位素的高度一致等特征.本文试探性提出新的月球起源分裂模型,能较好的解释某些月球特征,引起研究者们对月球起源新的思考.     

大陆板内构造环境的微量元素判别

通过对国内外大陆板内构造环境玄武岩的研究,提出了大陆板内不同构造环境玄武岩的微量元素判别方法及判别标志,即大陆板内玄武岩的Th/Nb>0.11,Nb/Zr>0.04,其中,大陆裂谷区玄武岩的Th/Nb=0.11~0.27,大陆拉张带玄武岩的Th/Nb>0.27(一般0.27~0.67),陆-陆碰撞带玄武岩的Th/Nb>0.67。     

Detrital zircon U–Pb ages along the Yarlung-Tsangpo suture zone, Tibet: Implications for oblique convergence and collision between India and Asia

Age-dating of detrital zircons from 22 samples collected along, and adjacent to, the Yarlung-Tsangpo suture zone, southern Tibet provides distinctive age-spectra that characterize important tectonostratigraphic units. Comparisons with data from Nepal, northern India and the Lhasa and Qiangtang terranes of central Tibet constrain possible sources of sediment, and the history of tectonic interactions.Sedimentary rocks in the Cretaceous–Paleogene Xigaze terrane exhibit strong Mesozoic detrital zircon peaks (120 and 170 Ma) together with considerable older inheritance in conglomeratic units. This forearc basin succession developed in association with a continental volcanic arc hinterland in response to Neotethyan subduction under the southern edge of the Eurasia. Conspicuous sediment/source hinterland mismatches suggest that plate convergence along this continental margin was oblique during the Late Cretaceous. The forearc region may have been translated > 500 km dextrally from an original location nearer to Myanmar.Tethyan Himalayan sediments on the other side of the Yarlung-Tsangpo suture zone reveal similar older inheritance and although Cretaceous sediments formed 1000s of km and across at least one plate boundary from those in the Xigaze terrane they too contain an appreciable mid-Early Cretaceous (123 Ma) component. In this case it is attributed to volcanism associated with Gondwana breakup.Sedimentary overlap assemblages reveal interactions between colliding terranes. Paleocene Liuqu conglomerates contain a cryptic record of Late Jurassic and Cretaceous rock units that appear to have foundered during a Paleocene collision event prior the main India–Asia collision. Detrital zircons as young as 37 Ma from the upper Oligocene post-collisional Gangrinboche conglomerates indicate that subduction-related convergent margin magmatism continued through until at least Middle and probably Late Eocene along the southern margin of Eurasia (Lhasa terrane).Although the ages of detrital zircons in some units appear compatible with more than one potential source with care other geological relationships can be used to further constrain some linkages and eliminate others. The results document various ocean closure and collision events and when combined with other geological information this new dataset permits a more refined understanding of the time–space evolution of the Cenozoic India–Asia collision system.     

Lithospheric dripping in a soft collision zone: Insights from late Paleozoic magmatism suites of the eastern Central Asian Orogenic Belt

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogen on the globe. The suture branches to the east to form the northern Hegenshan–Heihe Suture and the southern Solonker–Changchun Suture. The Hegenshan–Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction. Here we report results from an integrated study of the petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of the Early Carboniferous–Early Permian magmatic suite in the Hailar Basin of the Xing’an–Erguna Block. The Early Carboniferous igneous rocks are represented by 356–349 Ma andesitic tuffs, exhibiting typical subduction-related features, such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. These features, together with the relatively depleted Sr–Nd–Hf isotopic compositions, constant Nb/Y values, but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The Late Carboniferous–Early Permian magmatic suite (317–295 Ma) is characterized by high Sr contents (313–1080 ppm) and low Y contents (5–13 ppm), and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks. The calc-alkaline adakitic rocks have higher values of Sr/Y, (Sm/Yb)_{source normalized}, and Mg#, and lower values of Y, Yb_{source normalized}, and K₂O/Na₂O than the high-K calc-alkaline adakitic rocks, which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust. Based on our new data, in conjunction with those in previous studies, we conclude that the tectonic evolution of the Hegenshan–Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean, latest Early Carboniferous soft collision between the Xing’an–Erguna and Songliao blocks, and Late Carboniferous–Early Permian post-collisional extension. We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping, which results from combined effects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.     

Simulation on sand grain/bed collision mechanism: Cascade collision and ejection (1)

Ejection of sand grains from a sand surface is assumed to result from cascade collision caused by the impact of a saltating particle. Allowing for only two-body cascade collision and introducing new quantities such as the cross-section for sand grain–grain collisions and sand surface binding energy, the theoretical model for the cascade collision of ion particles is applied to simulate sand grain/bed collision processes. The results of simulations indicate that the collision cascade events caused by impacting particles can eventually lead to the ejection of grains taking place from the sand surface. The number of ejected grains at any surface point is found to be proportional to the fractional energy deposited at that point and inversely proportional to the sand surface binding energy. This cascade collision model also confirms that the peak in the spatial distribution of ejected grains does not appear at the impact point but is located downwind, and that the speed distribution of ejected grains at a fixed angle exhibits a peak having a value directly related to the ratio of surface binding energy to the mass of the ejected grain. The angular distribution at a certain ejection speed also exhibits a maximum at an ejection angle near 90°. The model also offers a new interpretation for the observed variability in the number of ejected grains under constant impact velocity; this variability is directly related to the wide distribution of the energy deposition in the surface layer of the sand bed.     

华北克拉通基底主要构造单元变质作用演化及其若干问题讨论

华北克拉通基底可分为三个太古宙微陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个早元古宙活动带(孔兹岩带、华北中部带和胶-辽-吉带)。这些构造单元具有不同的变质作用时间和P-T 演化特征。东部陆块和阴山陆块晚太古宙基底岩系的变质作用发生在~2.5Ga,变质演化以等压冷却(IBC)逆时针P-T 轨迹为特征,反映变质作用的成因与大规模地幔岩浆底侵有关。孔兹岩带主期变质作用发生在~1.95Ga,变质演化以近等温减压(ITD)顺时针P-T 轨迹为特征,反映阴山陆块与鄂尔多斯陆块碰撞形成西部陆块的热构造过程。华北中部带变质作用发生在~1.85Ga,变质演化同样以近等温减压(ITD)顺时针P-T 轨迹为特征,反映了西部陆块和东部陆块最终碰撞形成统一的华北克拉通基底的构造过程。早元古宙胶-辽-吉带变质作用表现‘双变质带’特征: 西北带的北辽河群、老岭群和粉子山群的变质作用以中压顺时针P-T 轨迹为特征,而东南带的南辽河群、吉安群和荆山群的变质作用以低压逆时针P-T 演化为特征。华北克拉通基底变质作用演化地质图能更好地反映上述不同构造单元的变质作用演化特征。尽管岩浆弧、大陆裂谷和地幔柱模式都能解释东部陆块晚太古宙基底变质作用所具有的近等压冷却(IBC)逆时针P-T 演化特征,地幔柱模式能够更合理解释东部陆块所存在的宽达800 千米而时代近于相同的晚太古代火成岩带、大量科马提质超镁铁质岩石和双峰式火山岩、广泛发育的穹窿构造等。华北克拉通变质基底中具有石榴石-单斜辉石-斜长石-石英组合的高压基性麻粒岩和具有蓝晶石-钾长石组合的高压泥质麻粒岩的出露只局限在早元古宙华北中部带的北段和胶-辽-吉带的南端; 这些高压麻粒岩形成在俯冲和陆-陆碰撞的构造环境中。西部陆块孔兹岩带含假蓝宝石麻粒岩是碰撞后(~1.92Ga)拉伸引发地幔岩浆底侵导致局部地带发生超高温(UHT)变质作用的产物。     

东天山东戈壁钼矿床辉钼矿Re-Os年龄及印支期成矿事件

东戈壁超大型钼矿床位于新疆境内的东天山觉罗塔格成矿带.8件辉钼矿样品Re-Os同位素年龄介于228.7±2.7Ma ～ 241.7±0.9Ma,等时线年龄为231.9±6.5Ma(95％置信度,MSWD =0.71),加权平均年龄为238.5±3.7Ma(95％置信度,MSWD=5.8).其中,最小的辉钼矿Re-Os年龄与矿区内斑状花岗岩年龄(227.6±1.3Ma)一致,表明成岩和成矿作用发生在三叠纪.7件辉钼矿样品Re含量为26.51×10-6～91.34×10-6,指示成矿物质主要来自古生代增生作用形成的不成熟大陆壳.已有成矿年龄显示了印支期成矿事件在东天山地区非常显著,发生于大陆碰撞造山体制.