Post-Collisional Transition from Subduction- to Intraplate-type Magmatism in the Westernmost Mediterranean: Evidence for Continental-Edge Delamination of Subcontinental Lithosphere

Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,Sr–Nd–Pb-isotope analysis and laser ⁴⁰Ar/³⁹Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8Ma), Si–K-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·3–0·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the Si–K-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. Izu–Bonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. Si–K-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways     

多积云参数化方案热带气旋路径集合预报试验

利用MM5模式采用5种积云参数化方案选项研究了2004～2006年共8个热带气旋路径的集合预报。结果表明,热带气旋路径预报对积云参数化方案的选取是敏感的,积云参数化方案之间优劣互补,而选择无积云参数化方案选项对热带气旋路径的预报有积极的贡献。采用3种路径集合预报方案研究的结果表明,集合方法对热带气旋路径预报有明显的改善,其中无积云参数化Grell、Betts—Miller和Kain—Fritsch4种积云参数化方案选项组合的集合预报效果最好,平均路径误差最小。     

桂西下三叠统牙形石序列的新认识

桂西下三叠统为海相沉积,根据其岩性、厚度、化石类型等诸方面的明显差异可以分为两种类型,即作登型和太平型。本文详细研究了桂西两种类型下三叠统的牙形石,认为不同类型的下三叠统具有不同的牙形石序列。在前人工作基础上,对桂西下三叠统牙形石序列予以重新厘定。补充了作登型下三叠统的Neogondolella carinta带;指出作登型下三叠统不宜建立Platyvillosus costatus带;在太平型下三叠统Dienerian阶—Spathian阶中新建了Pachvcladina erromera带,Platyvillosus costatus带,Pachycladina obliqua-Parachirognathus delicatulus带和Neospathodus triangularis带;指出了每一个牙形石带的性质并与作登型下三叠统牙形石序列进行了对比。本文还从事件地层学和生物地层学两个角度,对桂西二叠一三叠系界线提出了新的认识。     

长江口斜压诊断模式三维流场数值模拟

建立了σ坐标系下长江口斜压诊断模式三维流场数学模型,采用k-kl二方程紊流闭合模型求解垂向涡粘系数,计算域内恒定、非均匀盐度场反映了计算域密度斜压效应。验证结果表明,斜压诊断模式的模拟成果较切合实际。模型成功复演了长江口水域由往复流向旋转流过渡的流场特性;潮波从四条汊道传入后在分汊口附近相遇形成多个会潮点,其位置随着径流的大小、潮汐的强弱、汊道的形态等因素而变化。     

搅拌桩复合地基平面模拟的简化方法探讨

水泥土搅拌桩(DMM)在高速公路工程中得到广泛应用,人们也希望能对该复合地基在路堤荷载下的地基的变形深入了解,其数值分析是一种重要的方法。但是,采用三维有限元进行数值模拟在高速公路工程中容易造成数组过大,计算时间并且收敛性比较差,因此把三维问题简化为二维问题是众望所归。在把三维问题简化二维平面问题简化计算时,最重要的是参数的折减。基于水泥土搅拌桩复合地基的特性,结合理论分析提出了一种参数折减的方法。并依据连盐高速公路实测的数据验证了该参数简化方法的可行性。     

金塔绿洲风场与温湿场特征的数值模拟

使用美国NCAR新版MM5V3.6非静力平衡模式,采用三重嵌套的降尺度方法,模拟研究了甘肃河西走廊金塔绿洲风场与温湿场特征。结果表明:由于绿洲的存在,绿洲沙漠系统产生的次级环流对局地环流有一定影响;平流作用将沙漠中的干热空气送向绿洲,绿洲近地层会出现逆温,感热向地表输送;沙漠上由于临近绿洲的水汽平流作用,上层大气湿度比低层更大,这就是逆湿现象。     

Plastic Deformation and Recrystallization of Garnet: A Mechanism to Facilitate Diffusion Creep

Elongate and deformed garnets from Glenelg, NW Scotland, occurwithin a thin shear zone transecting an eclogite body that hasundergone partial retrogression to amphibolite facies at circa700°C. Optical microscopy, back-scattered electron imaging,electron probe microanalysis and electron back-scatter diffractionreveal garnet sub-structures that are developed as a functionof strain. Subgrains with low-angle misorientation boundariesoccur at low strain and garnet orientations are dispersed, aroundrational crystallographic axes, across these boundaries. Towardshigh-strain areas, boundary misorientations increase and thereis a loss of crystallographic control on misorientations, whichtend towards random. In high-strain areas, a polygonal garnetmicrostructure is developed. The garnet orientations are randomlydispersed around the original single-crystal orientation. Somegarnet grains are elongate and Ca-rich garnet occurs on thefaces of elongate grains oriented normal to the foliation. Commonly,the garnet grains are admixed with matrix minerals, and, wherein contact with other phases, garnet is well faceted. We suggestthat individual garnet porphyroclasts record an evolution fromlow-strain conditions, where dislocation creep and recoveryaccommodated deformation, through increasing strain, where dynamicrecrystallization occurred by subgrain rotation, to higheststrains, where recrystallized grains were able to deform bydiffusion creep assisted grain boundary sliding with associatedrotations. KEY WORDS: diffusion creep; EBSD; garnet; plastic deformation; recrystallization     

研究花岗岩类成因类型和幔壳成分比的同位素方法探讨

Sm-Nd法及其参数εNd（O）、εNd（T）、TCHUR^Nd、εDM^Nd及x,是测定花岗岩形成年龄及研究花岗岩成因类型、模式年龄及地幔物质百分比的重要手段,但因其测试费用高等原因,难以广泛使用。作者提出了Rb-Sm法的相应参数εSr（O）、εSr（T）、TUR^Sr、εDM^Sr及μ做为补充,在没有Sm-Nd同位素资料的情况下,Rb-Sr同位素参数基本上可以代替Sm-Nd同位素参数。经对比,求花岗岩中幔源物质百分比时,Rb-Sr法计算值与Sm-Nd法计算值相比,误差一般不超过10％。此外,作者还提出了计算花岗岩山幔源物质百分比的简化式,使该计算更为简便易行。     

Residence, Resorption and Recycling of Zircons in Devils Kitchen Rhyolite, Coso Volcanic Field, California

Zircons from the Devils Kitchen rhyolite in the PleistoceneCoso Volcanic field, California have been analyzed by in situPb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescenceimaging. The zircons yield common-Pb-corrected and disequilibrium-corrected²⁰⁶Pb/²³⁸U ages that predate a previously reported K–Arsanidine age by up to 200 kyr, and the range of ages exhibitedby the zircons is also approximately 200 kyr. Cathodoluminescenceimaging indicates that zircons formed in contrasting environments.Most zircons are euhedral, and a majority of the zircons areweakly zoned, but many also have anhedral, embayed cores, witheuhedral overgrowths and multiple internal surfaces that aretruncated by later crystal zones. Concentrations of U and Thvary by two orders of magnitude within the zircon population,and by 10–20 times between zones within some zircon crystals,indicating that zircons were transferred between contrastingchemical environments. A zircon saturation temperature of 750°Coverlaps within error a previously reported phenocryst equilibrationtemperature of 740 ± 25°C. Textures in zircons indicativeof repeated dissolution and subsequent regrowth are probablycaused by punctuated heating by mafic magma input into rhyolite.The overall span of ages and large variation in U and Th concentrations,combined with calculated zircon saturation temperatures andresorption times, are most compatible with crystallization inmagma bodies that were emplaced piecemeal in the crust at Cosoover 200 kyr prior to eruption, and that were periodically rejuvenatedor melted by subsequent basaltic injections. KEY WORDS: zircon geochronology; residence time; rhyolite; ion microprobe; California