Geochemistry of an island-arc plutonic suite: Wadi Dabr intrusive complex, Eastern Desert, Egypt     

Fawzy F. Abu El-Ela 《Journal of African Earth Sciences》1997,24(4):473-496

The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-horn blend e gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from An₇₅ to An₃₄ for the gabbroic varieties, except for the metagabbro which has An _11–18. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2–5 kb and 885–716°C.The intrusive rocks cover an extensive silica range (47.86–72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO₂, Al₂O₃, FeO^*, MgP, CaO, P₂O₅, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources.The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)_N=2.67−3.91] and depletion of heavy REE ((Tb/Yb)_N=1.42−1.47], which suggest the parent magma was of relatively primitive mantle source.The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb---Sr, K---Na---Ca, and to volcanic-arc granites in terms of Rb---and Nb---Y.The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes.  相似文献   

Redetermination of the transformation enthalpies of the xanthoconite–proustite, pyrostilpnite–pyrargyrite and trechmannite–smithite phase transitions     

B. Meyer  F. Scholz 《Physics and Chemistry of Minerals》1997,24(1):50-52

 We report here refined values of the transformation enthalpy for xanthoconite to proustite (54.81 kJ/mol) and pyrostilpnite to pyrargyrite (40.32 kJ/mol). Additionally, the enthalpy for the transformation of trechmannite to smithite (5.82 kJ/mol) has been determined. The refinement was possible by taking into account a previously unknown dependence of electrochemical signals on the amount of substance undergoing the reduction process. Received December 21, 1995/Revised, accepted June 29, 1996  相似文献   

ELNES spectroscopy of mixed Si coordination minerals     

B. Poe  F. Seifert  T. Sharp  Z. Wu 《Physics and Chemistry of Minerals》1997,24(7):477-487

 Si K- and L-edge ELNES spectroscopy and multiple-scattering (MS) calculations are used to examine mixed Si coordination compounds varying in Si^VI:Si^IV ratio. As in previous studies, the edges are influenced mainly by silicon coordination (tetrahedral vs. octahedral), as supported by the MS calculations. We demonstrate two methods semi-quantitatively to extract the value of Si^VI/(Si^VI+Si^IV): (1) A linear relationship between the L_2,3-L₁ splitting and Si^VI/(Si^VI+Si^IV) is observed, (2) a fitting method based on the coaddition of reference tetrahedral and octahedral Si spectra is applied to both Si K- and L-edge ELNES spectra. Received: February 10, 1997 / Revised, accepted: May 23, 1997  相似文献   

Distribution of isomorphous cations within octahedral sheets in montmorillonite from Camp-Bertaux     

F. Muller  G. Besson  A. Manceau  V.-A. Drits 《Physics and Chemistry of Minerals》1997,24(3):159-166

 Ni-saturated montmorillonite from Camp-Bertaux heated at different temperatures has been studied by X-ray powder diffraction, X-ray absorption (EXAFS) and vibration IR spectroscopy. Analysis of the experimental data has shown that heating of samples at temperatures higher than 150° C was accompanied by migration of Ni cations into vacant cis-octahedra of 2:1 layers. In the octahedral sheet the Ni cation has two “heavy” (Fe) and four “light” (Al and Mg) nearest octahedral cations. A model for the octahedral cation distribution in Camp-Bertaux montmorillonite was proposed in which Fe and Mg octahedral cations are segregated in small clusters. Received July 7, 1996 / Revised, accepted August 23, 1996  相似文献   

中国汽车工业──改革开放后的重整与国际化   总被引：4，自引：0，他引：4  

薛凤旋  刘卫东 《地理研究》1997,16(3):1-11

大规模的重整和广泛的国际化是改革开放后中国汽车工业发展的两个主要特征。作者首先从企业组织结构、生产能力与品种结构、规模经济和空间分布等四个方面分析了近二十年来中国汽车工业重整的效果及所形成的行业特征;其次根据利用的外资和技术的来源以及涉及的技术形态分析了其国际化的程度及在世界汽车工业中的位置;最后总结了中国汽车工业面临的若干问题,并提出在散乱的格局和跨国公司竞争的双重压力下,中国汽车工业正处于成长的关键时期。  相似文献   

Diatom assemblages from Costa Rican lakes: an initial ecological assessment     

Kurt A. Haberyan  Sally P. Horn  Brian F. Cumming 《Journal of Paleolimnology》1997,17(3):263-274

We compared the distributions of 59 diatom species in surface sediments of 25 Costa Rican lakes with 21 environmental variables using canonical correspondence analysis (CCA). The distribution of taxa was related to the chemical and physical characteristics of the lakes. The most influential chemical variables were cation concentrations (especially magnesium) and related variables such as water hardness, pH, and temperature. Lake area and lake depth were among the most important physical variables.A number of taxa were identified as potential environmental indicators. The diatoms Brachysira serians var. brachysira and Frustulia rhomboides seem to be associated with low values of alkalinity, hardness, Ca, Mg, and SiO2. Cymbella minuta var. silesiaca is associated with low to moderate values of alkalinity, hardness, Ca, and Mg. Nitzschia cf. amphibia may be an indicator of moderate-to-high concentrations of Mg. Pinnularia braunii var. amphicephala seems to prefer low values of hardness, Ca, Mg, and SiO2.In many closed lakes, these environmental variables (Mg/Ca/hardness/alkalinity) increase with effective evaporation. Consequently, these diatoms may be indirectly tracking P:E ratios. Results from this initial, small data set indicate the potential of diatoms for inferring lake paleochemistry, and perhaps P:E ratios, in Costa Rica.  相似文献   

Diatoms in sediments of perennially ice-covered Lake Hoare, and implications for interpreting lake history in the McMurdo Dry Valleys of Antarctica     

S. A. Spaulding  D. M. McKnight  E. F. Stoermer  P. T. Doran 《Journal of Paleolimnology》1997,17(4):403-420

Diatom assemblages in surficial sediments, sediment cores, sediment traps, and inflowing streams of perennially ice-covered Lake Hoare, South Victorialand, Antarctica were examined to determine the distribution of diatom taxa, and to ascertain if diatom species composition has changed over time. Lake Hoare is a closed-basin lake with an area of 1.8 km², maximum depth of 34 m, and mean depth of 14 m, although lake level has been rising at a rate of 0.09 m yr^-1 in recent decades. The lake has an unusual regime of sediment deposition: coarse grained sediments accumulate on the ice surface and are deposited episodically on the lake bottom. Benthic microbial mats are covered in situ by the coarse episodic deposits, and the new surfaces are recolonized. Ice cover prevents wind-induced mixing, creating the unique depositional environment in which sediment cores record the history of a particular site, rather than a lake-wide integration. Shallow-water (<1 m) diatom assemblages (Stauroneis anceps, Navicula molesta, Diadesmis contenta var. parallela, Navicula peraustralis) were distinct from mid-depth (4–16 m) assemblages (Diadesmis contenta, Luticola muticopsis fo. reducta, Stauroneis anceps, Diadesmis contenta var. parallela, Luticola murrayi) and deep-water (26–31 m) assemblages (Luticola murrayi, Luticola muticopsis fo. reducta, Navicula molesta). Analysis of a sediment core (30 cm long, from 11 m water depth) from Lake Hoare revealed two abrupt changes in diatom assemblages. The upper section of the sediment core contained the greatest biomass of benthic microbial mat, as well as the greatest total abundance and diversity of diatoms. Relative abundances of diatoms in this section are similar to the surficial samples from mid-depths. An intermediate zone contained less organic material and lower densities of diatoms. The bottom section of core contained the least amount of microbial mat and organic material, and the lowest density of diatoms. The dominant process influencing species composition and abundance of diatom assemblages in the benthic microbial mats is episodic deposition of coarse sediment from the ice surface.  相似文献   

A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district   总被引：1，自引：0，他引：1  

K. D. van der Post  F. Oldfield  E. Y. Haworth  P. R. J. Crooks  P. G. Appleby 《Journal of Paleolimnology》1997,18(2):103-120

Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing.  相似文献   

An auto-biographical approach to understanding migration: the case of Hong Kong emigrants   总被引：1，自引：0，他引：1  

A M Findlay  F L N Li 《Area》1997,29(1):34-44

Summary This paper seeks to make a methodological contribution by evaluating the autobiographical approach and its application to migration research. The migration act is seen as a manifestation of an individual's identity. This is moulded by many social influences over an individual's lifecourse. The paper explores the realm of practical consciousness (Giddens 1984) using a biographical approach in an attempt to identify the values underpinning the intentions of professional emigrants from Hong Kong.  相似文献   

Secondary ferrimagnetic minerals in Welsh soils: a comparison of mineral magnetic detection methods and implications for mineral formation   总被引：20，自引：0，他引：20  

J. A. Dearing  P. M. Bird  R. J. L. Dann  S. F. Benjamin 《Geophysical Journal International》1997,130(3):727-736

A study of mineral magnetic parameters was carried out on a Late Pleistocene and Holocene sedimentary sequence (of nearly 18 m) from Lake Bledowo (central Poland). Sediments of Lake Bledowo have already been analysed for bulk sediment mineralogy and biogenic materials. The mineral magnetic stratigraphy confirms the major changes in palaeo-environmental conditions that have been deduced from other methods. The most important mineral magnetic change results from the authigenetic formation of ferrimagnetic greigite, Fe₃S₄, during the beginning of lacustrine conditions (± 12 000 yr BP). Our data also indicate a detrital origin of overlying ferrimagnetic iron oxides. It is suggested that they originate from brown soils developed on the boulder clay constituting the west side of the lake shore. Variations of ferrimagnetic iron oxide size are related to the early diagenetic processes in the sediment. Larger particles are present in periods with early diagenesis of organic matter in anoxic conditions. This indicates the dissolution of fine magnetic particles by iron-oxide-reducing bacteria and results in homogeneous magnetic grain sizes, despite their origin from soils, characterized by a multimodal grain-size distribution.  相似文献