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891.
浑善达克沙地的形成时代与成因初步研究 总被引:30,自引:8,他引:22
根据较广泛的含风成砂地层的野外调查与有关室内样品分析结果,浑善达克沙地至少形成于晚第三纪。当时受暖干亚热带动力高压控制和较弱东亚季风影响,出现温暖干旱荒漠、半干旱草原及森林草原之间的环境变化,形成亚热带红色季风性沙漠沉积。第四纪,受东亚季风及其变迁影响,环境在温带荒漠草原至森林草原之间波动变化,出现一系列沙丘活化、沙漠扩展与沙丘固定、沙漠收缩的波动过程,形成温带黄色季风型沙漠沉积。 相似文献
892.
893.
Carlo Laj Herv Guillou Nadia Szeremeta Robert Coe 《Earth and Planetary Science Letters》1999,170(4):191-376
We have sampled a sequence of 107 lava flows in the Waianae series on Oahu, Hawaii, in two separate stretches. The first (51 flows) at Kaena Point extends from sea level to an altitude of 190 m, and the second (56 flows) follows the Satellite Tracking Station road from 218 m to 360 m above sea level. Thermal and af demagnetization yield very similar results, and reveal only normal polarities. K/Ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the Upper Mammoth and the Lower Kaena polarity transitions (3.22–3.11 Ma). Some of the flows are serially correlated. Filtering does not, however, drastically affect the final results. Inclinations are shallower than those expected from a centered dipole field. Although large (13°), the inclination anomaly is consistent with results from other sites at the same latitude. The scatter of the VGPs about the geographic pole is consistent with available data from other regions at similar latitudes. The scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation. Therefore, these results do not support the hypothesis of a Pacific dipole window. 相似文献
894.
A. Bollhfer N. Frank S. Rohloff A. Mangini J.C. Scholten 《Earth and Planetary Science Letters》1999,170(4):91-415
Two diagenetic manganese nodules from the Peru Basin were investigated by thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium. The TIMS concentrations for nodule 62KD (63KG) vary as follows: 0.12–1.01 ppb (0.06–0.59) 230Th, 0.51–1.98 ppm (0.43–1.40) 232Th, 0.13–0.80 ppb (0.09–0.49) 234U, and 1.95–13.47 ppm (1.66–8.24) 238U. Both nodules have average growth rates of 110 mm per million years. However, from the variations of excess 230Th with depth we estimate partial accumulation rates which range from 50 to 400 mm per million years. The δ234U dating method cannot be applied due to remobilization of U from the sediment and subsequent incorporation into the nodules' crystal lattice, reflected by decay corrected δ234U values far above the ocean water value. Sections of fast nodule growth are related to those layers having high Mn/Fe ratios (up to 200) and higher densities. As a possible explanation we develop a scenario that describes similar glacial/interglacial trends in both nodules as a record of regional changes of sediment and/or deep water chemistry. 相似文献
895.
达索普冰川海拔7100m处粒雪中空气的封闭 总被引:3,自引:1,他引:2
The air-bubble formation has been studied experimentally in the Dasuopu ice core from the northern slope of Mount Xixabangma. It was found that air-channel volume decreases gradually, but the firn remains permeable down to the depth above the transition of firn to ice, i. e., about the depth of the air-channels closing off to be isolated bubbles. Apparently, there exists an age difference between air trapped in bubbles and surrounding ice. Only when the air channels close off completely, the age of the air increases in the same rate as the surrounding ice. Air-bubbles formed in different depths at different sites depending on accumulation rate and mean annual temperature, but approximately at a mean density of 0.79~0.83 Mg·m -3 . The air-channels in the Dasuopu firn core closed rapidly in the depth between 40 and 47 m, and 50% of air-bubbles were formed in the depth of 45 m. It is calculated that the age difference between air and its surrounding ice is 59 a, and the bulk age distribution width of air trapped in ice bubbles is 11 a. 相似文献
896.
石榴石 Sm Nd 等时线法是近年来常用于较新年龄的测年方法。腾冲地块西缘新近发现的基性麻粒岩中石榴石和全岩的 Sm Nd 等时线年龄为 17~25 Ma, 是该麻粒岩经受后期变形变质作用的年龄, 与区内右旋走滑断裂活动时限相一致,为确证腾冲地区的右旋走滑断裂是印支地块逸出的西部边界提供了 Sm Nd 等时线方法的年龄约束。并推断石榴石 Sm Nd 体系的封闭温度较低, 应低于那邦变质基性岩晚期变质作用的640℃~720℃。 相似文献
897.
川西北地区马脑壳金矿床的成矿年代学 总被引:8,自引:0,他引:8
沉积容岩微细浸染型(SHDG)金矿床(或称卡林型金矿床),广泛分布于各大洲的许多古代和现代的地体(地块)周边,如美国的大盆及中国的黔西南和川西北的金矿床,是经济上非常重要的一种金矿类型。关于SHDG金矿床物质来源和成因一直存在着较大的争议,目前存在三... 相似文献
898.
江苏宁镇伏牛山岩体的铷锶等时线年龄 总被引:2,自引:0,他引:2
从伏牛山岩体采集了新鲜的花岗闪长斑岩作铷锶同位素年龄测定,得到了拟合度很好的RbSr岩体矿物等时线,获得111Ma±1Ma年龄和锶的初始比值(Sr87/Sr86)i为07058±00001。进一步印证该岩体形成于燕山晚期,为幔源岩浆受地壳物质混染后的产物,属同熔型成因系列 相似文献
899.
四川石棉地区的前震旦纪基底变质杂岩(“康定杂岩”) 呈断块产出,由中浅(深) 变质岩、变质侵入岩及混合岩组成。在片岩中获蓟县纪—青白口纪球藻类化石;采用锆石UPb 法和PbPb 法获得变粒岩的原岩年龄为1067Ma,变质砂岩的成岩年龄小于853Ma,变质侵入岩的成岩年龄为876 ~828Ma,混合岩的变成年龄为711Ma,结合早震旦世岩体侵蚀混合岩,故将基底变质杂岩的时代确定为蓟县纪—青白口纪。 相似文献
900.