从同位素到板块构造:化学地球动力学

化学地球动力学是地球化学的分支学科,它在研究地球内部化学组成和演化时,把地球视为一个完整的动力学系统而不是彼此孤立的地质集合体。通过研究地球各层圈内部的化学结构和过程以及不同层圈之间的化学相互作用,从本质上研究和认识发生在地球内部的各种地质作用。文中概括了化学地球动力学的特点和突出成果,分析了化学地球动力学研究的科学意义,并对在中国开展壳幔相互作用的化学地球动力学研究提出了建议。     

山西省五台山—恒山地区脉状金矿成矿的地球动力学

０　前　言（代序）该文集命名为“成矿的地球动力学”,表明它只是矿床学中一项较基础性的研究;把脉状金矿作为研究对象,则应属于金矿区域成矿学的范畴。展望２１世纪,资源、环境乃是人类生存与发展之本,因此矿床学所肩负的使命必将更加艰巨。虽然人类利用矿产资源已有数千年的历史,但作为地质科学的重要组成部分—矿床学到１６世纪中叶才萌芽,１９世纪才形成具有较完整理论的学科。如果说矿床学的基本任务是确定这种或那种矿床在地壳中的生成条件和分布规律,矿床的形成又非孤立和偶然的事件,它是一定地质构造背景和地质历史条件下…     

沉积盆地的地球动力学演化分类

阐述了沉积盆地分类的重要意义,论述了盆地所处的应力环境和地壳类型（大地构造背景）是控制盆地形成、演化的重要因素,并以此为基础提出了新的沉积盆地地球动力学演化分类。     

初始地壳的活化再造与金矿床密集区

前寒武纪古老变质基底是金矿床形成的物质基础,在初始地壳活化再造过程中受到过多次改造和再造,其中,金元素多次受到再分配。高级区花岗岩-绿岩带在活化改造和重熔再造的过程中表现出由麻粒岩相-角闪岩相-绿片岩相转化的退变质过程。这一过程使变质基底岩石中的金有一个预富集。在晚期（主要是燕山期）,岩浆-构造活动促使基底岩石中的金再次活化。在成矿围岩退化蚀变过程中,随岩石组分的转化,金得以再次聚集,并形成工业矿床。通过对几个金矿床密集区内的不同岩石类型含金丰度对比,表明地壳活化改造越强,金的矿化程度越高。     

CrustalP-wave velocity and velocity-ratio study in northeast India by a microearthquake survey

The upper crustal (20 km)P-wave velocity beneath the Shillong Plateau and Nowgong area has been studied by the time-distance plot method. TheP-arrival data of the shallow (20 km) microearthquakes from three temporary networks are used, and the average velocity is found to be 5.55 km/s. The velocity ratio (V _p /V _s ) for the upper crust (0–20 km) as well as for the lower crust (21–40 km) are determined by the Wadati-plot method and station-by-station method. The average value obtained by the two methods is compatible; theV _p /V _s ranges between 1.74 to 1.76. A generalized seismic velocity model of the area is suggested by this study, which has been very useful for microearthquake location.     

义敦型镁铁—超镁铁岩的主要特征及其与蛇绿岩的对比

“义敦型”相当于文献中通称的阿尔卑斯型,但却不是蛇绿岩。蛇绿岩代表洋壳和地慢的岩石组合,而义敦型镁铁-超镁铁岩体则是由陆壳下的地幔及其部分熔融的产物组成的。因此,蛇绿岩与义敦型岩体的构造含意不同,后者的分布与古缝合带无关。在造山带中区分开蛇绿岩和非蛇绿岩是至关重要的。义敦型镁铁-超镁铁岩组合的认定在理论上和找矿实践上都是有意义的。     

Determining the cooling history of in situ lower oceanic crust—Atlantis Bank, SW Indian Ridge

The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (∼850°C) drilled in Hole 735B is estimated at 11.93±0.14 Ma, based on U-Pb analyses of zircon by ion probe. This crystallization age is older than the ‘crustal age’ from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949-11.935 Ma). ⁴⁰Ar/³⁹Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42±0.21 Ma), implying cooling rates of >800°C/m.y. over the first 500,00 years to temperatures below ∼330-400°C. Fission-track ages on zircon (mean 9.35±1.2 Ma) and apatite reveal less rapid cooling to <110°C by ∼7 Ma, some 4-5 m.y. off axis.Comprehensive thermochronometric data from the structurally intact block of gabbro between ∼700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range ∼900-330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T≤280°C) are not consistent with these predictions and record anomalously high temperatures for crust >700 m below sea floor at 8-10 Ma (i.e. 2-4 m.y. off axis). We offer two hypotheses for this thermal anomaly:

(i)

Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from ∼19.5 to 7.5 Ma.

(ii)

Late detachment faulting, which led to significant crustal denudation (2.5-3 km removed), further from the ridge axis than conventionally thought.

     

The Mesoproterozoic basement in the southern Baltic Sea: insights from the G 14–1 off-shore borehole

The continuation of the Mesoproterozoic basement of the southern Fennoscandian Shield is documented in the G 14–1 off-shore borehole, northeast of the island of Rügen, where crystalline rocks of monzogranitic composition occur beneath flat-lying early Palaeozoic sediments at a depth of approximately 2,000 m. The greenish-grey, or partly reddish-grey, granites show a slightly porphyritic texture marked by plagioclase crystals or aggregates in a groundmass dominated by fresh microcline. Chloritized biotite occurs as a subordinate mafic phase. Ductile and brittle deformation is indicated by a weak foliation and the occurrence of several cataclastic zones, respectively. Major and trace element geochemistry suggest that these rocks are K-rich calc-alkaline granites and represent a restite-poor melting product of a granodioritic protolith. The low MgO, Cr, Ni and Co concentrations and relatively high content of accessory phases (apatite, zircon) point to formation of water undersaturated, high temperature (>900 °C) melts at low degrees of partial melting. Although the G 14 granite lacks hornblende, in contrast to most of the granites from Bornholm, it seems geochemically related to them. Analysed samples mostly fit an intermediate position between the Rønne and Vang granitoids, which both belong to a group of Mesoproterozoic intrusions showing partial ductile deformation. (Y+Nb) vs. Rb plots suggest that all these granitic rocks were generated in intracratonic conditions. A genetic relationship between them and contemporaneously intruded Karlshamn-group granites in Blekinge, eastern Scania and Småland is supported by intrusion age of 1,460±3 Ma obtained from Pb-Pb isotope ratios measured on single zircons of the G 14 granite.     

基于富钴结壳微地形采集头载荷波动性仿真研究

根据富钴结壳微地形的数字高程模型进行分类,得到某个特定高程差下9种地形模型。针对基于富钴结壳微地形的采集头切削过程,编制仿真程序进行模拟。仿真结果表明相同高程差地形模型(SS,MS,LS)、(SM,MM,LM)、(SL,ML,LL)的波动系数平均值对应为(0.35583,0.33142,0.32795)、(0.39355,0.41104,0.39326)、(0.48822,0.46608,0.50516),数据表明地形的高低对载荷波动性的影响不明显;不同类型地形模型(SS,SM,SL)、(MS,MM,ML)、(LS,LM,LL)的平均值对应为(0.35583,0.39355,0.48822)、(0.33142,0.411 04,0.466 08)(0.32795,0.39326,0.50516),数据表明地形起伏越大,对应的载荷波动系数也越大,采集头切削过程越不平稳。     

Fine-scale compositional variations in a Co-rich Mn crust from the Marcus-Wake Seamount cluster in the western Pacific based on electron microprobe analysis (EMPA)

The internal structure and composition of a 37-mm-thick Co-rich Mn crust from the Lamont Guyot in the Marcus-Wake Seamount cluster have been studied in detail by means of electron-probe microanalysis (EPMA). 184 point analyses for 16 elements were carried out at an average spacing of 0.20 mm and each point was dated using the Co-geochronometer method. Two types of variation in composition were observed in the crust: long-term trends and short-term erratic variations. The long-term trends were identified using the fifth order polynomial. Mn and Ni were shown to increase in concentration from 23.3 Ma to a maximum at about 20 Ma and then decline steadily to the Present, whereas Fe, P and Si showed the opposite trend. By contrast, Co displayed a double humped pattern with maxima at about 19 Ma and 3.5 Ma and minima at 23.3 Ma, about 11 Ma and at Present. The long-term trends in element concentrations in the crust lead us to suggest that much of the Fe in western Pacific Ocean ferromanganese crusts is aeolian in origin and derived from the deserts of central Asia. Cooling of the Asian mainland at about 20 Ma led to an increase in the flux of Fe to the oceans. This was supplemented by an additional input of Fe into the crusts as a consequence of the dissolution of biogenic CaCO₃ tests at 4.5−10.5 Ma. For the short-term erratic variations, three periods of 0.61, 0.96 and 1.65 m.y. were identified by spectral analysis. The second harmonics of these periods are 1.22, 1.92 and 3.30 m.y., which may correspond to the highest-order periods for eccentricity of 1.31, 2.04 and 3.47 m.y. This suggests the possibility that the high-frequency oscillations of the time series data are linked to climatic changes controlled by the highest-order periods of the Milankovitch cycles.