从江翁浪金矿中的构造变形特征研究

从江翁浪金矿是一个受构造控制明显的金矿床,金矿主要富集在受顺层剪切控制的强变形带内。本文通过对翁浪金矿控矿构造中变形岩石的宏观和微观变形特征分析,得出研究区顺层剪切带的运动学特征为逆冲型剪切,控制金矿产出的强变形带以韧性变形为主的认识。     

A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite

Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi₃ triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 Å from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to [001]. However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 Å with the OH vector oriented perpendicular to [001] as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be 330 kJ mol^–1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO₆ octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.     

A reappraisal of episodic burial metamorphism in the Andes of central Chile

The Jurassic to Miocene sequences of the central Andes, east of Santiago, reputedly show repeated cycles of episodic sub-greenschist facies, burial metamorphism that are identified by sharp breaks in metamorphic grade at major stratigraphic boundaries. This paper presents the first detailed petrochemical analysis of these low-grade metamorphic sequences by examining the progressive development of secondary minerals, reaction progress in mafic phyllosilicates, and topological variations in the low-grade assemblages as a means of testing this model. The results indicate a progressive increase from zeolite facies through to close to the onset of greenschist facies from Miocene to Jurassic rocks. Combined analysis of reaction progress in mafic phyllosilicates and petrochemical relationships of chlorite–pumpellyite–actinolite in metabasites provides no evidence for sharp metamorphic breaks at major stratigraphic boundaries. Integrating the results presented here with the most recent models of stratigraphic/tectonic development of the central Andes shows that the metamorphism took place in two episodes, and was not episodic on a 40-million-year cycle. An absence of sharp breaks in metamorphic grade in any part of the succession, as demonstrated here, shows that the original petrographic establishment of low-grade facies provided insufficient resolution of changes in metamorphic conditions to establish definitive evidence of such breaks. Accordingly, this study suggests that re-assessment of metamorphic breaks reputedly developed in other areas of the Andean Cordillera is imperative in order to resolve the questions raised here about the origin of the low-grade metamorphism.Editorial responsibility: B. Collins     

Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block,east Antarctica

Proterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet‐bearing aggregates in garnet‐absent groundmass. The garnet‐absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet‐bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K‐feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet‐bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) yield P–T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW‐ and N‐trending mafic dykes were emplaced at 1764 ± 25 and 1232 ± 12 Ma, respectively, whereas their metamorphic ages cluster between 957 ± 7 and 938 ± 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner orogen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lambert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes.     

西藏墨竹工卡县邦铺钼(铜)矿床辉钼矿稀土—微量元素特征及对成矿流体性质的指示

文章以西藏墨竹工卡县邦铺钼(铜)矿床辉钼矿为研究对象,采用高精度电感耦合等离子质谱( ICPMS)对辉钼矿进行了稀土和微量元素测试.测试结果显示辉钼矿具有轻稀土富集的右倾配分模式,轻重稀土内部分馏明显.辉钼矿稀土元素不同程度地出现铕负异常和铈负异常现象,分析得出其铕负异常可能是继承了成矿流体自身铕亏损的特征,而铈负异常...     

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

Samples of quartz-bearing rocks were heated above the α (trigonal)–β (hexagonal) phase transformation of quartz (625–950°C) to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron X-ray and electron backscatter diffraction. The trigonal–hexagonal phase transformation does not change the orientation of c- and a-axes, but positive and negative rhombs become equal in the hexagonal β-phase. In naturally deformed quartzites measured by neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable twinning and memory loss after the phase transformation. In experimentally deformed quartz rocks, where twinning was induced mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it does not account for the persistence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause of texture memory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal stresses are released during the phase transformation and the material returns to its original state prior to deformation. Similarly, on surfaces there are no tractions and thus texture memory is partially lost.     

化探背景与异常划分中的C型转换法

化学背景与异常在统计学上的差异同时表现在浓度和变差两个方面。在划分背景与异常时,把这两方面信息提取出来并作显化处理,使背景与异常的差别更大,以利划分,这就是Ｃ型转换的基本思想。介绍了Ｃ型转换法的基本原理及其在条件模拟和实际金矿区中的应用。     

A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO₂, MgO and Al₂O₃ (a_SiO2, a_MgO and a_Al2O3) buffered. At low a_SiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a_SiO2 and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100–1350 °C. Experiments at high a_SiO2 and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high a_SiO2 experiments were generally several hundred microns in length, but diffusion at low a_SiO2 was three orders of magnitude slower than in high a_SiO2 experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a_Al2O3)^1/2 and (a_SiO2)^3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al _4/3 ³⁺ vac_2/3SiO₄, whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl₂O₄. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high a_SiO2, even faster than Fe–Mg interdiffusion.     

长白山天池火山——多成因中央式火山

长白山天池火山属新生代多成因中央式火山,也是我国最大的一座具潜在危险的活火山。火山主体由早期玄武岩盾、中期粗面岩锥和晚期伊格尼姆岩席组成。天池火山布里尼喷发柱高度最高达25 km,柱体最大宽度为半径12～13 km。天池破火山口塌陷过程可以分为四期,分别位于造锥喷发阶段和造伊格尼姆岩喷发阶段。本文对天池火山未来可能的火山灾害类型及范围也作了初步预测。     

内蒙古赤峰地区早二叠世花岗岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

以赤峰地区铭山复式岩体、尖山子岩体为研究对象,通过锆石U-Pb年龄、地球化学研究,确定了其形成时代,探讨了岩石成因和岩浆源区性质及其形成的构造背景。LA-ICP-MS锆石~(206)Pb/~(238)U测年结果表明,铭山复式岩体中灰白色斑状黑云二长花岗岩、尖山子岩体中浅肉红色细粒正长花岗岩分别形成于284.4±7.9Ma、294.7±8.5Ma,为早二叠世。根据地质体间接触关系可知,铭山复式岩体中的二长花岗岩岩体和尖山子岩体均为复式岩体,有待进一步解体。斑状黑云二长花岗岩SiO_2含量较高,K_2O+Na_2O含量较高,A/CNK1,属于高钾钙碱性I型花岗岩。细粒正长花岗岩Al_2O_3含量为14.32%~15.14%,Na_2O/K_2O=0.71~0.99,A/CNK=1.17~1.20,在标准矿物中出现刚玉分子。二者微量元素特征相似,富集大离子亲石元素,亏损高场强元素Nb、Ta,具Sr、P、Ti的负异常,表现出岛弧岩浆岩类特征。结合岩石地球化学、区域地质特征,认为赤峰地区早二叠世岩体形成于岛弧/活动大陆边缘构造背景,其形成与古亚洲洋向南俯冲有关。