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1.
N. C. N. STEPHENSON 《Journal of Metamorphic Geology》1984,2(4):297-314
Coexisting Ca-poor and Ca-rich pyroxenes in granulites at Cape Riche, in the Precambrian Albany-Fraser Province, Western Australia, are dominantly chemically homogeneous within individual samples, suggesting a major episode of equilibration. However, occasional grains in a few samples contain exsolved domains interpreted as relics of an earlier, higher-T assemblage. Pyroxene pairs in ten, presumably isothermal, samples from a restricted area are used to (i) assess the suitability of several versions of the two-pyroxene thermometer for application to metamorphic rocks, and (ii) determine the thermal history of the Cape Riche pyroxenes. The various versions of the two-pyroxene thermometer applied to the well-equilibrated homogeneous pyroxene grains show poor to good precision and yield mean temperatures varying widely from 683° to 893°C, in the following order of increasing T: Lindsley (1983; opx version), 683°± 11°C; Kretz (1982; KD version), 705°± 19°C; Ross & Huebner (1975), 709°± 30°C; Kretz (1982; solvus version), 735°± 24°C; Fonarev & Graphchikov (1982; opx version), <750°C; Lindsley (1983; cpx version), 784°± 40°C; Fonarev & Graphchikov (1982; cpx version), ~820°± 30°C; Wood & Banno (1973), 849°± 16°C; Powell (1978), 854°± 23°C; Wells (1977), 893°± 10°C. Independent T estimates, based on mafic assemblages and garnet-biotite thermometry, suggest that the major episode of metamorphism occurred at 700-800°C (P ~ 5 kbar). Therefore the Wells, Powell, Wood & Banno and Fonarev & Graphchikov (cpx) temperatures are almost certainly too high. In the absence of a more precise independent T estimate it is difficult to assess the relative merits of the results obtained from the remaining versions of the two-pyroxene thermometer, none of which can be unequivocally demonstrated to be seriously in error, though the Lindsley (opx) T is probably too low. Other significant shortcomings evident in the results include the relatively poor precision obtained from the three methods based on purely graphical representation of the augite limb of the solvus (i.e., the Ross & Huebner, Fonarev & Graphchikov (cpx) and Lindsley (cpx) versions), and the apparent dependence of derived T on Mg/Fe2+ ratio for the Powell, Wood & Banno and Lindsley (cpx) methods. For the bulk compositions of exsolved domains, the different versions of the two-pyroxene thermometer yield mean temperatures 23° to 82°C (overall mean, 65°C) higher than for homogeneous grains in the same samples. These exsolved domains are interpreted as relics of a higher-T (peak?) metamorphic assemblage, rather than an igneous precursor. 相似文献
2.
A. BHATTACHARYA B. SPIERING S. K. SEN R. NATARAJAN A. C. MAZUMDAR 《Journal of Metamorphic Geology》1990,8(5):525-538
Manganese-rich and manganese-poor iron formations which occur as thin layers in the Halaguru-Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO2 contents. The rocks are of four petrographic varieties: (a) quartz-magnetite-orthopyroxene-clinopyroxene, (b) quartz-magnetite-orthopyroxene-clinopyroxene-garnet, (c) quartz-magnetite-clinopyroxene-garnet, and (d) quartz-magnetite-clinopyroxene-garnet-plagioclase. In the orthopyroxene-clinopyroxene pairs, Mn-Mg and Mn-Fe exchange is ideal irrespective of the MnSiO3 contents of orthopyroxenes (0.6–1.8 mol. % in Mn-poor and 15–25 mol. % in Mn-rich compositions). Mg-Fe exchange in the same pair is however non-ideal. Mn-Fe exchange in orthopyroxene-garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene-garnet and clinopyroxene-garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value. The equilibrium log ffo2 values in the iron formations, as calculated from the reaction 6FeSiO3+ O2= 2Fe3O4+ 6SiO2 are in the range of ?14.2 to ?15.5. Algebraic analysis of variations of fo2 with composition of phases indicates buffering of O2 in the rocks. The absence of grunerite in these assemblages is compatible with XH2O being less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C-O-H system, however, predict high XH2O values (>0.7) at ac= 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks. 相似文献
3.
G. J. Retallack 《Australian Journal of Earth Sciences》2013,60(4):383-399
The sea‐cliffs north of Sydney expose a complex of Triassic palaeosols, pedoliths, and sedimentary rocks. The most obvious and diagnostic features of the palaeosols are fossil roots in place and markedly leached or reddened, relatively massive, clay‐rich strata. Associated coaly layers and fossil plant remains in adjacent sediments show varying degrees of decomposition. The A horizons of some of these palaeosols have been silicified by plant opal and contain abundant insect, earthworm, and larger animal burrows, cradle knolls, and basket podzols. Many of the palaeosols have well‐preserved peds and their upper horizons slake more readily in water than their lower horizons. Their B horizons may consist of extensive layers of siderite nodules or red claystone with tubular grey mottles around old root channels. Less mature palaeosols show some relict sedimentary bedding and ripple marks within their profiles. More mature palaeosols, which appear massive in the field, may show anomalous grainsize variation in thin section, remaining from sedimentary bedding. Soils can be eroded and deposited as pedoliths. Conglomerates of palaeosol clay pebbles and siderite nodules are easy to recognize as pedoliths. Finer‐grained pedoliths have the distinctive mineralogy and some of the small structures, but not the larger structures and field relations, of palaeosols. Siderite crystals and nodules developed in the gleyed organic and A horizons of some of the clayey palaeosols shortly after they were covered by sediment and subsided below base level. With further compaction and dewatering, the ferric‐oxide minerals became redder by inverting to hematite, mineralized joints developed in some massive B horizons, some peds were accentuated by slickensides, and some root channels and coal cleat were filled with copper minerals. 相似文献
4.
通过对江汉平原周老孔第四纪岩芯的22个重砂样品中 0.125~0.063mm粒级的重矿物中的普通辉石进行实验分析,发现从钻孔深度104m开始向上,普通辉石的含量明显增多(辉石在该粒级沉积物中的丰度从平均3.2×10-4 增加到10.9×10-4),种类也明显多样(从单一的墨绿色普通辉石到多种颜色的普通辉石都有相当数量的出现),特别是含钛普通辉石(玫瑰红色调)首次出现。通过对江汉平原物源分析,特别是近源(三峡以下,含三峡)和远源(三峡以上)产出普通辉石的岩体的面积及类型的比较分析(近源与远源的基性、超基性岩分布明显不同),认为江汉平原沉积物中普通辉石的变化是由长江上游下金沙江-三峡贯通后远源物质进入所引起的,这个发生变化的钻孔深度(104m)的古地磁年龄为1.1Ma。 相似文献
5.
6.
福建马坑铁矿辉石的成因矿物学研究 总被引:2,自引:0,他引:2
本文研究了福建马坑铁矿床中辉石族矿物的时空分布、产状、种属及其晶体形态标型、物理标型、化学成分标型。通过不同成因的辉石(308个不同成因辉石)化学成分标型对比、图解等综合判别方法确定马坑铁矿中辉石有火山岩变质、沉积变质、接触交代和岩浆四种成因。矿体底部成矿建造为富Fe、K、Mn火山岩建造,中下部为富F、Mn硅铁质沉积建造,上部为富Ca、Mg、Mn、Fe、Si碳酸盐沉积建造。马坑矿床主矿体为海底火山沉积变质型铁矿。提出利用辉石的种属、含铁系数、化学标型判断有无矿体及其贫富的矿物学标志。 相似文献
7.
Hydrodynamic forces on a partly buried cylinder exposed to combined waves and current 总被引:1,自引:0,他引:1
The present study extends the investigations of the hydrodynamic forces on a cylinder, laid on, or partly buried in the bed. They were determined by measuring the pressure distribution on the cylinder surface in the case of steady current, waves and coexisting flow. The pressure distribution around the cylinder was measured by using pressure transducers, which were replaced in the cylinder. Force coefficients were obtained for the ranges of Re=0.8×104–1.5×104, for steady current, low KC numbers (KC<5) for wave alone case and, for current-to-wave velocity RATIO=0, 3, 6 and infinity (current) for coexisting flow. The forces were also determined for the various burial-depth-to-diameter ratios between 0 and 0.7 values of the cylinder. 相似文献
8.
A. Mottana T. Murata A. Marcelli Z. Y. Wu G. Cibin E. Paris G. Giuli 《Physics and Chemistry of Minerals》1999,27(1):20-33
X-ray absorption spectra at the Mg and Al K edges have been recorded using synchrotron radiation on synthetic end member
diopside (Di) and jadeite (Jd) and on a series of natural Fe-poor Ca-Na clinopyroxenes compositionally straddling the Jd-Di
join. The spectra of C2/c end members and intermediate members of the solid solution series (C-omphacites) are different from
those of the intermediate members having P2/n symmetry (P-omphacites). Differences can be interpreted and explained by comparing
the experimental spectra with theoretical spectra calculated via the full multiple-scattering formalism, starting from the atomic positional parameters determined by single-crystal X-ray
diffraction structure refinement on the same samples. Atomic clusters with at least 89 atoms, extending to more than 0.60 nm
away from the Mg or Al absorbers, are needed to reproduce the experimental spectra. This shows that in the clinopyroxene systems
XANES detects medium- rather than short-range order-disorder relationships. Theoretical spectra match the experimental ones
well for all features in the regions from 16 to 60 eV above threshold. Experimental near-edge features in the first 16 eV
are also reproduced, albeit less accurately. Certain near-edge features of C-omphacites reflect the octahedral arrangement
of the back-scattering six O atoms nearest neighbours of the probed atom (Mg or Al) located at site M1 of the crystal structure,
thus being indicators of short-range order. Others arise again from medium-range order. P-omphacites show more complicated
spectra than C-omphacites. Their additional features reflect the increased complexity of the structure and the greater local
disorder around the probed atom induced by the two alternative M1, M11 configurations of the six O atoms forming the first
coordination shells. Mg and Al are confirmed to be preferentially partitioned in the M1 and M11 site of the P-omphacite crystal
structure, however with a certain degree of local disorder. The relative heights of certain prominent features are directly
related to sample composition in terms of Di:Jd ratio in the Al K-edge spectra, whereas they show abrupt variations in the
Mg K-edge spectra. They demonstrate that XANES is directly related to composition and may be used to distinguish C- from P-omphacites.
Received: 24 November 1998 / Revised, accepted: 10 June 1999 相似文献
9.
四川盐边元古代蛇绿岩中辉石的成因矿物学及其大地构造意义 总被引:14,自引:0,他引:14
本文用电子探针分析了岩浆型残余辉石的化学成分,建立了辉石组分之间的相关关系和定量的类质同象交换式,并讨论了母岩浆成分、结晶条件(压力、冷凝速度)对辉石成分的影响。深成岩和火山岩中辉石的标型元素含量和相关参数十分相近,说明深成─火山岩系为同源岩浆成因。尽管一些火山岩样品中辉石的Al,Ti因快速冷凝而富集,但总体化学成分仍明显显示拉斑玄武岩特征,与世界上典型的显生宙蛇绿岩熔岩及现代洋中脊玄武岩中的辉石相似,但与岛弧有关的火山岩明显不同。这说明。盐边元古代蛇绿岩很可能形成于一个继大陆裂谷之后的海洋扩张环境。本文提供的成因矿物学资料有助于理解和重建扬子板块西缘元古代造山带的地质历史。 相似文献
10.
湖南金矿床中黄铁矿和毒砂的含金性探讨 总被引:5,自引:0,他引:5
(?)南的共生与(?)生金矿庆中的金,大部分为次显微金和(?)显微金.(?)中的黄铁矿和毒砂是金的主要载(?)矿物.金主要以机械混合物或微包体形式赋存,主要形成于硫(?)化物矿化(?)或晚期.矿石合金量的高低与载金矿物的晶形,(?)度,生成期及碎裂程度等有关.(?)黄铁矿、毒砂是金矿床的主要标型矿物.As是标型指示元素. 相似文献