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1.
CHRIS W. LEE 《Geology Today》1991,7(4):133-136
Each year many hundreds, if not thousands, of Earth-science students visit the coastline at Ogmore-by-Sea, mid-Glamorgan. They study the excellent exposures of Lower Carboniferous Limestone, with their well-preserved coral and other shelly faunas, together with the presumed late Triassic 'breccias'* that rest unconformably on top. A study of the cements within the breccias may give an insight into syndepositional mineralization and the formation of sedimentary basins linked to the opening of the Atlantic in early Mesozoic times. 相似文献
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Boulders of the assemblage rubysapphire corundum, chromianmuscovite, margarite, tourmaline (chromian chlorite, ZnMnchromite and MnTi magnetite) occur in glacial moraineand rivers of north Westland, South Island of New Zealand. Thelocation, Cr-rich composition of the boulders and the presenceof rare serpentinite rinds indicate that they are derived fromultramafic rocks (Pounamu Ultramafics) that occur within AlpineSchist of the Southern Alps. The largest sample is progressivelyzoned outwards from a corundummargarite core, throughan intermediate zone of Cr-muscovite, to an outer zone of Cr-chloritethat is in contact with serpentinite. Most finds consist oferosion-resistant corundum-rich cores. In the corundum, Cr2O3content ranges from 0.5 to 13%, with red coloration becomingmore intense with increasing Cr. In addition to the dominantCr3+ Al3+ substitution, those of (Fe, V)3+ Cr3+ and (Ti4++Fe2+) 2Cr3+ result in spectacular colour zoning from colourlessto deep ruby red-carmine and pale blue to dark blueviolet.Corundum has grown by replacement of the micaceous matrix thatconsists of chromian muscovite (0.104.10% Cr2O3) andchromian margarite (0.461.20% Cr2O3). Both micas containa significant paragonite component (up to 21.5% in muscoviteand up to 40.8% in margarite). Late phase muscovite is Ba richwith up to 4.77% BaO, and margarite has up to 0.66% SrO. Tourmalineoccurs as veins, vein outgrowths and larger poikilitic crystalsthat replace the mica matrix. Chromium content ranges between0.82 and 3.6% Cr2O3. High bulk rock Al (up to 78% Al2O3), K,Ca, Cr and Na, and low Si (14.523.1% SiO2), suggest thatthe corundumCr-silicate rocks are the products of extrememetasomatic alteration of quartzofeldspathic schist enclavesin serpentinite. Isocon analysis indicates that conversion ofthe schist to the micaceous matrix of the corundum rocks involvesconservation of Ca, Al, K, volatiles and Sr, a mass loss of59% and a volume reduction of 69% consequent on removal of 7080%Si and all other elements (most >80%), with enrichment ofbetween 900 and 1800% Cr. The formation of corundum from themica matrix involved a further massvolume reduction anddecrements in Si, Ca, K, volatiles and Sr from reaction sites.Concentric mineral zonation in single rock samples and zoningreplacementin minerals, e.g. Cr in corundum and chromite, Ti, Fe2+ in corundum,Ba in muscovite, Sr in margarite, and Mn and Zn in chromiteand magnetite, imply element redistribution during metasomatism.Experimental reaction between quartzofeldspathic schist andserpentinite at 450C and 2 kbar produced reaction sequencescontaining newly formed Ca-plagioclasephlogopitic micachloriteand muscovitechlorite that in terms of composition areanalogous with the observed (corundummargarite)muscovitechloritezonation. The temperature of metamorphism of garnet zone rocks(45020C) that contain the corundumCr-silicate rocksis well below that of the breakdown of muscovite and margariteto form corundum and indicates the importance of fluid composition,particularly the cationhydrogen variables aCa2+/H+, aK+/H+and aS1O2. Introduction of boron into the schist (from serpentinite),and boron released from the breakdown of original tourmalinein the schist, resulted in tourmaline veining and reaction ofthe mica matrix to form tourmaline that invoved both a massand volume increase and addition of Fe, Mg together with B. KEY WORDS: corundumCr-silicate rocks; metasomatism; New Zealand; Southern Alps
*Corresponding author. 相似文献
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Sustainability: A View From The Paddock 总被引:1,自引:0,他引:1
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DERECKE PALMER 《Geophysical Prospecting》1991,39(8):1031-1060
The depth to the surface of a refractor and the seismic velocity within the refractor are very often intimately related. In the shallow environment, increased thicknesses of weathering occur in areas of jointing, shearing or lithological variations, and these zones of deeper weathering can have lower subweathering refractor velocities. This association is important in geotechnical investigations and in the measurement of weathering thicknesses and sub-weathering velocities for statics corrections for reflection seismic surveys. Algorithms, which employ forward and reverse traveltime data and which explicitly accommodate the offset distance through the process known as refraction migration, are necessary if detailed structure on a refractor and rapid lateral variations of the seismic velocity within it are to be resolved. These requirements are satisfied with wavefront construction techniques, Hales’ method and the generalized reciprocal method (GRM). However, these methods employ refraction migration in fundamentally different manners. Most methods compute an offset distance with an often imprecise knowledge of the seismic velocities of the overlying layers. In contrast, the GRM uses a range of offset distances from less than to greater than the optimum value, with the optimum value being selected with a minimum-variance criterion. The approach of the GRM is essential where there are undetected layers and where there are rapid variations in the depth to a refractor and the seismic velocity within it. In the latter situations the offset distance necessary to define the seismic velocities can differ considerably from the value required to define depths. The efficacy of the GRM in resolving structure and seismic velocity is demonstrated with three model studies and two field examples. 相似文献
6.
Kenneth PALMER Paul C. KENNEDY Barry ROSER 《Geostandards and Geoanalytical Research》1983,7(2):301-303
The concentrations of 20 trace elements in 6 British Chemical Standards (BCS) reference materials and 5 European Federation of Refractories Producers (PRE) reference materials have been determined by X-ray fluorescence spectrometry. 相似文献
7.
ABSTRACT. Recent attempts by U.S. politicians to reform the nation'sschools have shifted the goal of education to school accountability as assessed in standardized testing. Such an emphasis undermines geographical education in schools because of geography'ssuperficial representation in tests and in the social studies curriculum. Fieldwork done in the classroom can point to means of circumventing this dilemma. Collaborative fieldwork between college faculty members and public‐school teachers has the potential for adding geography to the social studies curriculum in a substantive way. Work conducted jointly by Hartwick College and the Oneonta (New York) Middle School exemplifies such a partnership. 相似文献
8.
Crustal Contamination and Fluid-Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa 总被引:4,自引:0,他引:4
The Platreef is the main platinum group element (PGE)-bearingfacies of the northern limb of the Bushveld complex, but unlikethe Merensky Reef of the eastern and western limbs, it is indirect contact with the country rock. Mineral separate 相似文献
9.
U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins 总被引:6,自引:2,他引:6
PEATE DAVID W.; KOKFELT THOMAS F.; HAWKESWORTH CHRIS J.; VAN CALSTEREN PETER W.; HERGT JANET M.; PEARCE JULIAN A. 《Journal of Petrology》2001,42(8):1449-1470
New thermal ionization mass spectrometry U-series disequilibriumdata are presented for 24 basaltic to dacitic glasses from activespreading centres in the back-arc Lau Basin (SW Pacific), togetherwith additional inductively coupled plasma mass spectrometrytrace element analyses and SrNdPb isotope data.Valu Fa Ridge samples, adjacent to the arc front, have highU/Th and (230Th/238U) <1, implying a recent (<<350ka) addition of a U-rich slab-derived fluid. The Valu Fa datacan be combined with existing 230Th238U data for theCentral Tonga arc to infer a fluid addition event at 相似文献
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