PREHISTORY IN THE PACIFIC ISLANDS     

DEREK WILLIAMS 《New Zealand geographer》1991,47(1):41-41

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Dating metamorphic reactions and fluid flow: application to exhumation of high-P granulites in a crustal-scale shear zone, western Canadian Shield   总被引：3，自引：1，他引：3  

K. H. MAHAN  P. GONCALVES  M. L. WILLIAMS  M. J. JERCINOVIC 《Journal of Metamorphic Geology》2006,24(3):193-217

The Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–M_H2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H₂O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H₂O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.  相似文献   

Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield   总被引：1，自引：1，他引：1  

J. A. BALDWIN  R. POWELL  M. L. WILLIAMS  P. GONCALVES 《Journal of Metamorphic Geology》2007,25(9):953-974

A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year^?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.  相似文献   

Scanning electron microscope evidence for offshore-onshore sand transport at Fire Island, New York, USA   总被引：1，自引：0，他引：1  

A. T. WILLIAMS  P. MORGAN 《Sedimentology》1993,40(1):63-77

Balancing the sediment budget of the Atlantic shoreline of Long Island has proved to be problematic because more sediment has been found to be moving westward in littoral drift than was estimated to be supplied by wave erosion of the Ronkonkoma terminal moraine at Montauk Point. There is a need for the existence of an additional, possibly offshore source. As part of a larger Atlantic shore study, 11 beach samples from Fire Island were compared with samples representing the known glacial source at Montauk Point, and also with four offshore samples to the south. Comparisons were based upon quartz sand grain surface texture patterns derived by scanning electron microscope checklist analysis. Initial evidence of an offshore-onshore link was provided by qualitative results examining surface texture variability between samples, illustrated by photographic evidence and written notes. This evidence was confirmed consistently during quantitative analysis. In particular, canonical variate analysis grouped two offshore samples representing deposits of buried glacial to fluvioglacial lobes with beach samples from Fire Island in adjacent onshore locations or slightly downdrift. In addition, canonical variate analysis and factor analysis using the spssx package, together with cluster analysis using the ARTHUR package, depicted the main factors and texture variables largely responsible for sample discrimination. The significance of both the offshore-onshore link and the associated factors influencing quartz grain surface texture was revealed by the fact that they are strongly related to each other and to the regional geomorphology.  相似文献   

DETERMINATION OF TRACE ELEMENTS IN 11 CHINESE GEOLOGICAL REFERENCE MATERIALS BY ICP-MS   总被引：1，自引：0，他引：1  

Y.Q. TANG  K.E. JARVIS  J.G. WILLIAMS 《Geostandards and Geoanalytical Research》1992,16(1):61-70

More than 40 trace elements in 11 Chinese geological reference materials (stream sediments) GSD-1 to GSD-7 and GSD-9 to GSD-12 were determined by inductively coupled plasma-mass spectrometry (ICP-MS). Open acid digestions were used to prepare samples for the determination of most elements. However, for the measurement of Y, Zr and Hf, it was found necessary to use an alkali fusion in order to make fully quantitative measurements. The results obtained in this work are generally in good agreement with the recommended values from the sub-ppm level to > several hundred ppm and illustrate the versatility of the technique, particularly for some less well characterised elements.  相似文献   

Garnet Compositions at Broken Hill, New South Wales, as Indicators of Metamorphic Processes     

STANTON  R. L.; WILLIAMS  K. L. 《Journal of Petrology》1978,19(3):514-529

Electron microprobe analyses of garnets in the finely bedded‘banded iron formations’ (BIF) of the Willyama Complexat Broken Hill reveal marked compositional changes from onegarnet to the next on a scale of 1–2 mm. Further, systematicanalytical traverses across bedding and along bedding show thecompositions of the garnets to change markedly from one finebed to the next, but to remain extremely uniform within individualbeds. In view of the minuteness of the domains involved it appearsevident that compositional variation cannot be attributed tovariations in metamorphic pressures, temperatures or oxygenfugacities. Neither can they be attributed to variation in garnet-matrixpartition functions, as most of the garnets occur in one simplematrix—quartz. It is concluded that in spite of the high (sillimanite) gradeof the relevant metamorphism, any equilibration of garnet compositions,and hence any associated inter-grain metamorphic diffusion,has been restricted to a scale of less than 1 mm; that garnetcompositions here reflect original rock composition on an ultra-finescale, and have no connotations concerning metamorphic grade;that, hence, the garnets must derive from a single precursormaterial, earlier suggested to be a manganiferous chamositicseptechlorite; and that the between-bed variation: within-beduniformity of garnet composition reflects an original patternof chemical sedimentation—a pattern preserved with theutmost delicacy through a period of 1800 x 10⁶ years and a metamorphicepisode of sillimanite grade.  相似文献   

EM AND IP RESPONSE OF A STEEL WELL CASING FOR A FOUR-ELECTRODE SURFACE ARRAY. PART I: THEORY*     

J. R. WAIT  J. T. WILLIAMS 《Geophysical Prospecting》1985,33(5):723-735

The basic formulation for the mutual coupling between grounded circuits in the presence of a vertical conductor is used to examine the full electromagnetic and induced polarization response for a homogeneous half-space model. Both interfacial polarization at the steel/electrolyte boundary and induced eddy currents in the casing are accounted for in the analysis. It is shown that for mild steel casings the eddy currents, hitherto neglected, play an important role because of their strong frequency dependence. Calculated curves of the appar- ent complex resistivity as a function of frequency show that the overall response can be influenced by the state of corrosion at the surface of the casing.  相似文献   

Instrumental Neutron Activation Analysis of Fourteen Carbonate Reference Materials from the NBS and George Frederic Smith and Eleven Industrial Geological Samples from the Bureau of Analysed Samples     

P.J. POTTS  O. WILLIAMS THORPE  M.C. ISAACS  N.W. ROGERS 《Geostandards and Geoanalytical Research》1985,9(2):173-179

New data are presented for the analysis of the elements: REE, Na₂₀, Fe₂O₃, Cr, Co, Rb, Cs, Ba, Hf, Ta, Th and Il in twenty-five less well known carbonate and industrial geological reference materials by instrumental neutron activation analysis. The materials include NBS 1c, 15e, 88a and 120b from the National Bureau of Standards, Washington DC; GFS 400-403, 405, 407, 411, 412, 415 and 417 carbonate materials from George Fredrick Smith, Columbus, Ohio and BCS 267, 269, 309, 315, 319, 367, 368, 375, 376, 381 and 382/1 from the Bureau of Analysed Samples, Middles-borough, Cleveland, UK. Particular attention has been given to the correction of interferences due to the fission of uranium in the determination of the elements La, Ce, Nd and Zr.  相似文献   

Social and Cultural Aspects of Natural Hazards Perception and Response     

GEOFF GREGORY  ALISON LOVERIDGE  JANET GOUGH 《New Zealand geographer》1997,53(1):47-54

Focus groups conducted in two urban areas in New Zealand explored perceptions of natural hazards and disaster preparedness. The groups were structured by socio-economic status, gender, age, and cultural background. Participants were generally aware of likely major hazard events but few maintained emergency kits or had prepared emergency plans. People held an optimistic attitude, believing they could cope with disaster. Differences amongst the groups included the inability of lower socio-economic groups to afford emergency kits or insurance and evidence of strong supportive networking among Maori and Samoan groups.  相似文献   

Establishing the P–T path for Alpine Schist, Southern Alps near Hokitika, New Zealand     

J. K. VRY  R. POWELL  J. WILLIAMS 《Journal of Metamorphic Geology》2008,26(1):81-97

Pressure–temperature pseudosections for ‘greyschist’ (metamorphosed greywacke and argillite) from the Alpine Schist (Haast Schist group) near Hokitika (Southern Alps, New Zealand) are used to gain new insights into its metamorphic history. The rocks were metamorphosed at relatively low‐grade conditions associated with the first appearance and initial growth of garnet in the stability field of albite. The measured and predicted garnet compositional zoning data are used to construct an overall P–T path by combining P–T path results from nearby rocks that have a range of MnO contents. The P–T path obtained is steep from ～380 °C/2.5 kbar up to ～490 °C/8.5 kbar, then recurves sharply with garnet growth continuing during early decompression to ～500 °C/6.5 kbar. Most garnet growth in the study area took place in the stability field of albite, with oligoclase appearing only during decompression, when the peristerite gap was entered. On appearance of oligoclase, there is a marked decrease in the CaO content of garnet. The preservation of mineral assemblages from near‐peak temperature conditions can be understood in terms of the P–T path subsequently becoming tangential to water content contours, during cooling with further decompression.  相似文献