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Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA 总被引：3，自引：2，他引：1
Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H2O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 104 to 31 104 mol/cm2, and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA
*Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu 相似文献
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GREGORY J. McCABE JR 《水文科学杂志》2013,58(6):873-887
Abstract Winter mean 700-hectoPascal (hPa) height anomalies, representing the average atmospheric circulation during the snow season, are compared with annual streamflow measured at 140 streamgauges in the western United States. Correlation and anomaly pattern analyses are used to identify relationships between winter mean atmospheric circulation and temporal and spatial variability in annual streamflow. Results indicate that variability in winter mean 700-Hpa height anomalies accounts for a statistically significant portion of the temporal variability in annual streamflow in the western United States. In general, above-average annual streamflow is associated with negative winter mean 700-Hpa height anomalies over the eastern North Pacific Ocean and/or the western United States. The anomalies produce an anomalous flow of moist air from the eastern North Pacific Ocean into the western United States that increases winter precipitation and snowpack accumulations, and subsequently streamflow. Winter mean 700-hPa height anomalies also account for statistically significant differences in spatial distributions of annual streamflow. As part of this study, winter mean atmospheric circulation patterns for the 40 years analysed were classified into five winter mean 700-hPa height anomaly patterns. These patterns are related to statistically significant and physically meaningful differences in spatial distributions of annual streamflow. 相似文献
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PHILIP J. ASHWORTH GREGORY H. SAMBROOK SMITH JAMES L. BEST JOHN S. BRIDGE STUART N. LANE IAN. A. LUNT ARNOLD J. H. REESINK CHRISTOPHER J. SIMPSON ROBERT E. THOMAS 《Sedimentology》2011,58(7):1860-1883
The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture. 相似文献
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On the effect of moisture bonding forces in air-dry soils on threshold friction velocity of wind erosion 总被引：5，自引：0，他引：5
SUJITH RAVI TED M. ZOBECK† THOMAS M. OVER‡ GREGORY S. OKIN PAOLO D'ODORICO 《Sedimentology》2006,53(3):597-609
Wind erosion is a dominant geomorphological process in arid and semi-arid regions with major impacts on regional climate and desertification. The erosion process occurs when the wind speed exceeds a certain threshold value, which depends on a number of factors including surface soil moisture. The understanding and modelling of aeolian erosion requires a better understanding of the soil erodibility associated with different moisture conditions. In arid regions during the dry season, the atmospheric humidity plays an important role in determining the surface moisture content and the threshold shear velocity. By a series of wind tunnel tests and theoretical analyses, this dependence of threshold velocity on near surface air humidity is shown for three soils of different textures: sand, sandy loam, and clay loam. The results show that the threshold shear velocity decreases with increasing values of relative humidity for values of relative humidity between about 40% and 65%, while above and below this range the threshold shear velocity increases with air humidity. A theoretical framework is developed to explain these dependencies assuming an equilibrium between the surface soil moisture and the humidity of the overlying atmosphere. The conditions under which soil-atmosphere equilibrium occurs were tested experimentally in the laboratory for different soils in order to determine the effect of grain surface area and texture on the time required to reach equilibrium starting from different initial conditions. 相似文献
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Abstract Statistics of seasonal and daily rainfall in nine regions covering the United Kingdom are presented, together with the best currently available predictions of changes in rainfall and evaporation, based on various general circulation model (GCM) assessments of the climate 40 years ahead. On that basis, runoff sequences are generated for the region of northwest England with north Wales (NWE & NW, humid temperate) and for southeast England (SEE, temperate but less humid), both for the current climate and for the year 2030 estimate, the latter incorporating higher evaporation losses. The future annual runoff is reckoned as 8% less than the current average in the SEE region and 4% less than that in the NWE & NW region. Direct supply reservoir yield versus storage graphs are then derived in each case for a range of duration-based probability of emptiness, their yields (i.e. sustained draw-off rates) and storage capacities being expressed as proportions of current annual runoff. The year 2030 graphs fall systematically below those for the current climate, representing 8% to 15% loss of yield from existing storage in the SEE region and 4% to 25% loss of yield in the NWE & NW region. 相似文献
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Abstract Actual discharges from two experimental catchments are related to the corresponding measurements of the length of channel flow; within a single catchment this is equivalent to drainage density. Drainage densities ranging from less than 1.0 to nearly 10.0 are associated with discharges ranging from 0.15 to nearly 20.0 c.f.s. per square mile. In each catchment all discharge values (Q) and drainage density values (Dd) are related by a function approximately of the form Q ∝ Dd2, which is then discussed in relation to studies made of the variation of streamflow and drainage density between catchments. It is suggested that both peak flow and base flow are related to drainage density in the same way. Studies which include drainage density as a basin characteristic and which relate it to water yield should acknowledge that its value varies within any one catchment, and therefore that the drainage density values derived by a particular method can be related only to one type of flow. In Great Britain the watercourses shown on the 1:10,560 maps relate to specific low flow discharges. 相似文献