Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)   总被引：1，自引：0，他引：1  

Sadrack Flix Toteu  Joseph Penaye  Etienne Deloule  William Randall Van Schmus  Rigobert Tchameni 《Journal of African Earth Sciences》2006,44(4-5):428

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.  相似文献   

Evolved mare basalt magmatism, high Mg/Fe feldspathic crust, chondritic impactors, and the petrogenesis of Antarctic lunar breccia meteorites Meteorite Hills 01210 and Pecora Escarpment 02007     

James M.D. Day  Christine Floss  Lawrence A. Taylor  Mahesh Anand  Allan D. Patchen 《Geochimica et cosmochimica acta》2006,70(24):5957-5989

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Investigating the scale of structural controls on chlorinated hydrocarbon distributions in the fractured-porous unsaturated zone of a sandstone aquifer in the UK     

Adrian Lawrence  Marianne Stuart  Colin Cheney  Neil Jones  Richard Moss 《Hydrogeology Journal》2006,14(8):1470-1482

Contaminant migration behaviour in the unsaturated zone of a fractured porous aquifer is discussed in the context of a study site in Cheshire, UK. The site is situated on gently dipping sandstones, adjacent to a linear lagoon historically used to dispose of industrial wastes containing chlorinated solvents. Two cores of more than 100 m length were recovered and measurements of chlorinated hydrocarbons (CHCs), inorganic chemistry, lithology, fracturing and aquifer properties were made. The results show that selecting an appropriate vertical sampling density is crucial both to providing an understanding of contaminant pathways and distinguishing whether CHCs are present in the aqueous or non-aqueous phase. The spacing of such sampling should be on a similar scale to the heterogeneity that controls water and contaminant movement. For some sections of the Permo-Triassic aquifer, significant changes in lithology and permeability occur over vertical distances of less than 1 m and samples need to be collected at this interval, otherwise considerable resolution is lost, potentially leading to erroneous interpretation of data. At this site, although CHC concentrations were high, the consistent ratio of the two main components of the plume (tetrachloroethene and trichloroethene) provided evidence of movement in the aqueous phase rather than in dense non-aqueous phase liquid (DNAPL).  相似文献   

Basement-controlled faulting of Paleozoic strata in southern Ontario, Canada: new evidence from geophysical lineament mapping     

Joseph I. Boyce  William A. Morris 《Tectonophysics》2002,353(1-4)

Basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins. In southern Ontario, the basement consists of complexly structured mid-Proterozoic (ca. 1.2 Ga) crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of Paleozoic sedimentary strata. Reactivation of basement structures is suspected to control the location of Paleozoic fault and fracture systems, but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement. New aeromagnetic- and gravimetric-lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of Paleozoic fault and fracture systems. Lineament mapping was facilitated by reprocessing and digital image enhancement (micro-levelling, regional residual separation, derivative filtering) of existing regional gravity and aeromagnetic datasets. Reprocessed images identify new details of the structural fabric of the basement below southern Ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones. Linear zones parallel the projected trends of mid-Proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area, and are interpreted as zones of shearing and basement faulting. Mapped aeromagnetic and gravity lineaments show similar trends to Paleozoic faults and fracture networks and broad zones of seismicity in southern Ontario. These new data support an ‘inheritance model’ for Paleozoic faulting, involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata.  相似文献   

Gold deposits of the northern margin of the North China Craton: multiple late Paleozoic–Mesozoic mineralizing events   总被引：8，自引：0，他引：8  

Craig J. Hart  Richard J. Goldfarb  Yumin Qiu  Lawrence Snee  Lance D. Miller  Marti L. Miller 《Mineralium Deposita》2002,37(3-4):326-351

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Formation of refractory inclusions by evaporation of condensate precursors     

Lawrence Grossman  Denton S Ebel  Steven B Simon 《Geochimica et cosmochimica acta》2002,66(1):145-161

Berman’s (1983) activity-composition model for CaO-MgO-Al₂O₃-SiO₂ liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO₂, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of P_H2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at P_H2 = 10⁻⁶ bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO₂ evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO₂ evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO₂-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO₂ at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO₂ and <12% MgO require precursors with higher SiO₂/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.  相似文献   

The chemistry of lava-seawater interactions II: the elemental signature     

Joseph A Resing  Francis J Sansone 《Geochimica et cosmochimica acta》2002,66(11):1925-1941

The flow of lava into the ocean at the shoreline of Kilauea Volcano during the ongoing Pu’u O’o eruption has allowed a detailed study of the geochemical interaction between lava and seawater. This paper focuses on the chemistry of the major and minor elements in the fluids that resulted from this interaction. The elemental enrichments in these fluids are dominated by three processes: (1) evaporation of water from seawater, which creates solutions enriched in the major elements found in seawater, (2) congruent dissolution of the basalt glass matrix, which is limited by the solubility of some of the elements in seawater, and (3) removal of volatile phases from the lava on contact with seawater.Using a simple model of volatile emanation (using published emanation coefficients) and congruent dissolution, we are able to explain the concentrations observed for the majority of elements in precipitation from the steam plume at the shoreline lava entry and in water allowed to interact with molten lava in controlled experiments. Fe, Al, Ti, and some of the rare earth elements (REEs) in precipitation samples from the steam plume at the lava entry were > 10,000-times enriched over their ambient seawater concentrations, suggesting that these elements may be useful for identifying submarine eruptions. The flux of elements from the Kilauea ocean lava entry is greater than that from a typical midocean ridge hydrothermal vent field for Al, Cd, Co, and the REEs, whereas the opposite is true for the remainder of the elements studied.  相似文献   

Experimental silicate mineral/melt partition coefficients for beryllium and the crustal Be cycle from migmatite to pegmatite   总被引：1，自引：0，他引：1  

Joseph M Evensen  David London 《Geochimica et cosmochimica acta》2002,66(12):2239-2265

Partition coefficients (D_Be^mineral/melt) for beryllium between hydrous granitic melt and alkali feldspars, plagioclase feldspars, quartz, dark mica, and white mica were determined by experiment at 200 MPa H₂O as a function of temperature (650-900°C), activity of Be in melt (trace levels to beryl saturation), bulk composition, and thermal run direction. At trace levels, Be is compatible in plagioclase of An₃₁ (1.84 at 700°C) and muscovite (1.35 at 700°C) but incompatible in biotite (0.39-0.54 from 650-800°C), alkali feldspar (0.38-0.19 from 680-850°C), quartz (0.24 at 800°C), and albite (0.10 at 750°C). The partition coefficients are different at saturation of the melt in beryl: lower in the case of plagioclase of An₃₁ (0.89 at 700°C), muscovite (0.87 at 700°C), biotite (0.18-0.08 from 675-800°C), alkali feldspar (0.18-0.14 from 680-700°C), and quartz (0.17-0.08 from 750-800°C), but higher in the case of albite (0.37 at 750°C).With other data sources, these new partition coefficients were utilized to track, first, the distribution of Be between aluminous quartzofeldspathic source rocks and their anatectic melts, and second, the dispersion or concentration of Be in melt through igneous crystal fractionation of different magma types (e.g., S-type, I-type) up to beryl-saturated granitic pegmatites and, finally, into their hydrothermal aureoles. Among the rock-forming minerals, cordierite, calcic oligoclase, and muscovite (in this order) control the fate of Be because of the compatibility of Be in these phases. In general, beryl-bearing pegmatites can arise only after extended crystal fractionation of large magma batches (to F, fraction of melt remaining, ≤0.05); granitic magmas that originate from cordierite-bearing protoliths or that contain large modal quantities of calcic oligoclase will not achieve beryl saturation at any point in their evolution.  相似文献   

Determining the trophic guilds of fishes and macroinvertebrates in a seagrass food web     

Joseph J. Luczkovich  Garcy P. Ward  Jeffrey C. Johnson  Robert R. Christian  Daniel Baird  Hilary Neckles  William M. Rizzo 《Estuaries and Coasts》2002,25(6):1143-1163

We established trophic guilds of macroinvertebrate and fish taxa using correspondence analysis and a hierarchical clustering strategy for a seagrass food web in winter in the northeastern Gulf of Mexico. To create the diet matrix, we characterized the trophic linkages of macroinvertebrate and fish taxa present inHalodule wrightii seagrass habitat areas within the St. Marks National Wildlife Refuge (Florida) using binary data, combining dietary links obtained from relevant literature for macroinvertebrates with stomach analysis of common fishes collected during January and February of 1994. Heirarchical average-linkage cluster analysis of the 73 taxa of fishes and macroinvertebrates in the diet matrix yielded 14 clusters with diet similarity ≥ 0.60. We then used correspondence analysis with three factors to jointly plot the coordinates of the consumers (identified by cluster membership) and of the 33 food sources. Correspondence analysis served as a visualization tool for assigning each taxon to one of eight trophic guilds: herbivores, detritivores, suspension feeders, omnivores, molluscivores, meiobenthos consumers, macrobenthos consumers and piscivores. These trophic groups, corss-classified with major taxonomic groups, were further used to develop consumer compartments in a network analysis model of carbon flow in this seagrass ecosystem. The method presented here should greatly improve the development of future network models of food webs by providing an objective procedure for aggregating trophic groups.  相似文献   

Analytical Solution for Subsurface Gas Flow to a Well Induced by Surface Pressure Fluctuations   总被引：2，自引：0，他引：2  

Joseph Rossabi  Ronald W. Falta 《Ground water》2002,40(1):67-75

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