Geochemistry and petrogenesis of tertiary granitic rocks from the Island of Mull,Northwest Scotland

The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values for a comprehensive collection of granitic rocks from the 3 centres. The ¹⁸O values range from +4 to –6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of ¹⁸O with other major and trace element data and ⁸⁷Sr/⁸⁶Sr ratios are uniformly low, apart from Fe₂O₃. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks.These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO₂ value, the Centre 1 granites have higher Na₂O, MgO, P₂O₅, TiO₂, Sr, and V and lower Al₂O₃, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher Ce_N/Yb_N ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial ⁸⁷Sr/⁸⁶Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of ⁸⁷Sr/⁸⁶Sr with ⁸⁷Rb/⁸⁶Sr. The initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the ⁸⁷Sr/⁸⁶Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71003 ±36.Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively primitive chemical composition and high initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under open system conditions.With oxygen isotope analyses by J.J. Durham, Geochemical Division, Institute of Geological Sciences, 64–78 Grays Inn Road, London, WC1X 8NG, England     

Photographic evidence of rate of development of plant cover on the emergent island Anak Krakatau from 1971 to 1991 and implications for the effect of volcanism

Photographs of the vegetated North and East Forelands of Anak Krakatau, which emerged from the sea in 1930 and was totally devastated by volcanic activity in 1952/53 provide some evidence of rate of change of plant cover from grassland to woodland between 1971 and 1991. The series of photographs demostrate that vegetational stages of the two forelands have not been synchronous and suggest that the North Foreland is lagging the East Foreland by about 12 years.A recent hypothesis that Anak Krakatau's vegetation was destroyed, totally or almost so, by eruptions in 1972/73 and the island's vegetation has regenerated since that time is not easily accommodated by the photographic evidence, nor by the relative rates of change on Rakata (Krakatau's post-1883 remnant) and Anak Krakatau. The photographic evidence accords more closely with a hypothesis that the 1972/73 eruptions affected the North Foreland's vegetation only, or affected it to a much greater degree than that of the East Foreland, allowing the latter's post-1952/53 succession to proceed relatively unhampered.     

Soil pipes and their role in runoff generation and chemical denudation in a humid tropical catchment in dominica

Numerous soil pipes are reported from a small catchment in an area of kandoid (kaolin-rich) soils and 2375 mm annual rainfall in the humid tropical volcanic island of Dominica, West Indies. Two spot surveys in August 1982 at baseflow during the wet season indicated that pipes contributed at least 14–16 per cent of streamflow. Sampled pipeflow was of similar chemical composition to the baseflow-dominated streamwater. Specific conductance (249–420 μS cm^?1) and silica (75 mg l^?1) levels of pipeflow were high and suggested that the pipes tapped solute-rich water close to the soil-rock interface. The principal pipes flow perennially and showed little response to daily rainfalls of up to 43 mm during the monitoring period.     

High-Pressure (~2000 MPa) Kyanite- and Glaucophane-bearing Pelitic Schist and Eclogite from Cordillera de la Costa Belt, Venezuela

Cretaceous melange of the Cordillera de la Costa belt, north–centralVenezuela, there are knockers of eclogite, barroisite-bearingeclogite, and pelitic glaucophane schist. These occur in a metamorphicmelange matrix that locally consists of marble, serpentinite,amphibolite, actinolite schist, feldspathic schist and gneiss,graphitic schist, chloritoid schist, and garnet-bearing micaschist. The protoliths for these various rock types exhibita wide age range (Cambrian to Early Cretaceous?). Recently discoveredknockers of pelitic glaucophane schist contain Mg-glaucophane+ paragonite + kyanite + garnet + talc + graphite + rutile +quartz. The coexistence of kyanite and Mg-glaucophane suggestsminimum P 2000 MPa at T > 600°C. Eclogite knockers fromthe same outcrop contain garnet and clinopyroxene which yield500°C for cores, 700°C for rims, and P 1200 MPa. Theassemblage garnet–biotite–phengite–albitewithin schists of the melange matrix of this locality indicatesmetamorphic conditions of T = 450–520°C at P = 1800MPa. Because all lithologies in this outcrop record high-P conditions,this metamorphic melange formed before or during peak metamorphismin a mid-Cretaceous subduction zone. KEY WORDS: geothermobarometry; high-P pelitic schist; eclogite; Puerto Cabello; Venezuela     

Landscape Frontiers,Geography Frontiers: Lessons to Be Learned

Advancing ecotones, such as treelines and frontiers of human settlement, may share some characteristic dynamics because both include feedbacks between spatial pattern and process. Both might be examined as complex, self‐organizing systems in terms of complexity theory and thus be usefully compared. A cellular automaton of advancing alpine treeline in Montana shows attractors in power‐law frequency distributions of spatial and temporal pattern. Frontiers of study areas in the Amazonian region of Ecuador, analyzed using change detection of Landsat Thematic Mapper imagery, have power‐law distributions of advancing deforestation. Alternative approaches in self‐organized complexity, including self‐organized percolation, and the inverse cascade model, and an approach to complexity involving optimization, highly optimized tolerance, are considered. Some combination of these, based on their common ancestry in percolation theory (with its ties to geocomputation), might provide insights into population‐environment interactions at settlement frontiers and ecotones together, given comparisons drawn between the spatial feedbacks at alpine treeline and in Ecuador. GIScience and landscape ecology can develop synergies by building on this area of geocomputation and complexity theory, as in analysis of attractors in state spaces of spatial metrics from spatially explicit simulations and representing their uncertainty.     

The variation of cesium and 37 other elements in the Sardinian granite batholith,and the significance of cesium for granite petrogenesis

A suite of granites from the Hercynian batholith of Sardinia (Italy) was analysed for cesium and 37 other major and trace elements.Factor analysis was used to identify inter-element associations in this set of data. Cesium is shown to be closely associated with tin, lithium and rubidium, and less closely with beryllium and thallium, but it shows no correlation with any of the major elements. The role of crystal fractionation, magma mixing, and the process of partial melting in the magmatic source region are examined as possible controls of Cs distribution. The pattern of element associations revealed by factor analysis eliminates crystal fractionation and magma mixing as the main cause of magmatic variation. The variation in Cs is considered to be primary in origin, i.e. it originated in the magmatic source region. The relationship between Cs and other elements was mainly determined by the relative proportions of mica and feldspar entering the melt during anatexis.     

Habitat utilization by small flatfishes in a North Carolina estuary

Distribution and abundance of flatfish species (<150 mm standard length) were related to habitat characteristics in the Newport River and Back Sound estuaries in North Carolina. Salinity, turbidity, depth, distance from marsh edge, benthic composition, and grain size were used to describe the different shallow water habitats from April through October 1994. One Scophthalmidae, seven Paralichthyidae, one Achiridae, and one Cynoglossidae species were collected during the study including juvenileParalichthys albigutta (gulf flounder),P. dentatus (summer flounder), andP. lethostigma (southern flounder) along with multiple age classes ofCitharichthys spilopterus (bay whiff),Etropus crossotus (fringed flounder),Symphurus plagiusa (blackcheek tonguefish), andTrinetes maculatus (hogchoker). Incidental catches ofAncylopsetta quadrocellata (ocellated flounder),C. macrops (spotted whiff), andScopthalmus aquosus (windowpane) were also made. Flatfish distributions among habitats varied by species, size within species, and season. Regardless of season, the highest densities of flatfishes were found in the upper estuary. All habitats were used by one or more species and most species occurred at several habitats. Some species were significantly more abundant at specific habitat types. Ontogenetic shifts in habitat utilization were found for several species. High densities of smallP. lethostigma, C. spilopterus, S. plagiusa, andT. maculatus occurred in the upper estuary on muddy substrates. LargeP. dentatus, C. spilopterus, S. plagiusa, andT. maculatus utilized sand flats and channels in the lower estuary.     

Intermittent heating in a model of solar coronal loops

X-ray and EUV observations of the solar corona reveal a very complex and dynamic environment where there are many examples of structures that are believed to outline the Sun's magnetic field. In this present study, the authors investigate the temporal response of the temperature, density and pressure of a solar coronal plasma contained within a magnetic loop to an intermittent heating source generated by Ohmic dissipation. The energy input is produced by a one-dimensional MHD flare model. This model is able to reproduce some of the statistical properties derived from X-ray flare observations. In particular the heat deposition consists of both a sub-flaring background and much larger, singular dissipative events. Two different heating profiles are investigated: (a) the spatial average of the square of the current along the loop and (b) the maximum of the square of the current along the loop. For case (a), the plasma parameters appear to respond more to the global variations in the heat deposition about its average value rather than to each specific event. For case (b), the plasma quantities are more intermittent in their evolution. In both cases the density response is the least bursty signal. It is found that the time-dependent energy input can maintain the plasma at typical coronal temperatures. Implications of these results upon the latest coronal observations are discussed.     

Saharan dust and phosphatic fidelity: A three-dimensional biogeochemical model of Trichodesmium as a nutrient source for red tides on the West Florida Shelf

The availability of iron within the surface waters of the broad, oligotrophic West Florida Shelf (WFS) controls periodic blooms of the pelagic marine cyanobacterium Trichodesmium. Summer delivery of Saharan dust provided adequate iron (Fe) to shift limitation of growth to the availability of phosphorus (P). Florida's rivers drain Miocene phosphorus deposits to provide the WFS with freshwater nutrient supplies at molar dissolved inorganic nitrogen/phosphate (DIN/PO₄) ratios of <6. These diazotrophs draw upon ubiquitous stocks of dissolved nitrogen gas, once stimulated by Fe-deposition within P-replete waters of the WFS.