Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline–peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Aïr, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO₂ content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent.     

Identifying Optimum Locations for Tropical Testing of United States Army Materiel and Systems

This paper is an example of applied military geography focusing on a particular problem arising from the Panama Canal Treaty of 1977. One of the provisions of the treaty required the United States (US) military to completely withdraw from and relinquish its military and government facilities to the Republic of Panama prior to midnight on 31 December 1999. In so doing, the US army turned over to the Panama government all of its military bases, some of which had been used since World War I to test materiel, equipment and systems within a tropical environment. Given its global mission and responsibilities, the US army has a well‐established practice of testing its materiel, equipment and systems throughout the entire range of potential operating environments. The US experience in the Pacific in World War II and in Southeast Asia during the Vietnam War clearly demonstrated the need to test the performance of new equipment under the harsh conditions found in tropical jungles. Furthermore, since 1960, nearly 75 per cent of all international and internal armed conflicts have been in countries that are totally or partially in the tropics. This paper presents the findings of a scientific panel that was organised to propose a new location for testing US army materiel and systems within a tropical environment. Rather than simply choosing a location analogous to Panama, the panel sought to develop a methodology that could be employed for site selection based on any set of environmental criteria. This study is a classical locational analysis and reconfirms the integrative nature of geography and its utility for solving complex problems that cut across disciplinary boundaries.     

An overview of research on Eulerian–Lagrangian localized adjoint methods (ELLAM)

For problems of convection–diffusion type, Eulerian–Lagrangian localized adjoint methods provide a methodology that maintains the accuracy and efficiency of Eulerian–Lagrangian methods, while also conserving mass and systematically treating any type of boundary condition. In groundwater hydrology, this framework is useful for solute transport, as well as vadose-zone transport, multiphase transport, and reactive flows. The formulation was originated around 1990 by the authors, Herrera and Ewing, in a paper that appeared in Advances in Water Resources [Adv. Water Resour. 13 (1990) 187]. This paper reviews the progress in the development, analysis, and application of these methods since 1990, and suggests topics for future work.     

D/H ratios in speleothem fluid inclusions: A guide to variations in the isotopic composition of meteoric precipitation?

D/H ratios of fluid inclusion waters extracted from²³⁰Th/²³⁴U-dated speleothems that were originally deposited under conditions of isotopic equilibrium should provide a direct estimate of the hydrogen isotopic composition of ancient meteoric waters. We present here D/H ratios for 47 fluid inclusion samples from thirteen speleothems deposited over the past 250,000 years at cave sites in Iowa, West Virginia, Kentucky and Missouri. At each site glacial-age waters are depleted in deuterium relative to those of interglacial age. The average interglacial/glacial shift in the hydrogen isotopic composition of meteoric precipitation over ice-free areas of east-central North America is estimated to be ?12‰. This shift is consistent with the present climatic models and can be explained in terms of the prevailing pattern of atmospheric circulation and an increased ocean-continent temperature gradient during glacial times which more than compensated for the increase in deuterium content of the world ocean.     

Structural studies of imogolite and allophanes by aluminum-27 and silicon-29 nuclear magnetic resonance spectroscopy

High-resolution aluminum-27 and silicon-29 nuclear magnetic resonance spectra of natural and synthetic imogolites and allophanes obtained using high-field“magic-angle” sample-spinning (MASS) techniques indicate that the imogolite and protoimogolite components of allophanes are characterized by sharp (≈3 ppm) silicon-29 resonances at ?78±1 ppm from tetramethylsilane (in accord with Barron et al. 1982), and quite narrow (≈10 ppm at 11.7 Tesla) aluminum-27 resonances, at 5.2±1 ppm from Al(H₂O) ₆ ³⁺ (in accord with Wilson et al. 1984). However, the spectra of natural allophanes usually contain significant intensity arising from a less well defined material, characterized by a broad (≈20 ppm) silicon-29 resonance centered at ?90±2 ppm from tetramethylsilane, and a second relatively narrow (≈15 ppm at 11.7 Tesla) aluminum-27 resonance at 58.5±2 ppm from Al(H₂O) ₆ ³⁺ . Similar characteristic spectral features are exhibited by a synthetic amorphous Si:Al (1:1) gel, and presumably indicate the presence of framework aluminosilicate materials in the gel, and in most allophanes.     

Correlation of the Rockland ash bed,a 400,000-year-old stratigraphic marker in northern California and western Nevada,and implications for middle Pleistocene paleogeography of central California

Outcrops of an ash bed at several localities in northern California and western Nevada belong to a single air-fall ash layer, the informally named Rockland ash bed, dated at about 400,000 yr B.P. The informal Rockland pumice tuff breccia, a thick, coarse, compound tephra deposit southwest of Lassen Peak in northeastern California, is the near-source equivalent of the Rockland ash bed. Relations between initial thickness of the Rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the Mazama ash from Crater Lake, Oregon. Identification of the Rockland tephra allows temporal correlation of associated middle Pleistocene strata of diverse facies in separate depositional basins. Specifically, marine, littoral, estuarine, and fluvial strata of the Hookton and type Merced formations correlate with fluvial strata of the Santa Clara Formation and unnamed alluvium of Willits Valley and the Hollister area, in northwestern and west-central California, and with lacustrine beds of Mohawk Valley, fluvial deposits of the Red Bluff Formation of the eastern Sacramento Valley, and fluvial and glaciofluvial deposits of Fales Hot Spring, Carson City, and Washoe Valley areas in northeastern California and western Nevada. Stratigraphic relations of the Rockland ash bed and older tephra layers in the Great Valley and near San Francisco suggest that the southern Great Valley emerged above sea level about 2 my ago, that its southerly outlet to the ocean was closed sometime after about 2 my ago, and that drainage from the Great Valley to the ocean was established near the present, northerly outlet in the vicinity of San Francisco Bay about 0.6 my ago.     

Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to upper-mantle structure

Relative travel-time residuals computed from clear P-wave arrivals at fourteen seismograph stations in the North Island, New Zealand, from five deep-focus events in the Banda Sea region, show large spatial variations of up to 3 sec. The variations can be explained by higher than normal velocities in the oceanic lithosphere which is underthrust to depths of 350 km beneath the North Island. After correction for crustal structure, the residuals imply an average P-wave velocity about 11% higher than in the surrounding mantle. The lack of suitable source events at azimuths other than northwest prevents a more detailed investigation by this means.     

Winds in southeast Queensland and rain in Australia and their possible long-term relationship with sunspot number

Wind records at Brisbane indicate past changes in ambient weather systems that appear to be confirmed by observed changes in Australian rainfalls. Both wind and rainfall data support geological field evidence of a climatic change in southeast Queensland in the 1930s. At Brisbane the numbers of calms observed in January and July declined from 1887 to 1935, and then increased again. This variation was associated with changes in wind patterns, particularly in winter (July) from 1933 to 1937 and afterwards, but also in summer (January) during the 1930s. The changes in circulation were enough to explain the movement of beach and dune sands at that time. The changes in windiness show a close connection with the 80-year trend in sunspot number: the initial period of below average sunspot number coincided with increasing wind strength and more southeasterly winds. Rainfall trends show similar associations with sunspot trends, but the direction of rainfall change is different in different regions. In southeast Australia changes in rainfall are positively correlated with trends in sunspot number over the last 80 years, while the opposite holds for the far southwest and Cape York Peninsula. On the north coast negative correlations occur in summer, and positive correlations in winter, but in Queensland and western New South Wales the correlations are positive in summer and negative in winter. It is too soon to tell whether the connection between sunspots and weather is accidental or functional.     

A new interpretation of the structure of disordered α-cristobalite

Disordered -cristobalite which occurs extensively in bentonites, opals and particularly deep-sea cherts, has been previously interpreted in terms of a unidimensionally disordered structure in which cristobalite is interstratified with two-layer tridymite-like sheets. An alternative interpretation is that the structure is essentially tridymitic but that the sheets are stacked with random transverse displacement normal to the c axis, an arrangement similar to the translational, turbostratic stacking postulated for smectites. This interpretation was arrived at after a comparitive study of a silica phase in an Italian bentonite and a deep-sea chert, material which yielded an X-ray powder pattern almost identical with that of disordered -cristobalite, but electron diffraction patterns and infrared spectra more consistent with tridymite. It is suggested that this type of silica, which has been described almost universally as cristobalite, is more appropriately referred to as disordered -tridymite.Thanks are due to Mr R. H. S. Robertson, Pitlochry for the sample of bentonite and Mr D. S. Cassidy, Associate Curator, Antarctic Research Facility, Florida State University, U.S.A. for the sample of deep-sea chert.