The genesis of Variscan (Hercynian) plutonic rocks: Inferences from Sr,Pb, and O studies on the Maladeta igneous complex,central Pyrenees (Spain)

The Maladeta plutonic complex formed during the latest stages of the Variscan orogeny. It was emplaced into the Paleozoic sedimentary sequence of the Pyrenees. The eastern part, investigated in the present study, consists of an early intrusion of cumulate gabbronorites followed in order of emplacement by the main biotite-hornblende granodiorite, which was itself intruded by two small stocks of two-mica cordierite granite. An ⁸⁷Rb-⁸⁷Sr isochron dates the granodiorite at 277±7 m.y. with an initial (⁸⁷Sr/⁸⁶Sr)_o ratio of 0.7117±3. Gabbroic rocks have lower strontium initial ratios, down to 0.7092, while those of granite range from that of the granodiorite up to about 0.715. The three rock types have distinctive ¹⁸O values: 8.7 to 9.6 for the gabbronorites, 9.4 to 10.4 for the granodiorites and 10.3 to 11.8 for the granites. Lead isotopic compositions of rocks and feldspars are all radiogenic. Feldspars give consistent Pb model ages around 280 m.y., with and values of about 9.7 and 4.05, respectively. No pristine, mantle-derived magma was found among the investigated samples and the rocks cannot be related to one another by any simple mechanism of fractional crystallization. Some type of mixing process involving two end members seems to be required: a high-¹⁸O, high-⁸⁷Sr material that is clearly of crustal origin, and a lower-¹⁸O, lower-⁸⁷Sr end member derived from the mantle. Examination of various mixing models does not support magma mixing nor assimilation of crustal rocks by a mafic magma. The most acceptable model involves melting at different levels of a vertically-zoned source in the continental crust; this source was formed by mixing between mantle-derived magmas and crustal metasediments. This material was apparently thickened, tectonically downwarped and partially melted. None of the Maladeta magma-types appear to have been derived at a consuming plate boundary.Contribution Number 3280, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA     

Searching for the ‘Right’ Legend: The Impact of Legend Position on Legend Decoding in a Cartographic Memory Task

ABSTRACT

Map legends are key elements of thematic maps and cartographic communication. The question of how to style map legends is a topic which has often been addressed by cartographic academics and practitioners. Nevertheless, the question of where to position a map legend has only hardly been discussed. Principles of cognitive sciences allow the assumption that a legend positioned to the right of a map field can be read and decoded faster than a legend on the left side. This study investigates the impact of legend positioning on legend decoding. It involves an experiment based on a recognition memory paradigm and the registration of eye-movements. The results show that, in less time, a legend positioned to the right of the map field (compared to a left legend) can be decoded faster. The same accuracy of a cognitive representation of geographic space can be achieved in spatial memory.     

Trends in Extremes of Surface Humidity, Temperature, and Summertime Heat Stress in China

l. IntroductionPrevious studies have shown climatological and seasonal surface temperature and humld-ity dlstributlons over China (Wang and Gaffen, 200l ) and evaluated trends in seasonal meansof several surface humidity variables, rainfall, temperatures, and apparent temPerature for thelast half century (Easterling et al., 2000, Zhai et al., 1999, Zhai and Eskridge, l997, Zhai andRen, l997, Wang and Gaffen, 200l). However, trends in local temperatures, and particularlytrends in extreme he…     

Some implications of xenolith glasses for the mantle sources of alkaline mafic magmas

The assumption that mafic alkaline magmas are derived from mantle sources with a lherzolite mineralogy has become entrenched in the petrologic literature. Although it is commonly assumed that highly alkaline magmas require metasomatised mantle sources, there is little understanding of the spatial relation of such sources with respect to those of associated more Si-rich transitional magmas. Glasses developed in mantle xenoliths represent natural experiments which may provide some insight on this problem. Highly silica undersaturated glasses developed in the amphibole-garnet clinopyroxenite portion of a composite xenolith from Nunivak Island, Alaska, become quartz normative where they penetrate adjacent spinel lherzolite. A comparison of glass compositions in mantle pyroxenite and lherzolite xenoliths reveals that glasses developed in amphibole pyroxenite xenoliths are in general more silica undersaturated than those in lherzolite xenoliths. This suggests that some highly silica undersaturated magmas such as nephelinites may in fact be derived by the preferential melting of amphibole or amphibole-garnet pyroxenite veins and that the spectrum from nephelinite to transitional alkaline basalt that characterizes many individual alkaline volcanic suites is produced by mixing with melt derived from the host lherzolite as the degree of partial melting increases.     

Dispersion of spores released from an elevated line source within a wheat canopy

Turbulent dispersion of spores was studied near a source located inside a wheat canopy. Two colors of Lycopodium spores were released simultaneously at a steady rate from line sources at two heights (0.4–0.5 m and 0.7–0.8 m) in a 0.8 to 1.0 m tall crop. The number of spores of each color released was estimated by weighing the sources before and after each release. Aerial spore concentrations were measured at 2 and 4 m downwind of the sources using rotorods placed at four heights above the canopy and small suction traps at two heights inside the canopy. Concentrations near the ground were estimated from deposits on sticky glass microscope slides placed on the ground. Experiments were conducted on six different days. Friction velocities ranged from about 0.3 to 0.5 m s^–1. The average horizontal fluxes of spores were calculated as the product of the observed concentrations and average wind speeds. At a distance of 2 m downwind from the sources, more than 16 to 44% of the flux of spores released from the lower source and more than 41 to 50% of the flux of spores released from the upper source were estimated to be above the canopy. These fluxes were compared with fluxes calculated using both a K-theory model and a random-flight-fluid-parcel-trajectory simulation model. The fluxes predicted by the models were generally considerably less than the values determined experimentally.     

Response spectrum to strong aftershocks of the Lancang-Gengma earthquake

Near-field records of two strong aftershocks with magnitudeM _S=6.7 andM _S=6.3 in the Lancang-Gengma earthquakes sequence, Yunnan Province, November 1988, are used to calculate the response spectrum. The instruments, site conditions and the methods for computing are also illustrated in this paper. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 338–343, 1991. This project is supported by The Chinese Joint Seismological Science Foundation, SSB and the West Yunnan Earthquake Prediction Test Field, Yunnan Seismological Bureau.