Age of Jurassic continental tholeiites of French Guyana, Surinam and Guinea: Implications for the initial opening of the Central Atlantic Ocean     

Katja Deckart  Gilbert F  raud  Herv  Bertrand 《Earth and Planetary Science Letters》1997,150(3-4):205-220

A detailed ⁴⁰Ar/³⁹Ar study, of mineral separates from the Jurassic Atlantic Continental Tholeiites (JACT) of Guyana (French Guyana and Surinam, South America), and Guinea (West Africa) related to the initial opening of the Central Atlantic, has been carried out. In French Guyana, plateau ages of 196.0 ± 5.7 Ma and 196.1 ± 7.5 Ma were obtained on single, small amphibole grains from NNW—SSE trending dykes. In Guinea, single biotite grains from intrusive formations from the Kakoulima and Fouta Djalon areas yielded plateau ages of 200.4 ± 0.2 Ma and 194.8 ± 0.5 Ma, concordant with high temperature apparent ages on other biotites. The bulk plagioclase samples display disturbed age spectra due to alteration and excess argon. However, intermediate temperature, weighted mean plagioclase ages are similar in both regions of Guyana and Guinea, ranging from 200.2 ± 2.4 Ma to 188.7 ± 1.9 Ma, partly in agreement with the amphibole and biotite data.

These data, combined with previous ⁴⁰Ar/³⁹Ar and U/Pb results from the northern part of the Central Atlantic margins, indicate intense magmatic activity distributed over a large area from Iberia to Liberia (ca. 4500 km long) for a short period of time (204-195 Ma, perhaps less for the bulk of the magmatism) during the initial break-up of Pangea continent. These data do not support an initiation of the magmatism from a radial volcano-tectonic system centred in the south of the region, as suggested by May [1], and the initial break-up seems to affect the whole Central Atlantic during a period of 9 Ma.  相似文献   

Petrogenesis and source characteristics of metatholeiites from the Archean Ramagiri schist belt, eastern part of Dharwar craton, India     

John K. Zachariah  V. Rajamani  Gilbert N. Hanson 《Contributions to Mineralogy and Petrology》1997,129(1):87-104

The N–S trending, 2–4 km wide Ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks, separated and surrounded by granitic rocks of distinct characteristics. The metavolcanic rocks are tholeiitic in composition and are very similar in their major element composition as well as in their abundances of some trace elements. However, the rare earth elements (REE) require distinct sources. The rocks of the amphibolite facies eastern block have LREE depleted REE patterns ([Ce/Yb] = 0.7–0.9), requiring derivation from depleted mantle-like sources. The greenschist facies metatholeiitic rocks of the central block have LREE enriched REE patterns ([Ce/Yb] = 3–6), reflecting the nature of their source(s). The Nd isotopic data require that the LREE enriched nature could not have been attained significantly prior to its melting. The fine-grained, upper greenschist facies metatholeiites of the western block have flat to slightly LREE depleted patterns ([Ce/Yb] = 0.8–0.95). Minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of the three blocks. Different extents of partial melting of distinct mantle sources have played a dominant role in the generation of the parent magmas to the central versus eastern and western block metatholeiites. The geochemical data suggest that the mantle sources were non-lherzolitic, and that these sources may have seen previous episodes of melt addition and extraction prior to melting that gave rise to the parent melts to the rocks ∼2750 Ma ago. The REE data indicate that while the sources of the eastern and western block rocks were similar to depleted mantle (ɛ_{Nd( i )} about +2), the source of the central block rocks (ɛ_{Nd( i )} about +3.5) were enriched in large ion lithophile element (LILE)-rich fluids/melts probably derived from subducting oceanic crust. This and other trace element signatures point to magma extraction in tectonic settings similar to modern island arcs. Subsequent to magma emplacement and crystallization, all the three suites of rocks were affected by interaction with low-temperature, crustal derived fluids (ɛ_{Nd 2750Ma} of about −8 to −12), probably during the accretion of the three blocks of the belt in the present form. The inferred source characteristics, tectonic setting of magma generation and the crustal fluid processes seem to suggest that Phanerozoic-style tectonic processes may have been important in the generation of Archean crust in the Dharwar craton. Received: 31 July 1995 / Accepted: 12 May 1997  相似文献   

Gabbro and related rock emplacement beneath rifting continental crust: UPb geochronological and geochemical constraints for the Galicia passive margin (Spain)     

Urs Schrer  Jacques Kornprobst  Marie-Odile Beslier  Gilbert Boillot  Jacques Girardeau 《Earth and Planetary Science Letters》1995,130(1-4)

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites (‘peridotite ridge’) lying about 300 km off the coast in the North Atlantic ocean. From this ridge, a gabbro and a chlorite rock were studied in an attempt to substantiate rift-related subcontinental magmatism, occurring prior to sea-floor spreading. U-Pb dating of 13 different zircon fractions yields a precise age of 122.1 ± 0.3 Ma (2σ) for the emplacement of the chlorite rock protolith, from which more than 50% of Si and alkali-calc-alkali elements were lost during greenschist facies tectonometamorphism. Sr and Nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in LILE, relative to a chondritic reservoir. No evidence for the presence of continental material in the magma source regions can be observed. From the new zircon age of 122.1 ± 0.3 Ma, and earlier determined³⁹Ar⁴⁰Ar age of 122.0 ± 0.6 Ma for amphibole from the same locality, it can be documented that magma formation, solidification and unroofing of the mantle rocks occurred during a short period of time of about 3.4 Ma, which means that the peridotite ridge detached from the continent and rose to the surface immediately after, or even coevally with mantle melting.  相似文献   

Neural networks for quick earthquake damage estimation     

Gilbert L. Molas  Fumio Yamazaki 《地震工程与结构动力学》1995,24(4):505-516

This paper proposes the use of neural networks to predict damage due to earthquakes from the indices of recorded ground motion. Since the relationship between ground motion indices and resulting damage is difficult to express in mathematical form, neural networks are conveniently applied for this problem. Simulated earthquake ground motions are used to have a well-distributed data set and the ductility factor from non-linear analysis of two single-degree-of-freedom structural models is used to represent the damage. A sensitivity analysis procedure is described to identify qualitatively the input parameters that have a greater influence on the damage. The result of the trained neural network is then verified by using several recorded earthquake ground motions. It is found that some instability in the prediction can occur. Instability occurs when input values exceed the range of the training data. The neural network model using PGA and SI as input give the best performance in the recall tests using actual earthquake ground motion, demonstrating the usefulness of neural network models for the quick estimation of damage through earthquake intensity monitoring.  相似文献   

A distinction between ice-pushed and ice-lifted landforms on lacustrine and marine coasts     

Robert Gilbert 《地球表面变化过程与地形》1990,15(1):15-24

Study of a small lake, Second Roach Pond, in Maine, U.S.A. clarifies the distinction between shoreline features created by ice push which occurs on both lacustrine and marine coasts, and those created by ice lift related to tidal action in the marine environment. Ice lifting occurred as littoral sediments were frozen into the lake ice, followed by a rise in water level due to damming of the lake before the nival melt each spring. In the period 1905 to 1969 prominent barricades were constructed by this means. A classification based on this distinction is proposed. Ice-pushed landforms include ramparts, ridges in coarse and fine sediments, stone pavements, ice keel grooves, pushed boulder grooves, and tails off promontories. Ice-lift features include boulder barricades, perched stones, stone garlands, and ice keel depressions.  相似文献   

Effects of fluids on the interaction of granites with limestones: The Notch Peak stock,Utah     

Peter I. Nabelek  Gilbert N. Hanson  Theodore C. Labotka  James J. Papike 《Contributions to Mineralogy and Petrology》1988,99(1):49-61

In this paper we consider the mechanisms by which the mineralogy and composition of the margins of the Notch Peak granitic stock, Utah, were affected by calcareous argillite country rocks. The contact zone of the granite relative to the unaffected granite is enriched in: K₂O from about 4 to 10 wt.%, Rb from 250 to 510 ppm, Sr from 150 to 790 ppm and Ba from 250 to 2160 ppm. Locally, some of the intrusive rocks at the contact are nearly devoid of quartz and can be classified as syenites. The initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.7069 in the unaffected rocks to 0.7100 in the potassium-enriched samples, approaching the values of the calc-silicate country rocks of about 0.7110.Calculations show that the characteristics of the contact zone near the top of the stock are the result of a number of sequential processes. CO₂-rich fluids derived from the heated calcareous argillites apparently caused a shift in the phase boundaries in the magma, enhancing accumulation of K-feldspar and high-Ca augite at the expense of other phases. The accumulation resulted in the high Ba and Sr concentrations in some samples. However, the high K₂O and Rb concentrations and magmatic ¹⁸O values indicate infiltration of magmatic fluid emanating from the solidifying lower portions of the pluton subsequent to solidification of the cap. The minimum fluid-rock ratios of 4.6 by mass, calculated on the basis of K₂O and Rb concentration shifts, indicate that a substantial amount of the fluid was channeled through this contact zone. The desilication of the rocks forming the syenitic samples at the contact apparently occurred when a chemical potential gradient of silica between the granite and wall-rocks was established as quartz was consumed in the wall-rocks during calc-silicate reactions. The infiltrating magmatic fluid probably acted as a medium for transport of silica across the contact and perhaps exchange of Sr between the country rocks and the intrusion where up to 30% of strontium in the granitic and syenitic samples from the contact zone was derived from the calc-silicates. The syenitic rocks cannot be explained by desilication reactions involving assimilation of the calc-silicates by the granite magma. The results of this study show that fluids interacting with the country rocks need to be considered to explain the effects of country rocks on the composition of the margins of granitic intrusions.  相似文献   

Direct dating of Plio-Quaternary pumices by Ar/Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)     

Gilbert Fraud  Philippe Lo Bello  Chris M. Hall  Jean-Marie Cantagrel  Derek York  Michel Bernat 《Journal of Volcanology and Geothermal Research》1990,40(1)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give ⁴⁰Ar/³⁹Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the ⁴⁰Ar/³⁹Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.  相似文献   

Migration de la truite lacustre (Salmo trutta lacustris L.) dans le Léman et ses affluents     

Par Bernard Büttiker  Gilbert Matthey 《Aquatic Sciences - Research Across Boundaries》1986,48(2):153-160

A total of 2588 adult migratory brown trout (Salmo trutta lacustris) have been tagged from 1964 to 1974 on their spawning sites, in tributaries of the western part of Lake Geneva (Switzerland). Most of the fish (96.2%) recaptured during spawning seasons in the following years have been found in the same river where they had previously been caught. Most tagged fish recaptured by fishermen were found in the western part of the Lake, near their home rivers. This tendency is very strong in autumn and winter. It still remains highly significant in spring and summer.

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Dédié à Wolfgang Geiger  相似文献   

Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection     

R.P. Philp  T.D. Gilbert 《Geochimica et cosmochimica acta》1985,49(6):1421-1432

Biomarkers produced by microscale pyrolysis of extracted source rocks, kerogens and asphaltenes have been analysed directly by gas chromatography-mass spectrometry in the MID mode. A series of experiments have been undertaken. These include an investigation into the production of the hopanes at different pyrolysis temperatures and a comparison of results obtained from Curie point pyrolysis and the Chemical Data Systems pyroprobe. Steranes and triterpanes produced by pyrolysis of a series of vitrinites were investigated and correlated with maturity variations. Finally, the biomarkers produced from asphaltenes were compared with those produced from the extract and extracted rock in order to further investigate the theory that asphaltenes are a liquid kerogen. The results of these experiments showed that the biomarkers released by pyrolysis of source rocks have a potential use for source and maturity determinations. The method is advantageous in that it can be said with some certainty that the biomarkers are indigenous to the rock and not present in the rock as a possible result of migration.  相似文献   

Portable magnetometer theodolite with fluxgate sensor for earth's magnetic field component measurements     

J. Bitterly  J. M. Cantin  R. Schlich  J. Folques  D. Gilbert 《Surveys in Geophysics》1984,6(3-4):233-239

A new portable magnetometer theodolite with fluxgate sensor has been developed for measuring the components of the earth's magnetic field. With this instrument, used as a null detector, it is possible to make absolute measurements of declination (D) and inclination (I). The compensation of the main magnetic field allows direct intensity measurements (H, X, Y, Z components); in this case the calibration is made by reference to a proton magnetometer.The observation procedure is particularly easy and quick. The accuracy is better than 5 sec of arc forD andI and better than 1 nT for direct intensity measurements.  相似文献