The Late Precambrian Timna igneous complex, Southern Israel: Evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma   总被引：7，自引：0，他引：7  

Michael Beyth  Robert J. Stern  Rainer Altherr  Alfred Krner 《Lithos》1994,31(3-4):103-124

New data from a geochemical, geochronological and isotopic study of the Late Precambrian Timna igneous complex suggest the formation of alkali granites from a LIL-enriched, mantle derived, sanukitoid-type monzodiorite (a silica oversaturated rock with Mg# >60). These data also provide new insights into the petrology, timing and regional tectonic control of the transition from the calc-alkaline to the alkaline magmatic activity in the northern Arabian-Nubian Shield (ANS) during the Late Precambrian.

The Timna alkali granite was formed by fractional crystallization from the monzodioritic magma in a quasi-stratified magmatic cell which formed 610 Ma ago in the 625 Ma old calc-alkaline, porphyritic granite crust. These monzodiorites are mantle-derived, as demonstrated by their high Mg# (63), Cr (230 ppm), and Ni (120 ppm). They are characterized by initial ⁸⁷Sr/⁸⁶Sr of 0.7034, ε-Nd (610 Ma) = +3.4, and are enriched in K₂O (2.9%), Sr (840 ppm), Ba (1290 ppm) and LREE [(La/Lu)_n= 10–25]. The chemical characteristics and REE patterns of the monzodiorites and andesitic dykes of Timna are very similar to Dokhan andesites from northeastern Egypt and the Archean sanukitoids from Canada. The isotopic, geochemical and geochronologic data all indicate that Timna monzodiorites are comagmatic with the alkali granite. The alkali granite is a typical post-orogenic, borderline A-type granite. It is enriched in potassium (K₂O=4.68–6.64%), has a negative europium anomaly (Eu/Eu^*=0.058–0.38) and ε-Nd (610 Ma) of +3.9. The calc-alkaline granite is a typical I-type granite with a small positive europium anomaly (Eu/Eu^*=1.02–1.16). Its age and the Sr, Nd and Pb isotopic characteristics with ε-Nd (625 Ma) of +5.6 to +5.9 are significantly different from these of the alkali granite and monzodiorites, and indicate little interaction with the monzodiorite during the formation of the alkali granite.

The alkali granites are correlative with the post-collisional extensional granites in Jordan and Egypt while the porphyritic granites can be correlated with the late orogenic types. Crustal thickening associated with orogenic compression resulted in crustal anatexis to form the I-type granitic rocks, whereas crustal thinning associated with extension allowed LIL-enriched mantle melts to rise very near to the surface, where space was available for these to pond and fractionate to alkali granite.  相似文献   

Volatiles H2O,CO2, and Cl in a subduction related basalt     

David M. Harris  Alfred T. Anderson Jr. 《Contributions to Mineralogy and Petrology》1984,87(2):120-128

The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H₂O and CO₂ in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K₂O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO₂ from 51 to 54 wt.% SiO₂. The concentration of H₂O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H₂O in these inclusions. The gas saturation pressures for pure H₂O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H₂O)(Pa)=(SiO₂−48.5 wt.%) × 1.45 × 10⁷. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.  相似文献   

Present limitations of accurate satellite Doppler positioning for tectonics. An example: Djibouti     

Annie Souriau  Alfred Piuzzi  Micheline Etchegorry  Philippe Machetel 《Journal of Geodesy》1984,58(1):53-72

One of the possibilities of the Doppler positioning from satellite is to provide geodetic measurements continuous in time without the need for reference stations. If measurements of sufficient accuracy can be achieved they may be used to study local surface displacements in relation to tectonic activity. A Doppler receiver of the MEDOC network is located near Djibouti in the Ghoubhat-Asal rift region which corresponds to the accreting plate boundary between the Arabian and African plates. In November 1978, a seismic and volcanic crisis occured in this area. Surface geodetic measurements and levellings performed in 1973 and 1978–79, just after the crisis, reveal a 60–80 cm sinking of the graben floor and a lateral extension of about 2 meters. Here we analyse Doppler measurements for the period January 1977 to November 1980. Point positions are computed for 7 to 10 day intervals using a precise ephemeris, and a moving window analysis is applied to the data. An apparent 2 meter uplift preceding the November 1978 seismic crisis is detected at Djibouti, whereas no similar phenomenon is observed at the two closest stations, Pretoria and Uccle-Brussels. However, field observations rule out a tectonic origin for this uplift. In Djibouti, the correlation between the apparent vertical station position and the electron density in the ionospheric F-layer reveals that a bias may be induced by the third order term of the ionospheric refractive index not previously taken into account, or more probably by the ray curvature through the ionosphere. This bias is particularly strong for our data set, from a station located close to the magnetic equator, and related to a period of rapid increase in the solar activity. Although our analysis fails to detect any tectonic displacement related to the rift seismic activity, it shows that similar tectonic studies by the Doppler method will be possible once the residual ionospheric errors are removed, for example by use of higher radio frequencies.  相似文献   

Lower Cretaceous turbiditic sediments from the Rif chain (Northern Marocco) — palynology,stratigraphy and palaeogeographic setting     

Alfred A. Gübeli  Peter A. Hochuli  Walter Wildi 《International Journal of Earth Sciences》1984,73(3):1081-1114

The paper describes the palynology, stratigraphy, sedimentology and palaeogeographic setting of Lower Cretaceous turbiditic and terrigenous deep-sea sediments at the Tethyan margins of Africa and Alboran (External Domain of the Rif, and Flysch Nappes). During the Early Cretaceous terrigenous turbiditic deep-water sedimentation characterizes two different palaeogeographic domains in the southern part of the western Tethys:

In the northern area of the External Domain (Ketama Unit of the Rif) alternating turbiditic arenites and pelites are interpreted as sediments of a distal part of a fan system on the Tethyan margin of Africa. The main sources for the terrigenous material were situated in Central and Western Algeria; only little sand transited through the Prerif and the Mesorif zones of Morocco. The terrigenous sedimentation began in the Hauterivian, but the main turbiditic cycles are of Aptian to Lower and Middle Albian age.

The Flysch Domain probably was situated far to the Northeast with respect to the Rif basin, at the western and southern margins of the Alboran microplate. Relatively proximal turbidites form most of the Tisirène Nappe, whereas more distal turbidites constitute the series of the Melloussa and the Chouamat Nappes. The existence of two different source areas is demonstrated, one to the NE and the other to the NW of the depositional area. The turbidites probably were deposited on a E-W oriented fan system which progradated into a longitudinal trench-like trough. In the Central Rif area, the Tisirène Flysch is of Valanginian to Albian (pre-Vraconian) age. In the Western Rif and in the Melloussa and Chouamat Nappes of the whole area no sediments older than Aptian have been found.

The stratigraphy of the investigated series is based on a new tentative palynostratigraphic zonation, using pollen, spores and dinoflagellate cysts. Rich and well preserved assemblages have been found in the Western Rif only, whereas the samples of the Tisirène Nappe and the Chouamat Nappe in the Central and Eastern Rif have been affected by some thermal alteration. Black and mostly opaque palynomorphs from the Ketama Unit reflect the strong thermal influence in the External Domain of the Central and Eastern Rif.  相似文献   

Shallow seismicity at Montagne Pelée volcano,Martinique, Lesser Antilles     

Alfred Hirn  Nicole Girardin  Jean-Pierre Viodé  Sylvain Eschenbrenner 《Bulletin of Volcanology》1987,49(6):723-728

Continuous seismic monitoring at Martinique since the 1902 eruption of the Montagne Pelée volcano did not detect local earthquakes for the first 70 years. For the only eruption which occurred in this time span in 1929 the seismograph was 20 km away and of a standard type, not particularly suited for the detection of small-scale local seismicity. Improvement of the monitoring array over the last 15 years with the installation of sensors on the volcano itself allowed the detection of signals of local origin which were interpreted as being due to surface sources, such as rockfalls and landslides. Since December 1985 seismic sources in the volcano itself, i.e. small earthquakes at shallow depth, were identified and located with the aid of a temporary upgrading of the array close to these weak sources. Such an onset of local seismicity could not have been detected with previous seismic equipment; such episodes of seismicity in the volcano might have occurred in the past, apparently quiescent history of the volcano as the reinterpretation of seismograms of some events in 1976 would indicate, without evolving to more important volcanic phenomena. For seismographs on volcanoes the constant upgrading of observation capabilities is certainly perferred to a strict continuity of standard observations.  相似文献   

Polymetamorphic relations in iron ores from the Iron Quadrangle,Brazil: The correlation of oxygen isotope variations with deformation history   总被引：2，自引：0，他引：2  

Joachim Hoefs  Georg Müller  Alfred K. Schuster 《Contributions to Mineralogy and Petrology》1982,79(3):241-251

This study deals with the oxygen isotope composition of hematite-rich ore bodies in the Iron Quadrangle, Brazil. The area studied can be divided into two different regions: a western (W) zone of greenschist facies assemblages and an eastern (E) zone of amphibolite facies with transitions into granulite facies assemblages.The ¹⁸O values of 136 quartz-iron oxide pairs have been determined and temperatures of formation have been calculated. The ¹⁸O values of quartz vary between +6 and 20 except one value near +23, whereas the iron oxides fall between –4 and +10, with nearly 80% of the iron oxide values between –0.5 and 4.0. The regional distribution of the ¹⁸O values is as follows: in the W-region 85% of the quartz are >12, whereas in the E-region only 46% fall in this range, In contrast to quartz the iron oxides do not show any regional differences.The variation of oxygen isotope fractionations between quartz and iron oxides is obviously related to the complex deformation history of the iron ores. Samples with a primary schistosity (S₁) only represent peak metamorphic conditions. In the E-region the (S₁) high temperatures >700° C seem to correspond to orogenic events in the Archaen basement 2,700 m.y. ago. In the W-region S₁-temperatures between 460° and 560° C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 m.y. ago. Iron ores which have been overprinted by later deformation events are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering.The genetic processes associated with these hematite-rich ore bodies appear to be sedimentary-metamorphic rather than metasomatic processes. Furthermore, there is no evidence for secondary leaching by weathering solutions.  相似文献   

Alteration of oceanic crust and geologic cycling of chlorine and water   总被引：1，自引：0，他引：1  

Emi Ito  David M Harris  Alfred T Anderson 《Geochimica et cosmochimica acta》1983,47(9):1613-1624

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Petrology and geochemistry of eclogite xenoliths from the Rietfontein kimberlite,Northern Cape,South Africa     

C. M. Appleyard  D. R. Bell  A. P. le Roex 《Contributions to Mineralogy and Petrology》2007,154(3):309-333

Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with higher Al₂O₃ and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites. In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas. δ¹⁸O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this region.  相似文献   

Morphologies and emplacement mechanisms of the lava flows of the Faroe Islands Basalt Group,Faroe Islands,NE Atlantic Ocean     

Simon R. Passey  Brian R. Bell 《Bulletin of Volcanology》2007,70(2):139-156

The Palaeogene Faroe Islands Basalt Group (FIBG) comprises three eruptive sequences or formations, all emplaced into a subaerial environment during the development of the extensive continental flood basalt province that stretches from East Greenland through the Faroe Islands and into the Faroe-Shetland Basin. The Beinisvørð Formation, having a tabular-classic facies architecture, is composed of a sequence of simple flows each comprising a single sheet lobe. The Beinisvørð Formation is overlain by the distinctly contrasting Malinstindur Formation that has a compound-braided facies architecture. The Enni Formation occurs at the top of the sequence and consists of a mixture of simple and compound flows with tabular-classic and compound-braided facies architectures, respectively. Surface and internal characteristics of the sheet lobes of the Beinisvørð and Enni formations indicate emplacement through inflation, which is more obvious for the tube-fed compound flows of the Malinstindur and Enni formations. The difference between the simple and compound flow sequences of the FIBG is, most likely, linked to the manner in which the lava was supplied during the eruption and the eruptive style of the volcanic system. The sheet lobes were erupted over laterally extensive areas from fissure systems which had a continuous supply of lava, which contrasts with the tube-fed compound flows which were erupted in a gradual, piecemeal manner from point-sourced, low shield volcanoes with limited areal extents.  相似文献   

Stone: Building stone, rock fill and armourstone in construction, by M.R. Smith (Ed.). The Geological Society, London, UK, no 16, 1999. Hard cover, xx+478 pp. Price US$148. ISBN 1-86239-02-90     

F. G. Bell   《Engineering Geology》2001,62(4)

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