Geochronology, geochemistry and petrogenesis of mafic and ultramafic rocks from Southern Beishan area, NW China: Implications for crust–mantle interaction     

Yuanyuan Zhang  Jaroslav Dostal  Zehui Zhao  Chang Liu  Zhaojie Guo 《Gondwana Research》2011,20(4):816-830

The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian (~ 270–295 Ma) coeval with the basalts and the ultramafic rocks were emplaced at about 250 Ma. The basalts are within–plate tholeiites with slight enrichment in light rare earth elements (LREE) relative to heavy rare earths (HREE) and small negative anomalies of Nb and Ta. Gabbros including olivine gabbros, olivine gabbronorites and troctolites are grouped into two: the cumulate gabbros are depleted in LREE and show small negative Nb and Ta anomalies but distinct positive Sr and Eu anomalies; non–cumulate gabbros resemble tholeiitic basalts. Lamprophyres and cumulate ultramafic rocks are characterized by large enrichment of LREE relative to HREE with depletion in Nb and Ta. The enriched Sr–Nd isotopic trend from DM towards the EM II end member component implies that the lithospheric mantle was progressively enriched with depth by the involvement of subducted crustal material due to the delamination of thickened mantle lithosphere after collision. The digestion of subducted crustal material into the mantle resulting in the metasomatized and enriched mantle is inferred to be an important process during crust–mantle interaction.  相似文献   

Fotografie in der Meteorologie     

Jaroslav Kopáček 《Studia Geophysica et Geodaetica》1961,5(4):377-377

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Highly depleted oceanic lithosphere in the Rheic Ocean: Implications for Paleozoic plate reconstructions     

J. Brendan Murphy  Brian L. Cousens  James A. Braid  Rob A. Strachan  Jaroslav Dostal  J. Duncan Keppie  R. Damian Nance 《Lithos》2011,123(1-4):165-175

The Rheic Ocean formed at ca. 500 Ma, when several peri-Gondwanan terranes (e.g. Avalonia and Carolinia) drifted from the northern margin of Gondwana, and were consumed during the Late Carboniferous collision between Laurussia and Gondwana, a key event in the formation of Pangea. Several mafic complexes ranging in age from ca. 400–330 Ma preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by anomalously high εNd values that are typically either between or above the widely accepted model depleted mantle curves. These data indicate derivation from a highly depleted (HD) mantle and imply that (i) the mantle source of these complexes displays time-integrated depletion in Nd relative to Sm, and (ii) depletion is the result of an earlier melting event in the mantle from which basalt was extracted. The extent of mantle depletion indicates that this melting event occurred in the Neoproterozoic, possibly up to 500 million years before the Rheic Ocean formed. If so, the mantle lithosphere that gave rise to the Rheic Ocean mafic complexes must have been captured from an adjacent, older oceanic tract. The transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved. Possible Mesozoic–Cenozoic analogues include the capture of the Caribbean plate or the Scotia plate from the Pacific to the Atlantic oceanic realm. Our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the Rheic Ocean was consumed by subduction during Laurentia–Gondwana convergence.  相似文献   

Modelling topographic potential for erosion and deposition using GIS     

Helena Mitasova  Jaroslav Hofierka  Maros Zlocha  Louis R. Iverson 《International journal of geographical information science》2013,27(5):629-641

Abstract

Modelling of erosion and deposition in complex terrain within a geographical information system (GIS) requires a high resolution digital elevation model (DEM), reliable estimation of topographic parameters, and formulation of erosion models adequate for digital representation of spatially distributed parameters. Regularized spline with tension was integrated within a GIS for computation of DEMs and topographic parameters from digitized contours or other point elevation data. For construction of flow lines and computation of upslope contributing areas an algorithm based on vector-grid approach was developed. The spatial distribution of areas with topographic potential for erosion or deposition was then modelled using the approach based on the unit stream power and directional derivatives of surface representing the sediment transport capacity. The methods presented are illustrated on study areas in central Illinois and the Yakima Ridge, Washington.  相似文献   

A support for the existence of paleolakes and paleorivers buried under Saharan sand by means of “gravitational signal” from EIGEN 6C4     

Jaroslav Klokočník  Jan Kostelecký  Václav Cílek  Aleš Bezděk  Ivan Pešek 《Arabian Journal of Geosciences》2017,10(9):199

The goal of this study is to demonstrate that and how the recent gravitational and topographic data support the findings made by geologists and others as for the existence of the paleolakes and paleoriver systems, now buried under the sands of Sahara. It is always important and useful to have such an independent analysis supporting certain results, and this paper is such a case. We make use of the gravity disturbances (or anomalies), the Marussi tensor of the second derivatives of the disturbing geopotential, the gravitational invariants and their certain ratio, the strike angle and the virtual deformations. The geopotential is represented by the global combined (from satellite and terrestrial data) high-resolution gravity field model EIGEN 6C4 (till degree and order 2160 in spherical harmonic expansion). The topography is derived from the ASTER GDEM and ETOPO 1 models (both are used). With all these data, we confirm the existence of huge paleolakes or paleoriver systems under the Saharan sands known or anticipated in an independent way by geologists for the lakes MegaChad, Fazzan and Chotts; for Tamanrasset river valley; and Kufrah Basin, presumptive previous flow of the Nile River. Moreover, we suggest a part of the Grand Egyptian Sand Sea as another “candidate” for a paleolake and hence for a follow-up survey.  相似文献   

The impact of human activities on groundwater systems programs of the international association of hydrogeologists     

Jaroslav Vrba 《Environmental Geology》1983,5(1):9-9

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An Archean oceanic felsic dyke swarm in a nascent arc: the Hunter Mine Group, Abitibi Greenstone Belt, Canada     

J. Dostal  W. Mueller 《Journal of Volcanology and Geothermal Research》1996,72(1-2)

The 2730-Ma-old Hunter Mine Group (HMG), a dominantly felsic subaqueous volcanic sequence, was formed during early arc construction in the Abitibi greenstone belt (Quebec, Canada). The western part of the HMG contains a felsic dyke swarm up to 1.5 km wide and traceable up-section for 2.5 km. Five distinct generations were identified: (1) aphanitic to feldspar-phyric dykes; (2) quartz-feldspar-phyric dykes with < 5% quartz phenocrysts; (3) quartz-feldspar-phyric dykes with 10–25% quartz phenocrysts; (4) dacitic feldspar-phyric dykes; and (5) mafic dykes. The felsic dykes collectively constitute more than 90% of the dyke swarm. Geochemically, they resemble modern calc-alkaline dacites and rhyolites. Their mantle-normalized incompatible trace-element patterns display a moderate enrichment of Th and light REE relative to HFSE and heavy REE as well as negative Nb, Ta, Eu and Ti anomalies. Most of the major- and trace-element abundance variations in these rocks can be explained by crystallization of feldspars. Geochemical data including depleted mantle-like _Nd values suggest that an older sialic substrate was not involved in their genesis. We infer that the felsic rocks were generated by melting of mafic oceanic crust. The swarm was emplaced during nascent oceanic island-arc development and was related to rifting of the arc. The conformably overlying MORB-like basalts and basaltic komatiites of the Stoughton-Roquemaure Group used the same conduits and further indicate splitting of the arc. HMG and associated parts of the Abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western Pacific.  相似文献   

Long-term variations in geomagnetic and solar activities and secular variations of the geomagnetic field components     

Jaroslav Střeštík 《Studia Geophysica et Geodaetica》1991,35(1):1-6

Summary After the removal of the eleven-year periodicity, long-term patterns of the aa indices of geomagnetic activity and of Wolf's sunspot numbers are defined. The positions of maxima and minima exhibit the same regularities as the secular variations of the geomagnetic filed components. This result is associated with the motion of the Sun round the barycentre of the solar system.Presented at symposium Planet 88, Tihany, September 1988.  相似文献   

Investigation of the damping of geomagneticPi2 pulsations using generalized spectral analysis     

Jaroslav Střeštík  Reviewer M. Hvoždara 《Studia Geophysica et Geodaetica》1978,22(2):154-159

Summary The method of spectral analysis has been modified to render the spectrum not only a function of frequency, but also of damping. The generalized spectra of damped geomagnetic pulsations, recorded at the observatories of Budkov and Dymer, are computed. The degree of damping of these pulsations in the X and Y-components at both stations is determined. The results are compared with results obtained using other methods.  相似文献   

Petrology and geochemistry of the ignimbrites and associated lava domes from N.W. Sardinia     

C. Coulon  J. Dostal  C. Dupuy 《Contributions to Mineralogy and Petrology》1978,68(1):89-98

Cenozoic calc-alkaline ignimbrites and related lavas from N.W. Sardinia (Italy), which are closely associated with andesites, have dacitic to rhyolitic composition. The ignimbritic rocks underwent fractional crystallization dominated by plagioclase. The model calculations for REE, Ba, Sr, and Rb are consistent with the generation of ignimbritic magmas by partial melting of tonalitic rocks. P-T estimates for the crystallization of ignimbrites, which give temperatures in the range of 910 °–1070 ° C and P about 5 kb, imply a local increase of temperature in the crust, which is probably related to the ascending andesitic magma. The variations of trace elements indicate that associated andesitic rocks were contaminated by ignimbritic magma.  相似文献