Rare earth elements in high-grade metamorphic rocks from the western Alps     

J. Dostal  S. Capedri 《Lithos》1979,12(1):41-49

A sequence of amphibolite to granulite facies metasedimentary and mafic metaigneous rocks from the western Italian Alps has been analysed for rare earth elements (REE). In this sequence, the metasedimentary granulites have probably been affected by a melting event while the metaigneous granulites remained unaffected. Metasedimentary granulites have a less fractionated chondrite-normalized REE pattern than equivalent amphibolite facies rocks. The granulites tend to have a higher content of heavy REE and lower abundances of light REE (LREE). The leucosomes of migmatitic granulites have lower REE content than the melanocratic bands and both these rock types have variable relative abundances of Eu. The mafic granulites have LREE enriched patterns while the amphibolites are slightly depleted in LREE. The differences between the mafic granulites and amphibolites are probably of pre-metamorphic origin.  相似文献   

Geochemistry of basalts from central and southern New Hebrides arc: implication for their source rock composition     

C. Dupuy  J. Dostal  G. Marcelot  H. Bougault  J.L. Joron  M. Treuil 《Earth and Planetary Science Letters》1982,60(2):207-225

The basaltic rocks from the central and southern islands of the New Hebrides-Aneityum, Tanna, Erromango, Efate, Emae, Tongoa and Epi, have geochemical features typical of island arc volcanics. They are enriched in LILE and depleted in Zr, Hf, Nb and Ta compared to N-type MORB. The rocks were derived from a similar upper mantle source as N-type MORB but with a higher degree of partial melting. In addition their source was enriched in LILE (K, Rb, Sr, Ba and LREE) probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab. The basalts from Futuna island which is located farther from the trench, display characteristics typical of calc-alkaline rocks. The Futuna basalts were generated from a different LILE-enriched upper mantle source. It seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere.  相似文献   

Geochemistry of paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications     

J. M. Grajales-Nishimura  E. Centeno-Garcia  J. D. Keppie  J. Dostal 《Journal of South American Earth Sciences》1999,12(6):135

Analyses of Lower Permian or older basalts and associated dykes of the Juchatengo sequence indicate that they are rift tholeiites that formed in a continental rift or back-arc tectonic setting. Age constraints include a Middle Permian fossil recovered from the tectonically overlying sediments and a cross-cutting, post-tectonic pluton dated by K/Ar on hornblende at 282±6 Ma. A location adjacent to the Oaxacan Complex or other old continental crust is suggested by (1) an Nd_i isotopic value of −8.95 and a T_DM age of 1487 Ma in the overlying sediments, which are similar to the Oaxacan Complex; (2) enrichment of incompatible elements in the lavas, suggesting old crustal contamination; and (3) the presence of Permian–Triassic calc-alkaline plutons that stitch the Juchatengo–Oaxaca boundary. The possible tectonic models depend on the age of the Juchatengo basalts, which requires future geochronological work. If the Juchatengo basalts are Permo-Carboniferous, they could have formed near the eastern edge of a back-arc basin: the contemporaneous arc would have rifted away to the west. Eastward migration of the arc magmatism indicated by the Permian–Triassic calc-alkaline plutonism may reflect shallowing of the dip of the subduction zone, which probably also produced the deformation of the Juchatengo sequence.  相似文献   

Petrogenetic implications of uranium abundances in volcanic rocks from southern Peru     

C. Dupuy  J. Dostal  S. Capedri  C. Lefevre 《Bulletin of Volcanology》1975,39(3):363-370

The content of uranium of andesitic rocks of southern Peru varies according to the distance from the Peru-Chile trench, as has been observed for other trace elements geochemically associated with K. The highest contents found in these rocks may exceed the estimated U content of the upper crust. This enrichment may be related to a variable degree of partial melting and/or crustal contamination of the same source material.  相似文献   

Steady-state kinetics and equilibrium between ground water and granitic rock     

Tomas Paces 《Geochimica et cosmochimica acta》1973,37(12):2641-2663

A hypothesis is presented that the dissolution of albite includes the exchange of sodium for hydrogen ion in a surface layer of the mineral and the structural collapse of the residual anionic lattice of the layer. The ion exchange is described by the first law of diffusion (D_25°C = 3 × 10^?22 and 1.5 × 10^?20 cm²sec^?1 at P_CO2 = 0 and 26.2 atm, respectively). The surface residual layer reaches a steady-state thickness ranging from n × 10^?8 to n × 10^?5 cm according to the temperature and P_CO2. The increase in aqueous sodium with time in a continuous ground-water system is described by a simple exponential equation. The equation is used to estimate the percolation time of ground water from the data on the chemical composition of a water sample. The probable times range from 14 to 3840 days for various ground-water systems and are compared to the times of percolation calculated from the geothermal and hydraulic data. Both estimates are found to be in general agreement. The concentrations of Al and Si in cold water from granitic rocks are shown to be controlled by the chemical equilibrium with respect to an aged aluminosilicate. The aluminosilicate precipitates from ground water as an amorphous isoelectric solid. Its chemical composition is represented by a simplified stoichiometric formula [Al(OH)₃](_1?x)[SiO₂]_x and varies linearly with pH of the solution. The atoms of Al, O and H tend to occupy a fixed position in the solid given by the gibbsite structure upon aging in the field. The solubility product of the solid is estimated from the published data on experimental and field research into the dissolution of feldspars: logK = (1 ? x) × log [Al³⁺] + xlog [H₄SiO₄] ? (3 ? 3x) log [H⁺] = 8.56 ? 11.26x, where x is the molar fraction of silica in the aluminosilicate.  相似文献   

Inferred Palaeoproterozoic arc rifting along a consuming plate margin: insights from the stratigraphy,volcanology and geochemistry of the Kangerluluk sequence,southeast Greenland     

W. Mueller  J. Dostal  H. Stendal 《International Journal of Earth Sciences》2002,91(2):209-230

The 200- to 300-m-thick volcano-sedimentary sequence in the Kangerluluk Fjord, associated with penecontemporaneous and late-tectonic dykes, as well as a synvolcanic plutonic suite, represents an integral component of the Palaeoproterozoic Ketilidian Mobile Belt, south Greenland. The ca. 1808-Ma Kangerluluk supracrustal sequence contains four distinct mappable lithofacies: (a) a conglomerate-sandstone lithofacies; (b) a pyroclastic lithofacies; (c) a volcanic lithofacies; and (d) a peperite lithofacies. The volcanic lithofacies, up to 200 m thick, is characterized by shallow-water subaqueous brecciated and pillowed flows. Flows are either (a) feldspar-phyric, or (b) feldspar-pyroxene-phyric, with 0.2- to 3-cm-size plagioclase and 0.2- to 3-cm-size pyroxene that constitute between 20 and 30% (locally up to 50%) of the flows. Mafic dykes intruded wet unconsolidated pyroclastic lithofacies, resulting in the formation of peperites. Geochemically, the volcanic and pyroclastic units represent a distinct tholeiitic magmatic suite enriched in incompatible trace elements including Th, La, Yb, Zr and Nb, and exhibiting (La/Yb)_n~10. The plutonic suite and associated dykes display a calc-alkaline trend with enriched LREE and unfractionated flat HREE patterns, La_n ranging between 50 and 100, (La/Yb)_n ratios between 8 and 22, and negative Nb and Ti anomalies on the mantle-normalized, incompatible multi-element patterns. The pillowed flows lie in the continental flood basalt field on the Y-Nb-Zr discrimination diagram, and display a Nb anomaly and K₂O-enrichment that collectively suggest a crustal component and/or a subduction-modified mantle source. The geology, stratigraphy of the Kangerluluk area and geochemistry can be used to infer a change in magma genesis from arc to rift volcanism. The 1850- to 1800-Ma calc-alkaline Julianehåb batholith represents a magmatic arc that rifted during crustal extension, allowing for the ascent of mantle-derived mafic magma. The geochemistry of the mafic volcanic flows, synvolcanic dykes and pyroclastic deposits favours a crustal component in magma genesis and offers new insights into the tectonic evolution of the Ketilidian Mobile Belt.  相似文献   

Rare Earth Element Geochemistry of the Zigzag - Klondyke Sedimentary Rock Formations: Clues to the Evolution of the Baguio Mineral District (Luzon), Philippines     

Tomas A. Tam  III    Graciano P. Yumul  Jr    Estephanie Gera L. Ramos  Carla B. Dimalanta  Mei-Fu Zhou  Shigeyuki Suzuki 《Resource Geology》2005,55(3):217-224

Abstract. Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.  相似文献   

Reply to Marquart     

Tomas Nord  Lars E. Sjöberg 《Geophysical Journal International》1993,115(2):605-605

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Study of the Santa Catarina aquifer system (Mexico Basin) using magnetotelluric soundings     

Michel Chouteau  Stefka Krivochieva  Ramiro Rodriguez Castillo  Tomas Gonzalez Moran  Virginie Jouanne 《Journal of Applied Geophysics》1994,31(1-4)

A tensor magnetotelluric test survey was carried out in the region of Santa Catarina, located in the Chalco sub-basin of the Mexico Basin. The objective was to define the stratification at depth with an emphasis on the geometry of the main aquifer of that region which is partially known from DC resistivity soundings and drilling. High-quality magnetotelluric soundings could be recorded in the immediate vicinity of large urban zones because the sub-surface is very conductive. Interpretation shows that the solid bedrock is located at a depth of at least 800 m to the south and 1300 m to the north; it could, however, be much deeper. Using complementary DC resistivity sounding and well-logging data, three main layers have been defined overlying the bedrock. These layers are, from surface to bottom, an unsaturated zone of sand, volcanic ash and clay about 10 m thick, followed by a very conductive (1.5 ohm·m) 200 m thick layer of sand and ash with intercalated clay, saturated with highly mineralized water, and finally a zone with resistivity increasing gradually to 60 ohm·m. The investigated deep aquifer constitutes most of this third layer. It consists of a sequence of sand, gravel, pyroclastites and mainly fractured basalts. MT resistivity soundings and magnetic transfer functions also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This geologic feature is likely to be highly permeable fractured basaltic flows, which provide a channel by which water contaminated by the Santa Catarina landfill may leak into the basin.  相似文献   

Geochemistry of Precambrian ophiolites from Bou Azzer,Morocco     

J. L. Bodinier  C. Dupuy  J. Dostal 《Contributions to Mineralogy and Petrology》1984,87(1):43-50

The Upper Proterozoic ophiolite complex of Bou Azzer, Morocco, includes ultramafic rocks, cumulate gabbros, sheeted dykes, pillow lavas and diorite-quartz diorite intrusions and an overlying volcano-sedimentary sequence. The gabbroic cumulates, basaltic flows and dykes have compositions similar to recent ocean-floor rocks (N- and/or T-type). Among other features, they have comparable light REE-depleted patterns and relations of Ti-Zr and La-Nb. Although fractional crystallization played an important role in the evolution of these rocks, the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and/or by a dynamic partial melting process. Diorites, quartz diorites and the volcanic rocks of the overlying sequence are calc-alkaline, genetically unrelated to the tholeiitic suite and indicative of an island arc setting. A possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc.  相似文献