相似文献
7.
Petrological and geochemical relationships between pyroxene megacrysts and associated alkali-basalts from Massif Central (France) 总被引:2,自引:0,他引:2
Major, trace element, and REE analyses, as well as Sr isotopic ratios, have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali-basalt hosts from the French Massif Central. Equilibrium between crystals and host was examined based on petrographic and geochemical data.Two types of pyroxenes are recognized: the acmite-bearing clinopyroxenes, rich in incompatible elements and the salitic clinopyroxenes, poor in incompatible elements. 87Sr/ 86Sr isotopic data reveal no significant difference between clinopyroxenes and host lavas: they are in apparent isotopic equilibrium. The Sr isotopic ratios of the two types of pyroxenes are also quite similar. However pyroxene crystals from the first group are not in equilibrium with their host; they have crystallized at high-pressure from differentiated alkali-lavas and have been incorporated in a more primitive magma. Pyroxene crystals from the second group are in apparent equilibrium with their host lava; they have crystallized at various pressures. For the latter, distribution coefficients are proposed for compatible elements, trace elements and REE. 相似文献
8.
A scapolite+amphibole+clinopyroxene+Fe-Ti oxide+apatite association has been found as megacrysts in tephra from the Enval-Volvic volcanic line, east of the Chaîne des Puys (Massif Central, France) and in the cinder cone of the Segueïka volcano (Atakor, Algeria). In both kinds, lavahosts are basanites. Although never seen together in a single xenolith, a study of their inclusions indicates close genetic relationship between all 5 phases. This association must be considered as a paragenesis that crystallized within a narrow PT range. Volcanological, petrological and geochemical data suggest that these megacrysts are high-pressure phenocrysts rather than mechanically desintegrated fragments derived from coarse rocks. The composition of the amphibole suggests a pressure between 5 and 15 kb; Fe-Ti oxides imply a temperature close to 1,100° C.Crystallization of sulfur-rich scapolite involves a high fSO3, and therefore a high fO2, remarkable for such relatively undifferenciated alkalic magmas. Coexisting Fe-Ti oxides indicate a fO2 close to 10–6 bar. Such conditions imply that H2O was a major component of the gaseous phase. This high water content can be explained in terms of derivation from an hydrated upper mantle source. This is in agreement with other petrological evidences indicating that the basic magmas of the Massif Central crystallized under high fSO3 conditions. 相似文献
9.
Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen. 相似文献
10.
Representative samples of metagabbros, eclogites and a garnet peridotite from the Massif de Sauviat-sur-Vige (West Massif Central) have been analyzed for major and trace elements (including REE). The LREE enriched patterns are distinct from those of modern MORB but resemble those of continental or ocean island tholeiites. An intraplate tectoning setting is thus inferred. Geochemical modeling using REE and major element contents suggests that all the rock types might be related genetically by fractional crystallization, and, in some instances, by mixing of cumulates and differentiated liquid as in the case of the garnet peridotite. P, T estimates from microprobe analyses of coexisting garnet-clinopyroxene pairs yield about 820°C, and P > 15 kb for the eclogite facies metamorphism. 相似文献
11.
Elvira Bura-Naki Eric Viollier Didier Jzquel Alassane Thiam Irena Ciglene
ki 《Chemical Geology》2009,266(3-4):320-326
The vertical distribution of reduced sulfur species (RSS including H2S/HS−, S0, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H2SMBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSSV) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H2S/HS−, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeSam phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species. 相似文献
12.
《Tectonophysics》1986,125(4):357-370
The Hermitage Granite, situated in the northwest Massif Central (France) is a syntectonic Hercynian leucogranite emplaced along an active transcurrent shear zone. During emplacement and cooling, the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands (C-S mylonites). Increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline 〈a〉 slip and rotation recrystallization. Migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction. We discuss relevant deformation mechanisms and rheological implications for syntectonic plutons. In particular, we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid-state flow. 相似文献
13.
《Comptes Rendus Geoscience》2008,340(9-10):602-614
The long-term changes of the mammalian diversity of the Quercy and Limagne area (Massif Central, France) is quantitatively investigated through a ∼16-Myr time span running from the late Middle Eocene (∼38 Myr) to the Early Miocene (∼22 Myr). Within the specific context of ecosystem response to extreme climatic events, the goal of such a deep-time approach to local community and regional metacommunity evolution is to give time-scaled reference series with which extant changes can be objectively compared. The results indicate that local and regional ecological integration levels show very distinct evolutionary dynamics, involving at least partially independent sets of controlling parameters; those driving the metacommunity still remain largely to be better understood. Clearly, the analyzed data set constitutes a rather unique deep-time key into several major current macroecological debates. 相似文献
14.
We present dissolved load (< 0.45 μm) rare earth element (REE) patterns of small streams from a catchment basin in the Massif Central in order to characterize the individual fractionation stages for the dissolved REE from the source to the catchment outlet. The upper part of the catchment is located on a basalt plateau, followed downstream by deep and narrow valleys with granitic and orthogneissic bedrock. Stream water has basalt-like REE patterns on the basaltic plateau close to the source, followed by a continuous depletion in light REE (La-Sm, LREE) downstream. Strontium and neodymium isotope ratios of stream water demonstrate that the dissolved REE are essentially of basaltic origin, even in the lower, granitic and gneissic part of the catchment. Mixing with gneiss or granite derived REE thus cannot explain the observed evolution of the REE patterns. There seems also to be no link with the calculated speciation of the dissolved REE. In contrast, a correlation between saturation indexes for hematite and La/Yb ratios suggests that REE fractionation is mainly controlled by precipitation of Fe-oxide particles that preferentially remove LREE from solution. 相似文献
15.
Pavel Alexandrov Gilles Ruffet Alain Cheilletz 《Geochimica et cosmochimica acta》2002,66(10):1793-1807
Muscovites from the Blond granite (West French Massif Central) were dated by the 40Ar/39Ar single-grain method. The 40Ar/39Ar ages obtained vary from 305.5 ± 0.3 Ma to 311.3 ± 0.5 Ma, and most of the age spectra are slightly saddle-shaped. The analyzed muscovites show phengitic recrystallization under optical microscope observations, SEM images, and electron microprobe chemical analyses. It is proposed that the saddle-shaped age spectra result from a partial recrystallization, which produced three different isotopic reservoirs in the analyzed white mica single grains: domains of early muscovite, domains of neocrystallized muscovite formed by phengitic and Al-Fe substitutions, and “low-activation energy sites.” 相似文献
16.
Mingling of Immiscible Dolomite Carbonatite and Trachyte in Tuffs from the Massif Central, France 总被引:1,自引:0,他引:1
CHAZOT GILLES; BERTRAND HERVE; MERGOIL JULIETTE; SHEPPARD SIMON M. F. 《Journal of Petrology》2003,44(10):1917-1936
A pyroclastic tuff from the Velay volcanic province in the FrenchMassif Central contains blocks up to 30 cm long of local basementrocks, lava clasts, coarse-grained cumulates and pyroclasticfragments, with more or less diffuse boundaries with the hosttuff, which probably represent more consolidated parts of thetuff. All of the pyroclastics examined and approximately 10%of the cumulate xenoliths contain carbonates in variable amounts,textures and mineralogy. In some of the tuff samples, dolomiteoccurs in large amounts (up to 57%), principally as immiscibleglobules in trachytic melt (now glass), and represents the firstoccurrence of carbonatite reported from the Massif Central.The other carbonates, magnesiosiderite in the mafic cumulatesor occasionally in some tuffs, and calcite in the felsic cumulates,are always associated with a silicate glass of trachytic composition.Coexisting feldspars and carbonates in the various types ofsample are approximately in Sr isotopic equilibrium with aninitial ratio of about 0·7042. C- and O-isotopic compositionsof the carbonates covary and cover a very wide range of compositionfrom -2·9 to 3·9 相似文献
17.
In the Brévenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed,
together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma,
respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive
ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable
cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean
ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high
field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a
subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched
in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component
had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced
by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied
by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically
distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred
cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that
settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental
crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted
the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the
growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank
of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible
elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria
for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts.
Received: 21 April 1997 / Accepted: 30 June 1997 相似文献
18.
In the volcanic region of Velay (Massif Central, France), lake sediment sequences derived from maar craters situated close to one another (Ribains, Praclaux and Lac du Bouchet) have been correlated on the basis of tephrostratigraphical analyses. This has enabled the construction of a sequence (2980 pollen spectra) that begins during the glaciation preceding the Holsteinian and ends in the present. This sequence covers the period from 450 ka, i.e. from the end of marine oxygen isotope stage 12, to the present time, and includes five climatic cycles. The study of a second core from the Praclaux site completed the long Velay sequence. The biostratigraphy of the whole sequence is described in detail and illustrated by a synthetic and simplified pollen diagram. Thirteen temperate forest episodes (interglacials or interstadials) and as many cold periods (glacials or stadials) are defined. Each temperate episode is characterised by particular forest development and vegetation dynamics, and all of them, with the exception of the Holocene, begin and end with a Pinus forest. The vegetation dynamics observed during the Holocene resemble more closely those recorded during the interstadials than during the interglacials. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
19.
The high-grade Marvejols Group and the low-grade Albigeois-Cévennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their Nd---Sr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity.
High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cévennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cévennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone. “Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data. 相似文献 20.
J. Kalvoda O. Babek O. Fatka J. Leichmann R. Melichar S. Nehyba P. Spacek 《International Journal of Earth Sciences》2008,97(3):497-518
The Brunovistulian terrane represents a microcontinent of enigmatic Proterozoic provenance that was located at the southern
margin of Baltica in the early Paleozoic. During the Variscan orogeny, it represented the lower plate at the southern margin
of Laurussia, involved in the collision with the Armorican terrane assemblage. In this respect, it resembles the Avalonian
terrane in the west and the Istanbul Zone in the east. There is a growing evidence about the presence of a Devonian back-arc
at the margin of the Brunovistulian terrane. The early Variscan phase was characterized by the formation of Devonian extensional
basins with the within-plate volcanic activity and formation of narrow segments of oceanic crust. The oldest Viséan flysch
of the Rheic/Rhenohercynian remnant basin (Protivanov, Andelska Hora and Horní Benesov formations) forms the highest allochthonous
units and contains, together with slices of Silurian Bohemian facies, clastic micas from early Paleozoic crystalline rocks
that are presumably derived from terranes of Armorican affinity although provenance from an active Brunovistulian margin cannot
be fully excluded either. The development of the Moravo–Silesian late Paleozoic basin was terminated by coal-bearing paralic
and limnic sediments. The progressive Carboniferous stacking of nappes and their impingement on the Laurussian foreland led
to crustal thickening and shortening and a number of distinct deformational and folding events. The postorogenic extension
led to the formation of the terminal Carboniferous-early Permian Boskovice Graben located in the eastern part of the Brunovistulian
terrane, in front of the crystalline nappes. The highest, allochthonous westernmost flysch units, locally with the basal slices
of the Devonian and Silurian rocks thrusted over the Silesicum in the NW part of the Brunovistulian terrane, may share a similar
tectonic position with the Giessen–Harz nappes. The Silesicum represents the outermost margin of the Brunovistulian terrane
with many features in common with the Northern Phyllite Zone at the Avalonia–Armorica interface in Germany. 相似文献
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