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31.
Annie Vitrac-Michard Claude J. Allègre 《Contributions to Mineralogy and Petrology》1975,51(3):205-212
U-Pb ages for zircons of pyreneean gneisses and associated granites are comparable to whole rock Rb-Sr ages of 580, 320 m.y. and recent. Inherited radiogenic Pb in the granites limits the dating accuracy but points out an important contribution of crustal material in their formation. 相似文献
32.
Claude J. Allègre Francis Albarède Marc Grünenfelder Dr. Viktor Köppel 《Contributions to Mineralogy and Petrology》1974,43(3):163-194
Based on regular arrays in a U-Pb diagram of data points of zircons from polymetamorphic rocks of the Alps quantitative models are developed that include several episodic lead losses, continuous lead losses during discrete periods and combinations of both.The data from the Alps can be explained by assuming an initial age of the zircons of 2000 and/or 2500–2600 m.y. and three episodic lead losses 520–580 m.y., 300 m.y. and 30 m.y. ago with regionally varying intensities of the events as can be seen from the efficiency parameters which define the shape of the discordia. On the other hand, zircon data from North American basement rocks satisfy a model assuming a discrete interval of continuous lead loss that starts with the time of uplift of the rocks.The Th-U-Pb diagrams are valuable for recognising whether the history of the zircons and their host rock is a complex one or not which is not always noticeable in a U-Pb diagram. 相似文献
33.
In the East of Montagne Noire (South Massif Central), the granitic Mendic massif and the porphyroïds have been dated as 510±20 m.y. with a corresponding initial ratio of 0.706±0.001. Within the limits of the analytical errors, we cannot distinguish between two separate isochrons. The Mendic-porphyroïds represents a volcano-plutonic association. The age of 510 m.y. is somewhat younger than the one determined on the orthogneisses of the Central zone (530 m.y.). It can be connected to a post-orogenic magmatism of the cadomian orogenesis. At last the Sr of the minerals of the Mendic massif has been partially remobilised during the hercynian orogeny (285 m. y.). 相似文献
34.
Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements. 相似文献
35.
Bruno Hamelin Bernard Dupré Claude J. Allègre 《Earth and Planetary Science Letters》1984,67(3):340-350
We have determined the Pb and Sr isotopic compositions in a number of fresh young oceanic basalts from the East Pacific Rise (between 20°N and 21°S latitudes), and from the Mid-Atlantic Ridge (between 65°N and 10°N).The results confirm the PbSr isotopic correlation for mid-ocean ridges basalts obtained by Allègre et al. [1], Cohen et al. [2], Dupréand Allègre [3], and the correlation between isotopic variation and the compatible trace elements ratios variation [1].A comparison between the Atlantic and Pacific results reveals that there is a wider range of values for the Atlantic than for the Pacific. After filtering the short wavelengths, a good correlation is obtained between long-wavelength bathymetric and isotopic variations for the Atlantic.The preferred model proposed to explain these differences involves the constant presence of hot spots under ridges. On slow-spreading ridges like the Atlantic, the hot spots signature is clearly visible in both bathymetry and isotopic ratios. On fast-spreading centers, the hot spot signature in both the bathymetry and isotopic signature may be diluted by the rapid supply of material coming from the asthenosphere.However, an alternative explanation for which no hot spot influence is found on the East Pacific Rise cannot be definitely ruled out.In two occurrences, south of the Hayes fracture zone (Atlantic), large isotopic heterogeneities are observed within a single dredge. This does not contradict the concept of regional isotopic regularities, but suggests that blob injection and source mixing may be observed at very different scales under the ridges. 相似文献
36.
The Pb isotopic study of the Xigaze ophiolite (Tibet) shows that the tectonic (harzburgites) and magmatic (gabbros, dolerites, lavas) units bear different isotopic signatures, the harzburgitic rocks show much higher207Pb/204Pb values than the samples from the magmatic unit. Analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes. UPb analysis allowed determination of the crystallization age of the magmatites at 120 ± 10 m.y. This age is compatible with paleontological age determinations and thus demonstrates that UPb behaved more or less as a closed system since formation of the ophiolite. If an exchange with seawater has occurred, it must necessarily have happened shortly after crystallization of the Xigaze oceanic crust. Dolerite dykes crossing the harzburgites show similar Pb isotopic composition as the magmatites, indicating that they were emplaced when the ultramafic rocks were already brittle; however, the time difference of the emplacement was probably not larger than 280 m.y. This makes it evident that two sources with different isotopic signatures, one similar to normal MORB, the other with much higher207Pb/204Pb ratios, were active in the immediate neighborhood. One possible environment which can account for this association is a propagating ridge. 相似文献
37.
Application of the inversion technique described in Part I [1] to models in which the depleted (MORB) mantle is created by extraction of continental crust results in a stable and self-consistent set of model data, including estimates of uncertainties on all the parameters. The inversion shows that the mean age of the continents (and of the depleted mantle) is 2.14–2.46 b.y., and that the lower crust must be depleted in Rb by a factor of ～ 4 relative to the upper crust. The fraction of total mantle which has been depleted ranges over wide limits (30–90%) depending on: (a) whether the lower mantle is considered to be virgin, or somewhat depleted; (b) whether the depleted reservoir is represented in Sm/Nd and Rb/Sr systematics by average values of MORB, or by extreme values of MORB, and (c) whether the bulk earth Nd content is presumed to be similar to E or H chondrites, or is higher than all known meteorite classes. The usefulness of crust-mantle budget models in constraining such questions as whole mantle versus two-layer mantle convection will be enhanced by better data and understanding in these three key areas of geochemistry. 相似文献
38.
Gilbert Feraud Ishiro Kaneoka Claude Jean Allègre 《Earth and Planetary Science Letters》1980,46(2):275-286
We have dated by the K/Ar method the oldest volcanic formations known in the Azores archipelago. The results show a development of this oceanic island system from 5.5 m.y. to the present day.The age pattern is not compatible with a simple migration over a small fixed hot spot, but, plotting tectonic directions which control the volcanism versus age and distance to the Mid-Atlantic Ridge, a simple relationship seems to appear showing an anisotropic but fixed distribution of stress orientation at least for the last 0.7 m.y. 相似文献
39.
All A. Nowroozi 《Geophysical Journal International》1986,86(2):687-699
Summary. The statistical capability of the m b : M s discriminant for the discrimination of earthquake and explosion populations is examined by application of discriminant functions to a group of 83 explosions and 72 earthquakes in Eurasia. Equations are derived for the probability that an event is an earthquake or an explosion. The positive sign of DIS in the decision index equation, DIS i = 34.3383 – 11.9569 mb t + 7.1161 M si , indicates that the event i is an earthquake. Its negative sign indicates that event i is an explosion. The probability of correct classification for an event, P i , is related to its DIS i value, by P i = [1-exp (DIS i )]−1 , where a large, positive DIS indicates a high probability that an event is an earthquake and a large, negative DIS indicates a high probability that an event is an explosion. The discrimination line M s = 1.680 m b – 4.825, or m b = 0.595 M s + 2.872 very successfully separates the explosion population from the earthquake population. The points on this line have an equal chance of being an earthquake or an explosion; moreover, for any event, the distance parallel to the M s -axis from the point representing that event in the m b : M s plane to this line is a measure of the probability for the correct classification of that event. 相似文献
40.