Depending on the quality of the zircon grains available for analysis, two methods may be used to date igneous rock emplacement, namely U-Pb TIMS with isotope dilution or in situ U-Pb SIMS (SHRIMP). Both methods have been used to determine, in a precise and accurate manner, the emplacement age of the granitic protolith of the various orthogneisses in the Pyrenean Axial Zone. More specifically, four representative samples of G1, G2 and a “transition gneiss” yielded reliable datings with an average age of 473 ± 4 Ma for each sample. The surrounding sediments of the Canaveilles Group were constrained by zircon grains from interlayered metarhyodacite and dated at 581 ± 10 Ma using the SHRIMP method, clearly giving this group a late Proterozoic (Vendian) age. Finally, the Somail orthogneiss of the Montagne Noire, equivalent to that of the Canigou, yielded an age of 471 ± 4 Ma with the in situ U-Pb method, which is identical to the dating of the Pyrenean samples. In addition, most of the studied orthogneisses recorded a wide range of significant concordant inherited ages spanning from early Archaean (3.5 Ga) to Pan-African/Cadomian (600–800 Ma). Bearing in mind the calc-alkaline affinity of the studied rocks, this work demonstrates the huge contrast between the active Gondwana margin in the north (“South European terrane”) and the remarkably homogeneous continental plate that existed from Arabia to Morocco during the Ordovician.

We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilifies using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons. ]NTRODU•ONModeling the origin and emplacement of granite plutons is usually based on a structured study, beginning with the field characterization of the principal rock types and their spatial distribution and followed by detailed structural and/or petrological investigations. The pertinence of any petrogenetic or emplacement model is indeed dependent on the accuracy of the original field observations and laboratory characterizations of the rocks. Important field constraints include knowledge of whether the pluton is compositionally zoned and whether any such zoning is composite or gradational. Acquisition of such information is highly dependent on the quality of outcrop and the collection of spatially distributed samples for modal analysis. This stage of the investigation is often the most laborious part of the study, although essential data are usually generated. Unlike every other aspect of petrology and structural geology this facet of petrostructural research in granite plutons has not yet benefited significantly from the development of modem techniques. We contend that low field magnetic susceptibility measurements can contribute to the rapid and effective field mapping and petrographic characterization of certain types of granitoid plutons, particularly those of the ilmenite series, rather than the magnetite series granitoids [Ishihara, 1977; Takahashi et INow at G6oscience de l'Environement, URA 132 CNRSPaper number 9ZIB01590. 0148-0227/93/9ZIB-01590505.00.al., 1980]. The present approach, which therefore constitutes a basis for more detailed tectonophysical studies of graniteiris, naturally has strong potential to be a tool for geophysical exploration of a large fraction of the continental crust.Magnetic susceptibility measurement in low magnetic field (a few 10 -4 T) offers the promise of a simple and inexpensive technique. Two principal aspects can be explored with this technique, namely, structural aspects using the anisotropy of magnetic susceptib...

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ZusammenfassungDer Deckenbau am Ostrand der B6hmischen Masse erfolgte in zwei aufeinanderfolgenden Stadien, die sich sowohl in ihrer Kinematik als auch in ihrer Metamorphoseentwicklung deutlich voneinander unterschieden.Die AbstractThe entire pile of nappes in the eastern margin of the Bohemian massif is characterized by two stages of Variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution.The older emplacement (D1) probably occurred around 350-340 Ma ago and was synmetamorphic. The nappes show a typical systematic superposition of higher grade metamorphic units over lower grade ones. Thus, the crystalline complexes showing a HT-MP Barrovian imprint (Svratka allochthonous unit and Moldanubicum) were thrust over an intermediate unit affected by MT-MP recrystallization (Bfteg orthogneiss and its country rock), and at the base of the D1 nappe pile the Inner Phyllite Nappe (Bfl 3) Potok Unit) is characterized by LT/LP metamorphism.The second stage of tectonic evolution (D2) is characterized by a thin-skinned northward-oriented nappe emplacement that occurred under LT-LP conditions dated at 320-310 Ma. The whole nappe sequence formed during the first tectonometamor- phic period (D1) was transported northward over the autochthonous >>Deblin polymetamorphic and granitic complex<< of Upper Proterozoic age and its Devonian sedimentary cover with very low metamorphism. During this second tectonic event the Brno granite massif (580 Ma) was only marginally incorporated in the Variscan nappe tectonics which resulted in kilometer-scale cover and basement duplexes. The tectonic evolution of the nappe pile ended with stage D3, represented by large-to medium-scale east-vergent folds with limited displacement. R6sum6Uempilement des nappes a la bordure orientale du Massif de BohSme est caractrris6 par deux stades de mise en place prrsentant diffrrentes 6volutions cinrmatiques et mrtamorphiques.La tectonique majeure de mise en place des nappes crustales intervient lors d'un mdtamorphisme de type barrowien, cal6 autour de 350-340 Ma. Uempilement qui en rrsulte montre une superposition systrmatique d'unitds ~t fort degr6 de mrtamorphisme sur des unitds moins mrtamorphiques. Ainsi les complexes cristallins, montrant des reliques de mrtamorphisme de haute h moyenne pression-haute temprrature (unitrs cristallines de Svratka et du Moldanubien), chevauchent une unit6 intermrdiaire affectre par un mdtamorphisme de moyenne h basse pression-moyenne temprrature (l'orthogneiss de Biteg et son encaissant). A la base de cette pile 6difire durant la tectonique D1, l'unit6 des phyllites internes (unit6 de Bfl3~ potok) est caractdrisde par un mrtamorphisme de basse trmperature-basse pression.Le second stade D2 de l'rvolution tectonique est caractrris6 par une tectonique pelliculaire h vergence nord datre h 320-310 Ma. Uempilement rrsultant de D1 est ainsi transport6 vers le nord, au dessus du complexe autochtone d'~ge protrrozoique suprrieur (groupe de Deblin) et sa couverture sddimentaire drvonienne trrs faibleme...

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