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Géochronologie des socles alpins internes, implications géodynamiques

Abstract : The Western Alps comprise major outcrops of pre-Mesozoic massifs. Only the Upper Carboniferous units, rich in Coal Measures, were dated previously with some precision owed to their fossil content. Current progress in radiochronology, especially for the isotopic U-Pb on zircon dating method, has allowed advances regarding the age of the basement rocks. Six papers are reproduced, published between 1991 and 2002 (plus one on progress), where ages of rocks from seven basement massifs have been determined, most of them for the first time. The conventional method by isotope dilution in various laboratories (Brussels, Nancy, Zurich, Clermont- Ferrand) has yielded a number of results. Protolith age determinations by the conventional method were often hampered by inheritance problems in polycyclic orthogneisses and/or calc-alkaline metagranitoids. There, ion microprobes (SHRIMP at Perth and Canberra / ages marked by '*' hereafter, given within 1_____ errors ; Cameca IMS-1270 at Nancy / '**', id.) allowed establishing average 206Pb/238U zircon ages of polycyclic rocks. Comparative trials suggest a fair agreement between ages from both methods. __ External Crystalline Massifs.- The external-most 'Rameau Externe de Belledonne' micaschists yielded a minimum detrital zircon age of *570 Ma. Other authors have shown that the dominant rocks of the External basements are Carboniferous granites, while such formations are almost absent in the Internal zones. __ Internal Crystalline Massifs.- The Costa Citrin granite, apparently emplaced into the 'Zone briançonnaise' Upper Carboniferous series, has ages within error at the Visean-Namurian boundary (324 ± 17, 323 ± 8 Ma). In the 'Gneiss du Sapey', one orthogneiss (485 ± 50, *482 ± 5 Ma) and one metagranite (458 ± 41, *452 ± 5 Ma) are Ordovician in age. Four rocks of the Ruitor Massif (metavolcanics : 471 ± 5, 468 ± 22 Ma, augengneisses : **465 ± 11, 460 ± 7 Ma) also yielded Ordovician ages. In the Vanoise-Ambin Massifs two granophyres (507 ± 9, 511 ± 9 Ma) and one metarhyolite (484 ± 88, *500 ± 8 Ma) have late Cambrian to early Ordovician ages. Black metasediments of Vanoise have been intruded by the Cogne diorite during the Tournaisian (357 ± 24, *356 ± 3 Ma). The Gran Paradiso Massif felsic rocks have yielded only Permian ages : metavolcanics (271 ± 2.5, 279 ± 5 Ma), various generations of granite (272 ±32, *269 ± 6, 277.7 ± 3.3, 264-275, *271 ± 3, 274 ± 4 Ma) with one included enclave (281 ± 44, *263 ± 36 Ma). __ Similar ages have been found from some rocks of those units by other authors. Based on the available ages, the succession of Palaeozoic tectonic settings in the Penninic basements is different from the succession known in the External zones. More similarities are found with the Apulian plate (to the E) than with stable Europe (to the W). If confirmed, this statement might bring dramatic changes to the structural models of the Alps. Whatever the answer, the hypothesis of a Permocaboniferous age for all the Internal basements is definitely obsolete, while it was a basic tenet of the traditional Argand's model of 'Nappes de recouvrement'. Our current work questions alternate models, based on local stratigraphy and structures in the Vanoise and 'Zone houillère' realms. Late- to post-Eocene cover resedimentations might have occurred at the onset of subduction events. The main observable structures are related to vertical shortening during post-peak pressure exhumation.
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François Guillot. Géochronologie des socles alpins internes, implications géodynamiques. Planète et Univers [physics]. Université des Sciences et Technologies de Lille 1, 2005. ⟨tel-01670346⟩

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