Field and petrological evidence for the development of an ensialic marginal basin related to the Hercynian orogeny in the Massif Central,France |
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| Authors: | Herve Sider Maryse Ohnenstetter |
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| Institution: | 1. Laboratoire de Pétrologie structurale et Métallogénie, ERA CNRS no 1011-Pétrologie des granitoides, Université de Nancy I, B. P. no 239, 54506, Vandoeuvreles-Nancy, France
2. la rue de la Férollerie, CRSCM, GIS-BRGM-CNRS, 45045, Orléans Cédex, France
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| Abstract: | Six lithologic units in tectonic contact with each other have been defined during mapping of the Devonian in the Beaujolais area of the northeastern Massif Central. Five main igneous suites have been recognized: - A transitional basaltic suite restricted to a single unit.
- An acid volcanic-plutonic suite the members of which are related by fractional crystallization and magma mixing.
- Low-TiO2 volcanic rocks with calc-alkaline affinities.
- A TiO2-rich tholeiitic suite related to an ophiolitic complex.
- A plutonic suite with close resemblances to Alaskantype intrusions.
The transitional metabasalts (1) form the oldest igneous suite and could represent either an intraplate magmatic forerunner of rifting or tectonic slices of weakly metamorphosed rocks representing a pre-Acadian event. The next three suites may be related to a short-lived ensialic marginal basin that developed between the Acadian and Bretonian orogenies. The basin is asymmetric, with the ophiolite of the central part flanked by an acid ridge on one side and a passive continental margin on the other. Quartz-keratophyres (2) and calc-alkalic basic volcanic rocks (3) were intercalated in varying proportions to form a bimodal volcanic pile before the rifting event that led to the formation of the ophiolites (4). The acid ridge (2) may be due to the reactivation of a continental basement. Cumulate rocks with Alaskan-type affinities occur as olistoliths, emplaced before the formation of the rift basin, supporting a comparison of the Hercynian belt with accreted magmatic arc terranes. |
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