DATING OF A WIDESPIŒAD REMAGNETIZATION ON THE SOUTHEASTERN BORDER OF THE FRENCH MASSIF CENTRAL AND IMPLICATIONS FOR A REGIONAL FLUIDS MIGRATION RELATIVE TO PYRENEAN OROGENY
Henri ROUVIER(1), Bernard HENRY®, Maxime LE GOFF(2), Jean-Claude MACQUAR(1),
Jacques THIBIEROZ(1), Michael T. LEWCHUK(2), David LEACH(3)
(1) Laboratoire de Géologie Appliquée, Paris VI University, 4 place Jussieu, 75252 Paris Cedex 05, France
(2) Géomagnétisme et Paléomagnétisme, IPGP and CNRS, 4 avenue de Neptune, 94107 Saint-Maur Cedex, France
(3) US Geological Survey, Box 25046 Federal Center, Denver, CO 80225, USA
Remagnetizations have been pointed out in folded mountain ranges and foreland basins for at least twenty years. Usually they have been interpreted as resulting from fluid migration related to orogenesis in these mountain ranges. The geochemical properties of these fluids should be compatible with the formation or the transformation of ferrimagnetic minerals, thus allowing acquisition of remanent magnetization during fluid migration. Lead-zinc (+/-barite and fluorite) mineralization hosted by carbonate sedimentary rocks, usually called "MVT" (Mississippi Valley Type) have also been considered to be formed during migration of very important volumes of fluids and are commonly located in the foreland of mountains ranges. Radiometric dating on ore-stage mineralization in some MVT deposits has yielded ages that are coincident with that of the paleomagnetic remagnetization. This leads to the assumption of a similar origin for widespread carbonate remagnetization and MVT mineralization, and then to a new dating method for these ore deposits.
The Mesozoic sedimentary units of the Cévennes border host Pb-Zn-Ba-F mineralization that belongs to the MVT family. Paleomagnetic work was therefore performed to attempt to date of these deposits. However, the structural deformations in this area includes more or less local vertical axis rotations often affecting areas around the Cévennes fault. The results presented here are intended to provide a paleomagnetic reference direction on which further results can be based. Thus, they include barren rocks near mineralized areas, but only from those locations that belong to the most stable parts of this border: area of the Largentière mine (including Balazuc and Morte-Mérie boreholes) and area of the Saint Felix -de-Pallières mine.
A Characteristic Remanent Magnetization (ChRM) can often be isolated. For the data from the Balazuc and Morte-Mérie boreholes, the ChRM can be split into two different populations that vary according to the lithology. In the Triassic red beds formations, SSW reversed and antipodal NNE normal directions corresponds to the primary magnetization. This ChRM is not further considered here. In all other formations (dominated by carbonates) in the borehole and in all of the surface sites, the ChRM only of normal polarity with a steeper inclination, is a remagnetization (D = 2.9°, I = 55.4°, к = 3022 and ag 5 = 0.7°) acquired during or between two phases of deformation.
Comparison of the paleomagnetic pole for this ChRM (168. 1°E; 81.4°N, К = 2017; A95 = 0.8°) with the Apparent Polar Wander Path for stable Europe shows that it is Paleocene to Lower Eocene in age (between 60 and 50 Ma). The remagnetization is here not related to burial diagenetic processes (smectite-illite transfor¬ mation), but to migration of chemically active fluids. The fact that ChRM is only of normal polarity suggests a weak duration (lower than 1 Ma) of magnetization acquisition, i.e. of fluids migration. But, because of the chemical origin of the magnetization, only the dominant polarity during a sequence of opposite polarities can be obtained, and this duration could have been much more important.
The Paleocene to Lower Eocene age of the remagnetization and the fluid migration has to be compared to that of the known geological events in this area. It is coeval to the orogeny in the Pyrenean mountain range, located to the south of the Cévennes border, even though this is only the beginning of the Pyrenean tectonics in Cévennes itself. The Alpine orogeny, to the east, is younger (-30 to 10 Ma) and so it is not related to the remagnetization.
The relationship between the remagnetization and Pyrenean tectonics is very important because almost all prior research has considered that any fluids that were driven through the Cévennes border came from the east, through the Southeast basin. It was however difficult for fluids to migrate through this basin, because of the development of low porosity argillaceous facies in its central area. The fact that these fluids may be related to
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