The northernmost CAMP: 40Ar/39Ar age, petrology and Sr-Nd-Pb isotope geochemistry of the Kerforne dike, Brittany, France.

Fred Jourdan (1), Andrea Marzoli (1)*, Herve Bertrand (2), Michael Cosca (3), Denis Fontignie (1)

1  Section de Sciences de la Terre; Universite de Geneve; 13 Rue des Maraichers; 1211 Geneve 4, Switzerland;;;
2  Ecole Norrnale Superieure, Laboratoire des Sciences de la Terre,UMR-CNRS 5570 46, Allee d'Italie, 69364 Lyon Cedex 07, France;
3  Institute of Mineralogy and Geochemistry, University of Lausanne, BFSH 21015 Lausanne, Switzerland;

*corresponding author.


The Central Atlantic Magmatic Province (CAMP) is defined by tholeiitic basaltic flows and dikes associated with the initial breakup of Pangea and opening of the Atlantic Ocean at 200 Ma.  These tholeiites occur in once-contiguous part of North America, Africa, South America and Europe over a total area of about 7 million square km. The Kerforne dike, in Brittany (NW France), represents the northernmost outcrop of this province.  It is located more than 1500 km north of the main Atlantic rift.  Despite its distal position, this dike has an Ar/Ar age (193 ± 3 Ma, obtained on plagioclase separates) similar to the CAMP tholeiites.
      Kerforne dolerites are characterized by augite and minor plagioclase phenocrysts.  According to petrographic observation and microprobe analyses, some of these plagiociases contain resorbed high-An (An85) possibly xenocrystic cores which may be evidence of interaction with a mafic lower crust.  The low-Ti02 (1.0 wt %) tholeiitic Kerforne basalts are characterized by negativeNb anomalies, by a positive correlation between eSr, and eNd, by high radiogenic 207 Pb/204 Pb in comparison to relatively unradiogenic 206 Pb/204 Pb, and by an enrichment in LREE relative to HREE.  These chemical features, along with the mineralogic observations, are indicative of a minor contamination with mafic lower crust, like that represented by I.s. granulitic xenolithsof the Massif Central.  By contrast, contamination with the silicic upper crust (e.g., with the l.s. granitic basement) was negligible.
     The isotopic compositions of the least contaminated Kerforne basalts (e.g., enriched in radiogenic Sr and 207 Pb) are similar to those of most other CAMP low-Ti02 basalts, and are different from those of most oceanic basalts.  It is suggested that this enriched signature was inherited by interaction with metasomatized portions of the continental lithospheric mantle, similar to the sources of Variscan lamproites of Brittany.
     Tholeiitic basalts with similar age and isotopic and geochemical compositions crop out from central Brazil to northern France, implying a common origin. In view of their location, most of the CAMP basalts (e.g., Kerforne) cannot be considered as the magmatic expression of the Central Atlantic rifting.  A widespread thermal anomaly is required, which may have been generated by an incubating deep-seated mantle plume or by a long-time heat production and accumulation, favored by a shield effect, below the mega-continent Pangea.