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The fractionation of the trace elements Rb, Cs, Sr, Ba was measured by the radiotracers technique, between hydrothermal solution, Ab . Or feldspars and silicate melts ranging from Q₄₀Ab₆₀ to Q₄₀Or₆₀. The partition coefficient D between melts and solutions are close to the unit, and they are nearly independent from the composition. The values of D between crystals and solution are quite different from the unit, and are strongly dependent on the feldspar composition : alkali earth elements enter preferably the minerals, while alkali elements remain in the solutions. Some simplifications make it possible to calculate the fractionation coefficient between solids and melt, in the range of temperatures 700-800 °C, and for normative quartz values between 30 % and 40 %. The extreme values of D may reach 250 (D[Cs]melt/Ab) and 0.01 (D[Sr]melt/Or). For two liquids close to the Q.Ab and Q.Or sides of the Q.Ab.Or triangle, we discuss the variation of the bulk partition coefficient during a fractional crystallisation process, and the evolution of the trace elements contents in the residual melts.

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The dolerite dykes at the western end of Britanny (France), which is located on the North Atlantic margin, were emplaced in a NW–SE field of faults at the Trias–Lias time limit, between 210 and 195 Ma. These rocks are quartz tholeiites enriched in TiO2, with a trace element composition close to that of the Deccan basalts. The initial concentrations of Ba, K, and Rb, though slightly modified by the alteration, and of Nb seem to have been affected by crustal contamination during the magma transfer. These dykes are comparable, in age and composition, to the tholeiitic dykes exposed in the Iberian Peninsula, in the Moroccan Atlas and in the Northern Appalachians; they however have a different orientation. Furthermore, these various domains differ in their postintrusion history: the rifting leading to the opening of the Central Atlantic occurred soon after the intrusions of the dykes, whereas the opening of the North Atlantic began several tens of millions of years later, following the direction of the Armorican array of dykes. The study of the mode of emplacement of the intrusions shows evidence of two sets of dykes: "simple dikes", 5 to 10 m thick, resulting from a single magmatic injection and "multiple dykes", about 30 m thick, composed of five magmatic units that are individually 5 to 10 m wide. The western Armorican dyke field is the result of the intrusion of successive sets of dolerite having a uniform thickness and composition, suggesting a cyclic magmatic supply from homogeneous source. [Journal Translation]