Abstract
A chain of Late Devonian-Early Carboniferous intrusions on the western margin of the French Massif Central (the “Limousin tonalite belt”) consists of quartz-diorites which are unusual among French Hercynian granitoids in their low silica contents (53–68 wt% SiO 2) and abundant hornblende. They represent the earliest and most primitive plutonism post-dating the main phase of Hercynian metamorphism and have chemical characteristics strongly resembling andesitic lavas of calc-alkaline affinity. They differ from other Massif Central Hercynian granitoids in their low 87 Sr 86 Sr 360 (0.7047–0.7059) and high ϵNd 360 (−0.7 to + 1.8). Their initial Pb composition was rather homogeneous ( 206 Pb 204 Pb 360 = 18.15–18.38 ; 207 Pb 204 Pb 360 = 15.57–15.62 ; 208 Pb 204 Pb 360 = 38.06–38.26 ) and their 207 Pb 204 Pb i values are significantly lower than those of other Hercynian granitoids. The Limousin quartz-diorites are considered to have originated from an enriched mantle with characteristics modified by subduction of oceanic crust and terrigenous sediments. The primitive quartz-dioritic magmas were then contaminated in the lower continental crust by an AFC process, involving acid metaigneous or metasedimentary lithologies. The occurrence of such plutons in the earliest phase of Hercynian plutonism in the Massif Central is probably related to minor subduction at an active continental margin.
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