Petrogenesis of Hercynian leucogranites from the southern Armorican Massif: contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages

Abstract

REE geochemistry and isotopic (Pb, Sr, Nd, O) compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in South Brittany. Ages decrease from 340 Ma in the north to 300 Ma in the south whereas initial 87Sr/ 86Sr ratios increase from 0.706 to 0.710–0.717. REE abundances are relatively low (La N ∼ 20–80 × chondrites) with a strong enrichment of LREE relative to HREE (La N/Lu N= 13.4–26.6). Pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 Ma ago. 18O/ 16O ratios are high and relatively uniform at 10.7 to 12.8, mean 11.9. U Pb studies on zircon indicate the presence of an inherited radiogenic Pb component and that the sediment-dominated source region had been metamorphosed during the Siluro-Devonian (Ligerian). Negativeε Nd(T) values imply that the source region had been enriched in LREE prior to the melting episode. Based on the combined data, large-scale partial melting of crustal material, dominantly of sedimentary origin, is proposed. The model age of the source rocks is not very old (600–700 to 1200 Ma). The Pb and Sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites.