Genesis of lithium pegmatites from the leinster granite margin, southeast ireland: Geochemical constraints

Extensive and economically‐important lithium pegmatites have been described from the eastern flank of the large S‐type peraluminous Leinster Granite batholith in southeast Ireland. This flank area is also the locus of a major dip‐slip ductile shear zone which was active during emplacement of the Tullow Lowlands pluton of the batholith. All the known bedrock occurrences of lithium pegmatite are contained within this marginal deformation zone. Most previous studies have suggested derivation of the lithium pegmatites by fractionation from the Leinster Granite itself, though none are conclusive. More recently, an anatectic model has been proposed whereby the pegmatites were derived by melting of Li‐rich sediments adjacent to the batholithic contact. Our investigation has been concerned with a detailed study of the Rb‐Sr isotope systematics and trace element geochemistry of the pegmatites and their host granitoids in an attempt to resolve the problem of pegmatite genesis. The results suggest a strong geochemical and temporal coherence between the Leinster Granite and the lithium pegmatites. A model is proposed in which Li enrichment in residual magmas may be linked to the development of muscovite instead of biotite in the Leinster Granite itself. The Li excluded from entry to biotite then accumulates in highly mobile, low‐viscosity pegmatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower temperatures.