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

Abstract

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 pegrnatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower temperatures.

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

“…The Leinster pegmatite belt has been the target of exploration drilling and academic research since the 1970s (e.g. Steiger and von Knorring, 1974;Luecke, 1981;O'Connor et al, 1991). Barros and Menuge (2016) provide a lithological description of the pegmatites and their host rocks at Aclare and Moylisha.…”

Granitic pegmatites

“…the Li-rich and peraluminous Leinster Granite and pegmatites of SE Ireland with an initial 87 Sr/ 86 Sr of 0-705; O'Connor et al 1991). Multiple source rocks for cogenetic magmas of peraluminous granite-pegmatite systems (e.g.…”

Intra-Ordovician deformation in southeast Ireland: evidence from the geological setting, geochemical affinities and U—Pb zircon age of the Croghan Kinshelagh granite

“…The Leinster Granite was emplaced during shearing (McArdle & Kennedy, 1985). Foliated Li pegmatites within this deformation zone have been dated at 396 + 7 Ma (O'Connor, Gallagher & Kennan, 1991). Unlike the Leinster Granite and minor intrusions within the East Carlow Deformation Zone, however, the Croghan Kinshelagh granite is not strongly foliated.…”

“…In addition, recent Rb-Sr dating of the passively emplaced minor granitic and tonalitic sheets (e.g. Aughrim, Coolboy, Macreddin) shows them to be contemporaneous with the diorite suite (combined age 410+13 Ma) and perhaps to predate slightly the main batholith (O'Connor, Gallagher & Kennan, 1991). Brindley & Connor (1972) and Brindley & Gupta (1973) suggested that the Croghan Kinshelagh granite belonged to this suite of minor granitoids.…”