Cryptic Sr and Nd isotopic variation across the Leinster Granite, southeast Ireland

Abstract

Rb-Sr isotope whole-rock data from the end-Caledonian Leinster Batholith define an errorchron age of 464 ± 26 Ma, appreciably older than the accepted emplacement age of c. 405 Ma. This anomalously old age is the consequence of a highly variable initial Sr isotopic composition. Initial Nd ratios and T DM model ages also show a wide range of values. However, these isotopic variations are neither randomly distributed nor related to petrographic changes but instead reflect, in a simple way, geographic position about the axis of the batholith. The isotopic pattern revealed in the Leinster Batholith means that isochron ages obtained from S-type granites may be largely dependent on the sampling scheme adopted. The variable isotopic composition of the granite most likely resulted from either a heterogeneous metasedimentary source or hybridization of coeval, dominantly crustal melts.

The Origin of Spodumene Pegmatites Associated With the Leinster Granite In Southeast Ireland

“…This corresponds to an age of 402 ± 7 Ma when recalculated using the recent IUGS recommendation on 87 Rb half life (Villa et al 2015). The pegmatite errorchron initial Sr ratio of 0.705 ± 0.005 (O'Connor et al 1991) lies within the Leinster Granite range obtained by Mohr (1991). Moreover, results from fluid inclusion analysis (Whitworth & Rankin 1989) and mineral chemistry (Whitworth 1992) suggest a genetic link between the Leinster Granite and lithium pegmatites.…”

“…It has been assumed that all plutons intruded and crystallized simultaneously (Brindley 1973) and therefore that this age applies to the whole batholith. However a wide variability in initial Sr and Nd isotope ratios, both between and within plutons, has been interpreted as resulting from the generation of different magma batches by partial melting of an isotopically heterogeneous sedimentary source (Mohr 1991). In addition, disequilibrium textures in plagioclase phenocrysts indicate a multi-stage and multi-scale acid-acid mixing of magma batches during ascent and emplacement (Grogan & Reavy 2002).…”

“…Plotted geochemical data is scattered rather than representing a chemically closed or semi-closed continuous evolution system. Although the key elements Li and Cs have not been analysed in samples from elsewhere in the Leinster Granite, the chemical signature of granodiorites within the batholith varies between localities, probably reflecting the multi-intrusion character of the batholith and some variation in source rock composition (Mohr 1991, Sweetman 1987, Grogan & Reavy 2002.…”

Disequilibrium textures in the Leinster Granite Complex, SE Ireland: evidence for acid-acid magma mixing

“…It is thought that the textures of sample WG reflect mixing of melts from a heterogeneous source. Mohr (1991) reported on isotopic variability of the initial Sr and eNd values that are not consistent with a fractionation trend, thereby excluding a single homogeneous source. Sample LH displays a more complex magmatic history than sample WG and it is thought that it represents mixing during ascent with previously emplaced magma batches.…”

“…The granite complex is divided into five plutons, which comprise strongly peraluminous (Al 2 SiO 3 /[CaO + Na 2 O + K 2 O] > 1.1) one and two-mica granite variants. Geochemical and isotopic studies (Sweetman, 1987;Mohr, 1991) indicate derivation of the granite complex from the partial melting of a peraluminous immature flysch protolith. Structural studies have shown that the batholith was emplaced in an active extensional crustal lineament by a process of amalgamation of multiple magma batches, resulting in a sheeted complex (O'Mahony, 2001).…”