Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

Abstract

V p and V s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust. V p values for the metapelitic granulites range between 6.26 and 7.99 km s − 1 with a mean value of 7.09 ± 0.4 km s − 1 . Psammite and granitic orthogneiss samples have calculated V p values of 6.51 and 6.23 km s − 1 , respectively. V s values for the metapelites are between 3.86 and 4.34 km s − 1 , with a mean value of 4.1 ± 0.15 km s − 1 . The psammite and orthogneiss have calculated V s values of 3.95 and 3.97 km s − 1 , respectively. The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V s anisotropy ranges from 1% to 11%. Poisson's ratio ( σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation. When corrected for present-day lower crustal temperature, the experimentally determined V p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V p values of 6.85–6.9 and 6.9–8.0 km s − 1 , respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.