CRUSTAL STRUCTURE INFERRED FROM GRANULITE-FACIES XENOLITHS FROM CENTRAL IRELAND

The continental crust under central Ireland was assembled in the late Silurian and early Devonian towards the end of the Caledonian orogeny. Oblique collision of Laurentian and Avalonian lithosphere is marked by the Iapetus Suture Zone (ISZ), a ~ 50 km wide NE-SW-trending, NW-dipping structure, extending across Ireland and the UK. Caledonian deformation in central Ireland involved a major strike-slip component, which explains the absence of major crustal thickening and uplift and the lack of exposed deep basement rocks.

The only direct samples of the lower crust in central Ireland are xenoliths in Lower Carboniferous volcanic pipes near the ISZ entrained from depths of c. 22-28 km. These are considered to represent samples of the present day lower crust to within a few km. The xenoliths are predominantly metapelitic (khondalite) which is unusual compared with the mostly mafic composition of lower crustal xenoliths world- wide.

Laboratory measurements of xenolith seismic velocities provide important ground truth for deep crustal structure deduced from regional seismic data. Vp values for the khondalites are as high as 7.99 kms-1 and agree well with velocities calculated from modal mineralogy. Vp values also correspond to velocities from nearby seismic refraction lines allowing for dilution of the higher velocity khondalites by variable amounts of psammite and granitic orthogneiss. Some of the khondalites exhibit significant P-wave anisotropy, up to 14% largely accounted for by mineral L-S fabrics. Vs values for the khondalites range from 3.87 to 4.34 kms-1. Shear wave splitting (delta Vs) values from appropriately cored samples range from 0.05 to 0.12 kms-1, corresponding to anisotropy values of 1.3 to 2.9% but, overall, apparent anisotropy values range up to 11%.

Making reasonable assumptions about the orientation of the strain ellipsoid, these data provide fresh insight into the crustal structure and are testable using further field experiments.