Geophysical Studies of Irish Granites Using Magnetotelluric and Gravity Data

Abstract:

We present results of on-going geophysical studies of Caledonian radiothermal granite bodies in Ireland, which are being undertaken to investigate the volumetric depth extent and structural features of these granites. During three field seasons, magnetotelluric (MT) and audio-magnetotelluric (AMT) data were acquired at 156 sites targeting three separate granite bodies. These studies will contribute to a crustal-scale investigation of the geothermal energy potential of the granites and their contribution to the thermal field of the Irish crust.

Across the calc-alkaline Galway granite, located on the Irish west coast, MT and AMT data were acquired at 75 sites distributed in a grid. Preliminary 3D inversion reveals the presence of a resistor, thickest beneath the central block of the granite where it extends to depths of 11 – 12 km. The greater depth of the resistor beneath the central block is in contrast to previous thinking that proposed the central block granites to have shallower depth extent than those of the western block, based on Bouguer anomaly maps of the area in which the western block exhibited a more pronounced negative Bouguer anomaly than the central block.

At the S-type Leinster granite, in eastern Ireland and to the south of Dublin, MT and AMT data were acquired along two profiles (LGN – 27 sites and LGS – 32 sites). Preliminary 1D inversions of AMT data along profile LGN show the Northern Units of the Leinster granite to extend to a depth of 4.5 km and the Lugnaquilla pluton extending to 2.5 km depth.

MT and AMT data were acquired at 22 sites along a profile across the buried Kentstown granite, 35 km to the NW of Dublin. The Kentstown granite was intersected by two mineral exploration boreholes at depths of 492 m and 663 m. Preliminary 2D inversions do not yet satisfactorily resolve the top of the buried granite.

Inversion of MT and AMT data is continuing, with the electrical conductivity structures revealed by these inversions being used to constrain inversions of gravity data. The integration of MT and gravity data will provide an insight into the potential density distribution within the resistors associated with the granites and whether the granites, likely to have elevated heat-production (HP), are underlain by electrically resistive but denser, more mafic bodies, likely to be associated with lower HP.