Gamma-Ray Spectrometric Data
The spatial distribution of radioelements (K, U and Th) in the Galway Batholith was investigated by Feely and Madden (1986; 1987). A calibrated Geometrics GR 410A differential four-channel portable gamma-ray spectrometer was used to analyze granite exposures. Five two-minute counts were taken on flat exposures to estimate bedrock assays of K wt.%, e (= equivalent) U ppm and eTh ppm (where 1 ppm eU= 1 ppm U and 1 ppm eTh= 1 ppm Th, both in radioactive equilibrium with gamma-emitting daughters). Lovborg (1984), Madden (1987) and Madden and Feely (1987) outlined the calibration procedures used for the Galway Batholith survey. 700 gamma-ray stations were established and include both onshore and offshore exposures (Figure 3). Poor exposure in the central block left unavoidable gaps in the coverage. Hand contoured maps showing the spatial distribution of the radioelements and the associated surface heat production (HP) were published by Feely and Madden (1987) and highlighted a notable decrease in radioelement abundances going from west to east across the NE trending Shannawona Fault Zone (SFZ). The SFZ and Barna Fault Zone (BFZ) on the west of Galway City represent major structural delineations in the granite batholith. The region in between the aforementioned faults is referred to as the Central Block. From gravity-modeling studies, Madden (1987) determined that the spatial extent of the Galway Granite batholith displays a thickening of the western (8-12km) and eastern (8-10km) ends, separated by a relatively thinner (4-6km) central block. This thinner central block corresponds to the section bounded by the SFZ and BFZ (Figure 2). Leake (1978) suggested an upthrow of several kilometers on the eastern side of the SFZ. Gravity modeling studies carried out by Madden (1987) support this assertion, proposing an upthrow of about 2 km, and an estimated thickness of 2-3 km for the CMG. The most radioelement rich granite however, the leucocratic Costelloe Murvey Granite (CMG), lies to the east of the SFZ. This granite is reported by Feely et al. (2003) to represent the final emplacement of granite in the batholith at approximately 380 Ma. Radioelement concentrations increase with petrological evolution throughout the batholith (Feely and Madden 1986). Radioelement determinations are lowest in the least evolved mafic granites of the batholith (central block) and increase through the Carna Granite and Errisbeg Townland Granite (ETG). K, U and Th concentrations are highest in the evolved Murvey granites, particularly in the CMG (Feely and Madden 1986). Heat flow anomaly modeling of the batholith predicted surface heat flow values of approximately 72-79 mWm-2, with resultant temperature gradients of approximately 21-23ºC/km (Madden 1987).
