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New gravity measurements taken to the northeast of Builth Wells reveal, after removal of a regional field, a negative gravity anomaly of 28 gravity units magnitude and some 7 km × 5 km in extent. The anomaly can be modelled in terms of a near‐surface, three‐dimensional body some 1.7 km in thickness with a density contrast of − 0.30 Mg m−3, which corresponds to an actual density of 2.45 Mg m−3. We interpret this body as a granite whose low density is the result of the percolating fluids which it heated and ultimately gave rise to the hydrothermal system near Builth Wells. This is the first indication of the possible presence of a granite that is associated with the back‐arc basin that formed during the subduction of the lapetus Ocean beneath Eastern Avalonia.
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Goddard et al. (2002) studied this question.
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