The structure and petrology of the hypersthene-gabbro intrusion, Ardnamurchan, Argyllshire

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Abstract
Summary: The hypersthene-gabbro intrusion has an unusually broad outcrop compared with related ring-dykes. The outer wall has a moderate inclination away from centre 2, dipping in the same direction as the overlying sediments and lavas and to a similar degree. In contrast to this, banding in the gabbro is inclined in towards centre 2, with increasing steepness as the centre is approached. The banding or layer surfaces have therefore a roughly conical shape, and since there is some evidence that gravity accumulation of crystals was a factor in the production of layering it is suggested that the lower part of the intrusion has a conical form. The hypersthene-gabbro was probably forcefully intruded, and appears to bear little relationship to ring-dykes. The petrography of the hypersthene-gabbro and its varieties is described. A quartz-gabbro marginal variety may have consolidated from some magma remaining from a pre-hypersthene-gabbro phase of cone-sheet injection, this magma having been driven towards the upper surface of the hypersthene-gabbro intrusion during the forceful injection of the latter. The origin of various types of granular basic pyroxene-hornfels intrusions is discussed and criteria are suggested by means of which the types may be distinguished.

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