The Criffell-Dalbeattie granodiorite complex

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Abstract
Summary: The Criffell-Dalbeattie granodiorite complex is composed of three granodiorie' and associated quartz-dioritic rocks. Four-fifths of the area of exposed “ granitic ” rocks is occupied by the main Criffell-Dalbeattie granodiorite and the greater part of the paper is devoted to a study of the petrology, structure and emplacement of this granodiorite pluton. It is concluded that the main granodiorite was emplaced as a magma by the forceful displacement and some assimilation of the country rooks. The western end of the complex is considered in relation to the main pluton. The her-grained intermediate granodiorite was emplaced before the main pluton and itself intrudes the Bengairn quartz-diorite which flanks its north and west' margins. The Bengairn quartz-dioritic rocks are considered to have developed by the metasomatic transformation and mobilization of permeated hornfelsic material. It is suggested that the intermediate granodiorite magma may represent the h a 1 result of the metasomatic transformation of the country rooks, and it is postulated that a relatively small movement and internal circulation of the mobilized permeated material would be sufficient to give the essentially magmatic features exhibited by the intermediate granodiorite. The close association of the intermediate granodiorite with the Bengairn quartz-dioritio rocks may well be an indication of the possible origin of the granodioritic magmaa by the “ magmatization ” of permeated hornfelees in regions not much lower than the one now exposed in this area. The more basic composition of the later main granodiorite is attributed to the effect of the assimilation it accomplished during its longer and more active life as a magma. The porphyrite-porphyry minor intrusions and the fine-grained granodiorite may be related to various stages in the crystallization of the granodiorite magma which finally consolidated as the central porphyritic granodiorite.

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