The southern part of the Cruachan granitoid, Etive Complex, Argyllshire

Perkins, Stephen Guy (1986) The southern part of the Cruachan granitoid, Etive Complex, Argyllshire. (PhD thesis), Kingston Polytechnic, .


Mapping, petrography and geochemistry show that the Cruachan Granitoid, which forms the outer part of the Caledonian Etive Complex, consists of seven intrusive units, mostly quartz monzodiorites. They form three groups with chemically distinct magma sources. The first five units are a set of intersecting flat-topped bell-jar intrusions, which intrude to higher levels with decreased S.G., and whose centres migrate north-eastward. The other two are sets of inclined sheets. A NNE-SSW wrench fault bisects the complex, producing 0.8 km. displacements of the units. The emplacement of one set of sheets is seen to be related to the stress field of this fault. Subsidence of the centre of the complex subsequent to emplacement of the seven units has occurred. The magmas were mostly liquid when emplaced. Their PH[sub]20 values reached 1.0 kB during crystallisation. Oxygen fugacity of late-stage magmas in successive bell-jar units increased progressively regardless of their chemical composition. Increasing access to the surface is inferred. Variability within units is mainly due to in-situ crystal fractionation, but examples of magma mixing and probable contamination are also identified. A magma chamber evolving from basic to acid compositions throughout the life of the Complex supplied one group of Cruachan units. Intrusions of the other two groups, which are richer in K, La, Ce and Zr, intervene between stages of intrusion from this first source. One of the two formed quartz monzodiorites higher in Al than the first group; the other, quartz monzoni te with over 7000 ppm Ba. Three Lorne basalt compositions can yield all three magma groups independently by crystal fractionation and crustal granulite assimilation. It appears that the Etive Complex and the Lorne Lavas are contemporaneous. The work extends knowledge of regional chemical trends among Caledonian magmas which are significant for understanding their genesis.

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