The mineralogy and geochemistry of the Lochnagar complex
In this thesis determination of major,- trace,- and mineral chemistry are combined with field-work and petrography in an attempt to elucidate the petrogenesis and mode of emplacement of the Lochnagar granitic complex. The results show that this pluton is zoned-from an early, marginal quartz monzodiorite unit, through a granodiorite unit, (both forcefully emplaced), to a later, passive granite in the centre. The major-element chemistry is characteristic of a calc-alkaline series, and is consistent with the classical interpretation of the derivation of the Caledonian magma series by differentiation of a single, parental magma. This magma was fractionated at depth, and rose along a crustal dislocation as a series of discrete bodies of slightly differing compositions, to be emplaced in the crust. The first differentiate, a dry dioritic magma, was intruded shortly after by the granodiorite, both showing features of dilation and forceful emplacement. Continued regional uplift and denudation before the ascent of the granite units resulted in these units being passively emplaced in a high-level, brittle fracture environment. The granite has discordantly intruded the earlier units and a second thermal event is recorded in the metamorphic envelope to the S.E. In the N.W. of the complex however, where the granite has intruded the regional gneisses, clear evidence of stoping is seen, and only one thermal aureole is developed. Due to the concentration of water into residual melts, the granite magmas were considerably richer in water than earlier members. This resulted in a final phase of pegmatite development, autometasomatism and possibly the fissure eruption of volatiles. This late-stage metasomatism has tended to homogenize biotite chemistry throughout the pluton, regardless of rock-type. Correlation is made with other plutonic complexes in an attempt to confirm that the style, of emplacement is directly related to the localized physical conditions of the country-rocks, and depth of intrusion, and only indirectly to time of emplacement.