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Title: The northern contact of the Insch mafic igneous mass, Aberdeenshire : a geophysical and geochemical investigation
Author: Leslie, A. G.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1981
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A Total Field ground magnetic survey, supplemented by a programme of bedrock exposure in shallow pits, has enabled considerable revision to be made to the position of the north-eastern margin of the Inch mass. In addition it has revealed that the distribution and attitude of the basic igneous rocks and their associated hornfelses, and the nature of the external contact has been strongly influenced by zones of shearing deformation trending between E-W and NE-SW. Magnetic modelling has shown that the external contact of the mass, and the igneous rock units within the mass, dip steeply (60°-75°) southwards, and that the igneous rocks are limited to a depth of about 5km. New petrological information indicates that narrow but discrete zones of hornfelsic derivatives of the schistose and gneissose country rocks do exist, and that major trends and discontinuities observed on the magnetic map correlate with zones of shearing and mylonitization which generate the complex disruption of the basic mass and its country rocks. The external contacts are rarely undisturbed. Field relations in NE Scotland indicate a regionally important episode of shearing deformation about 470 ma in age. Later faulting on N or NW trends has further contributed to the disruption of the basic mass. Geochemical studies in the 'slate aureole' (c.f Read, 1923a) have shown the survival of variations in original sedimentary oxygen fugacities; these generate highly-oxidized and highly-reduced parageneses of hornfelsic rocks in the thermally metamorphosed derivatives of the Macduff slates. The compositions of the ferro-magnesium minerals in the hornfelsic rocks are precisely controlled by the oxidation ratio of their host rocks. Compositions of co-existing garnet, cordierite and biotite in rocks at or near the contact of the Insch mass imply temperatures of 750°C under a retaining pressure of 3-4kb (PH2O/P = 0.2-0.3) would have been experienced by rocks adjacent to the Insch mass. Chloritoid, representative of the post F2-pre F3 episode of regional metamorphic recrystallization, has been verified in Macduff slates immediately north of the Insch mass. Host rock composition (rock oxidation ratio is critical) controls the occurence of chloritoid-bearing assemblages in terrains where the P,T conditions are appropriate to chloritoid stability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available