An investigation of ground based and aeromagnetic data over the Cairngorm and other Caledonian granites in the UK

Stokes, T. M. (1995). An investigation of ground based and aeromagnetic data over the Cairngorm and other Caledonian granites in the UK. PhD thesis The Open University.



The aeromagnetic map of Great Britain shows a number of granites associated with magnetic anomalies. One of the largest of these granites, the Cairngorm pluton, has a complex overlying aeromagnetic anomaly, yet measurements show that the surface rocks have a magnetic susceptibility too low to account for this aeromagnetic field.

The purpose of this investigation was to determine the likely reasons for this discrepancy employing a range of computer based techniques, and using new land-based data.

Potential field data for the Cairngorm area, together with a region of the Southern Uplands, was subjected to frequency domain reduction and filtering. Hand specimens from the granite and host rock were collected for susceptibility measurements. The results have been integrated with profile models to explain the variations observed in the aeromagnetic and ground based magnetic field.

The Cairngorm granites appear to be zoned at depth, with a variable thickness cap of low density, low magnetic susceptibility granite. The presence of intermediate composition rock in the upper reaches of the granite mass provides evidence for stoping, whilst the presence of intermediate composition rock at deeper levels is thought to be due to multiple magma pulses and/or differentiation.

The Loch Doon granite has a low magnetic susceptibility subsurface core extending off its eastern flank. A number of correlations between the geophysics and geochemistry of the granites investigated were noted. The concept of gravrnag space has been developed and applied as a framework in which to explore the relationships between the granites of Scotland and northern England. Relationships between the background geophysics and the geochemistry of granites in the area of the proposed Iapetus Subduction zone have also been investigated.

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