The north-east Grampian Highlands : an investigation based on new geophysical and geological data
Modifications have been made to the boundaries of the 'Younger Basic' masses of the NE Grampian Highlands, with the addition of new areas of 'Younger Basic' rock. Numerous shear zones, some in excess of 2km wide, have been recognised in association with these masses, generally present along their margins, although some significant shear zones occur within the masses. It is considered, on the basis of gravity modelling, that the NE Grampians were intruded by a series of deep-seated plutons of basic and ultrabasic rock. Subsequent marginal shearing of these resistive basses resulted in the detachment of large bodies of rock (the 'Younger Basic' masses), which were transported into their present field setting whilst still hot enough to produce a thermal aureole in the country rocks, at a temperature above that of the Curie point for magnetite. The shearing deformation has also affected the distribution of the low-grade and high-grade metasediments, bringing gneisses into close proximity with slates and schists. These metasediments contain magnetite rich pelitic laminae, probably resulting from diagenetic changes within the original sediments, whilst near the earth's surface. Natural Remanent Magnetization measurements, made from samples of magnetic metasediments close to the Insch mass indicate that a narrow contact metamorphic aureole may exist along the southern deformed margin of the mass. The dehydration of the margins of biotite-rich 'Newer Granite' bodies by later pulses of granitic magma has resulted in the breakdown of biotite, to give magnetite and orthoclase, and hence produce 'aureoles' of magnetic granite. A series of ENE-WSW trending reversely magnetized quartz-dolerite dykes, belonging to the late-Carboniferous swarm, have intruded the region along pre-existing lines of weakness which possibly developed in response to the initial opening of the proto-N. Atlantic.