The geology of the Belhelvie mafic intrusion and its environs, Aberdeenshire

• Main Author: Boyd, R.
• Published: University of Aberdeen 1972
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592156

The Belhelvie mass has been remapped using shallow level drilling and interpretation of a detailed magnetic survey to augment outcrop mapping: this has shown the existence of the following sequence of rock units (west to east). a thin noritic zone (150 m. thick) serpentinized dunitic peridotite troctolite with several mappable dunitic bands a gabbroic unit with olivine-bearing rocks containing dunitic bands in the west and coarser noritic rocks to the east. a septum of country rock serpentinized dunitic peridotite with one large band of troctolite troctolite poorly exposed deformed gabbroic rocks The magnetic survey taken with published work (I.G.S., 1968) further suggests that the mass extends out to sea and that most of the southern part of the mass is ultramafic. Drilling has shown that in addition to the area of exposed deformed rocks in and near Belhelvie Quarry a narrow zone of deformation is present at all contacts affecting igneous and country rocks but, within the septum affecting only the latter. The sequence of rock types is in accord with a possible fractionation sequence in the system Forserite-Diopside-Anorthite-Silica (Coombs, 1963). It is suggested that the rocks formed by crystal settling in a nucleation zone near the base of a magma chamber at a depth of 15 to 30 km. and that before becoming completely solid the body was compressed, forcing the rock units into a vertical attitude, and uplifted to a depth of 7.5 km. at which point the remaining magma, having acted as a lubricant for the main mass, solidified forming zone and possibly other contact rocks. The mass was thereafter deformed. Mineralogical and chemical data does not confirm that the Belhelvie mass formed part of a basic sheet extending over much of N.E. Scotland but do suggest that it is. transitional between the 'alpine' and 'stratiform' types of mafic intrusion.