Structural dating of a Precambrian pegmatite in Moine rocks of northern Scotland and its bearing on the status of the 'Morarian Orogeny'

Abstract

A cross-cutting pegmatite emplaced into low grade, non-migmatitic, Proterozoic, Moine metasediments on the western sea-board of Inverness-shire, Scotland, gives Rb-Sr muscovite ages of 746–776 Ma which are interpreted as minimum estimates for the age of formation of the pegmatite. Intrusion followed at least one phase of folding and schistosity production in the country rocks and an early phase of syn-kinematic garnet growth, correlated with the regional D 2 tectono-metamorphic event. The pegmatite was folded and suffered considerable grain size reduction under metamorphic conditions during the regional D 3 deformation episode. The dilational character and low grade context of the pegmatite show that it was not a product of melting in place during metamorphism of the adjacent Moine Schists. Its lack of initial, syn-kinematic crystallization textures and fabrics, together with the absence of evidence that might relate its emplacement to any regional phase of compressive ductile deformation, would be compatible with a considerable time lapse between its intrusion and the earlier deformation and metamorphism of the host rocks. We have, however, no direct evidence for the extent of this interval. The relationship of the pegmatite to the earlier deformation and metamorphism of the host rocks provides clear evidence for substantial Precambrian orogenic activity but the age of this activity, whether 'Morarian' and, or, 'Grenvillian', remains a matter of interpretation.