recent evaluation of the Shetland ophiolite does not regard it to be oceanic
crust sensu stricto but marginal basin crust, thus a departure from the
‘standard’ ophiolite model for oceanic crust. It has been suggested that an
Upper Dalradian extensional basin (that was to become the Clift Hills Division)
developed to become a marginal spreading basin on the Laurentian Plate.
Eventually a period of eastward subduction of the basin crust was initiated but
switched to westward subduction as Baltica closed with Laurentia,
concluding with the obduction of the ophiolite nappes of Unst and Fetlar.
Thrusting, due to the docking of the volcanic arc, occurred under conditions of
regional medium to high-grade metamorphism so formed broad
ductile thrusts in the Moine rocks of Scotland characterised by broad belts of
re-crystallised mylonites. In Shetland it is interesting to note that Moine and
Dalradian successions are in tectonic contact via a ‘boundary zone’ of
near vertical tectonic slices of kyanite
grade metasediments and high grade gneisses. Narrow bands of distinctive
microcline-augen-gneiss mark the upper and lower edges of the Boundary Zone.
Peak regional metamorphism and crustal thickening in the Grampian Orogeny south
of the Great Glen Fault was reached in the Mid-Ordovician and was followed by
the emplacement of gabbros and foliated granites of north-east Scotland. None of
the major intrusions of Shetland are this old, although parts of the Moine and
Dalradian sequences and the Boundary Zone were migmatised and injected on
several occasions by pegmatites and aplites (The Scatsta Permeation Belt).