| The ancient suite of rocks we call the
Dalradian has long been a target for research as the original
sediments have since under gone nearly every geological process
possible to attain their present state. The knowledge gained by
many geologists has provided standards such as metamorphic
grades for mineral series in rocks. This has been applied to
many other areas that have been tectonically altered.
The Dalradian Supergroup rocks are now "sandwiched" between
the Great Glen Fault and the Highland Boundary Fault and cross
Scotland and the north west of Ireland.
The original sediments have been estimated to have attained
up to 13KM thickness over an older continental margin where
crustal extension created a series of basins which filled and
subsided. A change to a compressional regime thickened the
sediments and produced a series of nappes and shear zones which
were then intruded by magmas during several pre and post
tectonic events.
Dating of the original sediments, which contain few fossils,
has been taken from two distinctive marker horizons and these
have also given some indication of the tectonic relationships
for land masses of the time.
The Port Askaig Tillite, marking the Appin/Argyll boundary,
contains an assortment of limestones, schists and gneisses from
a glacial episode and these have been correlated to areas in
Scandinavia later found south of the Iapetus suture.
At the top of the Argyll Group the Tayvallich Volcanic
sequence has been dated to 590Ma. This event has been linked to
the opening stages of the Iapetus Ocean.
In north west Ireland Professor Hutton went to Donegal to
Horn Head where Appin Group rocks have been intruded by a series
of dolerite sheets. The upper and lower margins are accessible
so detailed mapping and thin sections were examined to trace the
history of the rocks.
It was hard to tell if the quartzite inclusions in the magma
were picked up as a previous fracture zone was exploited by the
intrusion or created when the rocks were first affected. The
thin sections showed crystals with previous stress patterns and
relic textures but those taken closer to the sheet margins had
recrystalised and lost these "fingerprints".
Further west into a quartz mylonite zone developed along a
thrust on the Horn Head Slide the dolerite had cut through and
ripped about some of the streaked out folds created from earlier
deformation.
A chance find on a fairly small exposure in some
conglomerates in the middle of the Argyll Group proved very
revealing. A basal conglomerate of the quartzites was found to
have been imposed on a much earlier conglomerate containing
highly cleaved pelites which must have been derived from another
source area during a previous period of rapid deposition.
Similar tectonic unconformities have been seen in the Jura
Quartzite which is a major formation on the Island of Jura. All
this work has added to the vast amount of information we now
have on the Dalradian and seems to suggest that the earliest
rocks are not typical of those of the Laurentian Platform but if
anything are nearer in style to some known to have originated on
Gondwanaland continental margins. They have since become
detached and later been "spliced" onto their current site after
a very chequered history.
We were all very grateful to Donny Hutton for his clear,
concise presentation of a very involved and detailed amount of
work, but as he commented, "there is a lot of information still
needed about this very complicated history". It added new
knowledge to the group on this area, we will remember this
lecture and perhaps "a chip off the old block" would have served
as an alternative title.
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