Subduction of Iapetus crust during the Caledonian orogeny has been seen by some authors as terminating at the end of Ordovician times and by others as continuing through the Silurian. We present petrological and chemical data which support a coeval arc origin for detritus in the Lettergesh Formation (Silurian), western Ireland. The arc, based on continental crust, lay to the north of the present outcrops of this formation, along the Laurentian margin of Iapetus, implying active oceanic subduction in the Silurian. This subduction may have been followed by some continental underplating around the middle of the Wenlock giving rise to a successor basin in Ireland and Scotland.
The Lettergesh Formation, Co. Galway, Ireland, comprises a 1.5 km thick sequence of turbidites deposited
c.
430 Ma ago during closure of the Iapetus Ocean. Petrographic and geochemical data show that the turbidites consist overwhelmingly of detritus derived from a young volcanic terrain. Palaeocurrent data indicate that the detritus was derived from the north. Modelling of the chemical compositions of the turbidites indicates provenance from a high-K calc-alkaline, subduction-related volcanic province built on continental crust. A subordinate ultrabasic component may represent detritus from either an ophiolite or tectonically emplaced serpentinite. The turbidites are characterized by low variability of initial ɛ
Nd
(−6.5 to −3.2) and highly variable initial
87
Sr/
86
Sr ratios. Modelling of the isotopic data suggests a mean crustal residence age of 1440–1600 Ma for the continental foundation of the volcanic terrain. Such a model age is consistent with a mixture of crust having
T
DM
ages of
c.
1.9 Ga and c. 1.3 Ga, both of which are known from the Annagh Gneiss Complex of NW Ireland.
The Silurian succession of North Galway is relatively well constrained
in terms of environmental
analysis and, in its lower half, palaeontologically. The initiation of
a late Llandovery marine
transgression can be demonstrated over fluviatile red sandstones. The deposition
of shallow-water
conglomerates at the base of a turbidite sequence within the succession
indicates the long-lived presence
of a channel system that was probably fault controlled. The back-stripping
method allows a subsidence
curve to be constructed for this succession. It demonstrates an initial
period of rapid
rift-related subsidence followed by a short-lived hiatus that may be due
to the cessation of subduction
in this part of the Caledonides. A comparison of the Galway subsidence
curve with that of the
Silurian succession at Girvan in Scotland shows strong, but diachronous,
similarities. This tends to
support an earlier suggestion that they formed part of a single but partitioned
basin throughout most
of the Silurian period. Although the eustatic fall in sea level at the
Llandovery–Wenlock boundary can
be recognized, the influence of tectonic regime and sedimentation rates
were the controlling factors in
determining relative sea levels.
Conglomerates from the Lettergesh Formation in the Silurian sequence of N Galway, Ireland, demonstrate palaeocurrents from the NE. Fabric studies demonstrate that the conglomerates were emplaced largely by traction currents and not by down-slope sliding or dispersive currents. Stratigraphic relationships demonstrate that the conglomerates were deposited in an environment which was adjacent to fluviatile, shallow-marine and deeper shelf facies at different times. A fan-delta environment of deposition is suggested, which requires a modification of previous Silurian sea-level curves at, or near, the base of the Wenlock.
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