Cleaved clasts in Dalradian conglomerates: possible evidence for Neoproterozoic compressional tectonism in Scotland and Ireland ?

Cleaved metasedimentary clasts are present in stratigraphically and geographically distinct conglomerates in the Argyll and Southern Highland Groups of the Neoproterozoic Dalradian succession in the SW Scottish Highlands and NW Ireland. The significance and relationships of these clasts are that: (1) they were unequivocally reworked and deposited by sedimentary processes; (2) their internal foliation is probably due to contractional deformation that pre-dates regional Caledonian fabrics; and (3) most of the cleaved clasts are only moderately deformed psammites and pelites and thus cannot be construed as having been derived from extensional mylonites. These conclusions, coupled with the generally accepted inferences that the Dalradian succession post-dates Grenville deformation (c. 1100-1000 Ma) and pre-dates early Palaeozoic Caledonian deformation (c. 470 Ma) and that the lowermost exposed Dalradian rocks, the Grampian Group, are truncated by a c. 806 Ma shear zone, imply that the clasts must have been foliated during an episode of mid-Neoproterozoic contractional deformation. The clasts may thus represent further evidence in support of the contentious c. 870-800 Ma Knoydartian orogeny and thereby further render as equivocal interpretations that the Neoproterozoic tectonostratigraphic evolution of the Scottish Highlands and MW Ireland is a record of long-lived 'episodic' extensional tectonism. Copyright (C) 2000 John Wiley & Sons, Ltd.