Is the Colonsay-west Islay block of SW Scotland an allochthonous terrane? Evidence from Dalradian tillite clasts

Abstract

In the Scottish Caledonides context, the Colonsay-west Islay terrane has seemed anomalous and therefore likely to be allochthonous. New studies suggest, however, that its basement is not necessarily exotic but represents the missing early Proterozoic link between Greenland and Scandinavia. Recent interpretations of the Scottish mainland deformation history imply that the age of the Grampian events there resembles that of the late Proterozoic deformation previously identified in the Colonsay Group cover succession. Reconnaisance studies of Dalradian tillite provenance suggest that the Colonsay-west Islay basement was a source of Dalradian sediment and perhaps floored that basin. There is, therefore, less reason now to regard the Colonsay-west Islay terrane as allochthonous, and recent hypotheses of its tectonic emplacement being the cause of the Grampian orogeny should be viewed with caution.

The Antrim–Galway Line: a resolution of the Highland Border Fault enigma of the Caledonides of Britain and Ireland

“…The Colonsay-west Islay block (which extends to Inishtrahull, Daly, Muir & Cliff, 1991), was previously thought to be exotic, but provenance links suggest that this type of basement could extend beneath the northern part of the Dalradian cover (Fitches et al 1990;Dickin & Bowes, 1991;Dickin, 1992;Muir, Fitches & Maltman, 1992 Strathmore Basin indicate derivation from the Grampian terrane to the north, but there were also sources within the Midland Valley whose diversity suggests that the Midland valley is a composite terrane with a variety of low grade cover sequences originally present (Haughton, 1988 Hawksworth, 1980;Upton, Aspen & Hunter, 1984;Halliday, 1984;Aftalion, van Breemen & Bowes, 1984). This suggests granulite-facies basement of Grenville age, consistent with the velocity structure (Davidson et al 1984;Barton, 1992) which also indicates that the basement is shallower than north of the Highland Border.…”

A Caledonian age for the Kiloran Bay appinite intrusion on Colonsay, Inner Hebrides

“…However, difficulties remain in matching their structural history. The late structures on Colonsay and in western Islay correlate most simply with the Islay Anticline, and related Grampian orogenic events in the Dalradian Block (Fitches and Maltman 1984). In contrast, the early structures in the Colonsay Group, which are also present in the Rhinns Complex, have no obvious counterparts in eastern Islay or in the SW Highlands.…”

“…The Group bears lithological similarities to the Dalradian Supergroup of the Grampian block, and also to the Torridonian rocks of the NW Highlands; correlations with the Moine Supergroup of the Northern Highlands are also possible. Various correlations between the Colonsay Group and the Neoproterozoic sequences exposed on the Scottish mainland have therefore been proposed (reviewed in Fitches and Maltman 1984), but none are conclusive.…”

“…Structural mapping (Fitches and Maltman 1984;Bentley 1986) has revealed that the Colonsay Group was involved in a major early deformational event not recorded in the Dalradian Supergroup to the east. This raised the possibility that the depostional history and at least part of the structural history of the Colonsay Group are unrelated to that of the mainland and that the Colonsay-W Islay block ( Fig.…”

“…1) was therefore a suspect terrane (Bentley et al 1988). More recently, however, Fitches et al (1990Fitches et al ( , 1996 have suggested that the Rhinns Complex was a local source of sediment during the deposition of the Appin Group Dalradian, and so the Colonsay-W Islay block is not 'suspect' within the SW Scottish Caledonides. Difficulties remain, nevertheless, in matching the structural and igneous history of the Colonsay Group with the mainland sequences.…”