A Morarian age for the ‘younger Moines’ of central and western Scotland
MUCH of our recent knowledge of the Moine metasediments1,2, which extend from the Moine Thrust across the Great Glen Fault to the boundary of the Dalradian Supergroup3,4 (Fig. 1), stems from the western Moine region. Here, late-Precambrian, Morarian ages (740–780 Myr) have been established from pegmatites in the Morar Division5, and a Grenvillian age (1,030 ±50 Myr) from the Granitic Gneiss of Ardgour in the Glenfinnan Division6. The ‘Central Highland Granulites’, generally regarded as ‘younger Moine’2,6, are little known geologically. Here we briefly outline the results of field (by M.A.J.P.) and radiometric investigations (by O.v.B.) in the Central Highland Granulites and the western Moines, which have fundamental implications for the evolution of these extensive metamorphic rocks, and for the nature of the Grenvillian–Caledonian interval in this sector of the North Atlantic Crust.
The Dalradian rocks of the Highland Border region of Scotland
The Highland Border region is defined here by the outcrop of the Southern Highland Group that lies north-west of the Highland Boundary Fault and runs from Stonehaven south-west to the Isle of Bute, and thence to the Campbeltown peninsula. The late-Neoproterozoic to early-Ordovician rocks of the Dalradian Supergroup in this region form a stratigraphical and structural entity that encompasses the >300 km-long surface traces of both the Tay Nappe (D1–D2) and the Highland Border Downbend (D4). The least deformed and metamorphosed Southern Highland Group rocks occur along the south-east margin of the region and are in continuity with the younger, newly recognized Trossachs Group, which has therefore been assigned to the Dalradian Supergroup. The earliest (D1) structures in the Dalradian rocks are dominant close to the Highland Boundary Fault but are successively overprinted northwards by D2, D3 and D4 structures and fabrics, here represented by a series of zones near-parallel to the Highland Boundary. Regional metamorphism increases progressively away from the Highland Boundary Fault and ranges from greenschist to upper amphibolite facies (sillimanite zone). Three fundamental features of deformed and regionally metamorphosed rocks worldwide were first recognized in this area: the ‘stretching lineation’ by Clough in 1897; the concept of regional metamorphic ‘Barrovian zones’ by Barrow in 1901; and the ‘facing direction’ of folds by Shackleton in 1958. The Highland Border region has acquired international recognition for research undertaken into the origin and mode of emplacement of the Tay Nappe, one of the largest recumbent folds known worldwide. This structure provides a framework for linking together most of the GCR sites in this paper.
The stratigraphy and structure of the Glen Orchy area, Argyllshire, Scotland
Synopsis The interpretation of Bailey and Macgregor (1912) and Bailey (1922) is revised. The presence of three major, isoclinal and recumbent folds is demonstrated. Their geometry and facing is described by reference to minor structures, sedimentary structures and the overall stratigraphy. The Glen Orchy Antiform, around which the primary structures are folded, possibly developed together with the primary folds. It is not associated with any minor secondary structures. The structure of the Glen Orchy area raises important questions in the interpretation of the overall structure of the Moine and Dalradian.