Age of the Dalradian Supergroup, Grampian Caledonides

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From: Stephenson, D, and Gould, D. 1995. British regional geology: Grampian Highlands. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Age of the Dalradian Supergroup[edit]

The age of the sediments which became the Dalradian Supergroup is poorly constrained. Fossils are rare, are poorly preserved and, where species have been identified, they invariably have a wide stratigraphical range. As yet few radiometric age determinations have been made on penecontemporaneous igneous rocks and most available dates relate to later tectonothermal events. Current estimates of age are based mainly upon attempts to fit this imprecise data into a framework of stratigraphical events established elsewhere in the Caledonides.

Geological sequence and events in the Grampian Highlands. P915452.

Palaeontology[edit]

The palaeontology of the Dalradian has been reviewed by Downie et al. (1971) and Downie (1975).

In the Appin Group, worm burrows have been recorded from quartzites (Peach and Horne, 1930) and algal stromatolites have been recorded from the Lismore and Islay limestones (Spencer and Spencer, 1972). The Islay Limestone has also yielded oncoliths. None are of biostratigraphical value.

In the Argyll Group, the Bonahaven Dolomite has yielded acritarchs and algal stromatolites (Hackman and Knill, 1962; Spencer and Spencer, 1972), including some which suggest a late Precambrian (Vendian, or Upper Riphean to Vendian) age (Downie, 1975). The Islay Quartzite contains worm burrows which indicate an age no older than Vendian, and the Easdale Slate contains long-ranging, Vendian to Cambrian acritarchs (Downie, 1975). Limestone clasts in the Selma Breccia Formation contain oncoliths, catagraphs and other calcareous fossils which have been assigned to the Vendian (Litherland, 1975). However, calcareous and burrowing algae which resemble Lower Cambrian and younger forms are also present (Downie, 1975). Acritarchs from the Tayvallich Limestone, which were originally thought to be Lower Cambrian (Downie et al., 1971), are now known to have a longer range, extending back into the Vendian.

In the Southern Highland Group the search for fossils has concentrated upon the weakly deformed, low-grade slates and limestones of the Highland Border area and Turriff Syncline. The Macduff Slates have yielded rare burrows, a few acritarchs and more widespread microfossils which resemble highly altered chitinozoa. Downie et al. (1971) tentatively identified one of the acritarchs as being of post-Cambrian age and possible chitinozoa which would suggest an early Ordovician age, probably Llanvirn or Llandeilo. These identifications remain highly controversial and have not gained general acceptance. An early record of Silurian graptolites is now attributed to a sample labelling error. Possibly one of the most striking features of these generally low-grade rocks is their lack of macrofossils and trace fossils which has led several workers to suggest that they must all be of Precambrian age. The ‘glacial’ dropstones and pebble beds at the top of the Macduff Slate Formation remain a problem since the next major global glacial period recorded after the basal Vendian (cf. the basal Argyll Group) was in late Ordovician times (Harland, 1972).

The fossiliferous limestone of Leny Quarry near Callander, which contains Lower Cambrian trilobites, was for many years regarded as the most reliable palaeontological indicator of the age of the youngest Dalradian strata in Scotland. This outcrop is now regarded as part of the Highland Border Complex (e.g. Curry et al., 1984), as are rocks reported as containing Middle Cambrian fossils in Clew Bay, Co. Mayo (Rushton and Philips, 1973; Harper et al., 1989). The structural relationships of these Cambrian rocks with the adjacent Dalradian and younger Highland Border Complex rocks, and hence their stratigraphical affinities, are still a matter of debate (see Chapter 9).

Radiometric dating[edit]

Note: Rb-Sr dates quoted in this section have all been recalculated using currently accepted decay constants and hence differ slightly from values quoted in the original publications

The oldest generally accepted radiometric dates from rocks of the Grampian Highlands are from pegmatite veins within the Grampian Slide Zone and other slides above and below the junction between the Central Highland Migmatite Complex and the Grampian Group. It is believed that these veins formed during early ductile shearing along the slides. Several dates have been obtained, by Rb-Sr methods on combined muscovite and whole-rock, which cluster quite tightly around 750 Ma (Piasecki and van Breemen, 1979; 1983) suggesting a correlation with the latest tectonothermal event to affect the Moine rocks north-west of the Great Glen in Knoydart and Morar (Fettes et al., 1986). Irrespective of whether the migmatites are regarded as stratigraphically and structurally contiguous with the Grampian Group (Lindsay et al., 1989) or as an earlier basement (Piasecki, 1980), this means that the migmatite complex and the lowest Grampian Group rocks must be more than 750 Ma old. There is no good evidence to indicate how much older than 750 Ma either unit may be but Soper and Anderton (1984) have speculated that the slides may be the result of lithospheric stretching in the early stages of development of the Dalradian basin. If this is so, then the earliest Grampian Group sediments may have been deposited only shortly prior to 750 Ma and hence may be assigned to the Upper Riphean Epoch in the late Precambrian.

Later parts of the Grampian Group have been shown to be stratigraphically continuous with the Lochaber Subgroup in several areas and in general there would then appear to be stratigraphical continuity throughout the Appin Group. There are no radiometric dates which cover this interval, and it is difficult to reconcile the deformation of the inferred lower part of the succession at 750 Ma with the ongoing sedimentation, with no obvious major tectonic break, which continued at least until the next available radiometric dates at the base of the Argyll Group.

It has been generally accepted, though by no means proven, that the tillites at the base of the Argyll Group can be correlated with the Varanger Tillite, which defines the base of the Vendian Epoch in Scandinavia. Metasedimentary rocks associated with the Varanger Tillite have been dated by Rb-Sr methods at 653 Ma (Pringle, 1972), but doubts have been cast on their validity and attempted correlations with the Scottish Dalradian should be regarded as extremely tenuous.

At the top of the Argyll Group, the Tayvallich Lavas have been dated by U-Pb on zircons at 595 Ma (Halliday et al., 1989) which is the latest Precambrian on all proposed time scales (Snelling, 1985).

Estimates of a minimum age for the top of the Dalradian depend upon the dating of post-sedimentary tectonothermal events, in particular early-metamorphic granites which should theoretically give ages nearest to the age of the sediments. The Portsoy Granite, intruded into the Argyll Group of the North-east Highlands, has been dated by Rb-Sr whole-rock methods at 655 Ma (Pankhurst, 1974). This is difficult to reconcile with the suggested 653 Ma depositional age of the base of the Argyll Group and, although it is possible that the granite was intruded shortly after deposition of the sediments (Pankhurst in Harris and Pitcher, 1975), both dates must now be regarded with caution. Much attention has centred upon the Ben Vuirich Granite near Pitlochry. Its host Appin and Argyll group rocks were considered to have been affected by both the D1 and D2 episodes of deformation prior to granite emplacement (Bradbury et al., 1976; Rogers et al., 1989). However, Tanner and Leslie (1994) have shown that the granite was emplaced between D1 and D2 and then subsequently deformed and metamorphosed during the D2 and D3 episodes. An age of intrusion of 590 Ma, obtained by U-Pb dating of carefully selected zircons from the granite (Rogers et al., 1989) and confirmed by ion-microprobe dates on individual zircons (Pidgeon and Compston, 1992), clearly has far-reaching implications for the history of Dalradian sedimentation and deformation. However, the fact that this granite had previously been dated at 514 Ma by a less precise U-Pb zircon study, and at 552 Ma by the Rb-Sr whole-rock method, emphasises the need for caution when comparing radiometric dates, in particular those obtained by different methods (Rogers and Pankhurst, 1993). Ideally more high-precision U-Pb dates are needed, both to check the currently available Rb-Sr dates of crucial pre-metamorphic intrusions and to provide new data points.

Conclusions[edit]

The high-precision U-Pb zircon age obtained from the Ben Vuirich Granite implies that most, if not all, of the Dalradian sedimentation occurred prior to 590 Ma, entirely during the Precambrian Era. All other radiometric and palaeontological data, although less precise, are compatible with at least this broad statement, although detailed conclusions are commonly confusing and contradictory.

The detailed chronological interpretation is constantly changing as more and better dates become available, but P915452 provides a useful ‘best-fit’ model at the time of writing. The Grampian and Appin groups together cover a period of at least 100 Ma in the upper Riphean from pre-750 Ma to 653 Ma but as yet there is no evidence to enable any chronological subdivision. If the tillites at the base of the Argyll Group are correlated with the Varanger Tillite at 653 Ma (itself not a particularly good quality date) then they may be considered to mark the base of the Vendian. The top of the Argyll Group is apparently well defined by the Tayvallich Lavas at about 595 Ma, implying that the group spans some 60 Ma, all well within the Vendian. The base of the Cambrian could lie within the Southern Highland Group, although there is no obvious stratigraphical horizon where it may be located. In fact, given the thickness of the group and the nature of the sediments, which suggest relatively rapid accumulation, it is possible that the whole of the Southern Highland Group, and hence the whole of the Dalradian, is Precambrian. Current interpretations of the sparse and imprecise palaeontological data are compatible with this overall interpretation.

The above model allows ample time for the deformation and metamorphism to reach a peak at 520 to 490 Ma in the early Ordovician (Chapter 7). However, problems arise over the timing of the earlier phases of deformation. For example, the Ben Vuirich Granite zircon age of 590 Ma implies that the granite was intruded only 5 Ma after the eruption of the Tayvallich Lavas and yet postdates at least one phase of deformation. If the analytical errors on the 595 and 590 Ma determinations are taken into account the two dates could even overlap and the suggestion by Rogers et al. (1989) that the true age of the Tayvallich Lavas may be slightly older than that published, eases this problem only slightly. Also, if the Tayvallich date is accepted, the D1 and D2 phases of deformation in the Southern Highlands must be younger than 595 Ma. However, in the Central Highlands D2 is dated by the pegmatites at 750 Ma, implying a gap of at least 150 Ma between the early phases of deformation in the two areas. Acceptance of these dates clearly requires that more careful consideration may have to be given to the possiblity of structural and metamorphic, as well as stratigraphical, diachronism between different areas or different levels in the sedimentary pile.