Abstract Ordovician orogeny affected the Laurentian margin of the Appalachian-Caledonian Belt from the Southern Appalachians to the British Isles and is dated, stratigraphically, as post-uppermost Lower Cambrian and pre-upper Llandovery. Geochronological data favour a short Grampian orogeny from c. 470 to 460 Ma during the late Arenig-mid-Llanvirn, which is supported by the late Arenig-earliest Llanvirn termination of the Laurentian rifted margin carbonate shelf. The likeliest plate tectonic model for the Grampian Orogeny is of a continent-facing oceanic (Laurentia) arc, created as an infant mafic arc during the Middle Cambrian, evolving into an Early Ordovician intermediate-silicic arc with suprasubduction-zone ophiolites. This arc collided with the Laurentian margin and forearc ophiolites were obducted across the margin creating an orogen and accreting the arc to the margin. Continued plate convergence was accommodated by a flip in subduction polarity that terminated late orogenic retrocharriage, and led to the rapid final stages of uplift and unroofing of the orogen and to subduction accretion in the new mid-Ordovician trench. Presented in this paper are definitive new detrital heavy mineral evidence that supports and enhances this model of a short-lived orogenic event involving ophiolite obduction, and the rapid development and unroofing of a Grampian Barrovian metamorphic complex over c. 10 Ma. High-resolution heavy mineral analysis (HRHMA) has been undertaken on most of the Ordovician and Silurian stratigraphical units in the South Mayo Trough in western Ireland. It is shown that ophiolite unroofing began during the Arenig and that, by the early Llanvirn, a Barrovian complex was being eroded. The Killadangan ‘Formation’, of the Clew Bay Complex is confirmed as part of a pre-Grampian early Ordovician accretionary prism by both HRHMA and by quartz-felspar-lithic (QFL) analysis, which show it to have been derived from Precambrian rocks of the Laurentian margin. QFL analysis also shows that the Ordovician provenance of the South Mayo Trough sequence evolves from an undissected arc through a dissected arc to a recycled orogenic detrital pattern, except for the earliest sediments (Letterbrock), which were derived from the transitional continental sediments of the Killadangan accretionary prism, which, in turn, were derived from Laurentia. The problem of timing the Grampian deformation and metamorphism of the Dalradian is now regarded as being solved. It was an Arenig-Llanvirn event lasting c. 10 Ma, which occurred during and is recorded, faithfully, by the detrital heavy mineral assemblage in the conformable Ordovician sequence of the South Mayo Trough. The Miocene evolution of New Guinea is strikingly similar to the Arenig-Llanvirn evolution of the Laurentian margin and was, analogously, the short-lived result of the collision of an arc with the north Australian margin followed by subduction polarity flip.

Petrography of Ordovician and Silurian sediments in the western Irish Caledonides: tracers of a short-lived Ordovician continent-arc collision orogeny and the evolution of the Laurentian Appalachian-Caledonian margin

The Kalak nappe complex of N Norway involves late Precambrian to Middle Cambrian sediments and a Precambrian gneiss basement on which the sediments were deposited. While the uppermost nappe in Finnmark was emplaced during the Silurian the members of the Kalak nappe complex were emplaced in late Cambrian/early Ordovician times during the Finnmarkian orogenic stage—probably the analogue of the Grampian stage of the British Caledonian. The tectonic-metamorphic events of the Finnmarkian stage were broadly coeval with the emplacement of basic and alkaline igneous bodies of the Seiland Igneous Province which were introduced from 552 ± 17 Ma (syn-D1) until 501 ± 27 Ma (late D2) and which themselves reflect magmatic evolution from tholeiitic to alkaline types.

The Finnmarkian phase of the Caledonian Orogeny

Eclogite, a garnet–pyroxene rock with basaltic chemistry, is a product of unusually high-pressure metamorphism. In Scotland, it occurs within the Caledonian orogenic belt close to the Moine thrust, where it forms partially amphibolitized, easterly-dipping layers, associated with metasediments in the eastern Glenelg Lewisian inlier1–4. In spite of its potential importance for an understanding of the geological history of north-west Scotland, its age has never been established more precisely than ‘probably Proterozoic’4. A K–Ar omphacite age of 1,515 Myr5 is rendered unreliable by the possible presence of excess argon. We report here the results of an attempt to date the eclogite metamorphism by the Sm–Nd method. Two samples of eclogite yielded mineral isochrons of just over 1,000 Myr. The results represent clear evidence of Grenville metamorphism in Scotland, and imply close proximity of its front to the Moine thrust near Glenelg. Also, as the local Moine cover has not apparently reached eclogite facies, it was presumably deposited on an eroded Grenville basement.

A Grenville Sm–Nd age for the Glenelg eclogite in north-west Scotland

The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin.

U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland

On the basis of new evidence, the Moine rocks SE of the Great Glen Fault are differentiated into two major units: the Central Highland Division, represented by a gneissose assemblage of about 1000 km 2 extent, and the Grampian Division, a younger and more extensive sequence of less deformed metasediments which is separated from the gneisses by a tectonic break, the Grampian Slide. Since the 2 Divisions have greatly differing tectonometamorphic histories, they are believed to correspond respectively to a basement complex and a cover sequence. The discovery of a suite of syntectonic Precambrian pegmatites ( c . 718 ± 19 Ma) equivalent in age to the Morarian pegmatites of Morar and Knoydart, permits the separation of Precambrian from Caledonian orogenic events. The Central Highland division is considered to be an eastward extension of the high-grade rocks which in part form the Glenfinnan Division W of the Great Glen Fault, widely held to be of Grenvillian age, whilst the Grampian Division is interpreted as early post-Grenvillian sediments affected first by a limited Morarian orogenic event, and subsequently by the Caledonian orogeny.

New light on the Moine rocks of the Central Highlands of Scotland

A 14 point Rb/Sr whole rock isochron obtained from the Morar Pelite in the Morar area of the Northern Highlands suggests an age of 1024± 96 Ma for the main metamorphism of the Morar Moines. Within error this age overlaps that recently given for the Ardgour Gneiss and thus confirms the existence of Grenvillian elements within the Northern Highland Block. It suggests that the tectono-metamorphic history of the southwestern Moines is more complex than hitherto thought.

M. Brook

Papers

Grenville events in Moine rocks of the Northern Highlands, Scotland