Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland

The Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An(1-3)), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism (T less than 270 °C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. 40Ar/39Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600-1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507 - 886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or straitigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

Late Precambrian U-Pb titanite age for peak regional metamorphism and deformation (Knoydartian orogeny) in the western Moine, Scotland

There has been controversy over the number and timing of orogenies in the Precambrian Moine block in the Scottish Caledonides since the earliest radiometric dating in the 1960s. This work challenges a recent hypothesis, that this sector of the Laurentian margin was subjected to continuous crustal extension between greater than 900 and 470 Ma. U-Pb dating (thermal ionization mass spectrometry) of titanite from a calcsilicate pod in the Moine (Morar Group) of the western Highlands gives an age of 737 ñ 5 Ma. The titanite grew from Fe-Ti-bearing detrital minerals during the main progressive, syn-D2, amphibolite-facies (sillimanite zone) regional metamorphism, thus demonstrating that a Neoproterozoic contractional tectonothermal event (Knoydartian orogeny) affected the Moine block following the rift-related emplacement of the West Highland granite gneiss at 873 Ma. We conclude that the Sgurr Beag Thrust, a major tectonic break separating the Morar and Glenfinnan groups of the Moine, is mainly of Neoproterozoic, not Caledonian, age. The early tectonothermal event was succeeded by the Grampian Phase (Caledonian orogeny) at 460-470 Ma

Current controversies in the Caledonides

No abstract available

Timing of deposition, orogenesis and glaciation within the Dalradian rocks of Scotland: constraints from U-Pb zircon ages

The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ñ 4 Ma), and on the late Grampian 'Newer Gabbros' (470 ñ 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 ñ 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected

Caledonian and Knoydartian overprinting of a Grenvillian inlier and the enclosing Morar Group rocks: structural evolution of the Precambrian Proto-Moine Nappe, Glenelg, NW Scotland

The Grenville and Caledonian orogens, fundamental to building Laurentia and Baltica, intersect in northern Scotland. The Precambrian Glenelg Inlier, within the Scottish Caledonides, preserves a record of Grenvillian, Knoydartian and Caledonian orogenesis. Based on new mapping and re-interpretation of previous mapping, we present a structural model for the evolution of the Glenelg Inlier. The inlier can be divided into Western Glenelg gneiss comprising orthogneiss with no record of Grenville-age metamorphism, and Eastern Glenelg gneiss with ortho- and paragneiss, affected by Grenvillian eclogite-facies metamorphism. The basement gneisses and their original cover of psammitic, Neoproterozoic Morar Group (Moine) rocks were deformed by three generations of major ductile folds (F1–F3). In medium-strain areas F2 and F3 folds are broadly coaxial and both face to the west; in higher strain areas F2 and F3 folds are oblique to each other. By restoring post-F1 folds and late faults, the Glenelg gneiss inliers are seen to form the core of a major recumbent SSE-facing F1 isoclinal fold nappe – the Proto-Moine Nappe. The upper limb of this nappe is a thick, right-way-up sequence of moderately strained Morar Group rocks whereas the lower, inverted limb comprises intensely deformed, migmatitic Morar Group rocks. Within the constraints of published geochronology, the Proto-Moine Nappe is likely Pre-Caledonian and may have originated during the early Neoproterozoic Knoydartian Orogeny