Explore the vast range of titles available.

Precambrian sedimentary successions are difficult to date and correlate. In the Scottish Highlands, potential correlations between the thick, undeformed siliciclastic \"Torridonian\" successions in the foreland of the Caledonian Orogen and the highly deformed and metamorphosed siliciclastic Moine succession within the Caledonian Orogen have long intrigued geologists. New and detailed mapping of the Neoproterozoic Altnaharra Formation (Morar Group, lowest Moine Supergroup) in Sutherland has discovered low-strain zones exhibiting well-preserved sedimentary features. The formation comprises 3-5 km of coarse, thick-bedded psammite with abundant nested trough and planar cross-bedding bedforms, defining metre-scale channels. Palaeocurrent directions are broadly unimodal to the NNE-ENE. We interpret the Altnaharra Formation as high-energy, braided fluvial deposits. The Altnaharra Formation and the unmetamorphosed, Neoproterozoic Applecross-Aultbea formations (Torridon Group) are similar in terms of lithology, stratigraphical thickness, sedimentology, geochemistry, detrital zircon ages and stratigraphical position on Archaean basement. Depositional age constraints for both successions overlap and are coeval with late Grenvillean orogenic activity. Detrital zircons imply similar source regions from the Grenville Orogen. The Morar and Torridon groups can thus be correlated across the Caledonian Moine Thrust and are best explained as parts of a single, large-scale, orogen-parallel foreland basin to the Grenville Orogen.

Detrital and inherited zircons from rocks of the Moine Supergroup from structural and stratigraphic positions above and below the Sgurr Beag and Naver thrusts have been dated by ion microprobe. 207Pb/206Pb ages for 65 detrital zircons from the Moine Nappe (Morar Group) range between 2707 and 947 Ma, with a bimodal distribution with clusters at c. 1650 and 1400 Ma. Five grains (8% of the analyses) were Archaean. Analyses of 97 inherited zircons from two Caledonian migmatites and two granites from the Naver Nappe yield 207Pb/206Pb ages between c. 2940 and 926 Ma. They have a dominant cluster at c. 1650 Ma with significant clusters at c. 1400 and c. 1050 Ma. Eight Archaean grains were discovered (8.5% of analyses). Three rocks above the Sgurr Beag Thrust have been examined and a total of 42 analyses obtained. 207Pb/206Pb ages for 35 inherited zircons from two outcrops of the essentially in situ West Highland granite gneiss within the Glenfinnan and Loch Eil Groups range from c. 1889 to 947 Ma. A sample of the Lochailort pelite provided a further seven detrital analyses. These data have a distribution with clusters at c. 1500 and 1100 Ma. The combined datasets indicate that the Moine Supergroup was deposited in a post-Grenvillian basin(s) with detritus derived predominantly from a late Palaeoproterozoic source, e.g. Gothian or Labradorian with only a very small proportion from older sources. The data indicate that the Naver and Sgurr Beag nappes had different sediment sources and so may not correlate. The Morar Group is dominated by late Palaeoproterozoic detritus with lesser amounts of Grenville-aged detritus. The Glenfinnan and Loch Eil Groups appear to have roughly equal proportions of detritus from the two sources. The few Archaean grains indicate that the source comprised comparatively little material of this age, precluding the Lewisian Complex or sub-Moine basement as a significant source and arguing against correlation of the Moine with the Torridonian.

Considerable debate exists over the tectonic regimes associated with mid-Neoproterozoic metamorphism of the Moine Supergroup, NW Scotland. Published pressure conditions imply burial to 35-40 km, a potential doubling of crustal thickness, and hence a substantial collisional event. Re-evaluation using updated thermodynamic software suggests more modest peak pressures of c. 7.5 kbar implying burial to c. 21 km. The revised P-T path has a comparatively flat clockwise evolution from early high geothermal gradient conditions. The revised P-T conditions suggest that c. 800 Ma crustal thickening within the Moine Supergroup was less significant than previously envisaged and possibly preceded by extension.

The Hebridean shield, the northwest foreland of the Caledonian Orogen of Scotland, is a small fragment of Laurentia detached during the Cenozoic opening of the North Atlantic Ocean and is now part of Europe. The shield was at the tip of a major promontory of the ancestral core of North America, between the Newfoundland (Appalachian) and Greenland (Caledonian) margins. Its history is important to understanding late Precambrian and early Paleozoic global paleogeography and tectonics. Isotopic ages and structural complexities in the Moine and Dalradian Supergroups of the Caledonian Orogen have been interpreted as reflecting Neoproterozoic orogenic episodes overprinted by early Paleozoic deformation and metamorphism. A critical body of rock in the Scottish Highlands, the West Highland Granite Gneiss, has been viewed as a synorogenic intrusion into Moine metasedimentary rocks, and its ∼870-Ma UPb zircon age as dating a Riphean \"Knoydartian\" orogeny. However, field evidence shows that the granitic protolith of the gneiss was emplaced before a regional suite of tholeiitic dikes was intruded into brittle fractures. The dikes carry all the ductile regional deformation. The zircon age thus reflects the crystallization of an anatectic melt, not its subsequent gneissification. Melting is thought to have resulted from advection of heat by emplacement of basaltic magma deep within the Moine sedimentary pile. In this new scenario, deformation and gneissification took place during the early (Grampian/Taconic) phase of the Caledonian Orogeny, not during the Neoproterozoic. Our interpretation is that all the Knoydartian events were extensional. This leads to a substantial simplification of the pre-Caledonian history of the Scottish Promontory of Laurentia. Protracted rifting in the Neoproterozoic was concentrated in two phases, with episodes of major extension and bimodal magmatism in the Riphean (∼900-750 Ma) and Vendian (∼600 Ma). These episodes coincide with the two-stage breakout of Laurentia as a discrete continent during the Neoproterozoic, hypothetically from the Rodinian and Pannotian supercontinents, respectively.

Within the Caledonides of Caithness, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup are intruded by minor sheets of strongly deformed granite. U-Pb sensitive high-resolution ion microprobe zircon ages of 599±9 Ma (Berriedale augen granite) and 588±8 Ma (Braeval augen granite) are interpreted to date emplacement during the late Neoproterozoic. These augen granites are therefore unrelated to either Knoydartian (c. 870-790 Ma) or Caledonian (c. 470-420 Ma) orogenesis in the Scottish Highlands. Intrusion was, however, broadly contemporaneous with late Neoproterozoic extension of the Laurentian margin during continental break-up and the opening of the Iapetus Ocean. In the context of evidence for rift-related mafic magmatism within the Dalradian basin in Scotland at c. 600 Ma, as well as contemporaneous anorogenic magmatism along the margins of the developing Iapetus Ocean in the Appalachians, we propose that the protoliths of the augen granites were probably emplaced during continental break-up. The new data broaden the extent of this late Neoproterozoic magmatic event in the Scottish Highlands: other deformed granites that are at present undated but have been assumed to be of Caledonian age therefore require reinvestigation.

Peak and retrograde P–T conditions of Grenville-age eclogites from the Glenelg–Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c. 20 kbar and 750–780°C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c. 13 kbar and 650–700°C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c. 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25°C/Ma, when compared with other parts of the Grenville orogeny and/or modern orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P–T–t data, together with petrology and the field relationships between the basement rocks of the Glenelg–Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.

The West Highland granite gneiss suite in Inverness-shire, Scotland, represents a series of S-type, anatectic granites formed by partial melting of host Neoproterozoic metasediments of the Moine Supergroup. U-Pb (SHRIMP) dating of zircons from a member of the suite, the Fort Augustus granite gneiss, indicates that the granitic protolith to the gneiss was intruded at 870±30 Ma. This is indistinguishable from the published age determined by the same method for the Ardgour granite gneiss at Glenfinnan, thus supporting the assumption that the various members of the West Highland granite gneiss are part of a single intrusive suite. The spread of ages from the zircon cores (1626-947 Ma) is interpreted to indicate a Proterozoic source terrain for the Moine sediments that were later melted to form the granitic protolith. A U-Pb age of 470±2 Ma obtained for titanite in the Fort Augustus granite gneiss is interpreted to date amphibolite-facies metamorphism during the early to mid-Ordovician Grampian Orogeny. The emerging similarity in the timing of this event either side of the Great Glen Fault implies that this structure does not juxtapose crustal blocks with significantly different histories with respect to the Grampian Orogeny.

Pre-tectonic metagabbroic rocks emplaced into the Glenfinnan and Loch Eil groups of the Moine Supergroup give a U-Pb zircon age of 873±6 Ma. This new age for the metagabbros confirms the absence of Grenvillian (c. 1.0 Ga) tectonic events in the Moine assemblage. The metagabbros are spatially associated with the Glen Doe body of the West Highland granite gneiss, and were emplaced soon after the granite gneiss protolith. The metagabbros have chemical characteristics indicating contamination with local country rocks. A regionally developed suite of tholeiitic metadolerite dykes post-dates the metagabbros. These dykes are geochemically similar to modern mid-ocean ridge basalt, albeit modified by interaction with metamorphic fluids. The presence of abundant MORB-like basaltic dykes, coupled with the lack of major compressional structures associated with the intrusive events, suggests that the c. 873 Ma event may have been dominated by extensional tectonics.

U-Pb monazite data for the Ardnish and Sgurr Breac pegmatites in the SW Moine block give Knoydartian ages of 827 ± 2 and 784 ± 1 m.y., respectively. Structural and metamorphic studies on the pegmatites and the local Moine metasediments suggest that pegmatite generation was a result of localized high-T shearing and metamorphism in the pelitic horizons; the local garnet-grade metamorphism is interpreted as being contemporaneous with the shearing and pegmatite generation, and was thus Knoydartian in age. Chemical zoning profiles in garnets are consistent with their growth during prograde regional metamorphic increases in P and T. The ∼ 45 m.y. difference between the pegmatite ages is taken to imply that the Knoydartian tectonothermal event was diachronous.

The Neoproterozoic (early Riphean) Moine Supergroup crops out extensively in the Scottish Caledonides north of the Great Glen and is characterized by structural complexity and monotonous siliciclastic lithology with a lack of biostratigraphic control. Despite these problems, it has become possible to combine locally defined successions into a regional stratigraphic framework. This permits an evaluation of the tectonic setting of Moine deposition, in a subsiding tract that became the Scottish part of the Laurentian margin when Iapetus opened in the Vendian. Two major rift-basins are inferred, each of half-graben type, controlled by east-facing normal faults. The earlier Morar Group basin received a thick fill of dominantly shallow-marine arkosic sandstones, the Upper Morar Psammite forming a major regressive sequence. The Glenfinnan Group consists of mixed and muddy deposits that are interpreted partly as distal equivalents of the Morar Group, partly as a post-Morar transgressive thermal re-equilibration sequence. The Loch Eil Group, dominated by shallow marine arkosic and siliceous psammites, was deposited in a second, more easterly rift-basin that was subsequently juxtaposed with the Morar basin during Caledonian thrusting. Sediment dispersal in the major sandy sequences was northwards, and there is some evidence that provenance was, at least in part, from a mid-Proterozoic basement terrain to the south. The extension-dominated depositional setting of the Moine Supergroup, and basin geometry, are similar to those inferred for the other major Riphean clastic sequences of Scotland, the \"Torridonian\" of the foreland and the Grampian Group to the southeast. The latter is followed by Upper Riphean transgressive strata of the Appin Group. Scottish Riphean stratigraphy records a major pre-Iapetan cycle of lithospheric extension and thermal recovery, with no regional scale unconformities yet identified that might constitute evidence for contemporary orogenesis.