Southern Highland Group Research Articles

Genetic diversity and distribution patterns of Ecuadorian capuli (Prunus serotina)

Prunus serotina subsp. capuli (Cav.) is an arboreal species with promising economic prospects in the timber, health-food and neutraceutical markets. Despite its cultural and commercial significance, limited information exists with regards to the degree of genetic variation and ecological history of P. serotina in Ecuador. The main objective of this study was to evaluate the degree of genetic diversity and population structure of Ecuadorian P. serotina, as a preliminary step towards understanding the distribution history of this species in Ecuador and establishing germplasm conservation programs. P. serotina samples (217, representing 8 provinces from the Ecuadorian highlands) were characterized using 8 heterologous SSR markers derived from related Prunus species. Expected heterozygosity across samples (He = 0.71) reveals a moderate level of genetic diversity for Ecuadorian P. serotina. Nevertheless, simple allele-count analysis indicates that Ecuadorian capuli has a narrower degree of allelic richness relative to collections from the species’ center of origin in North America. Furthermore, pairwise F statistics (0.0069 ≤ Fst ≤ 0.0319) and Nei genetic distance estimates (0.02 ≤ Ds ≤ 0.09) indicate minimal population differentiation within Ecuadorian capuli. However, Bayesian population structure analysis suggests a subtle genetic contrast between germplasm from the Northern and Southern highlands. Certainly, it is of interest to analyze whether this underlying genetic differentiation between the Northern- and Southern-Highland groups is also manifested in morphological, agronomic or other phenotypic characters that could indicate adaptive differences to divergent agro-ecosystems.

The Dalradian rocks of the central Grampian Highlands of Scotland

The central Grampian Highlands, as defined here, are bounded to the north-west by the Great Glen Fault, to the south-west by Loch Etive and the Pass of Brander Fault and to the south-east by the main outcrop of the Loch Tay Limestone Formation. The more arbitrary northern boundary runs north-west along the A9 road and westwards to Fort William. The detailed stratigraphy of the Dalradian Supergroup ranges from the uppermost Grampian Group through to the top of the Argyll Group, most notably seen in the two classic areas of Loch Leven–Appin and Schiehallion–Loch Tay; Southern Highland Group strata are preserved only in a small structural inlier south of Glen Lyon.Major F1 and F2 folds are complicated by co-axial northeast-trending F3 and F4 folding, as well as by locally important north- or NW-trending folds. In the Loch Leven area, nappe-like F1 folds verge to the north-west, whereas to the south-east the major recumbent F1/F2 Tay Nappe verges to the south-east. The trace of the upright Loch Awe Syncline lies between the opposing nappes, but in this region a large mass of late-Caledonian granitic rocks obscures their mutual relationship. Three tectonic ‘slides’ are identified that are certainly zones of high strain but which in part could be obscuring stratigraphical variations.The regional metamorphism ranges from greenschist facies on the western seaboard of Argyll to amphibolite facies in most of the remainder of the region. The study of garnets, together with kyanite and staurolite in the Schiehallion area, has enabled a detailed history of the metamorphism and structure to be unravelled.Stratabound mineralization occurs in the Easdale Subgroup, where there is also evidence of changes of sedimentary environment associated with volcanicity and lithospheric stretching. The region is dissected by a series of NE-trending, dominantly left-lateral, faults, subparallel to the Great Glen Fault, whose movement history is illustrated here by that of the Tyndrum Fault.

The Dalradian rocks of the north-east Grampian Highlands of Scotland

Abstract The north-east Grampian Highlands, as described here, are bounded to the north-west by the Grampian Group outcrop of the northern Grampian Highlands and to the south by the Southern Highland Group outcrop in the Highland Border region. The Dalradian succession therefore encompasses the whole of the Appin and Argyll groups, but also includes an extensive outlier of Southern Highland Group strata in the north of the region. The succession includes shallow-marine sequences, glacigenic deposits at two stratigraphical levels, the earliest evidence for volcanism in the Dalradian, a later major development of basaltic and picritic sub-marine lavas, and thick turbiditic sequences. In the south, the Grampian–Appin group boundary is a high-strain zone, with no obvious dislocation or stratigraphical excision, which was formerly termed the Boundary Slide. Shear-zones at higher structural levels are associated with pre-tectonic granites, such as the Ben Vuirich Granite, which have been dated at c. 600 Ma and hence place limits on the timing of sedimentation, deformation and metamorphism. The region is divided from north to south by a major zone of shearing and dislocation with associated igneous intrusions, termed the Portsoy Lineament. To the west of the lineament, the stratigraphy is more-or-less continuous along strike with that of the central Grampian Highlands. D1, D2 and D3 structures extend from the Tummel Steep Belt north-eastwards throughout this area. The stratigraphical succession is broadly continuous across the Portsoy Lineament but to the east, in the Buchan Block, correlations are more tenuous and do not extend below subgroup level. High-grade migmatitic paragneisses were once interpreted as pre-Dalradian basement but they are now assigned to the Crinan Subgroup, within the Dalradian succession. Within the Buchan Block the outcrop pattern is controlled by two broad, open, post-metamorphic folds, the Turriff Syncline and the Buchan Anticline. The Buchan Block is the international type area for the high-temperature/low-pressure Buchan-type regional metamorphism. To the south and west, this passes into higher pressure Barrovian-type metamorphism. South of Deeside, metamorphic conditions reached 820 °C and over 8 kbar, well into granulite facies and the highest recorded in the Grampian Terrane. The detailed relationship between the high heat-flow and the emplacement of large bodies of basic and silicic magma is a matter of ongoing research. Plutons of the north-east Grampian Basic Suite, emplaced at c. 474–470 Ma, during or shortly after the peak of metamorphism and the D3 deformation, provide key evidence for the timing of the Grampian orogenic event.

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 Dalradian rocks of the south-west Grampian Highlands of Scotland

The south-west Grampian Highlands, as defined here, include the Inner Hebridean islands of Islay and Jura, and extend north-east as far as Dalmally at the northern tip of Loch Awe. Due to a favourable combination of excellent coastal exposures and low tectonic strain, the late-Neoproterozoic rocks of the Dalradian Supergroup in this region are ideal for studying sedimentary structures. In addition, the diversity in protolith lithology from carbonate rocks to siliciclastic rocks of all grain sizes and volcanic rocks makes it possible to establish a very detailed lithostratigraphical succession and to recognize lateral facies changes. The stratigraphical range extends from the base of the Appin Group to the base of the Southern Highland Group and the area provides type localities for many regionally extensive formations of the Argyll Group. Rocks forming part of the basement to the Dalradian basins, the Rhinns Complex, are seen on Islay, where they are overlain by the Colonsay Group, a thick metasedimentary siliciclastic sequence of uncertain stratigraphical affinity.The structure of the Dalradian rocks in the south-west Grampian Highlands is controlled by early (D1) major folds (Islay Anticline, Loch Awe Syncline, and Ardrishaig Anticline), associated with a ubiquitous, penetrative, slaty or spaced cleavage. Most of the Dalradian rocks have been regionally metamorphosed under greenschist-facies conditions and amphibolite-facies (garnet zone) assemblages occur only in a narrow central zone, strongly affected by the D2 deformation.The area provides GCR sites of international importance for studying Neoproterozoic glacial deposits, splendidly preserved stromatolite bioherms and calcite pseudomorphs after gypsum. Deformed and undeformed sandstone dykes and interstratal dewatering structures are well displayed at several sites. Other features include thick sills of basic meta-igneous rock with unusual minerals such as stilpnomelane, and greenschist-facies rocks containing regional metamorphic kyanite. The area is of historical interest for the first recognition in Scotland, prior to 1910, of sedimentary way-up structures and pillow lavas in regionally deformed and metamorphosed rocks.

The Dalradian rocks of the Shetland Islands, Scotland

Metasedimentary and metavolcanic rocks to the east of the Walls Boundary Fault on Shetland have lithological similarities to those of the Dalradian Supergroup of the Scottish mainland. In particular, the middle part of the succession, termed the Whiteness Group, includes numerous metalimestones and associated pelites in a shallow-marine succession that recalls the upper parts of the Appin Group and the Argyll Group of mainland Scotland. Metavolcanic rocks within the deeper water turbiditic sequence of the succeeding Clift Hills Group might be broadly coeval with those of the Southern Highland Group of Scotland. Beyond that, correlations with the established Dalradian succession are tenuous and are not possible at formation level. A local succession immediately west of the Walls Boundary Fault is of even more-dubious Dalradian affinity.The dominant structure is the regional-scale, downward- and east-facing East Mainland Mega-monocline. This has a vertical western limb, which youngs to the east, and an eastern top limb that dips to the north-west at 20–30°. Strata on the eastern limb are inverted on Mainland, Whalsay and Out Skerries but are right way up on the west side of Unst, having been folded around the tight Valla Field Anticline. The Shetland Ophiolite-complex has been thrust over the inverted limb of the Valla Field Anticline on the east side of Unst. The regional monocline folds earlier small- to medium-scale, tight to isoclinal folds with associated planar and linear structures, which are all assigned to a single ‘Main Deformation’. It also post-dates the regional metamorphism, which ranges from chlorite to garnet grade, with localized development of staurolite-kyanite, gneissose fabrics, and the emplacement of schistose granitic sheets in the Colla Firth Permeation Belt.The GCR sites have been selected mainly to be representative of the East Mainland Succession with its associated structures and metamorphism. Highlights include well-preserved sedimentary structures, high-grade gneisses permeated by granitic material, basaltic pillow lavas and serpentinized ultramafic rocks. Some of the latter contain enigmatic skeletal pseudomorphs after olivine and have been interpreted as former high-magnesium lavas.

Discussion of ‘The Highland Boundary Fault and the Highland Border Complex’ by B. J. Bluck, Scottish Journal of Geology , 46 , 113–124

P.W. Geoff Tanner writes: In this paper, Bluck has failed to provide a balanced and accurate assessment of the progress that has been made over many years to find a robust, modern geological interpretation of the Highland Border. This is no trivial matter, for the rocks exposed in this narrow strip of ground retain many of the clues to understanding the causes, and subsequent development, of the Caledonian c . 470 Ma Grampian Event, and the role played by the Highland Boundary Fault in continental-scale plate-tectonic reconstructions (Strachan & Dewey 2003; Tanner 2008). The Highland Boundary Fault is considered by Bluck to define the contact between the Southern Highland Group (Dalradian) and the ‘Highland Border Complex’. However, at outcrop this boundary is variously seen as a fault (North Esk), a lithological transition (Keltie Water), or the sole of an ophiolite (Innellan), and there is no evidence of a major, through-going fault between Arran and Stonehaven, as conventionally depicted (Tanner 2008). Conflicting interpretations of Highland Border geology have generated two diametrically opposed models, essentially summarized below, variations notwithstanding, as A & B. In model A (Curry et al . 1984; Bluck 1985), the rocks between uncontested Southern Highland Group (Dalradian) to the NW and the unconformable Old Red Sandstone to the south, were assigned to the ‘Highland Border Complex’ (HBC). The latter was once ‘considerably separated’ from the Dalradian block, and acquired a northerly dip and younging direction before the two were amalgamated, following the Grampian Event, by movement on the Highland Boundary Fault. In model B (Tanner & Sutherland 2007), the HBC was divided into two parts: the Trossachs Group (now the youngest part of the Dalradian Supergroup), and an overlying, allochthonous unit, the Highland Border Ophiolite (HBO). The main differences between the two models are: all of the …

Chapter 63 The Neoproterozoic glaciogenic deposits of Scotland and Ireland

Abstract Of the three major Neoproterozoic supracrustal units in the Scottish and Irish Highlands (the Torridonian, Moine and Dalradian Supergroups), only the latter contains evidence of Neoproterozoic glaciations. The Dalradian is siliciclastic-dominated and constitutes much of the Scottish–Irish Highlands between the Great Glen and Highland Boundary Fault Zones, and their correlatives in Ireland. At the time of writing, three stratigraphically distinct glacial intervals in the Dalradian have been documented in the literature. The oldest is the Port Askaig Formation (Fm.) at the base of the Argyll Group (see Arnaud & Fairchild 2011 ). It ranges from several tens to many hundreds of metres in thickness and occurs in numerous localities in Scotland and the north of Ireland. A second glacial is recorded in the middle part of the Argyll Group (Easdale Subgroup) and consists of localised sedimentary breccias as well as pelites and schists containing dropstone/lonestone units inferred to be ice-rafted debris; these rocks are patchily preserved and typically a few metres or less in thickness. It is sharply overlain by a variably developed carbonate unit that is marked by a 1–7-m-thick, light-coloured, basal dolostone or dolomitic limestone interpreted as a cap carbonate. This succession is best preserved in Donegal, Ireland, as the Stralinchy–Reelan glacial and Cranford cap-carbonate sequence. A correlative cap carbonate, the Whiteness Limestone, has been identified in the Shetland Islands. The third and youngest glacial is represented by locally preserved dropstone and polymict diamictite beds ranging in thickness from several to a few tens of metres in thickness in the lower Southern Highland Group. These include the MacDuff and Loch na Cille Boulder Beds in, respectively, NE and SW Scotland, and the Inishowen Beds in Donegal, Ireland.

Basic volcanism contemporaneous with the Sturtian glacial episode in NE Scotland

ABSTRACTThe Dalradian Supergroup contains three distinct glacigenic units, formerly termed ‘Boulder Beds’, which are correlated with widespread Neoproterozoic glaciations. The oldest and thickest unit, the Port Askaig Formation, marks the Appin–Argyll group boundary of the Dalradian Supergroup and has been correlated with the Middle Cryogenian (Sturtian) glaciation. The Auchnahyle Formation, a diamictite-bearing sequence near Tomintoul in NE Scotland, exhibits strong lithological similarities to the Port Askaig Formation. Both these glacigenic ‘Boulder Bed’ units contain abundant dolomite clasts in their lower parts and more granitic material at higher levels. Both metadiamictite units are overlain by thick shallow-marine quartzite units. C isotope data from Appin Group carbonate strata below the Auchnahyle Formation support this correlation. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from the Auchnahyle Formation metadiamictite differ slightly from the Port Askaig Formation, but are similar to detrital zircon spectra obtained from the Macduff Formation, a diamictite unit in the younger Southern Highland Group of the Dalradian Supergroup; both apparently reflect derivation from local basement rocks. No detritus younger than 0·9 Ga is observed, so the data do not constrain significantly the depositional age of the glacial strata. A thin tholeiitic pillow basalt unit in the lower part of the Auchnahyle Formation is geochemically distinct from pre-tectonic metadolerite sills and from basic metavolcanic rocks up-section. A Sturtian (c. 720–700 Ma) age for the Auchnahyle Formation metadiamictite would imply that this basaltic volcanism represents the oldest recorded volcanic activity in the Dalradian Supergroup and is inferred to represent an early, local phase of proto-Iapetan rifting within the Rodinian supercontinent.

The tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head–Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages ( c . 670 °C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average 207 Pb/ 206 Pb zircon age of 467 ± 12 Ma whereas the main fabric yields a 40 Ar– 39 Ar biotite cooling age of 468 ± 1.4 Ma. The pegmatites yield 457 ± 7 Ma and 458 ± 7 Ma Rb–Sr muscovite–feldspar ages and 40 Ar– 39 Ar step-heating plateaux of 466 ± 1 Ma and 468 ± 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm–Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U–Pb analyses from a psammitic gneiss yield age populations at 1.05–1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

The Green Beds of the SW Highlands: deposition and origin of a basic igneous-rich sedimentary sequence in the Dalradian Supergroup of Scotland

Synopsis The Green Beds are distinctive chlorite- and epidote-rich metasedimentary rocks, which collectively represent a series of important lithostratigraphical marker units in the Southern Highland Group of the dominantly late Precambrian Dalradian Supergroup of Scotland. In a recent re-evaluation of the nature and origin of the Green Beds, integrated mapping, sedimentological, petrological and geochemical studies have been carried out. The Green Beds form relatively thin but laterally extensive units within thick sequences of siliciclastic metasedimentary rocks, and were deposited from high-density turbidity currents and debris flows in a deep-sea slope apron or ramp setting. They comprise a physical mixture of basic and siliciclastic material, representing periodic influxes of basic igneous detritus into the ‘background’ siliciclastic sedimentary system. The mineralogy of the Green Beds is largely the result of metamorphism, although rare relict detrital minerals from both granitic and basic sources are preserved. Geochemical studies show that the Green Beds and other Southern Highland Group metasedimentary rocks have a similar trace element signature, indicating a common siliciclastic source area. Two suites of Dalradian lavas, the Tayvallich and Loch Avich pillowlavas, were also analysed. Although it is unlikely that these particular volcanic units were the source of the Green Bed basic material, it is believed that the mafic component of the Green Beds was derived from episodes of near-contemporaneous volcanic activity.

Tephra-fall deposits in the Sleat Group (Torridonian), Isle of Skye, Scotland

Tephra-fall deposits in the Late Mesoproterozoic Sleat Group (Torridonian) from Skye, Scotland, are described for the first time. Two individual beds occur within the Loch na Dal Formation which represents sedimentation in a shallow marine environment. Each bed has a distinctive brown, crumbly, amorphous appearance in the field and has sharp contacts with its host metasediment. This unique lithology is identical to that of albitic schists described recently from the Southern Highland Group, Dalradian Supergroup, which were identified as air-fall tuffs.

More air-fall tuffs from the Southern Highland Group, Dalradian Supergroup, Scotland

Following the discovery of ten ‘brown beds’ near Birnam, Perthshire, and their identification as air-fall tuffs (Batchelor 2004), further fieldwork along the regional strike has yielded more examples of these unusual beds. Each bed has a distinctively brown amorphous appearance in the field, with sharp upper and lower contacts with its host metasediment, and thicknesses range from 5 mm to 100 mm. The geographical distribution of the localities is shown in Figure 1. Description The first locality SW of Birnam which yielded ‘brown beds’ is at a small cliff on the west side of the Sma’ Glen, near Crieff [NN894305]. This cliff lies about 20 m above the road and is located opposite field gates, near to Clach Ossian. The cliff comprises vertical beds of psammite and grit with quartz segregations. Five ‘brown beds” were identified, four of which were sampled (GH/RB/52–55). Two of these beds formed an elliptical pod, suggesting tectonic squeezing of a soft material. The second ‘brown bed’ locality SW of Birnam is at the south end of the Glen Lednock road in Comrie. An isolated bluff of psammites and pelites crops out amid trees on the NE side of the road, above the point where a signpost directs walkers to the “Devil’s Cauldron”. Within the restricted outcrop [NN77002225], three ‘brown beds’ were found (GH/RB/61–63). The third ‘brown bed’ locality, and the richest in terms of individual beds, is on the east side of Loch Lomond, below the road between Balmaha and Rowardennan. A track leaves this . . . This work was funded by the Carnegie Trust for the Scottish Universities as part of a large collaborative project under the direction of Dr Grahame Oliver (University of St Andrews). Angus Calder provided the XRF and XRD analyses and my wife Rosalind Garton assisted with fieldwork.

Zoned Ca-amphiboles and related P-T evolution in metabasites from the classical Barrovian metamorphic zones in Scotland

AbstractMetabasites in the Dalradian Argyll and Southern Highland Groups experienced multiphase deformation and coeval Barrovian-type prograde and retrograde metamorphism during the Caledonian Grampian orogeny. Metamorphic Ca-amphiboles crystallized with plagioclase,epidote,chlorite and quartz, and sometimes with garnet and clinopyroxene. The minor changes in metabasite mineral assemblages and systematic mineral-chemical variations in Ca-amphibole confirm the classical concept of increasing metamorphic grade in metapelitic Chlorite to Sillimanite zones. In the Chlorite zone, high-Ti brown amphibole enclosed by green Ca-amphibole is interpreted as a magmatic relic. In the Chlorite, Biotite,Garnet and Andalusite zones, Ca-amphibole displays zonation with actinolite in cores and magnesio-hornblende to tschermakite in rims. Poor amphibole zonations occur in the Kyanite and Sillimanite zones. Ca-amphibole zonations are best described in terms of IVAl, VIAl and Ti which semiquantitatively monitor temperature and pressure changes. Maximal IVAl in amphibole increase with metamorphic grade. Empirically calibrated amphibole equilibria enabled us to reconstruct coherent prograde P-T paths and maximal P and T from the mineral zones; with Tmax of 680±50°C in the Sillimanite zone and Pmax of 8±1.2 kbar in the Kyanite zone. Prograde P-T paths indicate that elevated geothermal gradients should be considered even at the beginning of the Grampian Barrovian metamorphism.

Air-fall tuffs in the Southern Highland Group, Dalradian Supergroup, at Birnam, Perthshire

Synopsis Ten ‘brown beds’ representing air-fall tuffs in the Southern Highland Group of the Dalradian Supergroup of Scotland are described for the first time from near Birnam, Perthshire, close to the Highland Boundary Fault. Their composition suggests an origin more silicic than the traditional Dalradian ‘green beds’. Expressions of volcanism in the upper Dalradian are preserved as the Tayvallich (Argyll Group) and Loch Avich (Southern Highland Group) basaltic lavas. The Tayvallich lavas are associated with minor quartz keratophyre intrusions and pink felsic tuffs. On the basis of the currently accepted stratigraphic correlation of the upper Dalradian, the ‘brown beds’ at Birnam would have been derived from eruptions of currently unidentified quartz keratophyric magmas in Southern Highland Group times. Alternatively, if the source of the ‘brown beds’ can be traced to the Tayvallich volcanism, then the upper Dalradian stratigraphy of the Birnam area would need to be revised.

Source of the Dalradian Supergroup constrained by U–Pb dating of detrital zircon and implications for the East Laurentian margin

Detrital zircons in siliciclastic units of the Dalradian Supergroup yield U–Pb ages ranging from 3.2 to 0.5 Ga. Detrital zircons from the sub-Grampian Group basement and the Grampian Group are predominantly of Palaeoproterozoic and Mesoproterozoic ages with Archaean grains absent or rare. In contrast, the overlying Appin, Argyll and Southern Highland groups contain a significant contribution of Archaean detrital zircon grains, some of which locally preserve evidence for late Palaeoproterozoic overgrowths dated at c . 1.8 Ga. In addition, on concordia plots Archaean grains are slightly discordant with a lower intercept at c . 1.8 Ga suggesting they were affected by a tectonothermal event at this time. Late Palaeoproterozoic and Mesoproterozoic grains also show evidence for overprinting by a tectonothermal event around 1.0–0.9 Ga. These tectonothermal events occurred in the source region before accumulation of the siliciclastic detritus. The analysed samples contained no definitive evidence for having been affected by a late Neoproterozoic (Knoydartian) event or of containing detritus derived from a source showing evidence for this event. The overall age range of detritus, combined with sparse palaeocurrent data, is consistent with derivation from the Laurentian foreland, especially the Labrador–Greenland region. Archaean detritus overlaps with that of Archaean cratons, notably the Superior, whereas Palaeoproterozoic detritus corresponds to the timing of suturing of Archaean cratons by a series of orogenic belts (Ketilidian–Makkovik, New Quebec, Nagssugtoqidian, Torngat belts). Mesoproterozoic detritus is consistent with derivation from the Grenville Orogen. The presence of a series of detrital age peaks in the late Palaeoproterozoic and early Mesoproterozoic (1.8–1.5 Ga), the paucity of mid-Mesoproterozoic detritus (1.4–1.2 Ga), and evidence for a tectonothermal event between 1.0 and 0.9 Ga is typical of the geological history of the Labrador–Greenland region of Laurentia.

Neoproterozoic glacial-rainout intervals: Observations and implications

Stratigraphically discrete glacigenic dropstone intervals have been identified within six separate Neoproterozoic glaciomarine successions: the ca. 720 Ma Chuos Formation (southwestern Congo craton) and Surprise Diamictite (southwestern Laurentia), the ca. 600 Ma Ghaub Formation (Congo craton), Blasskrans Formation (northwestern Kalahari craton) and Wildrose Diamictite (southwestern Laurentia), and the post-595 Ma lower Southern Highland Group (northeast Laurentia). These dropstone intervals are interstratified with hemipelagic, dropstone-free lithologies that record periods devoid of glacial rainout. Such episodic deposition requires, at a most rudimentary level, temporally compatible environmental fluctuations to generate melting of ice. The Ghaub Diamictite is particularly revealing in this regard, given that, according to the snowball Earth hypothesis, it had to be deposited either during glacial maxima (totally frozen seas) or melt back (geologically instantaneous) owing to its low-latitude setting and association with carbonates that record highly depleted δ 1 3 C values. The consistency of facies features shared by all six glaciomarine successions leads us to speculate that the temporally discrete glacial-rainout events (including diamictites deposited in low latitudes) record a dynamic glacial environment and that such conditions were characteristic of Neoproterozoic glaciations in general. These observations indicate that Neoproterozoic seas were not totally frozen and that the hydrologic cycle was functioning.

On the origins of layered calc-silicate rocks from the Buchan Dalradian and their bearing on the diagenetic history of the Southern Highland Group

Synopsis Calc-silicate rocks from the Southern Highland Group of the Dalradian at Banff are metamorphosed carbonate concretions. Many original sedimentary features are preserved even though the calc-silicates now have amphibolite facies mineral assemblages. Those occurring in the andalusite and staurolite zones contain layered mineral assemblages that are characterized by the presence of calcite, clinopyroxene, hornblende, cummingtonite, and biotite from centre to margin. It is unlikely that the layering formed principally by synmetamorphic diffusional processes since diffusion modelling demonstrates that the relative values of phenomenological diffusion coefficients required to reproduce this set of layers are inconsistent with known values. The layer order, with pyroxene adjacent to carbonate and amphibole adjacent to pelite, is also inconsistent with their formation by infiltration of an H 2 O-rich fluid into a pure carbonate body. The layers can be best explained as resulting from metamorphism of compositionally zoned, diagenetic carbonate concretions. If the metamorphism was essentially isochemical, then composition profiles present in the calc-silicates may be compared with those in unmetamorphosed concretions and interpreted in terms of known diagenetic processes. A decrease in CaO from centre to margin results from the progressive elimination of the primary porosity during compaction of the sediment, a conclusion supported by the presence of relict sedimentary structures. Such calc-silicates contain important information about the early burial history of the sedimentary basin where their host rocks were deposited prior to the onset of metamorphism. There is some evidence that the alkali profiles have been modified during metamorphism. However it is not appropriate to use such calc-silicates for studies of synmetamorphic diffusion or other synmetamorphic processes without taking pre-metamorphic composition gradients into account.

Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.

Two from Donegal: Neoproterozoic glacial episodes on the northeast margin of Laurentia

Two glacial episodes, the Sturtian (ca. 720 Ma) and Marinoan-Varangerian (ca. 590 Ma), characterize many Neoproterozoic successions worldwide. In contrast, northeast Laurentia is typically considered as containing only the latter; in Scotland and Ireland this is represented by the Port Askaig Tillite. Here we present evidence (ice-rafted debris) for a second, younger Neoproterozoic glacial event in the Dalradian Southern Highland Group of Donegal, Ireland. These deposits overlie strata correlative to ca. 595 Ma rocks in Scotland, making this heretofore unrecognized glacial episode a Marinoan-Varangerian equivalent. In that the Port Askaig occurs ∼3–5 km stratigraphically below the Southern Highland Group rocks, it is most likely Sturtian, not Marinoan-Varangerian, in age. Thus, the Dalradian succession in the Irish Caledonides contains strata representing both the Sturtian and Marinoan-Varanger glaciations.