Models for the tectonothermal evolution of the eastern Dalradian of Scotland

Abstract The preserved array of pressures in the eastern Dalradian indicates that considerable syn- to post-metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillimanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking syn-metamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.     

Thorium and uranium in some pelitic rocks from the Dalradian,Scotland

Instrumental neutron-activation analysis for the determination of Th and U in rocks is superior to the delayed-neutron method when a low-power reactor is used. Results using these methods on pelitic rocks from the Dalradian of Scotland indicate Th values higher and U values lower than sediments from elsewhere. There is no evidence of Th or U movement during metamorphism and the areal variations found are due to the different provenance of the rocks.     

Comparison of strain and magnetic fabrics in Dalradian rocks from the southwest Highlands of Scotland

The magnetic fabrics of 235 samples from 31 localities in Argyllshire, Scotland were determined to study the development of the Caledonian tectonic fabric in the southwest Highlands of Scotland. The regional fabric indicates a strong NE-SW compressional foliation due to the primary deformational phases, which in parts has been overprinted by secondary deformations. A detailed comparison of the anisotropy data and the available strain data shows that the two fabric ellipsoids are co-axial, and that their axial mean ratios seem to be related by an empirical power relationship of the type:

$\text{Xi}\text{Xj}\text{=}\text{li}\text{lj}{}^{\mathrm{a}}$

(for i = 1,2,3; j = 1,2,3 and i ≠ j) where χ_i and χ_j are orthogonal principal axes and l_i and l_j are the corresponding orthogonal principal strain axes. The exponent a for the sites from Scotland is 0.088 ± 0.017 compared with 0.142 ± 0.001 and 0.145 ± 0.005 found in the Caledonian slates of the English Lake District and the Welsh slate belt.     

Amphiboles from Dalradian metasedimentary rocks of NE Scotland: Environmental inferences and distinction from amphiboles of meta-igneous amphibolites

Summary Several formations within the lower part of the Dalradian Appin Group are characterised by calcareous and calc-silicate lithologies. Amphibole occurs in these metasedimentary units in a variety of habits, from fine-grained disseminated crystals, through radiating aggregates permeating the whole rock, to dark green layers consisting of over 90% amphibole. Electron microprobe analyses of these amphiboles exhibit simple compositional trends from tremolite, through tremolitic hornblende to magnesio-hornblende. The only significant substitutions are from tremolite towards alumino-tschermakite [MgSi Al^viAl^iv] and from tremolite towards ferri-tschermakite [MgSi Fe^viAl^iv].This is in marked contrast to amphiboles from nearby suites of metavolcanic rocks and intrusive metadolerites which exhibit four coupled substitutions, leading from tremolite towards the ferro-actinolite, edenite, alumino-tschermakite and ferritschermakite end-members. These distinctive compositional trends may aid the distinction between para- and ortho-amphibolites elsewhere.Tremolitic amphibole commonly constitutes a high proportion of the calc-silicate rocks, the only other phases present being quartz ± phlogopite. Carbonate minerals are notably absent. This suggests that the sedimentary protolith was high in Mg, such as a dolomitic marl, possibly containing some magnesite. Sedimentological considerations suggest deposition in tropical latitudes in restricted coastal lagoons forming part of an extensive stable shelf bordering a mature landscape.

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Amphibole in Metasedimenten der Dalradians, NE Schottland: Rückschlüsse auf prämetamorphe Ablagerungsbedingungen und Unterscheidung von Amphibolen aus orthomagmatischen Amphiboliten

Zusammenfassung Mehrere Formationen im unteren Teil der Appin Gruppe der Dalradians sind durch karbonatische und kalk-silikatische Lithologien gekennzeichnet. Amphibole treten in diesen metasedimentären Einheiten in verschiedener Ausbildung auf, als feinkörnige, disseminierte Kristalle, als radialstrahlige, die Gesteine durchwachsende Aggregate und in dunkelgrünen, zu mehr als 90% aus Amphibolen bestehenden Lagen. Elektronenstrahl-Mikrosondenuntersuchungen belegen einfache mineralchemische Zusammensetzungen, die Tremolite, tremolitische Hornblende und Magnesio-Hornblende umfaßt. Die einzige bedeutende Substitution ist die zwischen Tremolit und Alumino-Tschermakit [MgSi Al^viAl^iv] und zwischen Tremolit und Ferri-Tschermakit [MgSi Fe^viAl^iv].Im Gegensatz dazu zeigen Amphibole aus den nahegelegenen Metavulkaniten und intrusiven Metadoleriten vier Substitutionstrends von Tremolit zu Ferro-Akttinolith, Edenit, Alumino-Tschermakit und Ferri-Tschermakit. Diese unterschiedlichen Trends könnten auch anderswo bei der Unterscheidung zwischen Para- und Orthoamphiboliten behilflich sein. Tremolitische Amphibole machen normalerweise einen Großteil der kalk-silikatischen Gesteine aus, in denen ansonsten nur Quarz ± Phologpit auftritt. Karbonate fehlen bemerkenswerterweise. Dies weist auf Mg-reiche sedimentäre Protolithe, wie dolomitische Mergel mit möglicherweise etwas Magnesit, hin. Sedimentologische Überlegungen lassen die Ablagerung in abgeschnürten Küstenlagunen tropischer Breiten, die Teil eines weiträumigen stabilen Schelfes waren, vermuten.

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With 4 Figures     

Amphiboles from Dalradian metasedimentary rocks of NE Scotland: environmental inferences and distinction from amphiboles of meta-igneous amphibolites

Summary Several formations within the lower part of the Dalradian Appin Group are characterised by calcareous and calc-silicate lithologies. Amphibole occurs in these metasedimentary units in a variety of habits, from fine-grained disseminated crystals, through radiating aggregates permeating the whole rock, to dark green layers consisting of over 90% amphibole. Electron microprobe analyses of these amphiboles exhibit simple compositional trends from tremolite, through tremolitic hornblende to magnesio-hornblende. The only significant substitutions are from tremolite towards alumino-tschermakite [MgSi Al^viAl^iv] and from tremolite towards ferri-tschermakite [MgSi Fe^viAl^iv].This is in marked contrast to amphiboles from nearby suites of metavolcanic rocks and intrusive metadolerites which exhibit four coupled substitutions, leading from tremolite towards the ferro-actinolite, edenite, alumino-tschermakite and ferritschermakite end-members. These distinctive compositional trends may aid the distinction between para- and ortho-amphibolites elsewhere.Tremolitic amphibole commonly constitutes a high proportion of the calc-silicate rocks, the only other phases present being quartz ± phlogopite. Carbonate minerals are notably absent. This suggests that the sedimentary protolith was high in Mg, such as a dolomitic marl, possibly containing some magnesite. Sedimentological considerations suggest deposition in tropical latitudes in restricted coastal lagoons forming part of an extensive stable shelf bordering a mature landscape.

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Amphibole in Metasedimenten der Dalradians, NE Schottland: Rückschlüsse auf prämetamorphe Ablagerungsbedingungen und Unterscheidung von Amphibolen aus orthomagmatischen Amphiboliten

Zusammenfassung Mehrere Formationen im unteren Teil der Appin Gruppe der Dalradians sind durch karbonatische und kalk-silikatische Lithologien gekennzeichnet. Amphibole treten in diesen metasedimentären Einheiten in verschiedener Ausbildung auf, als feinkörnige, disseminierte Kristalle, als radialstrahlige, die Gesteine durchwachsende Aggregate und in dunkelgrünen, zu mehr als 90% aus Amphibolen bestehenden Lagen. Elektronenstrahl-Mikrosondenuntersuchungen belegen einfache mineralchemische Zusammensetzungen, die Tremolite, tremolitische Hornblende und Magnesio-Hornblende umfassen. Die einzige bedeutende Substitution ist die zwischen Tremolit und Alumino-Tschermakit [MgSi Al^viAl^iv] und zwischen Tremolit und Ferri-Tschermakit [MgSi = Fe^viAl^iv].Im Gegensatz dazu zeigen Amphibole aus den nahegelegenen Metavulkaniten und intrusiven Metadoleriten vier Substitutionstrends von Tremolit zu Ferro-Aktinolith, Edenit, Alumino-Tschermakit und Ferri-Tschermakit. Diese unterschiedlichen Trends könnten auch anderswo bei der Unterscheidung zwischen Para- und Orthoamphiboliten behilflich sein. Tremolitische Amphibole machen normalerweise einen Großteil der kalk-silikatischen Gesteine aus, in denen ansonsten nur Quarz ± Phlogopit auftritt. Karbonate fehlen bemerkenswerterweise. Dies weist auf Mg-reiche sedimentäre Protolithe, wie dolomitische Mergel mit möglicherweise etwas Magnesit, hin. Sedimentologische Überlegungen lassen die Ablagerung in abgeschnürten Küstenlagunen tropischer Breiten, die Teil eines weiträumigen stabilen Schelfes waren, vermuten.

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The Managing Editor regrets that the complete text of this paper has already been published in Mineral Petrol 49: 45–62 (1993)     

The boyndie syncline,a D1 structure in the Dalradian of Scotland

The structure of the Dalradian rocks of the Banffshire coast has been re-examined. It is shown that D₁ structures face upwards along the entire section and the major Boyndie Syncline (Sutton and Watson 1956) must also be of D₁, age and not D₃ as proposed by Johnson (1962). D₂ structures are developed in the west, most strongly in the Portsoy Group and Cowhythe Gneiss where they may be related to the Portsoy Thrust (Elles 1931). East of the Cowhythe Gneiss D₂ structures are not developed but D₃ structures are of local importance. Porphyroblast growth is considered to be post-D₁, pre-D₃ related to the ‘active’ D₂ deformation in the west (Barrovian) and to the ‘static’ D₂ interval in the east (Buchan). The wider implications of these conclusions for Dalradian structure are briefly considered.     

The Biotite Isograd and the Lower Greenschist Facies in the Dalradian Rocks of Scotland

Analyses of a series of coexisting minerals and their host rocks,from the Aberfoyle area, indicate that previously proposed biotite-formingreactions are inadequate to account for the appearance of biotitein the Dalradian greywackes and pelites. Initially biotite isformed as a product of the breakdown of the assemblage chlorite+microcline+phengiticmuscovite in greywackes of suitable bulk composition. With increasein grade the stability field of the biotite-bearing assemblageis enlarged through the reduction in the amount of celadonitesubstitution that can be tolerated by the white micas. The stabilityfield increases so that it includes progressively more aluminousrocks. The sequence of biotile-forming reactions is used to definethree zones of increasing metamorphic grade in the Dalradiangreywackes and pelites. The interrelationships of the assemblagesdeveloped in these zones are considered in detail. The assemblagesdeveloped in the Green Beds are explained on the assumptionthat the fugacity of CO₂ is a third intensive variable.     

A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe,SW Highlands,Scotland

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D₁ spaced cleavage was responsible for the formation of the D₂ spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D₂ microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D₁ microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D₂ flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.     

The structure of the Moine and Dalradian Rocks in the Dalmally District of Argyllshire,Scotland

Two phases of deformation are described from the area. D1 is related to the first phase movements widely recognised in the Dalradian; D2 is related to more localised modifications of the D1 fold pattern. Four major D2 folds are recognised from the evidence of minor structures. Three of these, the Beinn Chuirn, the Ben Lui and the Ra Chreag Folds, had previously been considered as D1 structures, whilst the fourth, the Dalmally Fold, was unrecognised. These folds provide continuity between the southeast facing of the D1 folds of the Moine and Ballachulish Succession of Glen Orchy (Thomas and Treagus 1968) and the southeast facing of the D1 Jay Nappe (Shacketon 1958) in the Iltay Succession above. The D2 deformation reaches a climax at the junction between the two Successions, the Iltay Boundary Slide, which is now regarded as a D2 structure. The regional implications of these results are briefly considered.     

Regional metamorphism of some Dalradian pelites in the Buchan area,N.E. Scotland

Regional metamorphic zones, based on mineral assemblages in pelites, are presented for the Dalradian rocks of Aberdeenshire and Banffshire, in the type area of Buchan metamorphism; electron microprobe analyses of cordierite (C), staurolite (S), chlorite (Chl), biotite (B) and white mica (Ms) are reported for rocks from the classic sections of the Banffshire coast and the valley of the Ythan.A low grade biotite zone, separates two NE-SW trending sets of higher grade zones, in which the sequence s defined by the entry of cordierite, andalusite (A) and (in the west only) staurolite. The zones are characterised by the assemblages (with quartz and muscovite) B-Chl, C-B-Chl, A-C-B and S-A-B.The western sequence contains a transition towards higher pressure, Barrovian type, metamorphism. The isograds arise from continuous reactions affecting rocks of restricted bulk composition (M/FM). With increasing grade, there is a regular trend towards more magnesian ranges of composition for the assemblages C-B-Chl, A-C-B and finally (as P increases in the west), S-A-B. The isograds form when these assemblages intersect the most Fe⁺⁺-rich rock compositions present which occurs in each case when the biotite M/FM=40. A complex of divariant equilibria, derived for the system KFMASH, is used to model the natural reactions.     

The variation in garnet,biotite and chlorite composition in medium grade pelitic rocks from the Dalradian,Scotland, with particular reference to the zonation in garnet

Garnet, biotite and host rock have been analysed along a traverse from the garnet isograd to the kyanite zone in the Dalradian of Central Perthshire, Scotland. FeO and MgO increase and MnO and CaO decrease in the garnet with increasing grade. Microprobe analyses of the garnets reveal zoning, which indicates that a garnet crystal as a whole does not equilibrate with the matrix during growth. Coexisting biotite varies in composition as a result of the abstraction of MnO, FeO etc. from the rook by the growing garnet, i.e. the mg/mg + fe ratio increases with grade. The microprobe analyses also reveal the size of the system from which garnet abstracted material varied from 0.100 to 2.000 g and the nucleation was frequently instantaneous. It also reveals the equilibrium or non-equilibrium nature of the assemblage, and explains the variation in garnet composition with grade in terms of a segregation model with a changing distribution coefficient. Primary chlorite was analysed from rocks near to the garnet isograd containing garnet and biotite. It has a similar mg/mg + fe value to the coexisting biotite. The results show that the three phase field defining the garnet isograd moves towards the mg corner with increasing grade. The higher grade fields lie to the mg rich side of the three phase field so that the sequence of mineral assemblages across the Barrovian zones in Perthshire, from the garnet isograd to the kyanite zone, can be summarized and displayed on a phase diagram.     

Sources of rare earth elements in Appin Group limestones,Dalradian, north-east Scotland

Summary Rare earth elements (REE) have been determined in 15 samples from three metamorphosed limestone units of the Appin Group of the Dalradian of north-east Scotland. Total REE contents range from about 7 to about 50 ppm and the light REEs are slightly enriched compared with REE in the North American Shale Composite (NASC). In the more impure limestones, the dominant control on REE content was the clay mineral content in siliciclastic impurities derived from terrestrial sources. In the purer limestones, the correlation between REEs and iron is consistent with the well-documented relationship between iron and REEs in the aqueous environment. REE associated with terrestrial, iron-rich, flocculated organic colloids or marine, authigenic iron-manganese coats may well have contributed significantly to the REE contents. Weak Ce anomalies on NASC-normalised, REE variation diagrams in two samples with very low REE contents suggest a marine influence on the REE distribution.

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Die Quelle von Seltenen Erden in Kalken der Appin Gruppe, Dalradians, NE Schottland

Zusammenfassung An 15 Proben aus metamorphen Kalken der Appin Gruppe der Dalradians NE Schottlands wurde die Zusammensetzung der Seltenen Erden (REE) bestimmt. Der Gesamtgehalt an REE variiert von 7 bis 50 ppm, wobei die leichten REE im Vergleich zum North American Shale Composite (NASC) leicht angereichert sind. In unreinen Kalken wird der REE-Gehalt vom Tonmineralanteil der silikatisch-klastischen Verunreinigungen, die einer terrestrischen Quelle entstammen, kontrolliert. In reinen Karbonatgesteinen besteht eine Korrelation zwischen den REE und Fe, eine Beziehung, die für ein wässriges Milieu bereits bereits gut dokumentiert ist. REEs, die mit terrrestrischen, Fe-reichen, ausgeflockten organischen Kolloiden oder marinen, authigen gebildeten Fe-Mn Überzügen assoziiert sind, könnten beträchtlich zu den REE Gehalten beitragen. Schwache, in auf NASC normalisierten REE-Diagrammen erkennbare Ce-Anomalien von zwei Proben, lassen einen marinen Einfluß vermuten.

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With 5 Figures     

Sources of rare earth elements in Appin Group limestones,Dalradian, north-east Scotland

Summary Rare earth elements (REE) have been determined in 15 samples from three metamorphosed limestone units of the Appin Group of the Dalradian of north-east Scotland. Total REE contents range from about 7 to about 50 ppm and the light REEs are slightly enriched compared with REE in the North American Shale Composite (NASC). In the more impure limestones, the dominant control on REE content was the clay mineral content in siliciclastic impurities derived from terrestrial sources. In the purer limestones, the correlation between REEs and iron is consistent with the well-documented relationship between iron and REEs in the aqueous environment. REE associated with terrestrial, iron-rich, flocculated organic colloids or marine, authigenic iron-manganese coats may well have contributed significantly to the REE contents. Weak Ce anomalies on NASC-normalised, REE variation diagrams in two samples with very low REE contents suggest a marine influence on the REE distribution.

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Die Quelle von seltenen erden in kalken der Appin Gruppe, Dalradians, NE Schottland

Zusammenfassung An 15 Proben aus metamorphen Kalken der Appin Gruppe der Dalradians NE Schottlands wurden die Seltenen Erden (REE) bestimmt. Der Gesamtgehalt an REE variiert von 7 bis 50 ppm, wobei die leichten REE im Vergleich zum North American Shale Composite (NASC) leicht angereichert sind. In unreinen Kalken wird der REE-Gehalt vom Tonmineralanteil der silikatisch-klastischen Verunreinigungen, die einer terrestrischen Quelle entstammen, kontrolliert. In reinen Karbonatgesteinen besteht eine Korrelation zwischen den REE und Fe, eine Beziehung, die für ein wässriges Milieu bereits bereits gut dokumentiert ist. REEs, die mit terrestrischen, Fe-reichen, ausgeflockten organischen Kolloiden oder marinen, authigen gebildeten Fe-Mn Überzügen assoziiert sind, könnten beträchtlich zu den REE Gehalten beitragen. Schwache, in auf NASC normalisierten REE-Diagrammen erkennbare Ce-Anomalien von zwei Proben, lassen einen marinen Einfluß vermuten.

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The Managing Editor regrets that the complete text of this paper has already been published in Mineral Petrol 49: 27–44 (1993)     

Regional fluid and metal mobility in the Dalradian metamorphic belt,Southern Grampian Highlands,Scotland

A prominent set of veins was formed during post-metamorphic deformation of the Caledonian Dalradian metamorphic belt. These veins are concentrated in dilational zones in fold hinges, but apophyses follow schistosity and fold axial surface fractures. The veins are most common in the cores of regional structures, especially the Dalradian Downbend and consist of quartz, calcite, chlorite and metallic sulphides and oxides. Metals, including gold, have been concentrated in the veins. The fluid which formed the veins was low salinity (1–5 wt% NaCl and KCl) CO₂-bearing (3–16 wt% CO₂) water of metamorphic origin. The fluid varies slightly in composition within and between samples, but is essentially uniform in composition over several hundred km². Vein formation occurred at about 350±50 °C and 200–300 MPa pressure. Further quartz mineralization occurred in some dilational zones at lower temperatures (160–180 °C). This later mineralization was accompanied by CO₂ immiscibility. Dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle-ductile transition. A similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby Tyndrum Fault at a later stage in the Dalradian uplift.     

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 δ ¹⁸O_(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. Received: 26 February 1999 / Accepted: 8 November 1999     

A Holocene tsunami deposit in eastern Scotland

A thin, regionally extensive, laterally persistent sand layer identified within the Holocene coastal sequences of eastern Scotland, dated to 7000 years BP, is suggested to be a tsunami deposit. The likely source of the tsunami wave is the earthquake induced second Storegga Slide on the Norwegian continental slope at least 750 km northeast of the deposit.     

Jurassic uraniferous rocks in the eastern Transbaikal region

The paper presents the results of lithological studies of Jurassic rocks in one of the largest basins in the eastern Transbaikal region (Olov depression). These results refined settings of the volcanosedimentary association and indicated that the Olov depression was formed in three stages: stage 1 related to tectonic activation manifested as cataclasis of granitoids in the Transbaikal region and incipience of numerous depressions (Olov included); stage 2 characterized by catastrophic events related to reactivation of tectonic motions, strong volcanism, and intense activity of geysers; and stage 3 marked by termination of volcanic activity and relative tectonic stability of the region that promoted the deposition of rocks of the middle and upper Ukurei subformations in the course of slow synsedimentary subsidence of the depression. Uranium mineralization in the rocks was governed mainly by the following conditions: (1) abundance of cataclased granitoids that delivered terrigenous material to the sedimentation basin; (2) favorable paleogeographic setting for the formation of both sedimentary and postsedimentary hydrothermal U-rich sequences. Field works in the Transbaikal region and analytical results reported in the present paper made it possible to make a videofilm shown in https://youtu.be/UOe9xzSKOEI.     

Metabasite amphiboles of the Scottish Dalradian

Blue-green hornblendes are observed in metabasite assemblages throughout the chlorite, biotite and garnet zones of the southwest Scottish Highlands. Actinolites are common in more Mg-rich metabasites in these zones. At low grade, hornblendes are relatively edenite-rich, and may sometimes occur together with a more Mg-rich, Al-poor actinolite. Within the garnet zone, hornblendes are pargasitic, showing extensive tschermakite substitution. Textural and chemical evidence do not indicate the presence of any miscibility gap between hornblende and actinolite within the chlorite to garnet zones in the southwest Highlands. The occurrence of hornblende-actinolite pairs in metabasites of the Scottish Dalradian, and perhaps also in other metamorphic terrains, is considered to reflect the incomplete chemical equilibration of lower grade actinolitic amphibole during prograde metamorphism, rather than a miscibility gap. The paucity of amphibole compositions intermediate between hornblende and actinolite in many metamorphic terrains is thought to reflect the rapid but continuous change of stable amphibole compositions in metabasites over a small range of increased metamorphic grade.     

A palaeo-ice stream of the British Ice Sheet in eastern Scotland

Terrestrial and marine subglacial landforms in eastern Scotland are used to evaluate previously unsubstantiated notions of ice streaming within the British Ice Sheet (BIS) in this area during the last glacial cycle. Employing both regional and local-scale data sets, we describe onshore landform-sediment assemblages, offshore geomorphology and stratigraphy, and reconstructed palaeo-ice flow patterns. The results and their glaciological significance are discussed in the context of stratigraphical and geomorphological frameworks established by earlier workers, and are compared with modelled reconstructions for the BIS in this area. We conclude that the Main Late Devensian ice sheet in eastern Scotland hosted a zone of fast-flowing ice at least 100 km long and 45 km wide, akin to a contemporary ice stream. This sector - the Strathmore Ice Stream - flowed through a combination of basal sliding on meltwater-lubricated rigid beds and by deforming unconsolidated basal substrates.