Laxfordian Deformation Research Articles

Constraints on the Palaeoproterozoic tectono‐metamorphic evolution of the Lewisian Gneiss Complex, NW Scotland: Implications for Nuna assembly

AbstractDespite extensive investigation, the tectono‐thermal evolution of the Archean crust in the Lewisian Gneiss Complex in NW Scotland (LGC) is debated. Most U–Pb zircon geochronological and metamorphic studies have focused on rocks from the central region of the mainland LGC, where granulite facies assemblages associated with the oldest (Badcallian) tectono‐metamorphic event at c. 2.75 Ga are overprinted by younger amphibolite facies assemblages related to the Inverian (c. 2.5 Ga) and subsequent Laxfordian (c. 1.9–1.65 Ga) tectono‐thermal events. In the southern and northern regions of the mainland LGC, deformation and metamorphism associated with the Laxfordian event are pervasive, although the timing and conditions are poorly constrained. Here, we present new field, petrographic and structural data, U–Pb zircon and titanite geochronology and phase equilibrium modelling of amphibolite samples from the northern and southern regions. Our field observations show that in both regions, pre‐Laxfordian structures are significantly reworked by steep NW‐striking fabrics that are themselves pervasively overprinted by co‐axial deformation and amphibolite facies metamorphism related to the Laxfordian event. In situ U–Pb titanite geochronology yields Laxfordian ages of 1853 ± 20 Ma in the southern region (P = 6–8 kbar and T = 640–690°C) and 1750 ± 20 Ma and 1776 ± 10 Ma in the northern region (P = 6–7.5 kbar and T = 740–760°C). While U–Pb dating of zircon rims from felsic gneisses in the central region shows a dominant Inverian metamorphic overprint at c. 2500 Ma, zircon rims in felsic gneisses from the northern and southern regions commonly yield Laxfordian dates as young as c. 1800 Ma. Combined, the results support the idea that, during the Palaeoproterozoic, the central region of the LGC acted as low‐strain domain, in which intense deformation and metamorphism were restricted to crustal‐scale shear zones. By contrast, in the southern and northern regions, early (c. 1.85 Ga) and late (c. 1.75 Ga) Laxfordian deformation and fluid‐mediated metamorphism were much more pervasive and at higher P–T conditions than previously proposed. The diachronous Laxfordian evolution of the southern and northern regions indicate that they reflect early and late snapshots of collisional to transpressional tectonics in the mainland LGC. The long‐lasting Laxfordian evolution documents the collision of the Rae and North Atlantic cratons during the Palaeoproterozoic amalgamation of the supercontinent Nuna, with implications for the palaeogeographic configuration of NW Scotland during Palaeoproterozoic Nuna.

Petrogenesis and geochemical halos of the amphibolite facies, Lower Proterozoic, Kerry Road volcanogenic massive sulfide deposit, Loch Maree Group, Gairloch, NW Scotland

The Palaeoproterozoic Kerry Road deposit is one of the oldest examples of volcanogenic massive sulfide (VMS) mineralization. This small VMS deposit (~500,000 tons grading at 1.2% Cu, 3.5% Zn) is hosted in amphibolite facies mafic-siliciclastic units of the c. 2.0 Ga Loch Maree Group, Scotland. Sulfide mineralization consists of pyrite and pyrrhotite with subordinate chalcopyrite and sphalerite, occurring in disseminated, vein and semi-massive to massive textures.The deposit was highly deformed and metamorphosed during the c. 1.8–1.7 Ga Laxfordian Orogeny. Textural relationships of deformed sulfide minerals, related to early Laxfordian deformation (D1/D2), indicate initial high pressure-low temperature (100 MPa, 150 °C) conditions before reaching peak amphibolite facies metamorphism, as evident from pyrrhotite crossing the brittle/ductile transition prior to chalcopyrite. Late Laxfordian deformation (D3/D4) is marked by local retrograde greenschist facies at low pressure and temperature (<1.2 MPa, <200 °C), recorded by late red sphalerite remobilization. δ34S values from all sulfide minerals have a homogeneous mean of 0.8 ± 0.7‰ (n = 21), consistent with interaction of hydrothermal fluids in the host oceanic basalt-island arc setting envisaged for deposition of the Loch Maree Group.Microprobe analyses of amphiboles record evidence of the original alteration halo associated with the Kerry Road deposit, with a systematic Mg- and Si- enrichment from ferrotschermakite (~150 m) to Mg-hornblende (~90 m) to actinolite (0 m) on approach to the VMS deposit. Furthermore, whole rock geochemistry records a progressive enrichment in Si, Cu, Co, and S, and depletion in Al, Ti, V, Cr, Y and Zr with proximity to the VMS system. These elemental trends, together with amphibole geochemistry, are potentially useful exploration vectors to VMS mineralization in the Loch Maree Group, and in similar highly deformed and metamorphosed terranes elsewhere.

Structural characteristics and Re–Os dating of quartz-pyrite veins in the Lewisian Gneiss Complex, NW Scotland: Evidence of an Early Paleoproterozoic hydrothermal regime during terrane amalgamation

In the Archaean basement rocks of the Assynt and Gruinard terranes of the mainland Lewisian Complex in NW Scotland, a regional suite of quartz-pyrite veins cross-cut regional Palaeoproterozoic (Badcallian, ca. 2700Ma; Inverian, ca. 2480Ma) fabrics and associated Scourie dykes. The quartz veins are overprinted by amphibolite-greenschist facies Laxfordian deformation fabrics (ca. 1760Ma) and later brittle faults. The hydrothermal mineral veins comprise a multimodal system of tensile/hybrid hydraulic fractures which are inferred to have formed during a regional phase of NW-SE extension. The almost orthogonal orientation of the quartz veins (NE-SW) to the Scourie dykes (NW-SE) are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events. This hypothesis is supported by Rhenium–Osmium dating of pyrite that yields an age of 2249±77Ma, placing the vein-hosted mineralisation event after the oldest published dates for the Scourie Dykes (2420Ma), but before the youngest ages (1990Ma). Sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle. The presence of the ca. 2250Ma quartz-pyrite veins in both the Assynt and Gruinard terranes confirms that these crustal units were amalgamated during or prior to Inverian deformation. The absence of the veins in the Rhiconich Terrane is consistent with the suggestion that it was not finally amalgamated to the Assynt Terrane until the Laxfordian.

Structure and evolution of the Lewisian Gairloch shear zone: variable movement directions in a strike-slip regime

Synopsis The Gairloch shear zone (GSZ) has long been regarded as a dextral strike-slip shear zone with a northwesterly plunging shear direction. However, the original strain analysis applies only to the northeastern third of the zone and the structure of the remaining two-thirds is considerably more complex. The 6.5 km wide GSZ was formed during late Laxfordian (D3) deformation at c . 1.7 Ga. It was a product of the interaction of D3 folding and dextral strike-slip shear under retrogressive greenschist-facies metamorphism on rocks previously intensely deformed by early Laxfordian (D2) deformation in amphibolite-facies. The GSZ is divided into four units across strike: the Northeastern shear zone with NW-plunging D3 linear fabric; the Flowerdale belt with SE-plunging D2 linear fabric and low D3 strain; the Ard belt with steeply plunging F3 folds; and the Southwestern shear zone, with SE-plunging D3 lineation. Thus the GSZ consists of two high-strain D3 shear zones on either side of a central low-strain belt. Although consistently dextral strike-slip throughout, the D3 shear regime varies from NE-up in the Northeastern shear zone through sub-horizontal strike-slip in the Ard belt to SW-up in the Southwestern shear zone. This variation may have been caused by local uplift or depression induced by variations in geometry of the walls of the strike-slip system. A revised very approximate minimum estimate of the horizontal component of dextral shear is c . 9.5 km with a net NE-up vertical component of c . 5.5 km. Similarities in structure between the GSZ and the South Harris belt strengthen the case for these two zones to have been part of the same structure.

Recording and analyzing geospatially accurate structural data through ‘digital mapping’ technique: A case study from the Canisp Shear Zone, NW Scotland

Spatial accuracy of structural data is an important factor for construction of a properly scaled model of crustal structures. Digital Geological Mapping methods can add the required spatial accuracy to the data, and also can improve the versatility of the geological/structural map in many ways, which are difficult to achieve through conventional mapping techniques. The present paper describes the methodology of digital mapping and discusses its applicability in structural analyses in the Canisp Shear Zone (CSZ) — a reactivated continental basement structure in the Precambrian Lewisian Gneiss complex of NW Scottish Highland. The CSZ is reinterpreted as a major dextral transpressional shear zone developed during Laxfordian deformation. It overprinted an earlier Inverian shear zone fabric, and was later reactivated and/or overprinted by brittle shearing of different phases and different scales. Spatial analyses within the ArcGISTM environment has helped bring out the geological relationships between different types of structural data in the shear zone, highlighting the partitioning of deformation into zones of high and low strain. A kinematic interpretation based on the geospatial data analysis, combined with conventional stereographic projection technique, is presented.

A major high-strain zone in the Lewisian Complex in the Loch Torridon area, NW Scotland: insights into deep crustal deformation

Abstract The Lewisian Complex is an Archaean-Proterozoic high-grade gneiss region with widespread amphibolite-facies fabrics. These fabrics have, on the 100 km scale, a subhorizontal enveloping surface and are interpreted as forming in gently dipping crustal-scale shear zones and steeper lateral ramp structures. The Scourie dyke swarm, of Proterozoic age, runs NW-SE subvertically outside such zones, but is transformed into subhorizontal concordant amphibolite sheets within them. The terms ‘Scourian’ and ‘Laxfordian’ are used to describe pre-dyke and post-dyke fabrics and events. In the southern mainland Lewisian at Loch Torridon, unmodified Scourian gneisses in the north pass southwards into lozenges bounded by anastomosing zones of Laxfordian deformation, and eventually to a region consisting entirely of Laxfordian fabrics. This geometry is compatible with a major Laxfordian shear zone that dips northeastwards beneath a Scourian hanging wall. Detailed maps show that structures on the metre to kilometre scale do not relate in a simple way to the crustal shear zone model for four reasons. First, many amphibolite-facies fabrics in the quartzo-feldspathic gneisses predate the dykes (they relate to a late Scourian or ‘Inverian’ event), so the dykes cannot be used to infer overall geometry and movement sense. A few structures along the Loch Roag Line, the most northeasterly high-strain belt in the Loch Torridon inliers, may indicate syntectonic dyke emplacement. Second, local shear with opposing senses is present in both the pre-dyke and post-dyke (Laxfordian) deformation. Third, foliation deflections on a variety of scales relate more to varying shear planes than to the deflection caused by increasing simple shear strain, and cannot be used to infer shear sense. Fourth, there is pure-shear stretching within the gneisses that masks the effects of simple shear. These structures can be incorporated into a crustal-scale shear zone model, but are not diagnostic of it. The scatter of foliation planes with a relatively constant lineation is a feature of other crustal-scale high-strain zones (e.g. Canisp, Nordre Strømfjord), although different explanations are available. Other high-strain zones (e.g. the Laxford Front) show a spatially varying lineation pattern, but this could relate more to a varying shear direction than to deflections of linear features into a single movement direction. The geometry of classic simple shear is not versatile enough to explain the kilometre-scale geometry of such high-strain zone, although it may work well in local subareas.

The Lewisian terrane model: a review

Synopsis The Lewisian terrane model of Friend and Kinny divides the Lewisian complex into nine separate terranes believed to have amalgamated during the Palaeoproterozoic. This paper analyses the rationale for the terrane model and suggests criteria to evaluate and refine it. Similarities and differences in Palaeoproterozoic structural, metamorphic and igneous features are used to divide the complex into 14 separate blocks of three types: type A (upper-plate) characterized by retrogressed high-pressure granulite-facies Archaean metamorphic rocks and localized Laxfordian deformation; type B (lower plate) characterized by amphibolite-facies Archaean metamorphic rocks originating at higher crustal levels, intense Laxfordian deformation, and swarms of Laxfordian granites and pegmatites; and third, Palaeoproterozoic complexes comprising oceanic and volcanic arc elements. The boundaries between the type A and type B blocks, where seen, are major shear zones believed to represent a deformed and disrupted collisional suture between two large continental plates, the central Greenland craton (CGC) to the NE and the North Atlantic craton (NAC) to the SW. The following sequence of events is proposed. The Palaeoproterozoic complexes were accreted to the upper plate (NAC) at c. 1.9 Ga. This was followed by collision with the CGC at c. 1.87 Ga, causing the intense early Laxfordian deformation and high-grade metamorphism. At c. 1.74 Ga a second collision took place with the 1.80 Ga ‘Malin’ volcanic arc to the south, resulting in reactivation and late Laxfordian deformation, producing NW-trending folds, dextral shear zones, granite sheets and pegmatites. These late Laxfordian movements are considered to have resulted in considerable disruption of the early Laxfordian terranes. They include substantial strike-slip movements that may have led to the creation of further displaced terranes, some of which may prove to be exotic.

Chapter 6 Laxfordian Structure and Metamorphism

The key to the Lewisian structural chronology in Gairloch, as elsewhere in the Lewisian complex, is the Scourie dyke swarm, which, following Peach et al. (1907) and Sutton &amp; Watson (1951), has been used to divide the complex into pre-dyke and post-dyke elements; the post-dyke structures and events are referred to the Laxfordian. However, as explained in Chapter 3, none of the dykes attributed to the Scourie dyke swarm in the Gairloch area, or in the Southern Region generally, have been dated (except for K-Ar dates interpreted as metamorphic cooling ages). Therefore they may correlate with either the 2.4 Ga or 2.0 Ga dykes of the Central Region (see Section 1.2.6), or could represent a third swarm of some other age. It is also possible that dykes of two or more different ages are represented. While the later Laxfordian structural and metamorphic elements can be correlated throughout the Lewisian complex, some of the earlier post-dyke structures assigned here to the Laxfordian may not correspond to similar structures elsewhere in the complex if the dykes themselves are not of the same age (see discussion on chronology in Section 1.3). Structures are also assigned to the Laxfordian if they affect the c. 2.0 Ga rocks of the Loch Maree Group (LMG). Although the Archaean gneisses share in the Laxfordian deformation, the following structural analysis depends critically on the evidence from the dykes and the LMG. Laxfordian deformation in the Southern Region of the mainland Lewisian is distributed through a wide belt over

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland (Published in Scottish Journal of Geology, 35(1), 1999, pp. 31–50)

R. G. Park and J. Tarney write: Droop et al . (1999) make a welcome addition to the Lewisian literature in providing the first rigorous published estimates of FT conditions at the Laxfordian metamorphic peak in the Loch Maree Group (530–630°C, 6.5 kbar, using various methods including calcite-dolomite and garnet-biotite geothermometers and garnet-plagioclase geobarometer). In their discussion of the implications of the relatively high pressure-temperature estimates, they note an apparent contradiction with the tectonic model for the early Laxfordian deformation presented by Coward and Park (1987) and Park et al . (1987). The 1987 tectonic model for the Gairloch area resulted from the recognition that the strong early Laxfordian deformation was associated with a WNW-ESE movement direction implying oblique slip on gently inclined planes, with both strike-slip and dip-slip components. The reason for choosing the dextral/normal-slip option for these movements then was because of the observation that certain of the more prominent shear zones exhibited dextral, NE-down movement-sense criteria. However subsequent more detailed work on the Gairloch shear zones (Lei and Park 1993) showed that the movement history on the shear zones was more complex than first thought, that both sinistral/normal and dextral/reverse movement criteria existed for the early shear zones, but that the main early movements involved overthrusting to WNW, i.e. compressional, in agreement with Droop et al .’s conclusions. Droop et al .’s estimates of the metamorphic conditions provide timely additional evidence in support of new work by ourselves, together with J.N. Connelly, recently accepted for publication (Park et al . in press), . . .

Linkage between mantle and crustal structures and its bearing on inherited structures in northwestern Scotland

Deep seismic reflection profiles in Scotland reveal mantle structures beneath a crust with a polyphase tectonic history that resulted in several generations of structures. Continuum mechanics suggests that coeval mantle and crustal structures must be kinematically linked. Inherited structures imply relative ages for the reflectors, ages that can be placed into the context of the geological history of the near-surface rocks of northern Scotland. Thus, some mantle reflectors are assigned Triassic ages related to the opening of the West Orkney and related marginal basins of the Atlantic Ocean. Other mantle reflectors are cut by late Caledonian structures associated with the Great Glen Fault Zone and therefore older than c. 400 Ma. Many of these structures also track the late Precambrian margin of Laurentia and may be related to either the opening (900–600 Ma) or closing (500–600 Ma) of the Iapetus Ocean. Some reflective structures may also be attributed to 1800–1700 Ma Laxfordian deformation that was part of a global-scale orogenic belt.

Laxfordian structure, strain distribution and kinematic interpretation of the Kenmore Inlier, Loch Torridon: anatomy of a major Lewisian shear zone

The Kenmore inlier exposes the most intensely deformed part of a major Laxfordian shear zone whose NE margin occurs at Diabaig, N of Loch Torridon. The Laxfordian (post-Scourie dyke) deformation commences with two very intense deformations D1 and D2, producing LS fabrics in dykes and gneisses associated with heterogeneous, locally very large, and typically prolate strains under amphibolite-facies metamorphism. D2 also produced widespread mesoscopic folds. The finite strain was measured at 135 localities using grain-aggregate shapes in Scourie dykes. The combined D1-D2 elongation lineation plunges gently to NW or SE, and the planar fabric appears to have been sub-horizontal before the D3 deformation, which produced NW-SE upright folds under retrogressive metamorphism.The D1-D2 structure is attributed to a major sub-horizontal shear zone with a top-to-NW movement, a minimum vertical width of 2·5 km and estimated minimum displacement of c. 19 km. The upper (NE) wall of the shear zone at Diabaig exhibits dextral-normal (NE-down) D1 movements. The D2 structures are attributed to superimposition of sub-horizontal movements on earlier inclined D1 fabrics as the shear zone evolves. The D3 refolding reflects a change to a dextral transpressive regime.

The evaluation of Laxfordian deformation in the carloway area, Isle of Lewis, Scotland

The paper outlines an investigation of methods of estimating variations in strain suffered during the tectonic evolution of a high-grade gneiss complex. A map has been produced representing the Laxfordian deformation shown by the Lewisian rocks of the Carloway area in Northwest Lewis. The map shows that modifications of the pre-Laxfordian Scourie Dykes — their loss of discordance, the nature of their contacts and their folding and boudinage — are paralleled by a series of modifications in the host rocks, “the grey gneisses”. Strain contours have been derived and tend to be oriented parallel to the strike of the rocks. The boundaries between high and low strain areas are transitional. One of the main values of such an analysis is that it aids the reconstruction of the pre-Laxfordian structural pattern in the area.

Scourian-Laxfordian relationships in the Barra Isles

The Lewisian rocks of Barra are a Laxfordian supra-structure of amphibolite facies gneisses and a Scourian infra-structure characterised by orthopyroxene bearing gneisses. Both units contain representatives of the Scourie Dyke suite; in the supra-structure they are highly deformed and folded; in the infra-structure they are relatively undeformed, unfolded, and retain original discordant relationships. The infra-structure also has several suites of intrusive igneous rocks earlier than the Scourie dyke suite, the most widespread being dykes of dioritic composition. The same sequence of Laxfordian deformation may be recognised in both units. Folding occurred in the supra-structure under amphibolite facies conditions, which continued after deformation ceased. In the infra-structure, evidence for a pre-Scourie dyke pyroxene granulite facies metamorphism is preserved. The Scourie dykes of the infra-structure have pyroxene granulite facies mineral assemblages resulting from intrusion into hot or dry country rock gneisses. Probably a very old cover to basement relationship is preserved in the area, which was first deformed in the Scourian orogeny, and then subsequent Laxfordian deformation of the interface between the two major units produced overall structure of the area.

DISCUSSION OF BOTH PAPERS BY FRANCIS COWARD

D R . R. G. P ARK congratulated both authors on their interesting accounts of the very complex geology of the southern Outer Hebrides. Lacking first-hand knowledge of that area, he was tempted, despite his sceptical approach to structure correlation in general, to try to match the structural sequence proposed by the authors with the sequence obtained by the speaker and others in their recent work on the southern Laxfordian belt of the Scottish mainland, but found considerable difficulty in doing so. A superficial similarity existed between the f 3 folds of the authors and the large-scale second Laxfordian folds on the mainland with a NW–SE trend and steep axial planes. The distribution of these structures showed a marked heterogeneity of deformation with narrow intensely deformed synformal belts (e.g. the Gairloch schist belt) separated by wide, relatively little-deformed zones. A characteristic feature of the Scourie dyke margins in the highly deformed zones was the presence of tight small-scale folds with relatively sharp antiformal and synformal hinges suggesting fairly ductile behaviour of dykes and gneiss, whereas no small-scale folding of dyke margins has been observed in the antiformal zones. In contrast, during the earlier Laxfordian deformation, small-scale folding of dyke margins is apparently absent throughout the region investigated and the dykes have developed a foliation only which is absent from the enclosing gneisses. In the light of these observations, could Dr. Coward comment on the role of small-scale f 1 and f 2 folding of the dyke margins in his area and on the physical conditions

The structure and origin of areas of anomalously low-intensity finite deformation in the basement gneiss complex of the outer hebrides

Several areas of anomalously low-intensity finite Laxfordian deformation occur in the southern Hebrides. Within these areas the Scourie-dykes are often still markedly discordant. Four phases of deformation have been recognized. In each phase, deformation was heterogeneous; in some phases almost the total deformation took place within narrow elongate zones of intense deformation. The variation in dihedral angle between dyke and gneissose foliation round Laxfordian folds, gives an idea of some of the mechanisms of deformation, and variations in mineral and lithological fabrics have been used to formulate an early Laxfordian strain path. The dominant linear fabric in these areas may record a finite strain resulting from the superimposition of two plane strains. The ductility contrast between dykes and gneiss varies and is related to syn-tectonic metamorphic phase changes. The areas of low-intensity deformation and the heterogeneous deformation within these areas may have resulted from a syn-tectonic change in ductility of the rock, wherein any original strain heterogeneities would be exaggerated by later deformation.

The Eastern Gneisses of South Uist

Synopsis The Eastern Gneisses of South Uist which overlie the Outer Isles Thrust consist of three units; the Corodale Gneiss mass which is interpreted as an igneous body; the Mashed Gneiss; a group of quartzo-feldspathic gneisses and an intervening zone of phyllonites, the Usinish Mylonites. The phyllonites appear to have been formed by intense deformation at the margin of the igneous body. Three phases of deformation have affected the Corodale Gneiss and are tentatively correlated with Laxfordian deformation phases affecting the rocks below the thrust. The igneous body is cut by dykes and is considered to be a member of a widespread group of basic to intermediate intrusions which post-date the Scourian gneiss-forming events, but pre-date the emplacement of Scourie dykes. There is evidence for an eastward increase in metamorphic grade during an early Laxfordian episode, prior to the main deformation, and an eastward decrease in metamorphic grade during the three main Laxfordian deformation-phases.

Large-scale Laxfordian structures of the outer hebrides in relation to those of the Scottish mainland

The deformation which took place during Laxfordian regeneration of an older gneiss complex within the Lewisian of the Outer Hebrides was strikingly inhomogeneous on all scales. Laxfordian deformation occurred in three, and locally four phases, the second and third (F 2 and F 3) being the most important. The large-scale structure of the southern islands is essentially a product of F 1 deformation. The folds of this phase have a cuspate form, the antiforms being typically broad open structures devoid of evidence of significant F 3 strain, while the complementary synforms are typically tight, highly deformed structures. The cuspate form of the major structures suggests that they may have resulted from the deformation of an interface between media of differing viscosities. If this is so, the more viscous medium may be represented by relatively dense and dry rocks exposed at low structural levels near the eastern side of the southern islands; while the less viscous medium would be the acid gneisses and associated metasedimentary relics which occupy the western side. Broad structural and petrological comparisons between the southern Hebrides and parts of the Scottish mainland suggest the existence of a plunge culmination and an area of possibly pre-Scourian basement beneath the younger sediments of The Minch. Remnants of a Scourian cover series may be incorporated in the portions of the gneiss complex which flank the culmination.

Discussion

Dr. J. V. W atson commented on the author's evidence that the rocks of adjacent regions had behaved differently in response to Laxfordian regeneration. Dr Park had confirmed Clough's view that the south-western limb of the Tollie antiform had been much more severely deformed than the north-eastern limb during the formation of the antiform. He had found that Laxfordian folds and fabrics of an earlier generation were also poorly developed in parts of the north-eastern limb, where Scourie dykes were not only discordant but also retained remnants of igneous minerals and textures. It therefore seemed that a contrast had been established at an early stage in Laxfordian regeneration between the north-eastern block, bordering Loch Maree, which was destined to remain relatively unaffected through successive episodes and the remainder of the region which was destined to be repeatedly deformed. Such a differentiation of active and inactive zones, initiated at an early stage and maintained through several phases of reworking, seemed to be rather characteristic of the Lewisian complex as a whole. T he A uthor , in reply to Dr Watson, agreed that the north-eastern and south-western halves of the Tollie area had behaved differently during the Laxfordian deformation and metamorphism, and that these differences had been initiated at an early stage. Although the pre-dyke history of the south-western limb was difficult to unravel, the differences between the central part of the area (sub-area 1) and the north-eastern part (sub-area 4) were probably initiated even before the intrusion of the dykes. Dr R. S t J.

Post-Scourian metadolerites in relation to Laxfordian deformation in Great Bernera, Outer Hebrides

Synopsis In Great Bernera, Lewis, many small basic bodies intruded after the formation of the Scourian gneiss complex were deformed and metamorphosed as a result of Laxfordian orogenic activity. These bodies, now represented by meta-dolerites, appear as contorted bands and lenticles whose outcrop-pattern is determined by the effects of two principal phases of folding. An attempt is made to deduce the initial forms of the intrusions and it is suggested that they were emplaced as members of a dyke-swarm trending roughly east and west. The structures produced by Laxfordian deformation of the metadolerites are compared with those exhibited by deformed metadolerites of the same generation in the Loch Laxford region of the Scottish mainland, and it is suggested that the apparent contrast may have a bearing on the orientation of the direction of Laxfordian compression.

Metamorphosed and deformed pegmatites and basic dykes in the Lewisian complex of the Outer Hebrides and their geological significance

The central zone of the Outer Hebrides is composed of a gneissic (‘grey-gneiss’) complex containing evidence of the earliest as well as many of the later events in the history of the Lewisian. In northern South Uist, an old metamorphic complex is intruded by pegmatites which were strongly deformed by a tectogenesis equated here with the Scourian of more than 2600 m.y. ago. It is concluded that much of the original material of the greygneiss may represent the products of a pre-Scourian orogenic cycle of possible Katarchaean (&gt;3000 m.y.) age. Widespread post-Scourian basic (tholeiitic) dykes which cut the grey gneiss are thought to be equivalent to the Scourie dykes of the Scottish Mainland. These intrusions were subjected to granulite-facies metamorphism, regarded as Early Laxfordian, followed by retrogressive amphibolite-facies metamorphism with local folding and migmatization, all of Late Laxfordian date (1500 to 1600 m.y.). A number of dykes described from Benbecula and South Uist illustrate stages of Laxfordian deformation in which perfectly preserved cross-cutting relationships are progressively obliterated, the final products in the southern zone of South Uist being highly deformed and disrupted amphibolite masses with concordant margins in mobilized Laxfordian migmatitic gneiss.