Lewisian Rocks Research Articles

Origin of vein-graphite derived from metamorphic fluids in Moine (Glenfinnan Group) rocks, NW Scotland

Synopsis Proterozoic metasediments at Glen Strathfarrar, Inverness-shire, host a deposit of vein-graphite that is unique within the Moinian stratigraphic sequence. Carbon isotopic analysis, Raman microspectroscopy, fluid inclusion studies and volatile gas analysis were used to constrain the origin of the vein-graphite. Carbon isotopic analysis shows that the carbon is heavier than that expected for sedimentary material (δ 13 C value of −14.35‰), suggesting that fractionation has taken place since sedimentation. Graphitization of sedimentary carbon in the Glenfinnan Group pelitic gneiss occurred in response to changes in both temperature and pressure arising from a series of regional metamorphic events. Scavenging of carbon by metamorphic fluids generated during amphibolite-grade metamorphism associated with the youngest, possibly Caledonian, event is the most probable source of the vein-graphite at Glen Strathfarrar. Similarities in gas composition with quartz veins occurring in Dalradian rocks show that carbonaceous sedimentary sequences consistently generate fluids rich in CO 2 and N 2 , making volatile gas analysis a valuable tool in determining the history of metamorphic fluids. The occurrence of crystalline graphite in the pelitic gneiss at Glen Strathfarrar makes it highly likely that the carbon-rich fluids have been generated from within the Glenfinnan Group sediments rather than from Lewisian rocks at depth. Despite the assertion that graphite is uncommon in Moine rocks, the high ammonium content of biotite in these metasediments and the consistently high N 2 values of fluid inclusions within the siliceous schist and quartz veins suggest that, prior to metamorphism, the sediments contained more organic matter than their present composition suggests. Supplementary material: The results of incremental crush analysis of vein samples from localities within the Moine, Dalradian and Old Red Sandstone are available at http://www.geolsoc.org.uk/SUP18499 .

Mike Coward's early days: the Lewisian, and what he made of it

Abstract Mike Coward began his career working on Lewisian rocks and readdressed the problems they pose several times during his life. His ability to extrapolate ideas from one area of geology to another and to straddle geological time scale enabled him to reappraise the regional Lewisian structure and evolution in the light of ideas evolving elsewhere, whether that was in his own head or in the geology of the wider world. It was done through an understanding of theoretical structural geology and small-scale example, yet at the same time it extrapolated to large scale and was significant not only in terms of the Lewisian and the Precambrian in general, but also in understanding the way in which the Earth's crust deforms. Coward's work rarely stayed within conventional bounds; and when it did, it pushed the limits. Our understanding of the world progresses through that sort of approach. It is seen too rarely nowadays.

The Use of the Common Pb Isotope Composition of Detrital K-Feldspar Grains as a Provenance Tool and Its Application to Upper Carboniferous Paleodrainage, Northern England

K-feldspar is a common detrital mineral in sandstones. It contains little U or Th, and hence the common Pb isotope composition of unaltered K-feldspar is that of the source rock. Pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the Pb signature of detrital K-feldspar grains a useful provenance tool. However, it is unclear how robust this signal is, and to what extent it can be modified by weathering, transport, burial diagenesis, and/or exposure to hydrothermal fluids. This study reports an evaluation of the technique, using two granite–arkose pairs (from Helmsdale, northern Scotland, and Shap, northwest England) where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived. The results of this test have informed a pilot study using the Pb isotope composition of detrital K-feldspar in Upper Carboniferous fluvial sedimentary rocks of northern England to constrain the sediment routing. Pb isotope compositions have been measured using an in situ high-resolution single-grain technique that can distinguish original composition, heterogeneity, and subsequent alteration within individual sand grains. The results for the granite–arkose pairs show that the common Pb isotope signature of the detrital K-feldspar grains matches the in situ feldspars in the source granites. The signal is independent of grain size (medium-grained sand to granules), but alteration along a paleo-regolith at Helmsdale has resulted in perturbations in Pb composition and an increase in the errors on the analyses. The application of the technique to arkosic Carboniferous (Serpukhovian–Bashkirian) Millstone Grit Group sandstones in the Pennine Basin reveals two discrete populations of K-feldspars that cannot be distinguished petrographically. Matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far-traveled (> 500 km) source of K-feldspar from the Archean–Paleoproterozoic Lewisian rocks of northwest Scotland or their extension to the north, but also an equally important proximal source emanating from the Southern Uplands Belt of Scotland in what would have been the lower part of the hinterland drainage basin of the Millstone Grit.

The geology of the UK Hatton-Rockall margin

Seismic, potential field, sea-bed sample and shallow drilling data have been used to investigate the geology of the UK part of the Rockall Plateau. Most of the Rockall Plateau is underlain by Early Proterozoic Rockall Bank/Islay terrane metamorphic basement although George Bligh Bank and the north-eastern end of Hatton Bank may be underlain, at depth, by Lewisian rocks. The basement crops out on Rockall Bank but occurs at a much deeper level beneath Hatton Bank and Hatton Basin. The widespread Palaeogene lavas of Rockall Plateau have been sourced from large central igneous complexes (Rockall, Swithin, Sandastre, Mammal, Lyonesse and Sandarro), local volcanoes and fissures. Some of the lavas may also have been derived from the incipient rift axis west of Hatton Bank at the time of continental break-up. Differences in seismic facies allow the extent of the lavas from the large centres to be mapped. Where the lavas are absent beneath parts of Hatton Bank, either due to non-emplacement or subsequent erosion during the Cenozoic, seismic data image the underlying Mesozoic (and ?older) rocks. Here BGS boreholes 99/1 and 99/2A have proved mid-Cretaceous (Albian) shales and sandstones respectively beneath a thin Cenozoic cover. The Mesozoic (and ?older) rocks appear to be quite extensive and are folded and faulted. Mesozoic strata are not observed on Rockall Bank in UK waters. The post-lava Cenozoic succession of Rockall Plateau can be sub-divided into three megasequences separated by two unconformities which, by analogy with Rockall Basin, have been designated ?C30 (intra Late Eocene) and ?C10 (intra Early Pliocene). The ages can be corroborated by DSDP/ODP boreholes in the Hatton Basin. The Cenozoic succession contains Eocene prograding fan sequences, contourite deposits on the flanks of the banks and shallow faulting, some of which affects the present day sea bed. Structurally, Rockall High appears mainly as a large basement block whereas Hatton High is more complex exhibiting half-grabens, inversion structures and reverse fault geometries due to Cenozoic tectonism. The northern end of Hatton High is a large anticline with its axis aligned ENE-WSW. The structural differences between the highs may be due to their locations with Rockall High exhibiting fewer tectonic effects due to its more landward, intraplate setting further away from plate-margin deformation.

Park, R.G. The Lewisian Geology of Gairloch, NW Scotland. Geological Society Memoir 26. Geological Society, London. 2003. 88 pp. Price £50 (US $83); Members (£42); ISBN 1-86239-116-5 (Paperback).

This is an account of a small part of the Precambrian Lewisian Complex around Gairloch, NW Scotland. It deals only with events between ~3 and 1.2 Ga. It is accompanied by a very clear and most useful coloured geological map, at a scale of 1:20,000 which extends about 13 km in a NE–SW direction, but does not show any Phanerozoic intrusions, such as Tertiary dykes, nor does it contain a cross-section. The text is well illustrated with 57 diagrams and magnificent field photographs of a very complex series of rocks. It is a summary of many years work in the field and laboratory by the author and brings together and updates much information scattered across the literature over the last 40 years. Since there has been no comprehensive summary of the Lewisian rocks since the much venerated 1907 Peach et al. NW Highland Geological Survey memoir, the present …

A regional geochemical assessment of some terrane relationships in the British Caledonides

Synopsis Regional geochemical data have been systematically collected across the British Caledonides and interpreted, using single and multielement image analysis, to provide geochemical constraints on terrane relationships. In the Hebridean Terrane, the Lewisian rocks show the characteristics of a large ion lithophile element (LILE) depleted lower crustal complex and seem unlikely to have been the source for the bulk of the overlying Torridonian clastic rocks. The psammite-dominated Moine Supergroup and the Dalradian rocks of the Grampian Group have closely similar evolved chemistries consistent with derivation from a higher crustal level than that of the currently exposed Lewisian complex. Across the Scottish Highland terranes the principal geochemical discontinuities coincide with the Moine Thrust Zone, the boundary between the Grampian Group and the Appin and Argyll Group Dalradian rocks, and the Highland Boundary Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary. South of the Southern Upland Fault, which also coincides with a major geochemical discontinuity, a range of elements indicates division of the Southern Uplands terrane into northern and southern zones. These are transitional between the Orlock Bridge and Moffat Valley faults and reflect a change in sedimentary provenance during the early Silurian. Basic and ultrabasic elements, especially Cr, show exceptionally high levels over early Llandovery strata possibly indicating derivation from Iapetus Ocean crust obducted further NE as the Scandian Orogen was initiated. The geochemical characteristics of the southern part of the Southern Uplands Terrane continue into the Windermere Supergroup of the English Lake District, emphasising the continuity of mid- to late-Silurian deposition across the Laurentia–Avalonia (Iapetus) suture zone.

A geochemical investigation of palaeosols developed on Lewisian rocks beneath the Torridonian Applecross Formation, NW Scotland

Synopsis Alteration zones are developed on biotite gneiss and amphibolite of the Lewisian Basement Complex beneath Neoproterozoic rocks of the Applecross Formation of the Torridon Group in northwestern Scotland. The age and origin of these altered rock s has been disputed but systematic sampling and chemical analyses reveal patterns in A1 2 O 3 − CaO + Na 2 O − K 2 O space that are identical to weathering trends in palaeosols elsewhere. The Lewisian palaeosols, like other ancient weathering profiles, display significant loss of Ca and Na and exhibit strong K-metasomatism. K/Rb ratios in the profiles exhibit systematic variations that are thought to be related in part to the nature of metasomatizing fluids, rather than reflecting their provenance. Chondrite-normalized REE patterns for the host rocks and palaeosols demonstrate the conservative nature of these elements. There are, however, some differences between REE patterns obtained from the fresh and altered basement rocks and those of mudstones of the Applecross Formation. These differences suggest that the provenance area for the Applecross sediments included rock types other than those of the immediately underlying Lewisian basement.

The influence of country rock structural architecture during pluton emplacement: the Loch Loyal syenites, Scotland

The late Caledonian Loch Loyal syenites comprise three related, but geographically separate, composite intrusions: the Ben Loyal, Ben Stumanadh and Cnoc nan Cuilean bodies. They were emplaced (c. 426 Ma) into Moine and Lewisian rocks of the Moine and Naver thrust sheets previously deformed by Caledonian NW-directed folding and thrusting at amphibolite facies metamorphic conditions. The intrusions lie mainly within a zone of large-scale cross-folds in which the regional foliation trends NW-SE at high angles to regional orogenic strike (NNE-SSW). The cross folds are underlain by a broad, ESE-dipping zone of high strain known as the Ben Blandy shear zone. The NW basal contact of the Ben Loyal body dips generally SE and strikes sub-parallel to the foliation in the underlying Ben Blandy shear zone. The higher parts of this syenite, together with those of the Ben Stumanadh and Cnoc nan Cuilean intrusions, form a series of variably coalesced dykes trending NW-SE sub-parallel to the foliation in the pre-existing zone of cross-folds. There is little evidence for significant country-rock deformation during emplacement of the syenites. Minor components of top-to-the-SE extension occurred along the NW boundary of the Ben Loyal body, whilst minor dextral shearing is associated with the emplacement of the NW-SE, vertical sheet-like units that are thought to form the SE part of this intrusion and the other Loch Loyal syenites. Generally contact-parallel magma tic-state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure-driven, internal deformation of the igneous bodies during and immediately after emplacement. The geometry of these fabrics and the Loch Loyal syenites as a whole has been directly controlled by the pre-existing structural architecture of the surrounding country rocks. This appears to be a previously overlooked feature of many plutons emplaced at mid- to upper crustal depths below the region influenced by the free surface. The siting and emplacement of the Loch Loyal syenites may have been initially facilitated by late Caledonian gravity-driven extension of the Moine Thrust sheets. Following the initial phase of intrusion, it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks. There is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle. The siting and ascent of the syenites at depth may be linked to the presence of steeply-dipping structures in the Lewisian autochthon buried beneath the Moine Nappe.

The seismic reflection Moho beneath the United Kingdom and adjacent areas

Onshore and offshore deep seismic reflection data have been integrated to produce two-way travel-time and depth maps of the seismic reflection Moho beneath the UK. Moho travel-times are mostly in the range 9.5–11.5 s, showing only weak correlation with specific upper crustal structural features, but decreasing generally northwestward towards the Atlantic margin. Moho depths show a much stronger correlation with upper crustal structure than do travel-times. There is pronounced crustal thinning (Moho depths <25 km) beneath the central and northern North Sea grabens and the basins of the northwest margin. Less marked crustal thinning is evident beneath the basins of the Irish Sea, English Channel and southern North Sea. Greatest Moho depths (>32 km) are found generally beneath basement massifs, particularly onshore. Short-wavelength topographic features on the Moho are quite rare, and are generally restricted to ancient zones of continental collision and/or subduction. Some of these ancient collision and/or subduction zones are spatially associated with a highly reflective Moho. This is particularly evident where low-angle shear-zones, in the lower crust or uppermost mantle, converge with or intersect the Moho. It is suggested that development of a Moho detachment surface can tectonically smooth the Moho and thereby markedly increase its reflectivity. A tendency in certain areas for the Moho to remain at roughly constant two-way travel-time (though not constant depth) beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium. Conversely, travel-time perturbations beneath smaller features (∼10 km across), indicate that these are at least partially uncompensated. The UK crust is therefore weak, but not sufficiently weak to attain local Airy isostasy. Basement massifs west of Scotland show systematically smaller Moho two-way travel-times than similar structures farther south. Within the constraints of regional isostasy, this can be interpreted in two main ways. Greater rigidity and strength of Lewisian rocks within the west Scottish massifs (manifest as higher seismic velocity relative to density) could be a cause of the observed travel-time differences. Alternatively, low-density material beneath the crust could buoy up the western Scottish massifs. If this buoyancy were the consequence of igneous underplating, then any underplated material would have to lie beneath the seismic reflection Moho.

Tourmaline in a sub-Cambrian palaeosol on the Proterozoic Lewisian rocks of NW Scotland

Tourmaline, not hitherto recorded in Lewisian rocks of Sutherland, nor reported as a secondary mineral in Precambrian palaeosols, occurs in a late Proterozoic, sub-Cambrian palaeosol in NW Scotland. From field and petrological criteria, the tourmaline is secondary and peculiarly associated with the alteration profile. Hence, it formed either during weathering or later in the profile’s history. A minor constituent of the palaeoregolith immediately below the unconformity, the tourmaline mostly occurs as sub-millimetre, radiating, acicular aggregates and is schorl in composition. Black, tourmaline+quartz veinlets cross-cut the profile. Stable isotope and fluid inclusion data suggest that the tourmaline formed from surface-derived fluids at c. 110ºC. It is concluded that boron was adsorbed onto precursor illitic clays of the soil profile during the Cambrian marine transgression. Thus the tourmaline formed subsequently after burial under the sedimentary pile, probably during the Caledonian orogeny, when temperatures were high enough to allow its formation at suitable sites of nucleation, e.g. adjacent to biotite. Its peculiarity as a palaeosol phase is due to the combination of a marine transgression over an exposed, permeable illitic soil and underlying regolith, followed by increase in temperature suffcient to allow the tourmaline to form.

Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1–6 m beneath fluvial red conglomerates and sandstones of the Applecross Formation of the early Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K 2 O and Fe 2 O 3 , and contain hematite as an alteration product of biotite and magnetite. Palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration. Thermal step demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a’ Mhurain and Sheigra reveal a stable, high-temperature component with a mean direction of remanence of D= 149.1°, I =57.2° (α 95 = 5.0°) (corrected for dip of the overlying Applecross strata) that gives a pole at 9.8°S, 199.2°E (dp = 5.3°, dm=7.3°). This direction differs significantly from that for little-weathered Lewisian at those two localities and also from directions for the late Mesoproterozoic Stoer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direction (D= 122.3°, I =50.5°, α 95 =10.8°) obtained by us from the thermal step demagnetization of 39 sandstone specimens of the Applecross Formation in the Cape Wrath area. Furthermore, the direction for the weathered rocks agrees closely with other dip-corrected results for the Torridon Group, such as the detrital remanent magnetization (DRM) mean direction (D= 148.3°, I =59.9°, α 95 =13.8°) for the Applecross Formation on Rum and that for the main outcrop of the Aultbea Formation ( D =139.7°, I =52.6°, α 95 = 14.5°). The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the Cape Wrath area acquired their remanence penecontemporaneously with deposition of the Applecross and Aultbea formations. The palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of Applecross age ( c. 980 Ma) that formed at a palaeolatitude of 38° and constitute the oldest known palaeosols in the British Isles—views that the weathering is substantially older or younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.

Late Proterozoic rift faults and basement—cover relationships within the Ben More thrust sheet, NW Scotland

Recently proposed models for the tectonic evolution of Northern Scotland, involving lithospheric extension during the late Precambrian, are supported by new discoveries and interpretations of early normal faults within the Moine Thrust Belt. Such structures control the preser­vation of Torridonian sediments beneath the sub-Cambrian unconform­ity. The Ben More thrust ramp from basement into the Cambrian cover may have been controlled by the early faults. Major synclines within the Ben More thrust sheet appear as tightenedhalf-grabens and thus are composite structures. This may require a re-examination of the field relationships between these folds and the apparently syn-tectonic alkaline intrusions that have been used to date thrust activity. The Ben More thrust sheet of the Moine Thrust Belt contains the most easterly outcrops of Torridonian Supergroup (late Proterozoic) in Scotland (Fig. 1). These sediments and their Lewisian basement are unconformably overlain by the Cambrian quartzites of the Eriboll Sandstone Group. The stratigraphic relationships of these units, with the two un­conformities (Cambrian on Torridonian, Torridonian on Lewisian) discordant so that the Cambrian sediments overstep from Toridonian onto Lewisian basement, have been known for over a century. However, the nature of the overstep and its implications for the Precambrian arrangement of Torridonian and Lewisian rocks, is less established. Soper &amp; Barber (1979) inferred the presence of major upright folds with wavelength in excess of 30 km so that Torridonian sediments lie preserved in synclines while the crests of the anticlines are eroded beneath the sub-Cambrian unconformity. This model for Precambrian basement–cover relationships has

Stochastic characterization and seismic response of upper and middle crustal rocks based on the Lewisian gneiss complex, Scotland

SUMMARY We have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the Lewisian gneiss complex, Scotland. the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques. We digitized two 1:10560 geologic maps of Laxfordian (Archean) age Lewisian rocks on a 26.8m grid. Both maps are believed to be representative of the upper and middle crust in north-western Scotland, and both are believed to provide cross-sectional views of parts of the crust. the digital maps were characterized by the statistics of their lithologic populations and by their 2-D spatial autocorrelation functions. Different lithologies were assigned seismic velocities appropriate for the mid-crust using petrophysical data. Three lithologies are dominant: silicic gneisses (Vp∼6.2 km s−1), mafic amphibolites (Vp— 6.75 kms−1), and intermediate composition schists (Vp—6.5kms−1). Both maps have self-affine spatial fabrics. The first map covers the core of a syncline. Its autocorrelation function defines a medium with a fractal dimension of 2.78, a horizontal characteristic length of about 244m and a vertical correlation of about 133m (aspect ratio is 2:1). It has an essentially trimodal velocity (lithology) population consisting of 37 per cent silicicgneiss, 43 per cent mafic amphibolites, and 20 per cent schists. This map is representative of 30-40 per cent of Laxfordian rocks. the second map is a plan view which can be rotated 90° about an axis perpendicular to strike to give a cross-section. This map is characterized by a fractal dimension of 2.55, with a horizontal correlation length of about 111m and a vertical correlation of about 38m (aspect ratio 3:1). It has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites. It is representative of 60-70 per cent of Laxfordian rocks. Lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2-D finite-difference synthetic seismograms. Short-offset shot records demonstrate that a Lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels. Measurements of transmission scattering Q, and coda decay rates confirm that seismic scattering in Lewisian-type crust is strong. the migrated CMP response of a Lewisian crustal model shows the characteristic ‘salt and pepper’ pattern often observed in the upper crust, and described, incorrectly, as ‘transparent'. We suggest that ‘translucent’ is a more appropriate descriptor.

LISPB revisited: a new look under the Caledonides of northern Britain

SUMMARY The Lithospheric Seismic Profile in Britain (LISPB), shot in 1974, included a 700 km long north-south profile of the crust and upper mantle of northern Britain. The wide-angle seismic data from LISPB was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile. This reinterpretation shows that the original crustal model published by Bamford et al. (1978) satisfies the observed first arrival traveltimes, but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface. The new lithospheric model is compatible with the observed gravity field when converted to appropriate densities, and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium. Profiles of the variation of velocity and density with depth from the new model are compared with measurements of Lewisian rocks made at confining pressures up to 8 kbar, allowing the seismic model to be interpreted geologically. A synthetic normal incidence profile of the new model is superimposed on the nearby NEC deep seismic reflection profile, and the features imaged by the two techniques are compared.

Shear zones at the Laxford front, NW Scotland and their significance in the interpretation of lower crustal structure

The Lewisian rocks in the Scourie-Laxford region, NW Scotland, show variations in shear strains and associated structures on the southern (upper) margin of a zone of intense Laxfordian reworking. The Scourian block was thrust a large distance obliquely northwards over the northern Laxfordian gneisses as part of a Laxfordian episode of crustal thickening. This was followed by crustal extension, during which time the Scourian block moved down to the south. Granite sheets were intruded along the main shear zone in a dilational region at the base of a rotational extensional block. The Laxfordian contractional and extensional deformation occurred under amphibolite facies conditions and analogies can be made with middle crustal structures inferred from deep seismic data. For example, structures produced during late Laxfordian crustal spreading can be compared to those seen on the BIRPS offshore Scottish data, which formed during Devonian basin development following Caledonian crustal thickening. The Scourie–Laxford structures give an idea of fault kinematics at the brittle-ductile transition in the middle to lower crust. Steep faults pass down into more gently dipping ductile shears and then to a zone of anastomosing shears which accommodate the strains and rotations at the base of the rigid upper crust. The shears may concentrate along a more planar zone of deformation, forming a detachment or decoupling zone, below which the extension may be variable, depending on the mode of lithospheric stretching.

Basement controls on sediment-hosted Zn-Pb deposits; a Pb isotope study of Carboniferous mineralization in central Ireland

Lead isotope ratios of galenas from carbonate-hosted Zn-Pb (+ or - Ba + or - Cd + or - Ag) deposits in central Ireland are homogeneous within individual deposits but vary between deposits. The variation correlates directly with geographic position, defining isoplumbs, and not with style of mineralization, age of host rock, or structural or physico-chemical factors. The least radiogenic Pb occurs in the northwest and the most radiogenic in the southeast. This pattern elegantly reflects Irish pre-Caledonian basement geology: Lewisian gneisses in the northwest and late Proterozoic, upper crustal rocks in the southeast. That the northwestern end member of the Irish Pb-mixing trend is ultimately Lewisian is demonstrated by the concordance of the Irish Pb isotope trend with the trends defined in Proterozoic to Tertiary rocks and ores of Scotland which contain Pb of demonstrably Lewisian derivation. Although geologic relations and Pb isotope ratios unambiguously implicate basement in the ore-forming process, we are unable to discriminate between Lewisian gneisses themselves and detritus derived from Lewisian rocks in the Caledonian metamorphic complex as the direct source of Pb.Comparison of the Irish deposits with McArthur River and Mississippi Valley deposits leads us to conclude that style of mineralization (syngenetic, syndiagenetic, epigenetic) is not a fundamental aspect of sediment-hosted Zn-Pb deposits but is a reflection of local stress regimes and physico-chemical conditions. The fundamental link between the types of sediment-hosted Zn-Pb deposits may be the onset of a condition, such as extension or compression, that provides a mechanism for driving fluids. In extensional regimes, seismic pumping processes may channel fluids toward the surface or into fractures or favorable pore spaces, resulting in a wide variety of styles of mineralization. The lack of dilatency in sediments in compressional regimes, however, may inhibit surface exhalation and favors epigenetic styles of mineralization.

The palaeomagnetic record in the Lewisian terrain

Summary Palaeomagnetic investigations of the Lewisian rocks identify a diverse remanence record which varies from zone to zone. The characteristic (A) remanence in the central mainland zone is a strong and stable NW downward-directed and magnetite-held component. This occurs sporadically elsewhere but is largely displaced in the northern mainland zone and in Lewis and Harris by lower blocking temperature/coercivity components with more variable directions; in addition hematite overprints occur sporadically in the central zone and in the Loch Torridon area of the southern mainland zone. Geological relationships and palaeomagnetic field tests suggest that all of these components were acquired during post-Laxfordian uplift and cooling of this terrain but a steep E downward-directed component is a feature of pre-Laxfordian belts in the south and may be considerably older. The collective data define migration of the geomagnetic field axis during uplift and cooling of the terrain and are linked in general terms to mineral ages in the range c. 1720–1500 Ma; further rock magnetic and 39 Ar/ 40 Ar investigations are in progress to characterize the areal significance of the components of remanence and to date them more precisely.

Physical properties of Lewisian rocks: implications for deep crustal structure

Summary Data on the densities, seismic velocities, magnetization, and permeability of Lewisian rocks are summarized. These properties have been used to estimate the shallow-depth extension of exposed boundaries within the Lewisian, and between the Lewisian and other rocks; and also to predict downward changes in porosity and electrical conductivity. Geophysical surveys of the deep crust can be interpreted in terms of subsurface distributions of Lewisian, or Lewisian-like, rocks. Among the more significant conclusions relevant to the interpretation of surface structure of the Lewisian is that the Kylesku Group rocks of the central belt in Sutherland may be isolated from deeper crustal rocks of similar composition which are characterized by P -wave velocities of 6.4 km s −1 and over. Such rocks may form the mid crust below NW Scotland, shallowing north of the mainland possibly with a sharp discontinuity upwards into more acid basement rocks. West of the mainland direct evidence of the 6.4 km s −1 rocks is missing; they appear to be absent from the upper crust and buried rather deeply, perhaps with a transitional top, in the middle or lower crust. Granulite-facies rocks occurring in the upper crust west of the mainland are likely to be more acid than average Kylesku Group rocks. The layered basic rocks of the central belt provide an analogue of the deepest crust in NW Britain. Permeability/porosity studies allow estimates of electrical conductivity to be made in an attempt to explain the high conductivity of the lower crust in terms of trapped saline fluids. Porosities in the deep crust may be high enough to account for deep crustal seismic reflections in areas where porous layers are sub-horizontal.

The evolution of the Scottish mineral sub-province

Synopsis The Lewisian rocks, or their equivalents to the northwest, were the ultimate source of most of the elements featuring in economic concentrations of industrial or metalliferous minerals in Scotland northwest of a line running through Loch Tay. Processes of derivation and concentration of these elements were: 1) mechanical weathering, particularly of the potassium-rich Laxfordian, to give the Moinian sub-arkoses and semipelites which were later attacked by convecting waters to release Ba, Pb, Zn and Ag for ore deposition in the Middle Dalradian (e.g. the sedimentary ores of Aberfeldy), 2) alkaline chemical weathering to produce smectite and illite bearing palaeosoils on the Lewisian; weathering which also released groundwaters rich in K, Al and Si for aluminosilicate formation by ‘reverse weathering’ in the seas around the Proterozoic-Phanerozoic boundary (e.g. K-feldspar in the Fucoid beds, Ba-feldspar at Aberfeldy and muscovite in the Dalradian), 3) direct partial melting to contribute molybdenum and minor gold to Caledonian granitoids. Mantle contributions to the crust, partly revealed by the Sm/Nd studies of O'Nions et al. (1983) and Halliday (1984) added Cr, Ni, Cu and Au to the Dalradian during rifting. Ba, Pb, Zn, Ag, Au, Pt, As and Sb were added during subduction and accretion of the Iapetus ocean floor and sediments in the Lower Palaeozoic. Certain associations of these elements were remobilized during the later stages of Caledonian magmatism (e.g. Cu and Au). Leaching of these source rocks also took place during early and late Carboniferous rifting when hydrothermal solutions produced the Pb + Zn veins at Leadhills and the Ag + Co + Cu + BaSO 4 mineralization in the Ochils respectively. Chemical weathering, particularly of the early Carboniferous basaltic pile in the reducing and acidic conditions of the Middle Carboniferous led to the generation of clay band ironstones and bauxitic clays.

Geometry of deformation adjacent to the Glencoul thrust at Loch Glencoul, Sutherland

Synopsis The Glencoul thrust at Loch Glencoul is on a ramp zone to which footwall imbricate faults are related. Lewisian rocks in the hanging wall are deformed against this ramp; dykes and the country rock foliation have been rotated into near-parallelism with the thrust plane. The degree of the deformation in the Lewisian gneiss and the dykes increases gradually towards the ramp surface.