Laxfordian Episode Research Articles

Palaeomagnetic properties of a Precambrian metamorphic terrane: The Lewisian complex of the Outer Hebrides, NW Scotland

The palaeomagnetism of a large area of Precambrian metamorphic basement comprising the Outer Hebrides archipelago in NW Scotland is described. A stable magnetite-held imprint characterises much of the complex. Dispersed easterly steep (‘A1’) and NE intermediate (‘C’) components are present in the northern sector but are subordinate to NW positive to NW negative (‘A2’ → ‘A5’, ‘B’) components. NW shallow (‘A4’) components predominate in the south of the outcrop area. ‘A1’, ‘A2’ and ‘C’ components are confined largely to areas which have escaped tectonism since Scourian (> 2650 Ma) times and are probably a relict record. The ‘A4’ components are most prominent in areas profoundly affected by the Laxfordian episode and record the peak of a later magnetising event. By correlation with APW from the Laurentian and Fennoscandian Shields, relict components are interpreted as a possible acquisition at 1820-1750 Ma close to the peak of Laxfordian tectonism. The younger components are linked to retrogressive amphibolite facies metamorphism and rapid crustal uplift at 1700-1630 Ma. Anisotropy of susceptibility is variable in magnitude and predominantly foliated. It appears to be controlled mainly by Laxfordian strain although the effect of the earlier Scourian episode is also recorded. A younger lamprophyre dyke suite yields a palaeomagnetic pole from nine sites (349° E, 47° S) dated as mid-Permian by reference to the Eurasian APW path. The spatial distribution of magnetisations in the Outer Hebrides basement mirrors the distribution observed in the Mainland outcrop comprising the NW foreland to the Caledonian orogen but is displaced laterally. The two distributions can be matched by adjustment for ca. 95 km of sinistral strike slip on the site of the Minches Basin between the two outcrops. This fault zone is the NW member of a suite of NE-SW trending transcurrent faults active in late Caledonian times and possesses the largest strike-slip offset.

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.

Provenance of the Old Red Sandstone: Pb isotope evidence from Arran, western Scotland

ABSTRACTOld Red Sandstone sediments from Arran in western Scotland yield an array of Pb isotope compositions whose slope corresponds to an age of 1830 ± 400 Ma. These compositions are not consistent with a significant contribution of material from the Southern Highlands, Midland Valley, or Southern Uplands, but are similar to the Torridonian sandstones of NW Scotland. Bearing in mind the probable Laxfordian provenance of the Torridonian assemblage, this points to a source for the Old Red Sandstone in a crustal block of early Proterozoic age corresponding to the age of the Laxfordian episode. Since such a source is not presently in a position to yield the Old Red Sandstone, strike-slip movement along the Highland Boundary fault is a strong possibility.

Rb-Sr isotopic studies near the major Precambrian junction, between Scourie and Loch Laxford, northwest Scotland.

The nature and significance of the Precambrian junction between Scourie and Loch Laxford, that separates pyroxene granulites and retrogressed equivalents of the Kylesku group to the south (Khoury 1968; Bowes et al. , 1971) from quartzofeldspathic gneiss of the Rhiconich group to the north (Dash 1969; Chowdhary and Bowes 1972; Gillen 1975), has been the subject of widely differing interpretations. However in all interpretations, chronological considerations relating to the c 2.5 km wide zone of highly deformed rocks immediately south of the Rubha Ruadh granite sheet (Gillen 1975) and the coincident ‘Ben Stack Line’ of Lambert and Holland (1972) play an important role. Development of the junction associated with gneiss formation during the Laxfordian episode ( c 1950–1700 m.y.) was suggested by Sutton and Watson (1951, 1962). Bowes (1969), on the other hand interpreted the evidence as indicative of major tectonic movements which deformed the gneisses of the Rhiconich group during the latter part of the Laxfordian episode. These differences are, in part, related to the chronological significance of basic minor intrusions in the zone. Formation of the junction during the Inverian episode ( c 2200 m.y.), before the emplacement of the Scourie dyke suite, was suggested by Lambert and Holland (1972), Bott et al. (1972) and Park (1973). Beach et al. (1973) also place the intense deformation shown in this zone before the emplacement of the Scourie dyke suite and after the granulite facies metamorphism of the Scourian episode that has been dated at 2700 ± 20 m.y. by Pidgeon and . . .

Rb-Sr whole rock isotopic studies of Lewisian metasediments and gneisses in the Loch Maree region, Ross-shire

Fifty-three whole rock Rb–Sr isotopic analyses are reported on two metasedimentary schist and four gneiss units in the Lewisian of the Loch Maree region. Isochron dates from the Gairloch metasediments–Ard gneisses (1980 ± 50 m.y.) and the Loch Maree gneisses (1965 ± 100 m.y.) date the main phase of metamorphism of the Laxfordian episode at 1975 ± 75 m.y. A 1745 ± 160m.y. whole rock isochron date on the Carnmore kyanite–biotite gneisses can be related to late tectonic pegmatite intrusions. Whole rock data points from the quartzofeldspathic gneisses from the Carnmore district do not define an isochron. A 1500 ± 95 m.y. isochron date for metasediments of the Loch Maree Group is interpreted in terms of final closure, during epeirogenic uplift, after a 200–300 m.y. period of isotopic exchange below an impervious cap of tectonically overthrusted hornblende schist and quartzofeldspathic gneiss. Initial 87 Sr/ 86 Sr considerations indicate both a crustal history for the gneisses of the Loch Maree and Carnmore districts going back to 2.7–2.8 b.y. and deposition of the Gairloch metasediments and Loch Maree Group after 2.2 b.y. ago, presumably unconformably on the older metamorphic complex. The Laxfordian orogenic cycle is defined, in the classical stratigraphic sense, to consist of a depositional episode 2.2–2.0 b.y. ago, an orogenic episode 2.0–1.7 b.y. ago and an epeirogenic episode 1.7–1.5 b.y. ago. There are marked similarities in the Laxfordian and Svecokarelian orogenic cycles, and the Lewisian chronology for the Loch Maree region shows striking parallelism with the Precambrian chronology of NW. Europe.

Basic minor intrusions north of Loch Laxford, Sutherland and their significance in Lewisian chronology

Synopsis Basic minor intrusions show cross-cutting relations to the regionally-developed Lewisian quartzo-feldspathic gneisses and amphibolites north of Loch Laxford. Their emplacement during the Laxfordian episode is indicated by the isotopic date of c. 1900 m.y. for the gneisses which they cut and the later emplacement of pegmatite and the development of crush-rock. Correspondence of chemical composition and mineralogy with other Lewisian basic igneous masses showing longer tectonic and metamorphic histories, together with difficulty of establishing whether concordance is the result of igneous or tectonic activity, makes such rock masses unsuitable for use as stratigraphic time-markers.

Geochronological investigation of the quartzofeldspathic rocks of the Lewisian of Rona, Inner Hebrides

Rona Island consists of Lewisian quartzofeldspathic gneisses and amphibolites that show the effects of polyphase deformation. The age of the gneisses, from a six point Rb-Sr whole rock isochron, with λ 87 Rb 1.39 × 10 −11 yr −1, is 2790 ± 210 m.y. This is not significantly different from the age of 2710 ± 20 m.y. determined from zircon fractions from a single block of the gneiss. These results are interpreted as reflecting amphibolite facies metamorphism and gneiss formation, associated with the first recognized fold phase, during the Scourian orogenic episode A Rb-Sr age for unaltered muscovite books from a pegmatite emplaced in the hinge zone of a fold of the third deformational phase is 1740 ± 10m.y. and a Rb-Sr whole rock isochron for a later granite sheet, emplaced between the fifth and sixth fold phases is 1680 ± 170 m.y. These igneous events, and the fold phases between them, belong to the later part of the Laxfordian orogenic episode. No isotopic disturbance of the Rb-Sr whole rock systems of the analysed gneisses has taken place during the Laxfordian episode and no isotopic events between the c 2700 m.y. and c 1700 m.y. episodes have been found in the quartzo, feldspathic rocks. K-Ar ages on the muscovite samples suggest minor opening of minerals to argon loss sometime after 1740 m.y.

Heterogeneous deformation in the development of the Laxfordian complex of South Uist, Outer Hebrides

Four phases of heterogeneous deformation ( f 1 to f 4 ) are distinguished within the Laxfordian episode of regeneration of the Lewisian gneiss complex of South Uist. The variation in amount of deformation has been examined with respect to three of the Laxfordian fold phases, especially f 3 and f 4 , and apparent relationships between the strain heterogeneities and both the original nature of the rock and the syntectonic metamorphic conditions have been observed. The heterogeneities of the f 3 deformation appear to be related to the forms of the large scale f 3 structures that are characterized by broad open antiforms and tight synforms. These owe their form, in part, to the strains set up during the buckling of an interface between competent rocks below and less competent rocks above. Heterogeneities in the f 4 deformation are closely related to variations in the intensity of the syntectonic metamorphism; intense f 4 deformation is closely associated with migmatisation and granitisation. Possibly intense f 4 strain may result from localised variations in ductility caused by a local thermal high.

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.