Regional setting of the Loch Maree Group

1. Introduction

The Lewisian of northwest NW Scotland formed part of a widespread Palaeoproterozoic system of orogenic belts throughout the North Atlantic region. A recent palaeomagnetic recon- struction Buchan et al., 2000; Fig. 1 of Lauren- tia and Baltica during the late Palaeoproterozoic to early Mesoproterozoic shows a close geometric relationship between the Nagssugtoqidian belt of Greenland and the Lapland – Kola belt of Baltica within the combined continent. In this reconstruc- tion, the Nagssugtoqidian belt of East and West Greenland appears to continue through the Lewisian complex of NW Scotland and has been correlated with it by previous workers e.g. Myers 1987. Comparisons of the tectonic histories of the Nagssugtoqidian and Lapland – Kola belts have been made by Bridgwater et al. 1990 and Kals- beek et al. 1993, and Park 1994, 1995 has noted that the kinematics associated with the peak temperatures in the two belts correspond best when the belts are approximately perpendicular as shown in Fig. 1. There is a general consensus that the Nagssug- toqidian and Lapland – Kola belts, together with the Torngat and New Quebec orogens of Lauren- tia, are broadly coeval and have resulted from the progressive accretion of a number of terranes during the period 1.90 – 1.83 Ga e.g. see refer- ences in Park, 1995. While most of the orogenic belts show evidence of a continental margin set- ting, including subduction and sedimentation, and eventual continent – continent collision, a similar historysetting has not been demonstrated for the Lewisian. This mainly reflects an apparent lack of magmatic arc rocks and Palaeoproterozoic conti- nental margin sediments and the similarity of the Archaean basements on either side of the Palaeoproterozoic belt. The pivotal central position of the Lewisian in Laurentia – Baltica reconstructions has warranted a re-evaluation of those lithotectonic units within the Lewisian that might imply a continent – conti- nent collisional setting, in spite of contrary histor- ical interpretations. This includes the Loch Maree Group LMG that was formerly thought to rep- resent sediments and volcanics accumulated in an intra-continental extensional basin Johnson et al., 1987. Utilising geochemistry and field rela- tions, the LMG is re-interpreted here as a marine sedimentary sequence laid down on oceanic plateau basalts. The Ard gneisses, comprising granitoid orthogneisses interbanded with the LMG along its southwest margin, were originally thought to be part of the Archaean basement. However, a new UPb Palaeoproterozoic mag- matic age is consistent with the existence of a previously unknown magmatic arc within the Lewisian sector of the Nagssugtoqidian – Lap- land – Kola collisional belt.

2. Regional setting of the Loch Maree Group

The Lewisian complex comprises mainly gneisses derived from igneous rocks and minor metasediments of various ages and compositions. The bulk of the gneisses are of Archaean age, formed in the period 2900 – 2700 Ma Hamilton et al., 1979; Whitehouse, 1988, 1989a,b and sub- jected to varying degrees of granulite-facies meta- Fig. 1. Reconstruction of Laurentia and Baltica during the Palaeoproterozoic based on the palaeomagnetic fit reported by Buchan et al. 2000. Note that the Lewisian lies within a continuous Palaeoproterozoic belt extending from the Torngat of Laurentia through the Nagssugtoqidian to the Lapland – Kola belt of Baltica. The arrows show inferred movement directions of various crustal segments relative to Laurentia. Table 1 Simplified Lewisian chronology. Source references: 1, Kinny Friend, 1997; 2, Corfu et al., 1998; 3, Corfu et al., 1994; 4, Heaman Tarney, 1989; 5, Whitehouse et al., 1997; 6, this paper; 7, Holland Lambert, 1995; 8, Moorbath Park 1971. morphism and deformation during three separate events: 1 Badcallian metamorphism that in- cludes pulses at : 2730 and : 2690 Ma Corfu et al., 1994, 1998; Kinny and Friend, 1997 Table 1; 2 the Inverian tectono-thermal event at the Archaean – Proterozoic boundary 2.5 – 2.4 Ga; and 3 the Laxfordian tectonothermal events be- tween 1.9 and 1.5 Ga. The lithotectonic units of the mainland Lewisian resulting from this com- plex history, which includes variable, polycyclic deformation and metamorphism, can be divided into three distinct domains, namely the northern, central and southern regions. Whereas the early Badcallian affected the exist- ing Archaean rocks across the entire Lewisian, the late Badcallian affected mainly the central region, which is relatively unaffected by subsequent Inve- rian and Laxfordian events. The deformation and metamorphism of the Inverian and Laxfordian periods predominantly reworked the northern and southern regions and produced features that are only readily distinguished in the field where the Scourie mafic dykes intruded the central and southern regions at 2.4 and 2.0 Ga. This paper focuses on the southern region as it contains the LMG, the only belt of Palaeopro- torozoic supracrustal rocks in the mainland Lewisian. This region is subdivided by a line extending southeast from Gruinard Bay: Ar- chaean structures and relict granulite-facies as- semblages dominate the northeast part of the region whereas NW – SE-trending Inverian and Laxfordian structures are common in the south- western part Fig. 2. NW – SE-trending Scourie dykes occur throughout this region and help dif- ferentiate Inverian and Laxfordian structures. The LMG occurs as two separate belts, one northeast of Loch Maree and the other at Gair- loch Fig. 2. In addition, a narrow zone of am- phibolite with minor siliceous schist south of Gruinard Bay Fig. 2 may also be of Palaeoproterozoic age Crane, 1978. The LMG outcrop at Gairloch, which is the subject of this paper, consists of a 3-km wide belt of metasedimentary and metavolcanic rocks Figs. 2 and 3 bounded on both sides by acid horn- blende- and biotite-gneisses cut by numerous am- phibolitised basic dykes belonging to the Scourie dyke suite see Park, 1991 for summary. The bounding gneisses are mostly Archaean in age but have been extensively reworked during the Inve- rian and Laxfordian see Table 1.

3. Lithology of the Loch Maree Group