Multi-episodic modification of high-grade terrane near Scourie and its significance in elucidating the history of the Lewisian Complex

Abstract

Synopsis The first detailed geological map made of the area south and east of Scourie since that of the pioneer mapping in the NW Highlands of Scotland by the Geological Survey in the latter part of the 19th century is used as a basis for establishing structural, metamorphic and igneous successions in orogenic belts whose ages have been determined from U–Pb isotopic data. The main metamorphic modification of the widespread ≥ 2710 Ma pyroxene granulites formed during an early phase in the development of the Scourian orogenic belt took place at 2490–2480 Ma in an early stage of the development of the Inverian orogenic belt. The predominant expressions of the latter orogen are generally NW-trending single and coalescing fold belts of hornblende – feldspar gneiss. These, and the intervening areas of granulites where there is patchy replacement of pyroxene by amphibole, are cut by a suite of c . 2450 Ma dolerite dykes emplaced into crust that was still relatively hot. There is no recorded expression of the c . 1.9–1.85 Ga Laxfordian orogenic belt in the domain around Scourie: it was not juxtaposed with the gneiss – amphibolite – granite domain around Loch Laxford until a collision of crustal segments c . 1750 Ma ago. The elongate, generally west-trending, mainly complex synformal fold – schist belts (‘pre-Torridonian lines of movement’) that cut across and modify the structural, metamorphic and igneous features of both the Scourian and Inverian orogenic belts are linked to bulk inhomogeneous shortening during the tectonothermal activity associated with this collision and are interpreted as tectonic ‘wrinkles’ on an overriding plate. These events were followed by sinistral transcurrent movements associated with the development of ductile shears and by limited dolerite dyke emplacement. Other weakly expressed metamorphic and structural features are linked to the 1690–1670 Ma mineral ages of a later episode. An interpretation of the Palaeoproterozoic histories of the domains around Scourie and Loch Laxford as being distinct depends upon the determination of consistent structural, metamorphic and igneous successions. These are based on evidence of the refolding, cross-cutting and overprinting relationships within the two domains, and on the integration of isotopic data into these successions. The availability of corresponding information in other parts of the Lewisian Complex is essential in order to establish the presence of other terranes in the Complex and to determine their areal extent.