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Intrusive metabasite belts within the Moine assemblage, west of Loch Ness, Scotland: evidence for metabasite modification by country rock interactions

British Geological Survey, West Mains Road. Edinburgh EH9 3LA, UK
^* Department of Environmental Sciences. The University. Lancaster LA1 4YQ, UK

Metabasites representing an extensive differentiated sill-complex (with minor dykes) now form two c. 10 km-wide NE-SW ‘belts’ in western Inverness-shire: a Western Belt, overlapping the ‘Quoich Line’, and an Eastern Belt, beside the Great Glen Fault. Regional evidence and, where interpretable. Rb-Sr isotope data, imply Grenvillian emplacement. More than one intrusive episode may be represented by contrasting adjacent discordant/unfoliated versus concordant/foliated bodies. although some such contrasts reflect later tectonism. Despite variations in form, size, internal homogeneity, paragenesis, grain-size and fabric, 120 newly analysed metabasites have much more uniformly tholeiitic chemistry than metabasites in the Moine of Ross-shire or Sutherland. Discriminant analysis nevertheless reveals that the Eastern Belt is compositionally invariable from NE to SW. whereas the Western differs between three sub-areas studied in detail, and shows more numerous evolved compositions. A single andesitic rock (c. 57% SiO₂) is the most evolved metabasite so far reported from the Moine. Metabasite–country rock interactions during igneous emplacement and/or regional metamorphism caused major chemical transfers. For example, host psammites and granite-gneisses gained Ca. causing hornblende to crystallize; metabasites in limestones gained Ca, Mg, Sr, and lost Fe, Ti, Rb, Rb, Y and Zr, totally suppressing garnet; metabasites in pelites lost Na, enhancing garnet relative to plagioclase; metabasites within psammites gained K and Rb, enhancing biotite; and metabasites within granite-gneisses gained Na, K, Mn, Rb and Sr, suppressing epidote. Overall chemical differences between metabasites with or without biotite, epidote and garnet may reflect the same interactions, rather than original magmatic variations. Even the ‘immobile’ element chemistry of high-grade metabasites may be an unreliable tracer of magmatic chemistry, unless country rock interactions are allowed for.

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