Progressive Regional Metamorphism of Thin Calcareous Bands from the Moinian Rocks of N.W. Scotland

Abstract

Bands and pods of calc-silicate rock a few centimetres thick are widely distributed throughout the Late Precambrian Moinian sequence in N.W. Scotland. They probably originated as late-diagenetic calcareous concretions and were subsequently affected by greenschist to upper middle amphibolite facies (Barrovian) regional metamorphism during the Caledonian (s.l.) orogeny. The calc-silicate rocks described here are from Inverness-shire in the Western Highlands of Scotland. Distinctive prograde mineral assemblages define four narrow zones which run broadly north-south and increase in grade eastwards. Plagioclase composition changes progressively from albite to near anorthite with increasing grade and, together with the presence or absence of zoisite biotite, and hornblende, is the basis of the zonal divisions. Zoisite Ca2(Al0.96, Fe0.04)3Si3O12(OH) with biotite is common in the lower zones and plagioclase ‘jumps’ in composition from calcic andesine to bytownite with the exit of all, or most of, the zoisite. Similarly biotite-bearing assemblages give way to those containing ferro-horn-blende and/or pyroxene. Almandine garnet with approximately 30–40 per cent grossular is present throughout and clinozoisite becomes more common in the higher grade assemblages. Three main episodes of folding (F1–F3) are recognized in the area, with the calc-silicate assemblages having developed during the second deformation (MS2) and immediately following it (MP2). The metamorphic zones (largely of MP2 age) are folded by major third folds and there is evidence of widespread superimposed ‘retrogression’ of probable MP3 age to the east. New analyses of 21 whole rocks by X.R.F., and of 20 minerals by microprobe, are presented. AI2O3 content varies over a small range in the whole rock analyses and aluminium is used as a standard for comparing variations in the other elements. CaO/Al2O3 ratios show little variation but a progressive decrease in Na2O/Al2O3 and K2O/Al2O3 with increasingly calcic plagioclase composition (as assessed by 2θ (131-13¯1) measurements on separated fractions), and hence with metamorphic grade, is noted. Coupled with the fact that the metamorphic zones cross-cut previously folded stratigraphic boundaries, this suggests that both Na2O and K2O have been in part lost from the higher grade calc-silicate assemblages during progressive metamorphism. Reactions are proposed for the observed mineral changes, and it is concluded that these are most likely achieved by cation exchange through the medium of the pore fluid and result in partial elimination from the local system of certain elements. No evidence is found that the reaction often quoted as that responsible for the exit of zoisite in semi-calcareous rocks, namely 4zo+qtz↑5an+gross+vap, has taken place.