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

Bands and pods of calc-silicate rock a few centimetres thickare widely distributed throughout the Late Precambrian Moiniansequence in N.W. Scotland. They probably originated as late-diageneticcalcareous concretions and were subsequently affected by greenschistto upper middle amphibolite facies (Barrovian) regional metamorphismduring the Caledonian (s.l.) orogeny. The calc-silicate rocks described here are from Inverness-shirein the Western Highlands of Scotland. Distinctive prograde mineralassemblages define four narrow zones which run broadly north-southand increase in grade eastwards. Plagioclase composition changesprogressively from albite to near anorthite with increasinggrade and, together with the presence or absence of zoisitebiotite, and hornblende, is the basis of the zonal divisions.Zoisite Ca₂(Al_0.96, Fe_0.04)₃Si₃O₁₂(OH) with biotite is commonin the lower zones and plagioclase ‘jumps’ in compositionfrom calcic andesine to bytownite with the exit of all, or mostof, the zoisite. Similarly biotite-bearing assemblages giveway to those containing ferro-horn-blende and/or pyroxene. Almandinegarnet with approximately 30–40 per cent grossular ispresent throughout and clinozoisite becomes more common in thehigher grade assemblages. Three main episodes of folding (F1–F3) are recognizedin the area, with the calc-silicate assemblages having developedduring the second deformation (MS2) and immediately followingit (MP2). The metamorphic zones (largely of MP2 age) are foldedby 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 mineralsby microprobe, are presented. AI₂O₃ content varies over a smallrange in the whole rock analyses and aluminium is used as astandard for comparing variations in the other elements. CaO/Al₂O₃ratios show little variation but a progressive decrease in Na₂O/Al₂O₃and K₂O/Al₂O₃ with increasingly calcic plagioclase composition(as assessed by measurements on separated fractions), and hence with metamorphic grade, isnoted. Coupled with the fact that the metamorphic zones cross-cutpreviously folded stratigraphic boundaries, this suggests thatboth Na₂O and K₂O have been in part lost from the higher gradecalc-silicate assemblages during progressive metamorphism. Reactions are proposed for the observed mineral changes, andit is concluded that these are most likely achieved by cationexchange through the medium of the pore fluid and result inpartial elimination from the local system of certain elements.No evidence is found that the reaction often quoted as thatresponsible for the exit of zoisite in semi-calcareous rocks,namely 4zo+qtz5an+gross+vap, has taken place.