Chemistry of the Dalradian (Vendian-Cambrian) metalimestones, British Isles

Abstract

Based on > 600 (∼ 300 new) analyses for up to 39 elements, chemical variations in all major Dalradian carbonate formations have been investigated. Based on actual SiO 2 and MgO distributions, samples are divided into pure carbonate rocks (< 25% SiO 2; 70% of analyses), impure carbonates (25–40% SiO 2; 22% of analyses) and skarns (Ca/Mg/Fe-silicate rocks; > 40% SiO 2; 8% of analyses). Pure carbonates are divided into limestones (< 7.5% MgO; 71% of analyses), dolostones (> 7.5% MgO; 27% of analyses) and ophicalcites (> 25% MgO; 2% of analyses). Impure carbonates and skarns have often been metasomatically altered from sediment compositions; only pure carbonates were assessed statistically. Element distributions are non-Gaussian (e.g., bimodal for Si, Ca, Sr and Mg; positively skewed near-Poisson for elements concentrated in accessory minerals). Outlying values are found for many (especially metallic) elements, probably reflecting local mineralisation. Graphical and non-parametric statistical techniques were therefore used to delineate the following stratigraphical and lateral variations: 1. (1) Except for thin dolostone beds in the Crinan and Islay Subgroups, dolostone/limestone ratios decrease up the succession, from ∼ 2:1 to near zero (Lower to Upper Dalradian). 2. (2) Dolostones have stochastically higher Si, Fe, V, Cr, Zn, Y, Zr, Ba and Pb than limestones, suggesting that dolomitisation has preferentially affected impure carbonate sediments. Dolostones have lower Sr because dolomite tolerates less Sr than calcite. 3. (3) Many supposed laterally equivalent carbonate formations are chemically heterogeneous — especially in the Blair Atholl Subgroup, where facies changes readily occurred in a shallow-water environment. 4. (4) In the Loch Tay Limestone, confidently traceable for 225 km along strike, Ca, Zn, Rb, Pb and Th tend to decrease, and Si and Mg to increase from SW to NE. Metamorphic grade increases slightly in the same direction, but since non-volatile element contents all increase with prograde decarbonation, these changes are likely to be pre-metamorphic. 5. (5) If ranks are assigned to major carbonate horizons, from 1 = oldest (e.g., Ballachulish Limestone), to 8 = youngest (e.g., Largie Limestone), V, Cr, ?Co, Ni, Zn and Pb contents of 90 dolostones are found to increase up the Dalradian sequence. 6. (6) By contrast, maximum contents of 16 elements (e.g., V, Rb, Al, Ti) in 350 limestones increase inwards from Lower and Upper towards less pure Middle Dalradian limestones. Observations (4) and (6) are consistent with the greater instability and incidence of volcanism in the Middle Dalradian basin. Observation (5) appears to reflect an association of dolomitisation with increasing, syngenetic Pb/Zn mineralisation. Many stratigraphical correlations suggested by field workers (e.g., Lismore/Blair Atholl, Culdaff/Tayvallich Limestones) are upheld by chemical comparison, whereas others may require revision (e.g., Sandend “group” may correlate with the Ballachulish, not Lismore Limestone). The Kinlochlaggan Limestone is most probably Lower Dalradian.