Structural Channelling of Metamorphic Fluids on Islay, Scotland: Implications for Paleoclimatic Reconstruction

Abstract

Analysis of the δ18O and δ13C values of carbonate rocks from Islay, Scotland reveals structural channelling of metamorphic fluids through the axial region of a major en echelon anticlinal fold system. Metamorphic fluid flow produced axial planar veins with higher vein density in the axial region of the fold. Fluid:rock ratios were more than 30:1 within this axial region, at least four times greater than the regional mean ratio of 7·6 ± 1·5:1 for carbonate rocks on Islay. This supports the interpretation that metamorphic fluids were channelled through the axial region of the Islay Anticline. Fluid:rock ratios were calculated using a model for coupled δ18O and δ13C exchange with a metamorphic fluid. The metamorphic fluid was calculated to have δ18O and δ13C values of 15·3‰ and –6·1‰, respectively and XCO2 of 0·2. This is in isotopic and chemical equilibrium with chlorite- and graphite-bearing metamudstones that are structurally below the folded metacarbonate rocks on Islay. Devolatilization of these metamudstones is therefore a likely source mechanism for this metamorphic fluid. Removal of the effects of metamorphic fluid flow on δ13C values recorded by metacarbonate rocks on Islay allows us to re-evaluate evidence used to reconstruct Neoproterozoic climate. This evidence includes a large negative δ13C excursion reported from the Lossit Limestone Formation. This unit underlies the Port Askaig Formation, which is dominated by diamictites that have been interpreted as glacial tillites. This ‘Islay anomaly’ has been correlated with other such anomalies worldwide and together with overlying tillites has been cited as evidence of major (worldwide) glaciation events. In this study, we show that the magnitude of this negative δ13C anomaly can partly be explained by exchange with metamorphic fluids. However, we also show that extremely negative δ13C values in the Bonahaven Dolomite Formation, which overlies the Port Askaig Formation and has been interpreted as a ‘cap carbonate’, cannot be attributed to metamorphic fluid flow.